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  • 1
    Digitale Medien
    Digitale Medien
    Springer
    Stochastic environmental research and risk assessment 10 (1996), S. 167-186 
    ISSN: 1436-3259
    Schlagwort(e): Reservoir stochastic theory ; reliability ; mean ; variance ; indicator function ; storage bounds ; nonlinear programming ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Energietechnik , Geographie , Geologie und Paläontologie
    Notizen: Abstract A new formulation is presented for the analysis of reservoir systems synthesizing concepts from the traditional stochastic theory of reservoir storage, moments analysis and reliability programming. The analysis is based on the development of the first and second moments for the stochastic storage state variable. These expressions include terms for the failure probabilities (probabilities of spill or deficit) and consider the storage bounds explicitly. Using this analysis, expected values of the storage state, variances of storage, optimal release policies and failure probabilities — useful information in the context of reservoir operations and design, can be obtained from a nonlinear programming solution. The solutions developed from studies of single reservoir operations on both an annual and monthly basis, compare favorably with those obtained from simulation. The presentation herein is directed to both traditional reservoir storage theorists who are interested in the design of a reservoir and modern reservoir analysts who are interested in the long term operation of reservoirs.
    Materialart: Digitale Medien
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  • 2
    Digitale Medien
    Digitale Medien
    Springer
    Stochastic environmental research and risk assessment 5 (1991), S. 125-134 
    ISSN: 1436-3259
    Schlagwort(e): Empirical Orthogonal Function analysis ; random fields ; simulation ; non-homogeneous fields
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Energietechnik , Geographie , Geologie und Paläontologie
    Notizen: Abstract In several fields of Geophysics, such as Hydrology, Meteorology or Oceanography, it is often useful to generate random fields, displaying the same variabilitity as the observed variables. Usually, these synthetic data are used as forcing fields into numerical models, to test the sensitivity of their outputs to the variability of the inputs. Examples can be found in subsurface or surface Hydrology and in Meteorology with General Circulation Models (GCM). Different techniques have already been proposed, often based on the spectral representation of the random process, with, usually, assumptions of stationarity. This paper suggests that Empirical Orthogonal Function (EOF) analysis, which leads to the decomposition of the covariance kernel on the set of its eigen-functions, is a possible answer to this problem. The convergence and accuracy of the method are shown to depend mainly on the number of EOFs retained in the expansion of the covariance kemel. This result is confirmed by a comparison with the turning band method and a matrix technique. Furthermore, a synthetic example of non-homogencous fields shows the interest of EOF analysis in the direct simulation of such fields.
    Materialart: Digitale Medien
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  • 3
    Digitale Medien
    Digitale Medien
    Springer
    Stochastic environmental research and risk assessment 12 (1998), S. 33-52 
    ISSN: 1436-3259
    Schlagwort(e): Keywords: Streamflow ; simulation ; nonparametric
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Energietechnik , Geographie , Geologie und Paläontologie
    Notizen: Abstract A new approach for streamflow simulation using nonparametric methods was described in a recent publication (Sharma et al. 1997). Use of nonparametric methods has the advantage that they avoid the issue of selecting a probability distribution and can represent nonlinear features, such as asymmetry and bimodality that hitherto were difficult to represent, in the probability structure of hydrologic variables such as streamflow and precipitation. The nonparametric method used was kernel density estimation, which requires the selection of bandwidth (smoothing) parameters. This study documents some of the tests that were conduced to evaluate the performance of bandwidth estimation methods for kernel density estimation. Issues related to selection of optimal smoothing parameters for kernel density estimation with small samples (200 or fewer data points) are examined. Both reference to a Gaussian density and data based specifications are applied to estimate bandwidths for samples from bivariate normal mixture densities. The three data based methods studied are Maximum Likelihood Cross Validation (MLCV), Least Square Cross Validation (LSCV) and Biased Cross Validation (BCV2). Modifications for estimating optimal local bandwidths using MLCV and LSCV are also examined. We found that the use of local bandwidths does not necessarily improve the density estimate with small samples. Of the global bandwidth estimators compared, we found that MLCV and LSCV are better because they show lower variability and higher accuracy while Biased Cross Validation suffers from multiple optimal bandwidths for samples from strongly bimodal densities. These results, of particular interest in stochastic hydrology where small samples are common, may have importance in other applications of nonparametric density estimation methods with similar sample sizes and distribution shapes.
    Materialart: Digitale Medien
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  • 4
    Digitale Medien
    Digitale Medien
    Springer
    Computing 52 (1994), S. 89-96 
    ISSN: 1436-5057
    Schlagwort(e): 65C10 ; 68C25 ; Random number generation ; log-concave distributions ; rejection method ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Beschreibung / Inhaltsverzeichnis: Zusammenfassung Wir erklären einen Algorithmus, der Stichproben von beliebigen log-konkaven Verteilungen (z.B. Binomial- und Hypergeometrische Verteilung) erzeugt. Er basiert auf Verwerfung von einer diskreten dominierenden Verteilung, die aus Teilen der geometrischen Verteilung zusammengesetzt wird. Der Algorithmus is gleichmäßig schnell für alle diskreten log-konkaven Verteilungen und nicht viel langsamer als Algorithmen, die nur für eine bestimmte Verteilung verwendet werden können.
    Notizen: Abstract We give an algorithm that can be used to sample from any discrete log-concave distribution (e.g. the binomial and hypergeometric distributions). It is based on rejection from a discrete dominating distribution that consists of parts of the geometric distribution. The algorithm is uniformly fast for all discrete log-concave distributions and not much slower than algorithms designed for a single distribution.
    Materialart: Digitale Medien
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  • 5
    Digitale Medien
    Digitale Medien
    Springer
    OR spectrum 19 (1997), S. 23-29 
    ISSN: 1436-6304
    Schlagwort(e): Job shop scheduling ; dispatching rules ; coordination ; look ahead information ; simulation ; Werkstattsteuerung ; Steuerungsregeln ; Koordination ; Vorausschauende Informationen ; Simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Beschreibung / Inhaltsverzeichnis: Zusammenfassung In diesem Beitrag wird ein neuartiger Ansatz zur Koordination dezentraler Werkstattsteuerungsregeln vorgestellt und mit Hilfe einer Simulationsstudie analysiert. Die Koordination basiert auf vorausschauenden Informationen und enthält einen Auftragsnachfrage-/-angebotsmechanismus. Die Simulations-experimente zeigen, daß durch den Einsatz des Koordinationsmechanismus die Leistung herkömmlicher Steuerungsregeln signifikant verbessert wird.
    Notizen: Abstract In this paper a new coordination approach for decentralized job shop scheduling rules is presented and analyzed in a simulation study. The coordination is based on look ahead information and contains a mechanism for demanding and supplying jobs. The simulation experiments show that the performance of conventional scheduling rules is significantly improved using the coordination mechanism.
    Materialart: Digitale Medien
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  • 6
    Digitale Medien
    Digitale Medien
    Springer
    OR spectrum 19 (1997), S. 23-29 
    ISSN: 1436-6304
    Schlagwort(e): Key words: Job shop scheduling ; dispatching rules ; coordination ; look ahead information ; simulation ; Schlüsselwörter: Werkstattsteuerung ; Steuerungsregeln ; Koordination ; Vorausschauende Informationen ; Simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Beschreibung / Inhaltsverzeichnis: Zusammenfassung. In diesem Beitrag wird ein neu-artiger Ansatz zur Koordination dezentraler Werkstattsteuerungsregeln vorgestellt und mit Hilfe einer Simulationsstudie analysiert. Die Koordination basiert auf vorausschauenden Informationen und enthält einen Auftragsnachfrage-/-angebotsmechanismus. Die Simulations- experimente zeigen, daß durch den Einsatz des Koordinationsmechanismus die Leistung herkömmlicher Steuerungsregeln signifikant verbessert wird.
    Notizen: Abstract. In this paper a new coordination approach for decentralized job shop scheduling rules is presented and analyzed in a simulation study. The coordination is based on look ahead information and contains a mechanism for demanding and supplying jobs. The simulation experiments show that the performance of conventional scheduling rules is significantly improved using the coordination mechanism.
    Materialart: Digitale Medien
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  • 7
    Digitale Medien
    Digitale Medien
    Springer
    Stochastic environmental research and risk assessment 4 (1990), S. 255-276 
    ISSN: 1436-3259
    Schlagwort(e): Stochastic hydrologic process ; daily discharges ; correlated generation ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Energietechnik , Geographie , Geologie und Paläontologie
    Notizen: Abstract A geomorphological study at the confluence of the Danube and the Isar in Bavaria required long series of daily discharges in both rivers. A model that generates simultaneous correlated streamflows in both rivers was developed and tested. The model is a modified shot noise model, first developed by Treiber (1975) for a single river, that was adapted to two rivers. It generates correlated pulses of events that produce flow for each river, and these pulses are then convoluted with a river specific systems function. The model, after being calibrated for the two rivers on the basis of 85 years of records, yields artificial series of discharges, in which the statistical properties of the historical records are reproduced. The performance of the model was tested with 20 generated series each 100 years long.
    Materialart: Digitale Medien
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  • 8
    Digitale Medien
    Digitale Medien
    Springer
    Rheologica acta 33 (1994), S. 485-505 
    ISSN: 1435-1528
    Schlagwort(e): Anchored bead-rod chain ; Brownian dynamics ; simulation ; shear flow ; surface bound polymer
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Abstract The behavior of a terminally anchored freely-jointed bead-rod chain, subjected to solvent shear flow, was investigated via Brownian dynamics simulations. Previous calculations have been improved by computing the segment density and fluid velocity profiles self-consistently. The segment density distributions, components of the radius of gyration, and chain attachment shear and normal stresses were found to be sensitive to low values of shear rate. Additionally, it was found that the thickness of a model polymer layer was a strong function of the shear rate, and that the functional dependence on shear rate changed dramatically as the chain length increased. For the longest chains studied, the thickness of the model polymer layer first increased as the shear rate increased, passed through a maximum, and then decreased at high shear rates, in accordance with experimental results in theta solvents. These results suggest that a dilute or semi-dilute layer model may explain hydrodynamic behavior previously thought to be due to the entanglements that occur in dense surface bound polymer layers.
    Materialart: Digitale Medien
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  • 9
    Digitale Medien
    Digitale Medien
    Springer
    Optical and quantum electronics 31 (1999), S. 1179-1188 
    ISSN: 1572-817X
    Schlagwort(e): CW operation ; nitride lasers ; simulation ; VCSELs
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Elektrotechnik, Elektronik, Nachrichtentechnik , Physik
    Notizen: Abstract A room-temperature (RT) continuous-wave (CW) operation of possible nitride vertical-cavity surface-emitting lasers (VCSELs) is considered in the present paper using a simple VCSEL simulation to give some essential guidelines for their proper designing. It is revealed that a substrate material has a critical influence on a possibility of reaching RT CW thresholds which practically excludes currently used sapphire substrates of relatively low thermal conductivity from this application. SQW nitride VCSELs are found to be very sensitive to an increase in temperature, which is followed by their inappropriate CW-operation characteristics. A moderate increase in a number of quantum wells in VCSEL active regions significantly improves their CW achievements, i.e. decreases RT CW thresholds as well as considerably widens their CW threshold ranges.
    Materialart: Digitale Medien
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  • 10
    Digitale Medien
    Digitale Medien
    Springer
    Optical and quantum electronics 31 (1999), S. 1009-1030 
    ISSN: 1572-817X
    Schlagwort(e): all-solid-source MBE ; high-power ; red ; simulation ; transverse structure
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Elektrotechnik, Elektronik, Nachrichtentechnik , Physik
    Notizen: Abstract The laser diode structures reported up to now in literature for the red wavelength range are still far from optimal – mostly because many of the desired characteristics are contradictory coupled. Some of the contradictory coupled laser diode characteristics are investigated and a novel transverse layer structure is proposed. Both optical simulation and a fully self-consistent model are used in a design optimization methodology and simple evaluation and optimization criteria are derived. A number of the analyzed high-power edge-emitting GazIn1−zP/(AlxGa1−x)yIn1−yP/GaAs quantum well laser structures were prepared using all-solid-source molecular beam epitaxy for layer growth and remarkable performances were obtained (continuous wave output powers of 3 W at 670 nm, 2 W at 650 nm, and 1 W at 630 nm; threshold current densities of 350–450 A/cm2 for 670 nm, 500–540 A/cm2 for 650 nm, and less than 700 A/cm2 for 630 nm). The good agreement between measurements and simulations for the prepared structures indicate that significant performance improvements – predicted by the simulations – are still possible. The presented novel structure and design optimisation procedure can also be applied to laser diode structure optimisation in other emission ranges – like, for example, in the case of 800 nm-range edge emitting AlxGa1−xAsyP1−y/GazIn1−zAswP1−w/GaAs laser diodes.
    Materialart: Digitale Medien
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  • 11
    Digitale Medien
    Digitale Medien
    Springer
    Formal methods in system design 6 (1995), S. 11-44 
    ISSN: 1572-8102
    Schlagwort(e): abstract interpretation ; simulation ; property preservation ; model-checking
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract We study property preserving transformations for reactive systems. The main idea is the use of simulations parameterized by Galois connections (α, γ), relating the lattices of properties of two systems. We propose and study a notion of preservation of properties expressed by formulas of a logic, by a function α mapping sets of states of a systemS into sets of states of a systemS'. We give results on the preservation of properties expressed in sublanguages of the branching time μ-calculus when two systemsS andS' are related via (α, γ)-simulations. They can be used to verify a property for a system by verifying the same property on a simpler system which is an abstraction of it. We show also under which conditions abstraction of concurrent systems can be computed from the abstraction of their components. This allows a compositional application of the proposed verification method. This is a revised version of the papers [2] and [16]; the results are fully developed in [28].
    Materialart: Digitale Medien
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  • 12
    Digitale Medien
    Digitale Medien
    Springer
    Adsorption 1 (1995), S. 213-231 
    ISSN: 1572-8757
    Schlagwort(e): adsorptive separation ; thermal parametric pumping ; modeling ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Physik , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Abstract A detailed model for the recuperative parametric pumping is presented. The model includes intraparticle mass transfer resistance, axial diffusion and non-linear equilibrium represented by Langmuir equation. The sensitivity studies shows that process performance strongly increases when cycle time increases and φ B /φ T ratio and particle size decreases. It also shows that bottom and top dead volumes do not influence much the process performance. Evolution of the histories of concentrations and temperatures, the bed performance from cycle to cycle and the bed dynamics at the cyclic steady state have been discussed. The model revealed itself as useful to simulate the behavior of the recuperative parametric pumping process and was applied to predict optimal experimental results for the system phenol-water/Duolite ES-861 (Part II).
    Materialart: Digitale Medien
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  • 13
    Digitale Medien
    Digitale Medien
    Springer
    Adsorption 1 (1995), S. 203-211 
    ISSN: 1572-8757
    Schlagwort(e): simulation ; pressure swing adsorption ; fixed bed system ; bulk separation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Physik , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Abstract Effects of variable feed composition on the performance of a pressure swing adsorption process are analyzed by simulation. Two scenarios are considered. The first, “increasing impurity,” case considers low impurity concentration in the feed followed by high impurity concentration in the feed. The second, “decreasing impurity,” case considers high impurity concentration in the feed followed by low impurity concentration in the feed. These results are compared against a case which has an impurity concentration in the feed at an average of the high and the low impurity concentrations. Simulations show that the increasing impurity scenario is expected to perform better, and the decreasing impurity scenario is expected to perform worse than the average feed concentration case.
    Materialart: Digitale Medien
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  • 14
    Digitale Medien
    Digitale Medien
    Springer
    Adsorption 1 (1995), S. 321-333 
    ISSN: 1572-8757
    Schlagwort(e): processes and applications—bulk separation ; simulation ; pressure swing adsorption
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Physik , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Abstract Gas separation of a binary gas mixture by various pressure swing adsorption (PSA) cycles was studied by a numerical simulation in order to provide a guidance in selecting PSA cycles. PSA cycles considered in this study are 3, 4-step cycles for production of only one component and a cycle with pressure equalization for production of a light component. 4 and 5-step cycles for simultaneous production of both components of a binary gas mixture are also considered. Separation of a CH4/CO2 gas mixture with zeolite 5A was chosen as a case study. Performances of cycles were examined and compared in view of purity, recovery and productivity. Their relative advantages were discussed. Inclusion of a purging step to a 3-step cycle for production of only one component improves a cycle performance. Further performance improvement of a cycle for production of a light component can be achieved by employing pressure equalization. Sircar's 4-step cycle with a recycle of effluent shows the best performance in view of purity and recovery among cycles for simultaneous production of both components.
    Materialart: Digitale Medien
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  • 15
    Digitale Medien
    Digitale Medien
    Springer
    Adsorption 2 (1996), S. 23-32 
    ISSN: 1572-8757
    Schlagwort(e): GCMC ; DFT ; adsorption ; MCM-41 ; buckytube ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Physik , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Abstract MCM-41 and buckytubes are novel porous materials with controllable pore sizes and narrow pore size distributions. Buckytubes are carbon tubes with internal diameters in the range 1–5 urn. The structure of each tube is thought to be similar to one or more graphite sheets rolled up in a helical manner. MCM-41 is one member of a new family of highly uniform mesoporous silicate materials produced by Mobil, whose pore size can be accurately controlled in the range 1.5–10 nm. We present grand canonical Monte Carlo (GCMC) simulations of single fluid and binary mixture adsorption in a model buckytube, and nonlocal density functional theory (DFT) calculations of trace pollutant separation in a range of buckytubes and MCM-41 pores. Three adsorbed fluids are considered; methane, nitrogen and propane. The GCMC studies show that the more strongly adsorbed pure fluid is adsorbed preferentially from an equimolar binary mixture. Ideal adsorbed solution theory (IAST) is shown to give good qualitative agreement with GCMC when predicting binary mixture separations. The DFT results demonstrate the very large increases in trace pollutant separation that can be achieved by tuning the pore size, structure, temperature and pressure of the MCM-41 and buckytube adsorbent systems to their optimal values.
    Materialart: Digitale Medien
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  • 16
    Digitale Medien
    Digitale Medien
    Springer
    Adsorption 2 (1996), S. 111-119 
    ISSN: 1572-8757
    Schlagwort(e): pressure swing adsorption ; bulk separation ; experiments ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Physik , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Abstract The piston-driven ultra rapid pressure swing adsorption (URPSA) equipment was developed and oxygen enrichment from air was examined as an example. The adsorbent bed is directly connected to the cylinder where a piston moves at high frequency. Thus pressurization and depressurization in the bed are driven by mechanical piston motion, which can achieve far more rapid cycles compared with the conventional pressure swing operation using valves. The cycle time is usually on the order of seconds or sub seconds. Oxygen enrichment from air up to about 60% or higher of oxygen concentration was achieved by small-scale equipment using zeolite 5A with a oxygen production capacity of 100 Nm3-product gas/m3-zeolite/hr, which is about ten times larger than those of commercialized PSAs for this purpose. A simplified numerical model describing the mass transfer taking place in URPSA was developed. The model could simulate fairly well the air separation characteristics in terms of oxygen concentration, oxygen production capacity and oxygen yield. The proposed model helps in the understanding of the basic nature of URPSA and possible applications. This novel PSA is promising as a compact yet high-capacity PSA to be utilized in a wide variety of applications.
    Materialart: Digitale Medien
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  • 17
    Digitale Medien
    Digitale Medien
    Springer
    Adsorption 2 (1996), S. 121-132 
    ISSN: 1572-8757
    Schlagwort(e): adsorption heat pump ; experimental kinetic data ; heat and mass transfer ; simulation ; zeolite
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Physik , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Abstract Heat and mass transfer during the adsorption of water on zeolite has been studied both theoretically and experimentally. A dynamic simulation model of a zeolite layer has been developed to estimate the predominant transport resistances and calculations were carried out to assist the simultaneous experimental investigations. On one hand, a metallic matrix was added to the compact zeolite layer to improve the heat transfer. On the other hand, pore-forming materials such as melamine or tartaric acid were used. These organic components are removed during drying of the zeolite so that the mass transfer inside the zeolite is significantly enhanced compared to a granulated zeolite bed. The experimental investigations show that the theoretically deduced possibilities of improving the adsorption process can be realized in the manufactured zeolites. The investigations described here are of interest for the development of adsorption heat pumps. Due to the thermodynamic characteristics the adsorption system zeolite-water is a promising working pair for this application. The investigations show that the main shortcoming of these machines, the thermal output, can be increased significantly.
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  • 18
    ISSN: 1572-8757
    Schlagwort(e): mixture adsorption ; cabon dioxide ; methane ; carbon pores ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Physik , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Abstract We have used the grand canonical Monte Carlo method to study the adsorption and selectivity of mixtures of carbon dioxide with methane and nitrogen at high (i.e., ambient) temperatures in model slit pores with graphitic surfaces. Experimental data, including new high pressure measurements for carbon dioxide and methane on a non-porous graphitic standard, were used to test the potential models. The mixture simulations predict that carbon dioxide is preferentially adsorbed in both systems. The results are discussed in terms of competing energetic and entropic effects and the underlying molecular mechanisms.
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  • 19
    Digitale Medien
    Digitale Medien
    Springer
    Adsorption 3 (1997), S. 41-54 
    ISSN: 1572-8757
    Schlagwort(e): liquid crystals ; moving bed system ; simulation ; purification
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Physik , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Abstract We investigate convective-diffusive transport of a solute through a medium with properties that can be externally modulated in space and time. In particular, we focus on the effect of a front—a sharp transition in the convective velocity (v) and diffusivity (D)—on the evolution of the solute concentration profile. Numerical results show that by suitably moving the front during the process an anti-dispersive effect may be realized, in which the solute accumulates in a thin region close to the moving boundary. Our computations take into account the realistic case of a front having a small but finite thickness, and we find that the width of the concentration profile scales as $$\left( {1/\sqrt {Pe} } \right)$$ , where Pe is the Péclet number. This is in sharp contrast to the 1/Pe scaling observed for the ideal case of the singular front assumed in previous work. The effect of the thickness of the front and the magnitude of the drop inv andD, on the solute concentration profile has also been studied. These results are relevant in order to implement and optimize protocols that apply an externally controlled moving boundary for the purpose of separation. We also present experimental results characterizing solute transport across a stationary front, expected to display many features needed in a model for moving fronts. The concentration profile of electrophoretically mobile BSA-FITC within the boundary layer at a polyacrylamde gel-buffer interface were visualized by epifluorescence microscopy. Measured boundary layer thickness exceeded that predicted for even a finite interface, indicating that the length scale associated with real boundaries is relevant to the modeling problem.
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  • 20
    Digitale Medien
    Digitale Medien
    Springer
    Adsorption 3 (1996), S. 151-163 
    ISSN: 1572-8757
    Schlagwort(e): pressure swing adsorption ; environmental ; activated carbon ; simulation ; factorial design
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Physik , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Abstract A two-level fractional factorial study was performed by computer simulation on the periodic state process performance of a pressure swing adsorption-solvent vapor recovery process (PSA-SVR). The goal was to investigate factor (parameter) interaction effects on the process performance, i.e., interaction effects that cannot be ascertained from the conventional “one-at-a-time” approach. Effects of seven factors, i.e., the purge to feed ratio, pressure level, pressure ratio, heat transfer coefficient, feed concentration, feed volumetric flow rate and bed length to diameter ratio, on the process performance were investigated. The results were judged in terms of the light product purity, heavy product enrichment (and relative enrichment) and recovery, and bed capacity factor. Only the purge to feed ratio, pressure ratio, and feed concentration had significant effects on the benzene vapor enrichment (and relative enrichment); and no two-factor and higher interactions were observed. The light product purity was affected by all seven factors; and the relative importance of the effect of each factor depended on the levels of the other factors, i.e., significant two-factor interaction effects existed. Two-factor interaction effects also existed on the benzene vapor recovery, although the effects of all seven factors and their interactions were relatively small. The bed capacity factor was affected mainly by the purge to feed ratio, the heat transfer coefficient and the feed concentration; two factor and higher order interaction effects were insignificant. Overall, this study demonstrated the utility of fractional factorial design for revealing factor interactions and their effects on the performance of a PSA-SVR process.
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  • 21
    Digitale Medien
    Digitale Medien
    Springer
    Adsorption 4 (1998), S. 149-158 
    ISSN: 1572-8757
    Schlagwort(e): pressure swing adsorption ; gas separation ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Physik , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Abstract In a multi-bed pressure swing adsorption (PSA) process, cycle steps with gas flow transferring from one bed to another such as equalization, purge, etc. are generally practiced to enhance the product recovery. However, if the flows for the connected beds in these steps are not balanced, the PSA process may not operate in a symmetrical manner. In the modeling of the PSA process, most of the simulations consider only one bed and assume that the rest of the beds would behave in a same way. In order to assess the impact of bed symmetry on the PSA performance, a new PSA model capable of studying bed symmetry in a two-bed system is developed. Experimental results from this paper show that uneven equalization flow can result in a lower product purity and a peculiar purity curve at different equalization levels. This phenomenon can be successfully predicted by this model. Simulation results also show that in large-scale PSA units, asymmetrical operation can cause drastically different temperature profiles in different adsorbers and hence a much lower performance. This paper demonstrates the importance of maintaining operation symmetry in PSA processes.
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  • 22
    ISSN: 1572-8757
    Schlagwort(e): PSA ; pressure equalization ; extent of equalization ; simulation ; experiment
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Physik , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Abstract Incomplete pressure equalization (PE) is practiced in a commercial oxygen concentrator for medical use by adopting simultaneous PE and feed-pressurization for pressurizing an adsorption bed. In such a cycle configuration, extent of equalization during PE affects process performance. In order to assess the effect, performance of pressure swing adsorption (PSA) process with incomplete PE was determined by both simulations and experiments. In simulations, an equilibrium model was used with the assumptions of multicomponent Langmuir isotherms, isothermal operation, and no pressure drop through a bed. The required parameters for simulations were measured in separate experiments. PSA experiments were performed for a two-bed cycle with PE. Two kinds of pressurization, feed and product, were examined. Effects of purge amount and extent of equalization on process performance were assessed in view of productivity and light-component recovery. From the obtained results performance contours were constructed. 95 oxygen mole percent production from air with zeolite 13× was considered as a case study. In both pressurizations, an optimal specific purge and an extent of equalization for the productivity and recovery were observed, but with a different level of equalization. For a maximum productivity feed-pressurization favored incomplete PE, while a maximum recovery occurred at complete PE for both pressurizations. The simulations depicted well existence of optimum conditions, though they showed quantitative disagreement with experiments.
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  • 23
    Digitale Medien
    Digitale Medien
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    Annals of operations research 39 (1992), S. 157-172 
    ISSN: 1572-9338
    Schlagwort(e): Score function method ; likelihood ratio ; sensitivity analysis ; simulation ; Monte Carlo methods ; conditioning
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Notizen: Abstract We consider first a discrete event static system that is to be simulated at values of a parameter or vector of parametersθ. The system is assumed driven by an inputX, where typicallyX is a vector of variables whose densityf θ (x) depends on the parameterθ. For the purpose of optimizing, finding roots, or graphing the expected performanceE θ L(X) for performance measureL, it is useful to estimate not only the expected value but also its gradient. An unbiased estimator for the latter is the score function estimator $$L(X)S(\theta ) = L(X)\frac{\partial }{{\partial \theta }}\ln f_\theta (x).$$ This estimator and likelihood ratio analogues typically require variance reduction, and we consider conditioning on the value of the score function for this purpose. The efficiency gains due to performing the Monte Carlo conditionally can be very large. Extension to discrete event dynamic systems such as theM/G/1 queue and other more complicated systems is considered.
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  • 24
    Digitale Medien
    Digitale Medien
    Springer
    Annals of operations research 53 (1994), S. 175-197 
    ISSN: 1572-9338
    Schlagwort(e): Variance reduction ; simulation ; control variates ; importance sampling ; common random numbers ; stratification ; conditioning ; efficiency
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Notizen: Abstract This paper provides an overview of the five most commonly used statistical techniques for improving the efficiency of stochastic simulations: control variates, common random numbers, importance sampling, conditional Monte Carlo, and stratification. The paper also describes a mathematical framework for discussion of efficiency issues that quantifies the trade-off between lower variance and higher computational time per observation.
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  • 25
    ISSN: 1572-9338
    Schlagwort(e): integer programming ; discriminant analysis ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Notizen: Abstract Although there have been several journal articles on the classificatory performance of mathematical programming approaches to the two-group discriminant problem, there has been no simulation study on the classificatory performance of mathematical programming approaches to the multiple-group problem reported in the literature. This study reports the results of a simulation experiment on the classificatory performance of a single-function and a multiple-function mathematical programming model relative to that of the standard parametric procedures for the three-group problem with small training samples. The effect of second-order terms on the classificatory performance of the mathematical programming models for the three-group problem is also investigated. Furthermore, this study theoretically examines the range of parameter values of a multiple-function mathematical programming model for which its number of misclassifications in the training sample cannot exceed that of a single-function model.
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  • 26
    Digitale Medien
    Digitale Medien
    Springer
    Annals of operations research 74 (1997), S. 321-332 
    ISSN: 1572-9338
    Schlagwort(e): simulation ; modeling ; aerospace ; air route structure ; free flight ; clustering
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Notizen: Abstract Recent work performed for the Federal Aviation Administration to support the development of future concepts of air traffic management has involved simulation modeling of patterns of airspace usage by commercial and business air traffic. The objective of these efforts has been to investigate the impacts of a pattern of airspace usage known as "free flight", whereby pilots and flight dispatchers have much more freedom to choose, say, direct or wind-optimal routing through airspace. One of the figures of merit investigated is a count of "convergence pairs" as a measure of the complexity of various traffic patterns. These are cases when aircraft in the simulation model fly close to each other. Interestingly, geographic plots of convergence pairs accumulated over time bring out certain features or patterns of congested air traffic flows or flight alignments. However, these plots are also thick with "noise" or extraneous convergence pairs, whose presence detracts from the ability to perceive congested air traffic flows. Cluster analysis has been found to be an effective method of filtering these displays so that the congested flow features are discernible. The process developed for this purpose is based on a two-pass clustering approach. The process has worked well for the simulation modeling performed to date. Classification of the locations of convergence pairs into congested flow corridors is visually appealing, and has helped distinguish differences in contrasting scenarios of airspace usage. The paper presents graphical results and describes the clustering algorithms employed.
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  • 27
    Digitale Medien
    Digitale Medien
    Springer
    Queueing systems 12 (1992), S. 369-389 
    ISSN: 1572-9443
    Schlagwort(e): Perturbation analysis ; stability ; stochastic difference equations ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract We investigate the stability of waiting-time derivatives when inputs to a queueing system-service times and interarrival times-depend on a parameter. We give conditions under which the sequence of waiting-time derivatives admits a stationary distribution, and under which the derivatives converge to the stationary regime from all initial conditions. Further hypotheses ensure that the expectation of a stationary waiting-time derivative is, in fact, the derivative of the expected stationary waiting time. This validates the use of simulation-based infinitesimal perturbation analysis estimates with a variety of queueing processes. We examine waiting-time sequences satisfying recursive equations. Our basic assumption is that the input and its derivatives are stationary and ergodic. Under monotonicity conditions, the method of Loynes establishes the convergence of the derivatives. Even without such conditions, the derivatives obey a linear difference equation with random coefficients, and we exploit this fact to find stability conditions.
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  • 28
    Digitale Medien
    Digitale Medien
    Springer
    Queueing systems 19 (1995), S. 169-192 
    ISSN: 1572-9443
    Schlagwort(e): Continuity ; rates of convergence ; robust estimation ; queueing systems ; simulation ; regenerative processes
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract Leta ands denote the inter arrival times and service times in aGI/GI/1 queue. Let a(n), s(n) be the r.v.s. with distributions as the estimated distributions ofa ands from iid samples ofa ands of sizesn. Letw be a r.v. with the stationary distribution π of the waiting times of the queue with input(a,s). We consider the problem of estimatingE[w α], α〉 0 and α via simulations when (a (n),s(n)) are used as input. Conditions for the accuracy of the asymptotic estimate, continuity of the asymptotic variance and uniformity in the rate of convergence to the estimate are obtained. We also obtain rates of convergence for sample moments, the empirical process and the quantile process for the regenerative processes. Robust estimates are also obtained when an outlier contaminated sample ofa ands is provided. In the process we obtain consistency, continuity and asymptotic normality of M-estimators for stationary sequences. Some robustness results for Markov processes are included.
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  • 29
    Digitale Medien
    Digitale Medien
    Springer
    Queueing systems 21 (1995), S. 391-413 
    ISSN: 1572-9443
    Schlagwort(e): Dynamic scheduling control ; queueing network ; fluid network ; queueing theory ; flow control ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract Motivated by dynamic scheduling control for queueing networks, Chen and Yao [8] developed a systematic method to generate dynamic scheduling control policies for a fluid network, a simple and highly aggregated model that approximates the queueing network. This study addresses the question of how good these fluid policies are as heuristic scheduling policies for queueing networks. Using simulation on some examples these heuristic policies are compared with traditional simple scheduling rules. The results show that the heuristic policies perform at least comparably to classical priority rules, regardless of the assumptions made about the traffic intensities and the arrival and service time distributions. However, they are certainly not always the best and, even when they are, the improvement is seldom dramatic. The comparative advantage of these policies may lie in their application to nonstationary situations such as might occur with unreliable machines or nonstationary demand patterns.
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  • 30
    Digitale Medien
    Digitale Medien
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    Queueing systems 12 (1992), S. 257-272 
    ISSN: 1572-9443
    Schlagwort(e): Scheduling ; simulation ; perturbation analysis ; stochastic approximation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract We consider the problem of scheduling the arrivals of a fixed number of customers to a stochastic service mechanism to minimize an expected cost associated with operating the system. We consider the special case of exponentially distributed service times and the problems in general associated with obtaining exact analytic solutions. For general service time distributions we obtain approximate numerical solutions using a stochastic version of gradient search employing Infinitesimal Perturbation Analysis estimates of the objective function gradient obtained via simulation.
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  • 31
    Digitale Medien
    Digitale Medien
    Springer
    Queueing systems 7 (1990), S. 101-124 
    ISSN: 1572-9443
    Schlagwort(e): Police ; fire ; queueing ; travel time ; simulation ; city government
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract Cities with under 100,000 in population expend a significant portion of their budgets on emergency services. One option that a number of these cities have considered for improving service and cutting costs is training personnel to handle both police and fire roles. In this paper we describe a hierarchy of models that we have used to assess the performance viability of a merger as well as to design specific deployment plans. The modeling environment is more complex than a traditional police or fire system. We need to model the response pattern of four or more patrol units along with the simultaneous dispatch of fire equipment from one or more fire stations. The major contribution of the paper is the manner in which a series of models is linked together to forecast a wide range of performance measures under differing dispatch assumptions. We use a queueing model of police patrol to calculate steady state probabilities and expected delays without preemption. We then model two types of preemptive dispatch strategies utilized in responding initially to a major fire by superimposing a binomial distribution on the basic queueing model. There is also a travel time simulation model to calculate conditional expected response time statistics. The queueing models and the travel time simulation are then combined to estimate unconditional expected values. Lastly, we describe a simulation model used to address transient performance issues that are of concern during a major fire.
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  • 32
    Digitale Medien
    Digitale Medien
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    Annals of operations research 53 (1994), S. 391-418 
    ISSN: 1572-9338
    Schlagwort(e): Autoregressive process ; confidence interval ; output analysis ; simulation ; statistics ; time series
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Notizen: Abstract We revisit and update the autoregressive-output-analysis method for constructing a confidence interval for the steady-state mean of a simulated process by using Rissanen's predictive least-squares criterion to estimate the autoregressive order of the process. This order estimator is strongly consistent when the output is autoregressive. The order estimator is combined with the standard autoregressive-output-analysis method to form a confidence-interval procedure. Alternatives for estimating the degrees of freedom for the procedure are investigated. The main result is an asymptotically valid confidence-interval procedure that, empirically, has good small-sample properties.
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  • 33
    Digitale Medien
    Digitale Medien
    Springer
    Annals of operations research 67 (1996), S. 183-210 
    ISSN: 1572-9338
    Schlagwort(e): Capacity planning ; emergency services ; health care ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Notizen: Abstract Due to its highly stochastic nature and complex interaction between services involved, health care has been a demanding area of application for computer simulation. This paper includes details and results of a simulation study realized in the Surgical Emergency Department at Istanbul University School of Medicine. The purpose is to suggest new bed capacities to improve the current system, and also to provide the management with guidelines for their expansion plans. For this aim, arrival rates, treatment procedures, inpatient admittance, and service durations have been carefully analyzed and modeled. The model, coded in SLAM-II simulation language, has been run under several bed capacity scenarios, and resulting queueing and waiting patterns have been discussed in detail.
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  • 34
    Digitale Medien
    Digitale Medien
    Springer
    Transportation 26 (1999), S. 399-416 
    ISSN: 1572-9435
    Schlagwort(e): cellular telephones ; incident detection ; simulation ; traffic modeling ; wireless communications
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung
    Notizen: Abstract This research study was designed to assess by simulation the efficacy of incident detection by cellular phone call-in programs. The assessment was conducted by varying the proportion of drivers with cellular phones on the highway so as to mirror the cellular industry statistics that show a continued growth of ownership of cellular phones in the United States. An analytical model, which combined simulation and the limited field data available in the literature, was used to determine measures of effectiveness of the cellular phone-based detection system. The results showed that a cellular phone detection system offers fast incident detection times and higher detection rates for both shoulder and lane blocking incidents. For example, in moderate traffic flow (i.e. 1,550 vehicles per hour per lane), 90 percent of incidents blocking two lanes were detected in 1.5 minutes when the proportion of drivers with cellular phones was one out of 10 drivers, even with only 20 percent of them willing to report incidents. When the current proportion of cellular ownership, i.e. 1 out of 3, was used in the simulation, the detection time improved to 0.8 minutes. The simulation analysis of incident detection by cellular phones also showed that there is a direct relationship between the probability of detection and the detection time; that is, the specification of a higher detection rate resulted in slower detection times. This is in sharp contrast with the results of field study of automatic incident detection (AID) systems which demonstrated an inverse relationship between probability of detection and detection time.
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  • 35
    Digitale Medien
    Digitale Medien
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    Queueing systems 11 (1992), S. 307-333 
    ISSN: 1572-9443
    Schlagwort(e): Sensitivity analysis ; perturbation analysis ; discrete event dynamical systems ; simulation ; likelihood ratio ; point processes
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract In this article, we introduce a new method for obtaining the ersatz sample derivatives useful in sensitivity analysis: the maximal coupling RPA method.
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  • 36
    Digitale Medien
    Digitale Medien
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    Queueing systems 27 (1997), S. 37-54 
    ISSN: 1572-9443
    Schlagwort(e): massive parallel processing ; simulation ; MIMD and SIMD parallel computers ; scalable algorithm ; G/G/1 queue ; longest path ; queueing networks
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract This paper presents several basic algorithms for the parallel simulation of G/G/1 queueing systems and certain networks of such systems. The coverage includes systems subject to manufacturing or communication blocking, or to loss of customer due to capacity constraints. The key idea is that the customer departure times are represented by longest-path distance in directed graphs instead of by the usual recursive equations. This representation leads to scalable algorithms with a high degree of parallelism that can be implemented on either MIMD or SIMD parallel computers.
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  • 37
    Digitale Medien
    Digitale Medien
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    Queueing systems 31 (1999), S. 43-58 
    ISSN: 1572-9443
    Schlagwort(e): perturbation analysis ; sample path analysis ; queueing theory ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract We study infinitesimal perturbation analysis (IPA) for queueing networks with general service time distributions. By “general” we mean that the distributions may have discrete components. We show that in the presence of service time distributions with discrete components commuting condition (CC) is no longer sufficient for unbiasedness of IPA. To overcome this difficulty, we introduce the notion of separability of real‐valued random variables, and show that separability of service times together with (CC) establishes unbiasedness of IPA for queueing systems with general service time distributions. It turns out that the piecewise analyticity of service times is a sufficient condition for separability.
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  • 38
    Digitale Medien
    Digitale Medien
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    Queueing systems 6 (1990), S. 89-108 
    ISSN: 1572-9443
    Schlagwort(e): Queueing network ; approximation ; retrials ; telecommunication ; simulation ; overflow process ; departure process ; superposition process
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract The tandem behavior of a telecommunication system with finite buffers and repeated calls is modeled by the performance of a finite capacityG/M/1 queueing system with general interarrival time distribution, exponentially distributed service time, the first-come-first-served queueing discipline and retrials. In this system a fraction of the units which on arrival at a node of the system find it busy, may retry to be processed, by merging with the incoming arrival units in that node, after a fixed delay time. The performance of this system in steady state is modeled by a queueing network and is approximated by a recursive algorithm based on the isolation method. The approximation outcomes are compared against those from a simulation study. Our numerical results indicate that in steady state the non-renewal superposition arrival process, the non-renewal overflow process, and the non-renewal departure process of the above system can be approximated with compatible renewal processes.
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  • 39
    Digitale Medien
    Digitale Medien
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    Queueing systems 8 (1991), S. 265-277 
    ISSN: 1572-9443
    Schlagwort(e): GI/G/1 queue ; birth-death queue ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract We study a class of infinitesimal perturbation analysis (IPA) algorithms for queueing systems with load-dependent service and/or arrival rates. Such IPA algorithms were originally motivated by applications to large queueing systems in conjunction with aggregation algorithms. We prove strong consistency of these estimators through a type of birth and death queue.
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  • 40
    Digitale Medien
    Digitale Medien
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    Annals of operations research 94 (2000), S. 139-162 
    ISSN: 1572-9338
    Schlagwort(e): irrigated system ; modeling ; multi‐agent system ; simulation ; social networks ; coordination
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Notizen: Abstract The viability of irrigated systems in the Senegal River Valley is being brought into question today due to their under‐utilization. We assume that their viability depends largely on the way their different components behave and interact. We therefore sought to examine in greater depth today's knowledge of the structure of these systems and activities performed within them. This led to the development of a multi‐agent system model, a kind of virtual irrigated system, with a special focus on rules in use for access to credit, water allocation and cropping season assessment as well as organization and coordination of farmers. The purpose of this paper is to show how this kind of tool is relevant to the study of irrigated systems' viability. As an example it is used to examine the influence of existing social networks on the viability of irrigated systems.
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  • 41
    Digitale Medien
    Digitale Medien
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    Annals of operations research 74 (1997), S. 239-257 
    ISSN: 1572-9338
    Schlagwort(e): autocorrelation ; L 1 regression ; least absolute deviations ; robust regression ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Notizen: Abstract Least absolute value (LAV) regression provides a robust alternative to least squares, particularly when the disturbances follow distributions that are nonnormal and subject to outliers. While inference in least squares estimation is well-understood, inferential procedures in the context of LAV estimation have not been studied as extensively, particularly in the presence of non-independent disturbances. In this work, we study three alternative significance test procedures in LAV regression, along with two approaches used to correct for serial correlation. The study is based on large-scale Monte Carlo simulations, and comparisons are made based on both observed significance levels and power.
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  • 42
    Digitale Medien
    Digitale Medien
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    Queueing systems 14 (1993), S. 57-78 
    ISSN: 1572-9443
    Schlagwort(e): Perturbation analysis ; sample path analysis ; finite buffer queues ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract Applying the technique of smoothed perturbation analysis (SPA) to theGI/G/1/K queue, we derive gradient estimators for two performance measures: the mean steady-state system time of a served customer and the probability that an arriving customer is rejected. Unbiasedness of the estimators follows from results of a previous general framework on SPA estimators. However, in that framework, the estimators often require the simulation of numerous additional sample subpaths, possibly making the technique practically infeasible in applications. We exploit some of the special structure of theGI/G/1/K queue to come up with an estimator which requires at most the simulation of a single additional sample subpath. By establishing certain regenerative properties, we provide a strong consistency proof for the estimator.
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  • 43
    Digitale Medien
    Digitale Medien
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    Queueing systems 19 (1995), S. 247-268 
    ISSN: 1572-9443
    Schlagwort(e): Sensitivity coefficient ; score function ; driving sequence ; queuing model ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract Queuing models described as regenerative processes driven by several sequences are under investigation. Occurrence times in these sequences are supposed to be asynchronous, in general. Sensitivity coefficients that are derivatives of some stationary performance measures with respect to definite parameters as well as derivatives of higher orders are derived for such queuing models. These sensitivity coefficients can be evaluated by simulation.
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  • 44
    Digitale Medien
    Digitale Medien
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    Annals of operations research 39 (1992), S. 1-39 
    ISSN: 1572-9338
    Schlagwort(e): Stochastic programming ; stochastic quasigradient methods ; discrete event systems ; simulation ; concurrent approximation and optimization
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Notizen: Abstract In this paper, stochastic programming techniques are adapted and further developed for applications to discrete event systems. We consider cases where the sample path of the system depends discontinuously on control parameters (e.g. modeling of failures, several competing processes), which could make the computation of estimates of the gradient difficult. Methods which use only samples of the performance criterion are developed, in particular finite differences with reduced variance and concurrent approximation and optimization algorithms. Optimization of the stationary behavior is also considered. Results of numerical experiments and convergence results are reported.
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  • 45
    Digitale Medien
    Digitale Medien
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    Annals of operations research 52 (1994), S. 107-129 
    ISSN: 1572-9338
    Schlagwort(e): DSS ; intelligent front-ends ; knowledge-based systems ; MADM ; outranking ; Scheduling ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Notizen: Abstract Knowledge-based systems (KBS) can help to make simulation available to a large group of users. We want to exemplify this by describing a decision support system (DSS) for short term rescheduling in manufacturing called SIMULEX. It couples expert systems, simulation, and a multiattribute decision making (MADM) procedure to assist the production manager. After an introduction to simulation as a problem solving tool, the current problems in production control and the goals of the project are described. Then, the various components of SIMULEX are explained in some detail. Some results and a short outlook conclude the article.
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  • 46
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    Annals of operations research 73 (1997), S. 233-252 
    ISSN: 1572-9338
    Schlagwort(e): Data Envelopment Analysis ; model specification ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Notizen: Abstract The use of Data Envelopment Analysis for estimating comparative efficiency has become widespread, and there has been considerable academic attention paid to the development of variants of the basic DEA model. However, one of the principal weaknesses of DEA is that - unlike statistically based methods - it yields no diagnostics to help the user determine whether or not the chosen model is appropriate. In particular, the choice of inputs and out-puts depends solely on the judgement of the user. The purpose of this paper is to examine the implications for efficiency scores of using a misspecified model. A simple production process is set up. Simulation models are then used to explore the effects of applying misspecified DEA models to this process. The phenomena investigated are: the omission of significant variables; the inclusion of irrelevant variables; and the adoption of an inappropriate variable returns to scale assumption. The robustness of the results is investigated in relation to sample size; variations in the number of inputs; correlation between inputs; and variations in the importance of inputs. The paper concludes that the dangers of misspecification are most serious when simple models are used and sample sizes are small. In such circumstances, it is concluded that it will usually be to the modeller's advantage to err on the side of including possibly irrelevant variables rather than run the risk of excluding a potentially important variable from the model.
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  • 47
    ISSN: 1349-9432
    Schlagwort(e): photolithography ; simulation ; topography scattering ; numerical absorbing boundary condition ; finite-element method
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract A method for simulating latent image formation in a photoresist illuminated by an arbitrary imaging system is presented. A variational formulation for light scattering, which does not depend on a specific configuration of the imaging system, is derived and solved using the finite-element method. The perfectly matched layer absorbing boundary condition is applied to take wave propagation in the infinite region surrounding the photoresist into account. The validity of the method is examined by comparing the results with those made by the vertical propagation model and the previous two-dimensional models.
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  • 48
    Digitale Medien
    Digitale Medien
    Springer
    The VLDB journal 2 (1993), S. 1-37 
    ISSN: 0949-877X
    Schlagwort(e): Buffer management ; query optimization ; simulated annealing ; join methods ; queueing model ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract In a multi-query environment, the marginal utilities of allocating additional buffer to the various queries can be vastly different. The conventional approach examines each query in isolation to determine the optimal access plan and the corresponding locality set. This can lead to performance that is far from optimal. As each query can have different access plans with dissimilar locality sets and sensitivities to memory requirement, we employ the concepts of memory consumption and return on consumption (ROC) as the basis for memory allocations. Memory consumption of a query is its space-time product, while ROC is a measure of the effectiveness of response-time reduction through additional memory consumption. A global optimization strategy using simulated annealing is developed, which minimizes the average response over all queries under the constraint that the total memory consumption rate has to be less than the buffer size. It selects the optimal join method and memory allocation for all query types simultaneously. By analyzing the way the optimal strategy makes memory allocations, a heuristic threshold strategy is then proposed. The threshold strategy is based on the concept of ROC. As the memory consumption rate by all queries is limited by the buffer size, the strategy tries to allocate the memory so as to make sure that a certain level of ROC is achieved. A simulation model is developed to demonstrate that the heuristic strategy yields performance that is very close to the optimal strategy and is far superior to the conventional allocation strategy.
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  • 49
    Digitale Medien
    Digitale Medien
    Springer
    Computing 53 (1994), S. 13-31 
    ISSN: 1436-5057
    Schlagwort(e): Disk cache ; disk I/O ; I/O architecture ; parallel computers ; parallel I/O ; performance evaluation ; simulation ; transaction processing
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Beschreibung / Inhaltsverzeichnis: Zusammenfassung Während sich die Rechenleistung heutiger Computersysteme beständig erhöht, entwickelt sich die Platten-Ein/Ausgabe zu einem Engpaß. Dies wird besonders in hochparallelen Maschinen deutlich, wo das Mißverhältnis zwischen Rechen- und Ein/Ausgabeleistung häufig besonders groß ist. Diese Situation kann—in Analogie zur Multiprozessortechnologie—durch Einführung paralleler Ein/Ausgabe-Systeme entschärft werden. Ausgehend von einer solchen verteilten Ein/Ausgabe-Architektur für Parallelrechner, schlagen wir die Benutzung von Plattencaches auf verschiedenen Architekturebenen vor. Diese Cachestrukturen haben wir durch Simulationen näher untersucht. Wir beschreiben zunächst den Ansatz zur Cachemodellierung und das Lastmodell der Ein/Ausgabe, das von Transaktionsverarbeitung und General-Purpose-Lasten abgeleitet ist. Dann diskutieren wir die Ergebnisse für den Fall von Caches, die sich auf jeweils einer einzigen Architekturebene befinden, und für solche auf mehreren Architekturebenen gleichzeitig. Es ergibt sich, daß große Caches auf den Ein/Ausgabeprozessoren in Verbindung mit kleineren auf den Verarbeitungselementen die optimale Struktur darstellen. Zusätzlich können Hardwarecaches auf Plattenebene zur weiteren Leistungssteigerung eingesetzt werden. Bezüglich der Schreiboperationen vom Cache auf die Platte ergibt sich ein verzögerter Schreibmodus (delayed write) als beste Strategie.
    Notizen: Abstract I/O in computer systems is prone to become a bottleneck. This is a particular severe problem in highly parallel machines where some applications are fully I/O bound if only one or few conventional I/O paths exist. Similar to the use of multiprocessor technology for increasing processing performance, disk I/O performance can be substantially improved by employing parallel I/O schemes. Based on a distributed I/O architecture for parallel computers, we propose to use disk caches on several architectural levels, and confirm this by simulations of various structural options. In this paper, we describe the cache modelling approach and the I/O load model which has been derived from transaction-processing and general-purpose applications. Then we discuss the results for caches on single and multiple architecture levels. Large caches on I/O processors in combination with small caches on processing elements turn out to be the preferable structure. In addition, hardware caches can be employed at disk level for further performance improvement. For write operations, a delayed write strategy is shown to be superior to other modes.
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  • 50
    Digitale Medien
    Digitale Medien
    Springer
    OR spectrum 13 (1991), S. 159-166 
    ISSN: 1436-6304
    Schlagwort(e): Facility location for obnoxious facilities ; Gaussian model ; simulation ; Standortplanung für umweltbelastende Anlagen ; Gauß-Modell ; Simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Beschreibung / Inhaltsverzeichnis: Zusammenfassung In dieser Arbeit werden zunächst die in neuerer Zeit entwickelten wind-diskreten Modelle und Verfahren zur Standortplanung für unerwünschte, umweltbelastende Anlagen analysiert und die Grenzen dieser Ansätze für die Lösung praktischer Problemstellungen aufgezeigt. Basierend auf diesen Erkenntnissen wird ein simulations-basiertes Verfahren für ein neues wind-stetiges Modell vorgestellt, das diese Schwachstellen überwindet. Abschließend wird das Basis-Standortmodell durch Einbeziehung der Emissionshöhe als Entscheidungsvariable erweitert.
    Notizen: Summary The present paper regards the location of obnoxious facilities in the context of a realistic environment. Efforts have been made in the past towards this direction, without much success as fas as the modeling is concerned. The prevailing meteorological conditions of the area under consideration, that evidently play an important role in the dispersion of pollution, either were completely ignored or inadequately considered. In this work, the most recent wind-discrete models and solution methods are presented and analyzed and their limitations are identified. A new simulation-aided wind-continuous model is introduced capable of remedying the above limitations. Finally, a generalization is proposed and discussed, regarding the introduction of the stack height as a decision variable in the relative model.
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  • 51
    Digitale Medien
    Digitale Medien
    Springer
    Meccanica 30 (1995), S. 105-124 
    ISSN: 1572-9648
    Schlagwort(e): Artificial ground motion ; probabilistic models ; random processes ; simulation ; earthquake engineering
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Beschreibung / Inhaltsverzeichnis: Sommario Sono analizzati criticamente i modelli probabilistici del moto sismico. Tale rassegna comprende processi modulati uniformemente, oscillatori, modulati in ampiezza e fase, modelli ARMA, ed altri. Inoltre, viene presentato un modello gaussiano non-stazionario che risulta innovativo e di validità generale. Il modello consiste nella sovrapposizione di armoniche con ampiezze gaussiane correlate. Le frequenze delle armoniche e le caratteristiche probabilistiche delle ampiezze dipendono rispettivamente dalla durata e dalla correlazione del moto sismico di riferimento. Il modello può essere generalizzato per rappresentare il moto sismico in due o più siti. Sono anche presentati alcuni metodi per la generazione di campioni artificiali del moto sismico. Tali metodi sono inoltre applicati alla generazione di sismi artificiali e alla valutazione dell'efficienza dei vari modelli probabilistici.
    Notizen: Abstract Probabilistic models of seismic ground acceleration are reviewed. The review includes uniformly modulated, oscillatory, amplitude and phase modulated, ARMA, and other processes. Moreover, a novel and general nonstationary Gaussian model is presented. The model consists of a superposition of harmonics with correlated Gaussian amplitudes. The harmonic frequencies and the amplitude probabilistic characteristics depend on the duration and the correlation of the target seismic ground motion, respectively. The model can be generalized to represent the seismic ground motion at two or more sites. Methods are also presented for generating realizations of the probabilistic models in the paper. The methods are applied to demonstrate the generation of artificial seismic ground motions and to evaluate the usefulness of various probabilistic models in earthquake engineering.
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  • 52
    Digitale Medien
    Digitale Medien
    Springer
    Journal of intelligent and robotic systems 4 (1991), S. 129-143 
    ISSN: 1573-0409
    Schlagwort(e): CAD/CAM ; off-line programming ; industrial vision ; pattern recognition ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik , Maschinenbau
    Notizen: Abstract CAD/CAM tools are essential components of the computer-integrated factory. Up to now, they have been used for tasks such as the simulation and path programming of numerically controlled machine tools, and sometimes industrial robots. The CAD-vision interconnection described here enables us to program parts learning on the workstation, to download piece features in the vision system for inspection on the production line, to simulate the recognition process on a set of parts stored in the computer, and to update vision files after modifications in the CAD system database.
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  • 53
    Digitale Medien
    Digitale Medien
    Springer
    The journal of supercomputing 17 (2000), S. 299-310 
    ISSN: 1573-0484
    Schlagwort(e): parallel program analysis ; simulation ; performance analysis ; software reengineering
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract In this paper we present a novel approach, based on the integration of static program analysis and simulation techniques, for the performance prediction of message passing programs. PS, a simulator of PVM applications developed in the last years by our research group, is fed with traces collected by executing the parallel program to be analyzed in quasi-concurrent mode on a single workstation. Since this process is typically a non negligible part of the simulation complexity, we have devised a technique based on static analysis and code restructuring for significantly speeding up the trace generation. We show how, by statically analyzing and restructuring the program, it is possible to obtain a simplified code (shrinked code) to be run for collecting a reduced version of the traces.
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  • 54
    Digitale Medien
    Digitale Medien
    Springer
    Journal of intelligent and robotic systems 5 (1992), S. 49-62 
    ISSN: 1573-0409
    Schlagwort(e): Off-line programming ; robot programming ; simulation ; computer graphics ; sensor programming ; sensor modeling ; robotics ; world modeling ; calibration
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik , Maschinenbau
    Notizen: Abstract The programming of robots is slowly evolving from traditional teach pendant methods to graphical Off-Line Programming (OLP) methods. Graphical simulation tools, such as OLP, are very useful for developing and testing robot programs before they are run on real industrial equipment. OLP systems are also used to develop task level programs. Traditional OLP systems, however, suffer from the limitations of using only position control which does not account for inherent robot inaccuracies and dynamic environments. This paper describes our work on improving and supplementing traditional position control programming methods. A baseline OLP system was implemented at NIST's Automated Manufacturing Research Facility (AMRF). Experience gained in implementing this system showed that an effective OLP system must accurately simulate the real world and must support sensor programming to compensate for real-world changes that cannot be simulated. The developed OLP geometric world model is calibrated using robot mounted ultrasound ranging sensors. This measurement capability produces a baseline geometric model of relatively good static accuracy for off-line programming. The graphical environment must also provide representations of sensor features. For this specific application, force is simulated in order to include force based commands in our robot programs. These sensor based programs are able to run reliably and safely in an unpredictable industrial environment. The last portion of this paper extends OLP and describes the functionality of a complete system for programming complex robot tasks.
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  • 55
    Digitale Medien
    Digitale Medien
    Springer
    Journal of intelligent and robotic systems 9 (1994), S. 193-207 
    ISSN: 1573-0409
    Schlagwort(e): Model-based diagnosis ; robotics ; simulation ; DEVS ; high autonomy systems
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik , Maschinenbau
    Notizen: Abstract Deep reasoning diagnostic procedures are model-based, inferring single or multiple faults from the knowledge of faulty behavior of component models and their causal structure. The overall goal of this paper is to develop a hierarchical diagnostic system that exploit knowledge of structure and behavior. To do this, we use a hierarchical architecture including local and global diagnosers. Such a diagnostic system for high autonomy systems has been implemented and tested on several examples in the domain of robot-managed fluid-handling laboratory.
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  • 56
    Digitale Medien
    Digitale Medien
    Springer
    Journal of productivity analysis 8 (1997), S. 167-182 
    ISSN: 1573-0441
    Schlagwort(e): Identifiability ; least squares ; likelihood ; profile ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Wirtschaftswissenschaften
    Notizen: Abstract Although conceptually pleasing, normal-gamma frontier models lead to difficult estimation problems. It is shown here that unless the sample size reaches several thousands of observations the shape parameter of the gamma density is hard to estimate, and that this carries over to estimates of the stochastic frontier, the individual inefficiencies, and the allocation of the overall variance to the stochastic frontier and to the inefficiencies.
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  • 57
    Digitale Medien
    Digitale Medien
    Springer
    The journal of supercomputing 5 (1991), S. 57-71 
    ISSN: 1573-0484
    Schlagwort(e): Mesh ; torus ; neighboring communication ; simulation ; routing ; embedding
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract Given a d-dimensional square mesh or square torus G and a c-dimensional square mesh or square torus H such that G and H are of the same size but may differ in dimensions and shapes, we study the problem of simulating in H parallel neighboring communications in G. We assume that the nodes in H have only unit-size buffers associated with the links, and that packets can be sent and received simultaneously from all outbound links and inbound links of the nodes. For permutation-type parallel neighboring communications, for all the combinations of graph types and graph shapes of G and H, except for the case in which d 〈 c and c is not divisible by d, we show that H can simulate G either optimally or optimally up to a constant multiplicative factor for fixed values of d and c. For scatter-type parallel neighboring communications, for some special cases of G and H, we also show that H can optimally simulate G. All these simulation times are smaller than the diameter of H, the lower bound on the routing complexity to support general data permutations in H.
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  • 58
    Digitale Medien
    Digitale Medien
    Springer
    Higher-order and symbolic computation 10 (1998), S. 237-271 
    ISSN: 1573-0557
    Schlagwort(e): abstract interpretation ; operational semantics ; collecting semantics ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract We present trace-based abstract interpretation, a unification of severallines of research on applying Cousot-Cousot-style abstract interpretation a.i. tooperational semantics definitions (such as flowchart, big-step, and small-step semantics)that express a program‘s semantics as a concrete computation tree of trace paths. Aprogram‘s trace-based a.i. is also a computation tree whose nodes contain abstractions ofstate and whose paths simulate the paths in the program‘s concrete computation tree.Using such computation trees, we provide a simple explanation of the central concept of collecting semantics, and we distinguish concrete from abstract collectingsemantics and state-based from path-based collecting semantics. We also expose therelationship between collecting semantics extraction and results garnered from flow-analytic and model-checking-based analysis techniques. We adapt concepts fromconcurrency theory to formalize “safe” and “live” a.i.‘s for computation trees; in particular, coinduction techniques help extend fundamental results to infinite computation trees. Problems specific to the various operational semantics methodologies are discussed: Big-step semantics cannot express divergence, so we employ a mixture of induction andcoinduction in response; small-step semantics generate sequences of programconfigurations unbounded in size, so we abstractly interpret source language syntax.Applications of trace-based a.i. to data-flow analysis, model checking, closure analysis,and concurrency theory are demonstrated.
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  • 59
    Digitale Medien
    Digitale Medien
    Springer
    The journal of supercomputing 11 (1997), S. 255-278 
    ISSN: 1573-0484
    Schlagwort(e): Ewald ; parallel ; T3E ; simulation ; electrostatic ; molecular dynamics ; PME
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract We report our work to parallelize the Particle Mesh Ewald (PME) method to compute the long-range electrostatic interactions in the molecular dynamics program AMBER and to extend the scalability of the PME method to hundreds of processors.
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  • 60
    Digitale Medien
    Digitale Medien
    Springer
    Journal of intelligent and robotic systems 8 (1993), S. 267-284 
    ISSN: 1573-0409
    Schlagwort(e): Intelligent control ; simulation ; agriculture ; robotics ; melon harvesting ; intelligent systems ; planning
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik , Maschinenbau
    Notizen: Abstract An intelligent control system for an agricultural robot which performs in an uncertain and unstructured environment was modelled as distributed, autonomous computing modules that communicate through globally accessible blackboard structures. The control architecture was implemented for a robotic harvester of melons. A CAD workstation was used to plan, model, simulate and evaluate the robot and gripper motions using 3-D, real-time animation. The intelligent control structure was verified by simulating the dynamic data flow scenarios of melon harvesting. Control algorithms were evaluated on measured melon locations. Picking time was reduced by 49% by applying the traveling salesman algorithm to define the picking sequence. Picking speeds can be increased by a continuous mode of operation. However, this decreases harvest efficiency. Therefore, an algorithm was developed to attain 100% harvest efficiency by varying the vehicle's forward speed. By comparing different motion control algorithms through animated visual simulation, the best was selected and thereby the performance improved.
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  • 61
    Digitale Medien
    Digitale Medien
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    Marketing letters 7 (1996), S. 77-94 
    ISSN: 1573-059X
    Schlagwort(e): promotion ; scanner data ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Wirtschaftswissenschaften
    Notizen: Abstract We summarize and critique seven theories that might explain the lack of a postpromotion dip in sales in the weeks following a promotion. We then propose and provide empirical support for a new explanation. We argue that in markets where the consumer category purchase decision is not strongly influenced by inventory levels, the displacement effect of accelerated sales will tend to be distributed fairly uniformly into the future such that clearly defined dips are not observed. We utilize a simulation based on real data to investigate this explanation. The simulation shows that given the degree to which inventory influences the purchase decision, we would not expect to see postpromotion dips, even though promotion influences the purchase decision. However, the simulation shows that if inventory had a greater influence on the purchase decision, we would expect to see postpromotion dips. We conclude with implications for both researchers and managers.
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  • 62
    ISSN: 1573-059X
    Schlagwort(e): multinomial probit ; semiparametric estimation ; Bayesian estimation ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Wirtschaftswissenschaften
    Notizen: Abstract In recent years, major advances have taken place in three areas of random utility modeling: (1) semiparametric estimation, (2) computational methods for multinomial probit models, and (3) computational methods for Bayesian estimation. This paper summarizes these developments and discusses their implications for practice.
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  • 63
    Digitale Medien
    Digitale Medien
    Springer
    Natural hazards 20 (1999), S. 215-229 
    ISSN: 1573-0840
    Schlagwort(e): lava ; Cellular Automata ; simulation ; hazard ; risk
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Energietechnik , Geographie , Geologie und Paläontologie
    Notizen: Abstract The model SCIARA, based on the “Cellular Automata” paradigm, is a versatile instrument whose scope is to analyse volcanic risk from lava flows. The possible fields of intervention are: [(a)] Long term forecasting of the flow direction at various eruption rates and points of emission by locating potential risk areas and permitting the creation of detailed maps of risk; [(b)] The possibility to follow the progress of an event and predict its evolution; [(c)] The verification of the possible effects of human intervention on real or simulated flows in stream deviation. A risk scenario has been developed for the Etnean territories of the towns of Nicolosi, Pedara and S. Alfio, simulating possible episodes with different vent locations along the fracture opened in the 1989 eruption and successively activated in the 1991–1993 eruption. The main characteristics of lava flows, that might be dangerous to the inhabited areas, have been analysed on the basis of the carried out Cellular Automata.
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  • 64
    Digitale Medien
    Digitale Medien
    Springer
    Journal of statistical physics 81 (1995), S. 681-715 
    ISSN: 1572-9613
    Schlagwort(e): Crystal growth ; Markov process ; stationary distribution ; growth rate ; thermodynamic limit ; cluster expansion ; coincidence probability ; simulation ; solid-on-solid
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract A new Markov process describing crystal growth in three dimensions is introduced. States of the process are configurations of the crystal surface, which has a terrace-edge-kink structure. The states are continuous along edges but discrete across edges, in accordance with the very different rates for the two types of captures of particles. Stationary distributions, describing steady crystal growth, are found in general. To our knowledge, these are the first examples of stationary distributions for layered crystal growth in three dimensions. The steady growth rate and other quantities are obtained explicitly for two interacting edges. For many interacting edges, growth behavior is determined (a) in various asymptotic regimes including thermodynamic limits, (b) via simulations, and (c) using series (cluster) expansions in the slope of the surface, the first three coefficients being computed. The theoretical growth rates show a marked dependence on surface dimensions. This may contribute to the size dependence and dispersion in the observed growth rate of small crystals.
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  • 65
    Digitale Medien
    Digitale Medien
    Springer
    Experimental astronomy 3 (1994), S. 121-122 
    ISSN: 1572-9508
    Schlagwort(e): VLT ; CONICA ; simulation ; SIMCO
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract CONICA is an acronym for COudé Near Infrared Camera. It is one of the four currently planned Infrared instruments for ESO's Very Large Telescope (VLT) in Chile. This multimode instrument is to be installed at the Coudé-focus (of Unit Telescope no. 1), where adaptive optics and speckle interferometry will also be available. High angular resolution imaging (to the diffraction limit) will be possible in the 1–5 μm range, as well as spectroscopy with low- and medium resolution and polarimetry by means of Wollaston prism and wiregrid analysers. Various softwares are developed for this instrument. One of them: the Simulation Software has a threefold aim: 1. provide the user with feedback information on his/her choices of observational parameters. This is be achieved by displaying the calculated performance and throughput of the combined Source-Atmosphere-Telescope-Camera-Detector system in various formats, such as images, tables, isophotical images ... for point as well as for extended sources (also annular, double, etc.); 2. verify if his expectations are realistic, before actually using CONICA itself. 3. give feedback on the design to the developers of the instrument.
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  • 66
    Digitale Medien
    Digitale Medien
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    Journal of intelligent and robotic systems 17 (1996), S. 309-325 
    ISSN: 1573-0409
    Schlagwort(e): modeling ; camera ; CCD ; subpixel ; simulation ; vision ; image ; diffusion ; CAD ; CIM ; bias
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik , Maschinenbau
    Notizen: Abstract In this paper we propose a modeling of an acquisition line made up of a CCD camera, a lens and a frame grabber card. The purpose of this modeling is to simulate the acquisition process in order to obtain images of virtual objects. The response time has to be short enough to permit interactive simulation. All the stages are modelised: in the first phase, we present a geometric model which supplies a point to point transformation that provides, for a space point in the camera field, the corresponding point on the plane of the CCD sensor. The second phase consists of modeling the discrete space which implies passing from the continous known object view to a discrete image, in accordance with the different orgin of the contrast loss. In the third phase, the video signal is reconstituted in order to be sampled by the frame grabber card. The practical results are close to reality when compared to image processing. This tool makes it possible to obtain a short computation time simulation of a vision sensor. This enables interactivity either with the user or with software for the design/simulation of an industrial workshop equipped with a vision system. It makes testing possible and validates the choice of sensor placement and image processing and analysis. Thanks to this simulation tool, we can control perfectly the position of the object image placed under the camera and in this way, we can characterise the performance of subpixel accuracy determining methods for object positioning.
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  • 67
    Digitale Medien
    Digitale Medien
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    Journal of intelligent and robotic systems 24 (1999), S. 125-149 
    ISSN: 1573-0409
    Schlagwort(e): planning ; control ; multi-fingered robot hand ; optimization ; internal force ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik , Maschinenbau
    Notizen: Abstract In this paper, the problem of controlling multi-fingered robot hands with rolling and sliding contacts is addressed. Several issues are explored. These issues involve the kinematic analysis and modeling, the dynamic analysis and control, and the coordination of a multi-fingered robot hand system. Based on a hand-object system in which the contacts are allowed to both roll and slide, a kinematic model is derived and analyzed. Also, the dynamic model of the hand-object system with relative motion contacts is studied. A control law is proposed to guarantee the asymptotic tracking of the object trajectory together with the desired rolling and/or sliding motions along the surface of the object. A planning approach is then introduced to minimize the contact forces so that the desired motion of the object and the relative motions between the fingers and the object can be achieved. Simulation results which support the theoretical development are presented.
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  • 68
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    Digitale Medien
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    Pure and applied geophysics 135 (1991), S. 559-576 
    ISSN: 1420-9136
    Schlagwort(e): Historical seismicity ; North China ; stress release ; point processes ; risk forecasting ; Markov processes ; earthquake models ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract Univariate and multivariate stress release models are fitted to historical earthquake data from North China. It is shown that a better fit is obtained by treating separately the Eastern part of the region, including the North China Plain and Bohai Sea, and the Western part of the region, including the Ordos Plateau and its Eastern boundary. Further improvement is obtained by fitting the large events (M≥7.6) and smaller events in the Western region by different stress release models. The comparisons are made by computing the likelihoods of the fitted models and discounting the number of parameters used by Akaike's AIC criterion. The models are used to develop long-term risk scenarios for the East and West regions.
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  • 69
    ISSN: 1349-9432
    Schlagwort(e): Fraunhofer diffraction ; diffraction pattern ; slit aperture ; knife edge ; smooth surface ; metal cylinder ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract The Fraunhofer diffraction pattern of an aperture formed between a knife edge and a smooth surface of a turned metal cylinder or half cylinder is different from the diffraction pattern of an ideal slit aperture. The diffraction pattern is simulated by adding the light reflected by the cylinder surface to the pattern formed by a slit, and is measured using a cylinder that has been partly cut away so that it can represent either a full or half cylinder. The theoretical and measured patterns agree well with each other.
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  • 70
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    Optical review 5 (1998), S. 247-251 
    ISSN: 1349-9432
    Schlagwort(e): simulation ; Sagnac interferometer ; gravitational wave ; coupled cavities ; dynamical cavities
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract A digital filtering approach to fast simulation for interferometric gravitational wave detectors of Michelson topology involving dynamical coupled cavities was developed in a recent paper. The present paper describes its application to detectors which might be of Sagnac topology with ring-cavities in arms and thus would be characterised by a different and more complicated nature of coupling among fields in various constituent cavities. The simulation results show that digital filtering approach is successful in developing a fast and reasonably accurate algorithm for the computation of dynamical evolution of fields in such a detector.
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  • 71
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    Acta mechanica solida Sinica 10 (1997), S. 283-298 
    ISSN: 0894-9166
    Schlagwort(e): fracture mechanics ; finite element ; simulation ; curved beam ; pipeline ; arrest
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract Mechanical arrestors are frequently used in gas transmission pipelines to prevent the possibility of long running cracks. These arrestors have the effect of reducing the pipe from opening as the crack passes. This decreases the available crack driving force and, as a result, crack arrest may take place. This essentially is a second line of defence against catastrophic failure in the event crack initiation cannot always be prevented. A novel analyzing method presented in this paper has been developed to investigate the suitability of these crack arrestors. This is based on a fluid/structure/fracture interaction package, PFRAC. A dynamic finite element approach has been developed for a three dimension curved beam with initial plane curvature, which has been implemented in PFRAC to simulate the behaviour of the arrestor. The contact conditions between the pipe wall and the arrestor, along with the various computational procedures, are described. Several numerical results for a cracked pipe with arrestors are presented and compared with those for pipes without arrestors.
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  • 72
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    Finance and stochastics 1 (1996), S. 25-41 
    ISSN: 1432-1122
    Schlagwort(e): Key words: Martingale estimating function ; option pricing ; quasi-likelihood ; simulation ; stochastic differential equation ; volatility. ; JEL classification: G12 ; Mathematics Subject Classification (1991): 62M05
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Notizen: Abstract. In the present paper we consider a model for stock prices which is a generalization of the model behind the Black–Scholes formula for pricing European call options. We model the log-price as a deterministic linear trend plus a diffusion process with drift zero and with a diffusion coefficient (volatility) which depends in a particular way on the instantaneous stock price. It is shown that the model possesses a number of properties encountered in empirical studies of stock prices. In particular the distribution of the adjusted log-price is hyperbolic rather than normal. The model is rather successfully fitted to two different stock price data sets. Finally, the question of option pricing based on our model is discussed and comparison to the Black–Scholes formula is made. The paper also introduces a simple general way of constructing a zero-drift diffusion with a given marginal distribution, by which other models that are potentially useful in mathematical finance can be developed.
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  • 73
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    Journal of nanoparticle research 1 (1999), S. 267-276 
    ISSN: 1572-896X
    Schlagwort(e): nanoparticles ; microemulsions ; reaction engineering ; formation model ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik , Technik allgemein
    Notizen: Abstract Engineering aspects of the preparation of palladium nanoparticles in non-ionic w/o-microemulsions are examined. In order to achieve reproducible synthesis conditions a semi-batch reactor with a standardized design is used. Influences of the stirring rate and of different ways of concentration control on the product properties are observed. For reproducible synthesis it is important to establish appropriate and defined preparation conditions. Monodisperse palladium particles of around 5 nm size are obtained by adding the microemulsion containing the palladium salt at a constant feed rate to the precharged microemulsion containing the reducing agent. A quantitative kinetic model is proposed to describe particle formation in microemulsions. Unknown parameters of the model have been estimated by independent examinations or can be achieved by fitting to the experimental data.
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  • 74
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    Transport in porous media 6 (1991), S. 407-433 
    ISSN: 1573-1634
    Schlagwort(e): Network modelling ; relative permeability ; simulation ; viscous fingering ; capillary forces
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Technik allgemein
    Notizen: Abstract We present a computer study of two-phase flow in a porous medium. The porous medium is represented by an isotropic network of up to 80 000 randomly placed nodes connected by thin tubes. We then simulate two-fluid displacements in this network and are able to demonstrate the effects of viscous and capillary forces. We use the local average flow rates and pressures to calculate effective saturation dependent relative pemeabilities, fractional flows and capillary pressures. Using a radial Buckley-Leverett theory, the mean saturation profile can be inferred from the solution of the fractional flow equation, which is consistent with the computed saturation. We show that the relative permeability may be a function of both viscosity ratio and capillary number.
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  • 75
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    Journal of fluorescence 10 (2000), S. 217-217 
    ISSN: 1573-4994
    Schlagwort(e): Corrected spectra ; simulation ; polyaromatics ; dyes
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract A new teaching method, using multimedia software, is in progress at our university. Running on a PC computer, it allows self-formation in molecular spectroscopy. A new software package devoted to photoluminescence (ISPL) is currently in progress. The concepts are introduced by using numerous graphs, animations, and simulations. The software allows electronic book facilities (search function, glossary, and bookmarks, for instance) and self-assessment. Tedious and time-consuming training is always necessary to acquire experimental skill. This is particularly true in the case of luminescence because numerous artifacts are possible and the number of parameters to be optimized is very large. In this view, a spectrofluorometer simulator appears to be a valuable tool that makes this training easy and fast: after choosing a set of experimental parameters, the computer displays immediately the corresponding spectrum. The influence of every experimental parameter is simulated: monochromator slit width, excitation or emission wavelengths, xenon source type, photomultiplier tube high-voltage, etc. The nature of solvents, with first- and second-order Rayleigh and Raman scattering, is managed. Sample geometry and concentration, with a corresponding inner filter, are also simulated. Obviously, there is no risk of damaging the virtual photomultiplier tube.
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  • 76
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    Acta mechanica Sinica 14 (1998), S. 147-156 
    ISSN: 1614-3116
    Schlagwort(e): fracture mechanics ; finite element ; simulation ; pipeline ; soil
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract This paper describes the use of the finite element method to simulate crack propagation in a pipeline in the presence of soil backfill. Since typical values for soil modulus are known, a spring element can be used to simulate the effects of backfilled soil on crack propagation. This consists of the relationship between the soil property and the stiffness of the spring element, and the effect of backfill depth on the crack propagation in the pipes. By equating crack driving force,G, in the presence of soil, to the dynamic fracture toughness of the pipeline material, a theoretical predictive capability is obtained for the first time for the gas transmission and distribution pipelines. Numerical calculations have also been carried out for the cracked pipe with backfill and without backfill.
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  • 77
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    Acta mechanica Sinica 13 (1997), S. 63-80 
    ISSN: 1614-3116
    Schlagwort(e): fracture mechanics ; pipeline ; experiment ; finite element ; simulation ; fracture toughness ; critical pressure
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract The Small-Scale Steady State (S4) test has been recently developed in order to assess the fracture behaviour of polyethylene (PE) gas distribution pipe material during rapid axial crack propagation. Based on an investigation of the S4 test, a simulation model of S4 test has been developed. This paper describes the use of the results obtained from the S4 test and program modified from PFRAC (Pipeline Fracture Analysis Code) to evaluate the fracture toughness of the material,G, which could not be directly obtained from the test, and to predict critical pressure,p c , for rapid crack propagation (RCP) in a full scale PE pipe. The algorithms for contact conditions are developed to consider the opening pipe wall impact against a series containment rings and the capabilities of PFRAC are also extended. WhenG d is evaluated, investigations are made on the effect of temperature, wall thickness and crack velocity. In addition, procedures to evaluate the critical pressure for the S4 test pipe are also discussed.
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  • 78
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    Computational economics 13 (1999), S. 93-101 
    ISSN: 1572-9974
    Schlagwort(e): efficiency ; multivariate probit ; quasi maximum likelihood ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik , Wirtschaftswissenschaften
    Notizen: Abstract This paper discusses the most efficient estimator among Quasi Maximum Likelihood Estimators using at most two levels of numerical integration, for the multivariate probit model. Simulations show that this estimator is more efficient but not more costly than the second-best alternative. However, its added efficiency depends on the correlation structure.
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  • 79
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    The journal of real estate finance and economics 11 (1995), S. 177-187 
    ISSN: 1573-045X
    Schlagwort(e): asset valuation ; simulation ; risk ; stochastic rent
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Wirtschaftswissenschaften
    Notizen: Abstract Introduction of the Present Value Distribution Model (PVD) offers an alternative method for the valuation of projects yielding intertemporal stochastic rents. A combination of concepts from many areas of the literature yields the given model. The base procedure relies on Monte Carlo Simulation with the application of recently established theories on stochastic rents, path dependent cash flow trajectories, and period dependent discount rates. Among the benefits of the (PVD) are exogenous risk discounting and consistent distribution determination. Risk discounting is applied to the resultant of the distribution model rather than within the model. Furthermore, the present value distributions are independent of analyst's perceptions yielding an objective single period gamble.
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  • 80
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    Review of quantitative finance and accounting 13 (1999), S. 189-207 
    ISSN: 1573-7179
    Schlagwort(e): simulation ; financial statement ; monthly statement
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Wirtschaftswissenschaften
    Notizen: Abstract Many accounting and finance studies investigate the time-series properties of historical accounting records from corporate financial statements. Some of them have recognized the potential benefits of using disaggregated monthly accounting records. Disaggregated data are beneficial because one can use more data points within a relatively short period of time, thus reducing the chance of structural change. The added data points and reduction of the number of variables needed to accommodate potential structural changes can enhance the statistical power of any subsequent analysis. The use of disaggregated data may also improve the predictive ability of time-series analytic approaches. In order to systematically assess various financial indicators and investigate the effects of different organizational characteristics, a large number of monthly statements with certain predetermined characteristics are desirable. However, such statements are not readily available. At best, monthly statements can be obtained from a few volunteer companies. Under this circumstance, simulation of controlled financial statements seems to be a reasonable solution. This research explores a methodology for simulating complete monthly financial statements based on actual company quarterly financial statements. The methodology incorporates the interrelationships among accounting numbers and the effects of exogenous variables. To test the empirical validity and whether the monthly results derived from the quarterly data can accurately track the real monthly figures, we compare the results simulated by the proposed method and those generated by a naive random walk model. We test both complete financial statements for three companies and sales statistics from the retail industry. The results of both tests demonstrate the superiority of the method proposed by this study over a naive random walk model. The proposed simulation method provides an opportunity for researchers to examine the time-series properties of financial statement elements by using the monthly data of a large number of companies. In addition, the simulation approach allows researchers to perform cross sectional comparisons on companies with different characteristics (e.g., sales behavior patterns and degrees of stability) in their financial and economic activities. Moreover, it enables the researchers to manipulate some of these characteristics to test various hypotheses.
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  • 81
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    Review of quantitative finance and accounting 14 (2000), S. 45-65 
    ISSN: 1573-7179
    Schlagwort(e): bid-ask spread ; event study methods ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Wirtschaftswissenschaften
    Notizen: Abstract This study examines empirical issues associated with the use of bid-ask spreads in event studies. The simulation results indicate that the distribution of average standardized abnormal spread shows little deviation from normality. Simulation results also indicate that the widely used percent spread metric results in test statistics with low power. In contrast, use of a standardized raw spread metric and a simple mean-adjusted expectation model results in well specified and reasonably powerful Patell and Brown-Warner type test statistics. As the abnormal spread series is characterized by high first order serial correlation, it is important to adjust for this serial correlation when using multi-day event windows.
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  • 82
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    Review of quantitative finance and accounting 7 (1996), S. 289-298 
    ISSN: 1573-7179
    Schlagwort(e): time diversification ; stochastic dominance ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Wirtschaftswissenschaften
    Notizen: Abstract We use stochastic dominance to test whether investor should prefer riskier securities as the investment horizon lengthens. Return distributions for stocks, bonds, and U.S. Treasury bills are generated for holding periods of one to 25 years by simulation. For each holding period, stochastic dominance tests are run to establish preferences between the alternative security classes. Contrary to previous mean-variance based studies, we find no evidence that high-risk securities (stocks) dominate low-risk securities (bonds, Treasury bills) as the investment horizon lengthens. However, we do find that corporate bonds systematically dominate government bonds.
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  • 83
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    Computational economics 10 (1997), S. 107-118 
    ISSN: 1572-9974
    Schlagwort(e): visualisation ; simulation ; linear quadratic control ; observer ; Kalmanfilter
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik , Wirtschaftswissenschaften
    Notizen: Abstract Simulation of economic models is frequently used in the investigation of economic policy. Yet one of the problems with simulation is that it can be difficult to appreciate the model properties due to the nature of the simulation process. Stochastic simulation, for example, can produce large quantities of output which can be difficult to comprehend. Further, when mathematically sophisticated techniques such as the use of optimal control and Kalman Filtering are applied to models, the simulation process can become even more complex. Visualisation techniques in model building, simulation, and analysis of simulation output can help reduce the complexity. This is especially the case with interactive simulation. In this paper we investigate the use of visualisation in simulation by examining the application of optimal control techniques to a stochastic forward looking analytic economic model. We also use interactive object oriented simulation software where objects, such as components of models or graphs of outputs, can be visually manipulated to form simulation systems. We find that the use of visualisation can make the investigation of policy analysis issues with such models more comprehensible.
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  • 84
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    Mathematical geology 22 (1990), S. 175-188 
    ISSN: 1573-8868
    Schlagwort(e): auto-correlation ; autoregressive model ; cross-correlation ; space ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Mathematik
    Notizen: Abstract Analytical formulation of a general simulation model for generation of anisotropic as well as isotropic synthetic patterns in one, two, or three dimensions is proposed. It has significance for the purpose of modeling geologic properties such as ore grades, reservoir porosity, mineral distribution, fracture spacings, aperture, orientations, etc. General procedures for such a simulation by the autoregressive process are given for model parameters estimation and synthetic pattern generation. The model works on the square net basis and generates sequential patterns first along any desired direction for unidimensional simulation and then two-dimensional patterns are constructed with reference to two orthogonal unidimensional sequences. Applications to synthetic two-dimensional pattern are shown for isotropic cases with different model parameters. The extension of the model to three-dimensional space is readily available.
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  • 85
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    Mathematical geology 22 (1990), S. 151-173 
    ISSN: 1573-8868
    Schlagwort(e): information theory ; multidimensional normality ; profile log-likelihood ; prediction ; simulation ; statistical inference
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Mathematik
    Notizen: Abstract The analysis of, and from, models of spatial data usually proceeds under the assumption, often implicit, that the correct model has been specified. However, any model identification procedures based on sample data are subject to error, and consequences of such errors then permeate subsequent analysis. Thus, an attempt to quantify some of these consequences is of interest. A standard framework for analysis is extended here, by introduction of information theory, to permit the study of effects of model misspecification on maximum likelihood estimators of parameters of model covariance. Asymptotically valid theoretical results are presented, and the relevance of these results to samples of finite sizes met in practice is assessed in a series of simulation experiments. The effect of model misspecification, and use of estimators of parameters of misspecified covariance models, on the practical problem of prediction at a previously unsampled location is considered briefly, and further areas for possible investigation are outlined.
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  • 86
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    Mathematical geology 28 (1996), S. 723-734 
    ISSN: 1573-8868
    Schlagwort(e): simulation ; probability field ; uniform transform ; filtering ; histogram reproduction
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Mathematik
    Notizen: Abstract A combination of factorial kriging and probability field simulation is proposed to correct realizations resulting from any simulation algorithm for either too high nugget effect (noise) or poor histogram reproduction. First, a factorial kriging is done to filter out the noise from the noisy realization. Second, the uniform scores of the filtered realization are used as probability field to sample the local probability distributions conditional to the same dataset used to generate the original realization. This second step allows to restore the data variance. The result is a corrected realization which reproduces better target variogram and histogram models, yet honoring the conditioning data.
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  • 87
    ISSN: 1573-8868
    Schlagwort(e): simulation ; nonlinear conditioning ; optimization ; FFT ; local perturbation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Mathematik
    Notizen: Abstract A fast Fourier transform (FFT) moving average (FFT-MA) method for generating Gaussian stochastic processes is derived. Using discrete Fourier transforms makes the calculations easy and fast so that large random fields can be produced. On the other hand, the basic moving average frame allows us to uncouple the random numbers from the structural parameters (mean, variance, correlation length, ... ), but also to draw the randomness components in spatial domain. Such features impart great flexibility to the FFT-MA generator. For instance, changing only the random numbers gives distinct realizations all having the same covariance function. Similarly, several realizations can be built from the same random number set, but from different structural parameters. Integrating the FFT-MA generator into an optimization procedure provides a tool theoretically capable to determine the random numbers identifying the Gaussian field as well as the structural parameters from dynamic data. Moreover, all or only some of the random numbers can be perturbed so that realizations produced using the FFT-MA generator can be locally updated through an optimization process.
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  • 88
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    International journal of parallel programming 21 (1992), S. 193-224 
    ISSN: 1573-7640
    Schlagwort(e): Program unification ; priority queue ; parallel ; vector ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract In this experimental study, we apply the technique of program unification to priority queues. We examine the performance of a variety of unified priority queue implementations on a Cray Y-MP. The scope of the study is restricted to determining if different implementations of priority queues exhibit markedly different performance characteristics under program unification. We found this to be true. In a larger view, this result has interesting consequences in the application of program unification to discrete event simulations on vector or SIMD machines. We find the heap to be a promising data structure in the program unification paradigm.
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  • 89
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    Journal of network and systems management 3 (1995), S. 371-380 
    ISSN: 1573-7705
    Schlagwort(e): Telephone traffic ; network management ; control theory ; dynamic flows ; stability ; routing algorithms ; broadband networks ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract The control of telephony traffic is the task of network management and routing algorithms. In this paper, a study of two trunk groups carrying telephony traffic is used to show that instabilities can arise if there is a delay in getting feedback information for a network controller. The network controller seeks to balance the traffic in the two trunk groups, which may represent two paths from a source to a destination. An analysis shows how factors such as holding time, controller gain and feedback delay influence stability. Simulation of a two service case is also carried out to show that the same instabilities can arise.
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  • 90
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    Journal of intelligent information systems 14 (2000), S. 85-94 
    ISSN: 1573-7675
    Schlagwort(e): decision-support ; information systems ; simulation ; prediction ; interfaces
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract Decision makers are expected to make decisions which have a positive effect on the future of their enterprises. We expect that intelligent information systems support their activities. Today, databases and web-based resources, accessed through effective communications, make information about the past rapidly available. To project the future the decision makers either have to use intuition or employ tool for prediction, and initialize such tools with information obtained from an information system to such tools. An effective information system should integrate forecasting the future, and because there choices have been made, such a system must also support the comparative assessment of the effects of alternate decisions. The complexity of an information system handling the past, and multiple futures will be great, and must be modularized with effective interfaces. We recommend the use of an SQL-like interface language to access existing tools to assess the future, as spreadsheets and simulations. Making results of simulations accessible as another contribution to integrated information systems has the potential of greatly augmenting their effectiveness and really support decision-making.
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  • 91
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    Applied intelligence 8 (1998), S. 195-218 
    ISSN: 1573-7497
    Schlagwort(e): modelling ; simulation ; teams ; coordination ; organizations
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik
    Notizen: Abstract The problem of modelling air missions is part of a larger problem—simulating possible war-like scenarios in the air, sea, and on land. In modelling such military systems one is required to model the behaviour of various actors and the resources that are available to them. One aspect of this problem is the modelling of a group of actors as a team and then modelling the coordinated behaviour of such a team to achieve a joint goal. In the domain of air mission modelling the actors are pilots who control aircraft and their behaviour is referred to as tactics. In this paper we present the approach we adopted in modelling teams and team tactics as part of the development of the Smart Whole AiR Mission Model (SWARMM) for the Air Operations Division of the Australian Defence Science and Technology Organization. In our approach teams are composed of sub-teams and adopt organizational structures. Such structures define the responsibilities of the sub-teams towards the mission to be achieved as well as towards the control and coordination of the sub-teams. We also describe how communication is used when adopting a variety of control and coordination strategies and how one could reason about the choice of organizational structures for a given mission and situation.
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  • 92
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    Autonomous robots 3 (1996), S. 253-268 
    ISSN: 1573-7527
    Schlagwort(e): underwater robotics ; hydrodynamics ; simulation ; articulated mechanisms ; tree topologies ; object-oriented design
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik , Maschinenbau
    Notizen: Abstract This paper presents a computational framework for efficiently simulating the dynamics and hydrodynamics of Underwater Robotic Vehicle (URV) systems. Through the use of object-oriented mechanisms, a very general yet efficient version of the Articulated-Body (AB) algorithm has been implemented. An efficient solution to branching within chains is developed in the paper so that the algorithm can be used to compute the dynamics for the entire class of open-chain, tree-structured mechanisms. By including compliant contacts with the environment, most closed-chain systems can also be modeled. URV systems with an extended set of topologies can be simulated including proposed underwater walking machines with intra-body powered articulations. Using the encapsulation inherent in C++, the hydrodynamics code has been confined to a single class, thereby explicitly defining this framework and providing an environment for readily implementing desired hydrodynamics algorithms. Resulting simulations are very efficient and can be used in a number of applications both in the development and use of URV systems.
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  • 93
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    Autonomous robots 6 (1999), S. 281-292 
    ISSN: 1573-7527
    Schlagwort(e): mobile robotics ; reinforcement learning ; artificial neural networks ; simulation ; real world
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik , Maschinenbau
    Notizen: Abstract We present a case study of reinforcement learning on a real robot that learns how to back up a trailer and discuss the lessons learned about the importance of proper experimental procedure and design. We identify areas of particular concern to the experimental robotics community at large. In particular, we address concerns pertinent to robotics simulation research, implementing learning algorithms on real robotic hardware, and the difficulties involved with transferring research between the two.
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    Springer
    Natural resources research 8 (1999), S. 49-58 
    ISSN: 1573-8981
    Schlagwort(e): Resource assessment ; anchored method ; Geo-anchored method ; Horvitz–Thompson estimator ; sensitivity test ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie
    Notizen: Abstract The Geo-anchored method, based on a moment-type estimator, has been developed for estimating parent population properties from a successive sample of discoveries. By substituting the expectation of the waiting time z (n+1) of the (n + 1) th discovery to occurrence for an unknown parameter λ in the anchored method, the Geo-anchored method allows estimation of inclusion probabilities directly from observed data, thus eliminating the need for a priori selection of a value of N, R, or some other feature of the parent population. Because direct estimation of N and R requires an ordered sample, the Geo-anchored method is more sensitive to the data-generating process than the anchored method. This paper presents a sensitivity study on the Geo-anchored method. The test is based on simulated discovery sequences with different assumptions regarding discovery efficiency, exploration maturity, and the shape of the parent field size distribution. As a reference for comparison, estimates from the Horvitz–Thompson estimator also are presented.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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  • 95
    Digitale Medien
    Digitale Medien
    Springer
    Water resources management 11 (1997), S. 395-406 
    ISSN: 1573-1650
    Schlagwort(e): irrigation reservoirs ; operation ; water deficit ; yield response ; multicrop environment ; stochastic dynamicprogramming ; soil moisture ; water allocation ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Geographie
    Notizen: Abstract A two-phase stochastic dynamic programming model is developed for optimal operation of irrigation reservoirs under a multicrop environment. Under a multicrop environment, the crops compete for the available water whenever the water available is less than the irrigation demands. The performance of the reservoir depends on how the deficit is allocated among the competing crops. The proposed model integrates reservoir release decisions with water allocation decisions. The water requirements of crops vary from period to period and are determined from the soil moisture balance equation taking into consideration the contribution of soil moisture and rainfall for the water requirements of the crops. The model is demonstrated over an existing reservoir and the performance of the reservoir under the operating policy derived using the model is evaluated through simulation.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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  • 96
    Digitale Medien
    Digitale Medien
    Springer
    Water resources management 11 (1997), S. 437-444 
    ISSN: 1573-1650
    Schlagwort(e): water first and second drainage ; layered unsaturated soil ; experiment ; γ-radiation ; simulation ; finite differences ; periodically changing head
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Geographie
    Notizen: Abstract Experimental results during first and second drainage in a vertical column of saturated layered soil were compared to those predicted from simulation. The sample was composed of a sandyloam soil overlying a fine sand. The soil water content was measured by using γ-ray absorption method (241Am) and the water pressure through tensiometers, arranged vertically along the column and connected to pressure transducers. From the evaluation of moisture and pressure versus time, the characteristic curves Ψ(θ) of the layers were obtained and approximated by van Genuchten's analytical equation. The relationship K(θ) between hydraulic conductivity and moisture was estimated by van Genuchten's prediction model. Ψ(θ) and K(θ) equations were used as inputs in the numerical model. The drainage of water was simulated by Richard's partial differential equation, which was solved with the finite differences computational scheme type Laasonen. The upper and lower boundary conditions were zero flux and a periodically changing head respectively. Numerical results show a good agreement with experimental data, with small deviations for certain hours.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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  • 97
    Digitale Medien
    Digitale Medien
    Springer
    Water resources management 12 (1998), S. 13-30 
    ISSN: 1573-1650
    Schlagwort(e): risk ; reliability ; resiliency ; vulnerability ; drought risk index ; drought damage index ; simulation ; water supply ; reservoir operation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Geographie
    Notizen: Abstract In this study, simulation is used to evaluate the performance of the municipal water system in Fukuoka city. In combination with daily simulation model, a kind of risk model incorporating water demand prediction is presented. This model applies five risk indices: reliability, resiliency, vulnerability, drought risk index (DRI) and drought damage index (DDI). They aid in the identification of operation policies for the municipal water system, and the planning and operational policies obtained are aimed at achieving minimum risk for a given scenario of operation. In this paper, the performance risk of the municipal water system is investigated under three alternatives: (1) the existing system operation when available supply from the Chikugo river is decreased; (2) water restrictions for different percentages of reduction are implemented; and (3) available water supply increases when desalination of sea water is implemented. The results obtained show that savings of between 5 and 12% of water consumption from May 1, or increasing of daily desalination of sea water about 30 000 m3 or more, may efficiently decrease the performance risk of the Fukuoka water supply system. Potentials also exist for further increase of reservoir storage by more rational operation. The measure that more attention should be paid to increasing the water supply from stable sources is recommended as well.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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  • 98
    Digitale Medien
    Digitale Medien
    Springer
    Water resources management 12 (1998), S. 285-293 
    ISSN: 1573-1650
    Schlagwort(e): agrometeorological models ; PRZM ; simulation ; SWATRER ; water balance
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Geographie
    Notizen: Abstract Two methods widely used in agrometeorogical simulation models for describing field soil water regimes are compared and evaluated based on their ability to predict experimental data from a single field study. The first method is based on a numerical solution to the Richards' equation for describing water movement in the soil profile, while the second method is based on soil-water capacity terms. The evaluation of the performance of the two methods is based on goodness-of-fit statistics and graphical analysis. Results indicate that the first method is more accurate for both the upper and the lower soil zones; the second method, despite its low performance in the upper soil zone can adequately describe soil moisture conditions in the lower soil zones. Recommendations are made for model use based on comparison results, the intended model use, and the difficulty in the acquisition of required model inputs.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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  • 99
    Digitale Medien
    Digitale Medien
    Springer
    Water resources management 6 (1992), S. 149-159 
    ISSN: 1573-1650
    Schlagwort(e): Manning's equation ; vegetated open channels ; submerged aquatic weeds ; simulation ; Egyptian canals
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Geographie
    Notizen: Abstract Modified formulas for Manning's equation are developed for use in the design of earthen open channels with submerged aquatic weeds. The proposed relations have their basis in field and experimental data. A multiple regression analysis was used to develop a relationship between the mean velocity, hydraulic radius, and water surface slope. The new formulas are dependent on the distribution of weeds along the channel's wetted perimeter and the percentage of vegetation density with respect to the cross-section area of the waterway.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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  • 100
    Digitale Medien
    Digitale Medien
    Springer
    Water resources management 6 (1992), S. 101-115 
    ISSN: 1573-1650
    Schlagwort(e): Staged disaggregation ; parameter estimation ; Mejia-Rousselle model ; moment preservation ; simulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Geographie
    Notizen: Abstract The problem of moment preservation associated with the use of the Mejia and Rousselle model in a staged disaggregation scheme is addressed in this paper. General parameter estimation equations are derived. Different from the original parameter estimation equations, the equations proposed herein employ two corrected historical moments which are shown to be conditional on the stage and the annual input generation model. With parameters estimated from the proposed equations, the modeling can exactly preserve the historical moments of interest and maintain the linear relationship between the input and output of each disaggregation process.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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