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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Experiments in fluids 10 (1991), S. 181-193 
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Digital particle image velocimetry (DPIV) is the digital counterpart of conventional laser speckle velocitmetry (LSV) and particle image velocimetry (PIV) techniques. In this novel, two-dimensional technique, digitally recorded video images are analyzed computationally, removing both the photographic and opto-mechanical processing steps inherent to PIV and LSV. The directional ambiguity generally associated with PIV and LSV is resolved by implementing local spatial cross-correlations between two sequential single-exposed particle images. The images are recorded at video rate (30 Hz or slower) which currently limits the application of the technique to low speed flows until digital, high resolution video systems with higher framing rates become more economically feasible. Sequential imaging makes it possible to study unsteady phenomena like the temporal evolution of a vortex ring described in this paper. The spatial velocity measurements are compared with data obtained by direct measurement of the separation of individual particle pairs. Recovered velocity data are used to compute the spatial and temporal vorticity distribution and the circulation of the vortex ring.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract An experimental study has been conducted to investigate the effect of velocity ratio on the approach of a plane mixing layer to self-similarity. Plane mixing layers with five different velocity ratios (0.5, 0.6, 0.7, 0.8 and 0.9) were generated in a newly designed mixing layer wind tunnel with both initial boundary layers tripped. For each velocity ratio, mean flow and turbulence measurements were obtained at eight streamwise locations with a single cross-wire probe. The results indicate that the splitter plate wake plays a very dominant and, in some cases, a lasting role in the development of the mixing layer. For velocity ratios between 0.5 and 0.7, self-similarity of the mixing layer was observed with the asymptotic states comparable. Mixing layers with the higher velocity ratios failed to achieve a self-similar state within the measurement domain, although a slow approach to it was apparent. The development distance decreased with increasing velocity ratio up to 0.7, after which it appeared to increase. Almost all of the observed effects may be attributed to the presence of the splitter plate wake and its complex interaction with the mixing layer.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract A rapid time series of photographs of the horizontal cross-sections of several y + locations were taken of a turbulent open-channel water flow with Re d = 3,900. A pair of photographic images were obtained with a time difference of 1.3 v/u τ 2 at each y + locations. The pictures were digitized into 8 bit data with a spatial resolution of 2.5 viscous scales. Instead of identifying discrete particles, a variable interval spatial correlation technique was used to extract the velocity components. With this technique, two-dimensional spatial cross-correlations of the illumination intensities were taken between a pair of picture images. The correlations were taken over small areas and the peak of the correlation coefficients were used to obtain the convection velocity yielding the u and w components of velocity. Some statistical properties were calculated and are shown to be comparable with previous data. Spatial correlations of the velocity components revealed some unique characteristics related to the structure of turbulence.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract This paper reports the investigation of mean and turbulent flow characteristics of a two-dimensional plane diffuser. Both experimental and theoretical details are considered. The experimental investigation consists of the measurement of mean velocity profiles, wall static pressure and turbulence stresses. Theoretical study involves the prediction of downstream velocity profiles and the distribution of turbulence kinetic energy using a well tested finite difference procedure. Two models, viz., Prandtl's mixing length hypothesis and k-ɛ model of turbulence, have been used and compared. The nondimensional static pressure distribution, the longitudinal pressure gradient, the pressure recovery coefficient, percentage recovery of static pressure, the variation of U max/U bar along the length of the diffuser and the blockage factor have been valuated from the predicted results and compared with the experimental data. Further, the predicted and the measured value of kinetic energy of turbulence have also been compared. It is seen that for the prediction of mean flow characteristics and to evaluate the performance of the diffuser, a simple turbulence model like Prandtl's mixing length hypothesis is quite adequate.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Experiments in fluids 10 (1991), S. 294-296 
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract This investigation addresses simulation of flow visualization of vortical structures, accounting for both the circulation and the degree of concentration of vorticity of the vortices via the exact nonlinear solution of Stuart for an unsteady mixing layer. At a fixed value of circulation, an increased concentration of vorticity (which corresponds to decrease in the area containing most of the vorticity) actually spreads the visualization marker over an increased area of the flow. Moreover, different combinations of vorticity concentration and circulation give essentially the same flow patterns.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Experiments in fluids 10 (1991), S. 281-284 
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Further experimental results on transition of boundary layer flows in the presence of streamwise counter-rotating Goertler vortices were obtained on concave test surface of 3.0 m radius of curvature. The test surface was mounted in a perspex (plexiglass) curved rectangular section duct connected to a low speed, blow down type, wind tunnel for a free-stream velocity range of 2.0 to 13.1 m/s. Velocity measurements were made using a single sensor hot-wire anemometer and boundary layer flow transitions were detected by a frequency spectrum method. The experimental results show a simple linear relationship between transition start position and free-stream velocity, and lie well between the limit lines of Goertler number transition criteria. It is found that the onset of transition at the flow upwash region occurs when the Goertler number based on the boundary layer momentum thickness reaches a value of about 7.5.
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Experiments in fluids 10 (1991), S. 296-298 
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Experiments in fluids 10 (1991), S. 299-300 
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Conclusions Flow visualizations obtained in a two-phase jet flow with 80 μm particles at a mass loading of 5% revealed the following. 1. Particles exited the jet tube in straight trajectories with “fan-spreading” angles of up to ±30°. The velocities of those particles having large angles were considerably lower (40% or more) than the mean particle velocity of the bulk flow. 2. Reducing the mean particle velocity at the exit from 25 m/s to 7 m/s reduced the “fan-spreading” effect with a maximum angle of around 10°.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Experiments in fluids 10 (1991), S. 322-332 
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract An experimental investigation was made of the flow over a grooved circular cylinder of different aspect ratios. Based on the drag coefficient, Strouhal number and mean shear stress, three flow regimes of subcritical, critical and supercritical were found, all of which are below the subcritical Reynolds number of a smooth cylinder. Boundary layer characteristics within the different flow regimes were measured. The shift of the boundary layer away from the grooves and an estimated change in the virtual origin are used to establish the similarity of the flow characteristics of grooved and smooth cylinders.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract High resolution particle image velocimetry is used to measure the turbulent velocity field for fully developed flow (Re = 2,872) in an enclosed channel. Photographs of particle displacement are obtained in a plane that is parallel to the flow and perpendicular to the walls. These are analyzed to give simultaneous measurements of two components of the velocity at more than 10,000 points. Maps of velocity vectors, spanwise vorticity and Reynolds stress reveal structural aspects of the turbulence. In particular, internal shear layers are observed, in agreement with predictions of direct numerical simulation. Ensemble-averaging of a number of photographs yields statistical properties of the velocity in good agreement with laser-Doppler velocimeter measurements, and with direct numerical simulations.
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