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  • 1
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have extended the mechanical equilibrium theory of J. W. Matthews and A. E. Blakeslee [J. Cryst. Growth 27, 118 (1974)] (MB) for determining the critical thickness in semiconducting heteroepitaxial films by including the effect of the Peierls barrier. The new formulation allows an evaluation of the dependence of critical thickness on the orientation of epithreading dislocation, and a comparison of theoretical predictions with measurements indicates that a knowledge of the epithreading dislocation orientation is necessary in predicting critical thicknesses in heteroepitaxial structures. In this formulation, the effect of the Peierls barrier is to bring the theoretical critical thicknesses closer to experimental values as compared to the predictions of the MB theory.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2018-12-01
    Description: The unsteady, three-dimensional flow field of a helicopter rotor blade encountering a passing vortex is calculated by solving the Euler/thin layer Navier-Stokes equations by a finite-difference numerical procedure. A prescribed vortex method is adopted to preserve the structure of the interacting vortex. The cases considered for computation correspond to the experimental model rotor test conditions of Caradonna, et al. and consist of parallel and oblique interactions. Comparison of the numerical results with test data show good agreement for both parallel and oblique interactions at subsonic and transonic tip speeds.
    Keywords: AERODYNAMICS
    Type: AIAA PAPER 89-1848
    Format: text
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  • 3
    Publication Date: 2011-08-24
    Description: The viscous, three-dimensional, flowfields of UH60 and BERP rotors are calculated for lifting hover configurations using a Navier-Stokes computational fluid dynamics method with a view to understand the importance of planform effects on the airloads. In this method, the induced effects of the wake, including the interaction of tip vortices with successive blades, are captured as a part of the overall flowfield solution without prescribing any wake models. Numerical results in the form of surface pressures, hover performance parameters, surface skin friction and tip vortex patterns, and vortex wake trajectory are presented at two thrust conditions for UH60 and BERP rotors. Comparison of results for the UH60 model rotor show good agreement with experiments at moderate thrust conditions. Comparison of results with equivalent rectangular UH60 blade and BERP blade indicates that the BERP blade, with an unconventional planform, gives more thrust at the cost of more power and a reduced figure of merit. The high thrust conditions considered produce severe shock-induced flow separation for UH60 blade, while the BERP blade develops more thrust and minimal separation. The BERP blade produces a tighter tip vortex structure compared with the UH60 blade. These results and the discussion presented bring out the similarities and differences between the two rotors.
    Keywords: AERODYNAMICS
    Type: In: AHS and Royal Aeronautical Society, Technical Specialists' Meeting on Rotorcraft Acoustics(Fluid Dynamics, Philadelphia, PA, Oct. 15-17, 1991, Proceedings (A93-29401 10-71); 18 p.
    Format: text
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  • 4
    Publication Date: 2013-08-31
    Description: Thin layer Navier-Stokes and Euler equations are numerically solved using a multi-block zonal approach to simulate the formation and roll up of tip vortices of wings in subsonic and transonic flows. Several wing planforms were considered to examine the influence of tip-cap shape, planform geometry and free stream Mach number on the formation process. A good definition of the formation and qualitative roll up of tip vortices was achieved.
    Keywords: AERODYNAMICS
    Type: AD-A172312 , A-86415 , NAS 1.15:88334 , USAAVSCOM-TM-86-A-4 , NASA-TM-88334
    Format: application/pdf
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  • 5
    Publication Date: 2013-08-31
    Description: Unsteady interactions of strong concentrated vortices, distributed gusts, and sharp-edged gusts with stationary airfoils were analyzed in two-dimensional transonic flow. A simple and efficient method for introducing such vortical disturbances was implemented in numerical codes that range from inviscid transonic small disturbance to thin-layer Navier Stokes. The numerical results demonstrate the large distortions in the overall flow field and in the surface air loads that are produced by various vortical interactions. The results of the different codes are in excellent qualitative agreement, but, as might expected, the transonic small-disturbance calculations are deficient in the important region near the leading edge.
    Keywords: AERODYNAMICS
    Type: REPT-85075 , NAS 1.15:86658 , NASA-TM-86658 , USAAVSCOM-TM-84-A-10 , AD-A152417
    Format: application/pdf
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  • 6
    Publication Date: 2013-08-31
    Description: A procedure for calculating the interaction of a vortex with a two dimensional airfoil in a uniform free stream is presented along with results for several test cases. A Lamb like anaytical vortex having a finite core and convect in a uniform free stream interacts with the flowfield of NACA 0012 or NACA 64A006 airfoil in transonic and subsonic flow. Euler and thin layer Navier-Stokes solutions are computed and the results are compared with the results from transonic small disturbance code and available experimental results. For interactions within the limits of transonic small disturbance assumptions, the three methods gave qualitatively similar results of a two bladed helicopter rotor and suggest that the time lag effects of the free stream velocity approaching the blade may be important and should be considered in the analysis. In general, the results show a tremendous influence of the interacting vortex on the flowfield around the airfoil. This is particularly true when the vortex is stationary. For a convecting vortex, the most dramatic changes in the flowfield seem to occur when the vortex is within one chord of the airfoil.
    Keywords: AERODYNAMICS
    Type: NASA-CR-3885 , FSI-84-01 , NAS 1.26:3885
    Format: application/pdf
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 62 (1987), S. 414-418 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A parameter free, microscopic model has been formulated to study the energy deposition due to protons bombarding silicon samples. The model is based on a Monte Carlo technique and uses two sets of essential and fundamental quantities from nuclear physics and condensed matter physics, the energy spectra and angular distributions of light fragments and heavy recoil nuclei from proton induced reactions, and the stopping power of ions through condensed media. Our model calculations are found to agree very well with the pulse-height spectra measured by McNulty, Farrell, and Tucker [IEEE Trans. Nucl. Sci. NS-28, 4007 (1981)] and by Farrell and McNulty [IEEE Trans. Nucl. Sci. NS-29, 2012 (1982)] for thin slabs of silicon (8.7–200 μm) exposed to protons at various energies (86–158 MeV). We present a critical analysis of the contributions of various spallation products to the total spectra and discuss the physical implications of such results for semiconductor devices.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 4574-4582 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a methodology for calculating the thermodynamic and configurational properties of a classical system of N interacting particles. For solids we show how the method can be used to determine the structural properties in the equilibrium state. As an application we analyze the formation of misfit dislocations in a nickel-gold bicrystal with a (100) interface. We show that the dislocation exists in a state of minimum Helmholtz free energy. The equilibrium structure and energy are also determined through Monte Carlo calculations and they show good agreement with the results obtained from our method.
    Type of Medium: Electronic Resource
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  • 9
    Publication Date: 2018-12-01
    Description: A perturbation form of an implicit conservative, noniterative numerical algorithm for the two-dimensional thin layer Navier-Stokes and Euler equations is used to compute the interaction flow-field of a vortex with stationary airfoil. A Lamb-like analytical vortex having a finite core is chosen to interact with a thick (NACA 0012) and a thin (NACA 64A006) airfoil independently in transonic flow. Two different configurations of vortex interaction are studied, viz., (1) when the vortex is fixed at one location in the flowfield, and (2) when the vortex is convecting past the airfoil at freestream velocity. Parallel computations of this interacting flowfield are also done using a version of the Transonic Small Disturbance Code (ATRAN2). A special treatment of the leading edge region for thin airfoils is included in this code. With this, the three methods gave qualitatively similar results for the weaker interactions considered in this study. However, the strongest interactions considered proved to be beyond the capabilities of the small disturbance code. The results also show a far greater influence of the vortex on the airfoil flowfield when the vortex is stationary than when it is convecting with the flow.
    Keywords: AERODYNAMICS
    Type: AIAA PAPER 84-0254
    Format: text
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  • 10
    Publication Date: 2017-10-02
    Description: Unsteady interactions of strong concentrated vortices, distributed gusts, and sharp-edged gusts with stationary airfoils were analyzed in two-dimensional transonic flow. A simple and efficient method for introducing such vortical disturbances was implemented in numerical codes that range from inviscid, transonic small-disturbance to thin-layer Navier Stokes. The numerical results demonstrate the large distortions in the overall flow field and in the surface air loads that are produced by various vortical interactions. The results of the different codes are in excellent qualitative agreement, but, as might be expected, the transonic small-disturbance calculations are deficient in the important region near the leading edge.
    Keywords: AERODYNAMICS
    Type: AGARD Unsteady Aerodynamics-Fundamentals and Applications to Aircraft Dynamics; 13 p
    Format: text
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