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  • 1990-1994  (34)
  • 1
    Electronic Resource
    Electronic Resource
    Interaction of zones of flow separation in a centrifugal impeller-stationary vane system (1994)
    Springer
    Experiments in fluids 17 (1994), S. 427-433 
    Details
    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 In a radial flow pump operating in off-design conditions, regions of stall can exist on the rotating impeller blade and on the downstream diffuser blade, vane or tongue. Interaction of these stall zones can generate complex patterns of vorticity concentrations. In turn, these vorticity concentrations are related to sources of unsteady stagnation enthalpy. The form of these patterns is strongly dependent on the instantaneous location of the impeller trailing-edge relative to the leading-edge of the vane. Comparison of instantaneous with ensemble-averaged images shows that the flow structure in the gap region between the impeller and the vane is highly repetitive. Away from this region, in particular in the separated shear layer from the vane, the nonrepetitive nature of the vorticity field is manifested in substantial reduction of peak levels of vorticity in the ensemble-averaged image, relative to the instantaneous image. The three-dimensional flow structure resulting from these separation zone interactions was characterized via end views of the flow patterns. Particularly pronounced concentrations of vorticity can occur in this plane. They tend to be located in the shear layer at the outer edge of the large-scale separation zone. These vorticity concentrations are, however, highly non-stationary for successive passages of the impeller blade. Ensemble-averaging reveals that they persist primarily on the endwalls of the diffuser.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01877045
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  • 2
    Electronic Resource
    Electronic Resource
    Period doubling in the wake of a three-dimensional cylinder (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1477-1478 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Forced excitation of a long cylinder having a mild variation of diameter along its midspan generates period-doubled vortex formation. The onset of this localized, period-doubled response involves formation of diamond-shaped vortical structures during every other cycle of the cylinder oscillation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857983
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  • 3
    Electronic Resource
    Electronic Resource
    Timing of vortex formation from an oscillating cylinder (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 3677-3682 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The instantaneous structure of the near-wake of a cylinder subjected to forced oscillations is examined using particle imaging, which leads to representations of the streamline patterns and distributions of vorticity. As the frequency of excitation of the cylinder is increased relative to the inherent vortex formation frequency, the initially formed concentration of vorticity moves closer to the cylinder until a limiting position is reached; at this position, the vorticity concentration abruptly switches to the opposite side of the cylinder. This process induces abrupt changes of the topology of the corresponding streamline patterns; such topological patterns alone, however, do not properly suggest the existence and rearrangement of the vorticity concentrations. Moreover, this vorticity-switching concept persists to high values of Reynolds number, where the values of the mean base pressure coefficient and vortex formation length differ substantially from those at low Reynolds number. The switching mechanism is not significantly altered, either in an instantaneous or ensemble-averaged sense, by the presence of small-scale Kelvin–Helmholtz vortices that coexist with the large-scale (Kármán) vortices.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868424
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  • 4
    Electronic Resource
    Electronic Resource
    Control of the spanwise structure of a bluff-body wake by changes in boundary-layer thickness at separation (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 509-511 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Flow separation from a blunt trailing-edge gives rise to inclined vortex formation in a manner similar to the well-known structure from a circular cylinder. By extracting a portion of the boundary layer on one side of the trailing edge, it is possible to generate large-scale vortex formation having a high degree of spanwise uniformity. This two-dimensional, large-scale vortex formation is accompanied by a substantial decrease in the near-wake fluctuation level.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858876
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  • 5
    Electronic Resource
    Electronic Resource
    Instantaneous structure of a breaking wave (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 2877-2879 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The quantitative, instantaneous structure of a stationary wave undergoing breaking in the spilling mode is characterized using high-image-density particle image velocimetry. The breaker originates from a sharp trough of the free surface. The essentially discontinuous slope of the surface, in the presence of flow separation beneath it, serves as a source of vorticity, giving rise to vorticity concentrations in a separated mixing layer. The region between this vorticity layer and the free surface is turbulent; it has insignificant levels of vorticity compared to the mixing layer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868113
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  • 6
    Electronic Resource
    Electronic Resource
    Flow structure from an oscillating nonuniform cylinder: Generation of patterned vorticity concentrations (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 531-536 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Forced oscillation of a mildly nonuniform cylinder generates patterns of vorticity concentrations oriented orthogonally to its axis. These complex, but ordered, patterns are repeatable at subharmonics of the cylinder oscillation frequency; they therefore provide a means of identifying the first and second period-doubled states of the wake response.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868348
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  • 7
    Electronic Resource
    Electronic Resource
    Destabilization of the Karman vortex street by frequency-modulated excitation (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 723-725 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A two-dimensional cylinder is subjected to frequency-modulated excitation in the cross-stream direction. Transition from a locked-in response to a completely destabilized response can be attained by lowering the value of modulation frequency while all other parameters are maintained constant. This transition involves progression through a series of identifiable states including lock-in at the modulation frequency, period doubling at the modulation frequency, and spectral-broadened response.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858004
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  • 8
    Electronic Resource
    Electronic Resource
    High image-density particle image velocimetry using laser scanning techniques (1993)
    Springer
    Experiments in fluids 14 (1993), S. 181-192 
    Details
    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 Laser scanning, corresponding to time-dependent deflections of laser beam across a field of interest, can provide relatively high illumination intensity of small particles, thereby allowing implementation of high image-density particle image velocimetry (PIV). Scanning techniques employing a rotating (multi-faceted) mirror, an oscillating mirror, and an acousto-optic deflector are addressed. Issues of illumination intensity and exposure, rate of scan of the laser beam, and retrace time of the scanning beam are assessed. Representative classes of unsteady separated flows investigated with laser-scanning PIV are described.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00189509
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  • 9
    Electronic Resource
    Electronic Resource
    Construction of three-dimensional images of flow structure via particle tracking techniques (1993)
    Springer
    Experiments in fluids 14 (1993), S. 257-270 
    Details
    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 Construction of three-dimensional images of flow structure, based on the quantitative velocity field, is assessed for cases where experimental data are obtained using particle tracking technique. The experimental data are in the form of contiguous planes of particle images. These contiguous data planes are assumed to correspond to successive spatial realizations in steady flow, or to phase-referenced realizations in an unsteady flow. Given the particle images on contiguous planes, the in-plane velocity fields are determined. Then, the out-of-plane velocity field is obtained using a spectral interpolation method. Application of this method allows, in principle, construction of the three-dimensional vorticity field and the streamline patterns. A critical assessment is made of the uncertainties arising from the in-plane interpolation of the velocity field obtained from particle tracking and the evaluation of the out-of-plane velocity component. The consequences of such uncertainties on the reconstructed vorticity distributions and streamline patterns are addressed for two basic types of vortex flows: a columnar vortex, for which the streamlines are not closed and are spatially periodic in the streamwise direction; and for a spherical (Hill's) vortex exhibiting closed streamline patterns, and no spatial periodicity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00194017
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  • 10
    Electronic Resource
    Electronic Resource
    Cinematographic system for high-image-density particle image velocimetry (1994)
    Springer
    Experiments in fluids 17 (1994), S. 110-114 
    Details
    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 cinematographic system, which integrates the concepts of high-image-density PIV, laser scanning, and framing photography, allows temporal resolution of the order of one percent of the time scale of the largest vortical structures in the turbulent wake from a cylinder at a Reynolds number of 10,000. With this resolution in time, it is possible to track, in a continuous fashion, the patterns of streamwise vorticity in the near-wake.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02412812
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