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  • 1
    Electronic Resource
    Electronic Resource
    Turbulent spots in plane Poiseuille flow — Measurements of the velocity field (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 2183-2195 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An experimental study on the development of turbulent spots in plane Poiseuille flow at a Reynolds number of 1600 has been carried out with the aim of achieving a better understanding of the transition to and maintenance of turbulence at low Reynolds numbers. Spots were triggered by a loudspeaker-induced jet of high velocity. The initial disturbance was found to undergo a first stage of rapid expansion, in which sharp internal shear layers form at locations away from the symmetry plane and precede the transition to turbulence. After this initial stage, a nearly self-similar structure develops with the typical features of a turbulent spot. The general features, turbulent properties, and spanwise spreading of the spot were investigated and compared both to previous experimental data and to numerical simulations. High-frequency fluctuations are absent at the front of the spot, whereas the turbulence is apparently self-sustained at the rear and displays features similar to fully turbulent Poiseuille flow at much higher Reynolds numbers. The waves accompanying the wing tips of the spot extend well within the spot, and reach amplitudes far in excess of those previously found outside the spot. The high level of random fluctuations in this part of the spot indicates that the breakdown of the waves is important for the spanwise propagation of the turbulent region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857805
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  • 2
    Electronic Resource
    Electronic Resource
    Turbulent spots in plane Poiseuille flow–flow visualization (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 1328-1331 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The structure and spreading of turbulent spots in plane Poiseuille flow were studied through flow visualization, in the Reynolds number range 1100 to 2200 (based on centerline velocity and channel half-height). The spot spreading half-angle varied from 6° for the lowest Re to 12° for the highest Re. The propagation velocity of the rear laminar–turbulent interface was slightly above 50% of the centerline velocity. Spotsplitting, as reported in earlier studies, seems to occur only at low Re.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865884
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  • 3
    Electronic Resource
    Electronic Resource
    Direct drag measurements for a flat plate with passive boundary layer manipulators (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 696-700 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Drag measurements for a flat plate equipped with passive boundary layer turbulence manipulators, the so-called Large Eddy Break Up (LEBU) devices, have been carried out in a towing tank at plate length Reynolds numbers of 5–20 million. The towing tank facility allowed direct drag measurements as opposed to indirect methods based on, e.g., measurement of momentum loss thickness of the boundary layer. The LEBU devices consisted of two hydrofoils arranged in tandem, with a spacing of five to eight boundary layer thicknesses at the manipulator position (δm) and a height in the boundary layer of 0.5δm–0.9δm. Four different manipulator configurations were tested, with and without a trip wire on the test plate. The LEBU foil section was 10% thick and symmetric. The ratio between the LEBU chord and δm ranged from 0.8 to 1.1, and the chord Reynolds number ranged from 25 000 to 100 000. Only for the lowest Reynolds number was a 2% drag reduction obtained. However, the main body of the experiments showed no drag reduction in contrast to previously reported experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865921
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  • 4
    Electronic Resource
    Electronic Resource
    Numerical and experimental results for developing curved channel flow (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1473-1476 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Three-dimensional finite-volume simulations of the spatial development of centrifugally unstable flow in a curved channel have been performed, and compared with detailed hot-wire measurements. Both approaches revealed spatially developing streamwise vortex pairs at the concave wall, which gave rise to regions of alternating negative and positive streamwise perturbation velocities. The computational box size was chosen large enough so that the flow was allowed to naturally select the spanwise wave number. The computed velocity field was found to be in excellent agreement with the experimentally determined one.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857982
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  • 5
    Electronic Resource
    Electronic Resource
    Symmetry properties of developing three-dimensional laminar disturbances in plane Poiseuille flow (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 1618-1620 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The evolution of a finite-amplitude point-like laminar disturbance in plane Poiseuille flow is investigated using hot-wire anemometry. In contrast to earlier experiments, the initial disturbance was introduced simultaneously through both the upper and lower wall of the channel, resulting in an antisymmetric disturbance in the normal velocity. Although the initial disturbance mainly generated symmetrical streamwise velocity modes, the subsequent development of the perturbation showed a marked tendency for antisymmetry.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868276
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  • 6
    Electronic Resource
    Electronic Resource
    Experiments on the stability of streamwise streaks in plane Poiseuille flow (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 11 (1999), S. 915-930 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The development and stability of streamwise streaks are studied in an air-flow channel experiment at subcritical Reynolds numbers. The streaks were generated by continuous suction through small slots at the wall. The streak amplitude first grows algebraically, and if the amplitude exceeds a certain threshold secondary instability in the form of travelling waves is observed. These waves give rise to high urms values in the region of large spanwise mean flow gradient. Measurements with two hot-wire probes indicate that velocity fluctuations are 180° out of phase at two neighboring peaks at each side of a low velocity region and implies the existence of a sinuous type instability. Measurements were also made with controlled disturbances where earphones were used to force the secondary instability. Phase averaged data clearly show the oscillation of the low velocity region and also provides the growth rate, phase speed as well as amplitude and phase distributions of the secondary instability. Several of these features suggest that the instability is of inflectional origin. Finally the disturbance breaks down and the flow undergoes transition to turbulence. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869962
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  • 7
    Electronic Resource
    Electronic Resource
    Experiments on instabilities in curved channel flow (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 1666-1676 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental results are reported from hot-wire measurements in a narrow gap, curved air channel with a spanwise aspect ratio of 29. The Reynolds numbers (Re) covered a range from 2 to 6.5 times the critical Re, i.e., the Re for which the flow becomes centrifugally unstable according to linear stability theory. For the lowest Re studied, the measurements showed a regular flow pattern of streamwise vortices, whereas at higher Re interaction occurred between vortex pairs. The streamwise disturbance velocity increased rapidly near the inlet section, thereafter the disturbance amplitude overshot before it finally reached a saturated level of the nonlinear stage. At even higher Re a secondary instability in the form of traveling waves appeared on top of the primary instability and was found to be localized between one pair of vortices at the inflow region from the concave wall.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858388
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  • 8
    Electronic Resource
    Electronic Resource
    The fluctuating wall-shear stress and the velocity field in the viscous sublayer (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 1026-1033 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The fluctuating wall-shear stress was measured with various types of hot-wire and hot-film sensors in turbulent boundary-layer and channel flows. The rms level of the streamwise wall-shear stress fluctuations was found to be 40% of the mean value, which was substantiated by measurements of the streamwise velocity fluctuations in the viscous sublayer. Heat transfer to the fluid via the probe substrate was found to give significant differences between the static and dynamic response for standard flush-mounted hot-film probes with air or oil as the flow medium, whereas measurements in water were shown to be essentially unaffected by this problem.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866783
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  • 9
    Electronic Resource
    Electronic Resource
    The possibility of drag reduction by outer layer manipulators in turbulent boundary layers (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 2814-2820 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments were carried out with the aim of investigating the possibility of obtaining a net drag reduction on a finite body by manipulating the outer layer structure of the turbulent boundary layer. The experiments were carried out in a 260 m long towing tank, where large eddy breakup devices (LEBU's) were used in single and tandem configurations on a large flat plate. The total drag was measured directly by a force gauge, and the geometrical configuration parameters, as well as the chord Reynolds number, were varied over wide ranges. The highest chord Reynolds number tested was 260 000, which is within the range of interest for practical applications. Also, the device drag was measured directly in flight, which enabled evaluation of the performance of the manipulators, as well as determination of the total skin friction reduction. Despite a substantial skin friction reduction for certain configurations (7% averaged over the part downstream from the manipulators) as well as a low device drag, no drag reduction was found. The present results, and a critical evaluation of previously reported data, make any substantial net drag reduction by use of LEBU's at high Reynolds numbers seem implausible.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866989
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  • 10
    Electronic Resource
    Electronic Resource
    Experimental study of heat and momentum transfer in rotating channel flow (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 8 (1996), S. 2964-2973 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The enhancement of momentum and heat transfer caused by stationary streamwise vortices due to a Coriolis instability in a rotating straight channel is examined. It is shown that the changes in skin friction and Nusselt number depend on changes in the spanwise averaged mean flow and temperature distributions. Hot wire anemometry was used to experimentally determine the streamwise velocity and temperature distributions in a cross stream plane 68 channel widths downstream of the inlet. A technique to accurately compensate the velocity readings for the varying temperature in the channel was developed. It is shown that the streamwise vortices give rise to disturbance profiles which are close to those obtained from linear theory for small rotation numbers and that in this region there is no enhancement of either the averaged momentum or heat transfer. However, even for a disturbance amplitude in the streamwise velocity of the order of 20%, the disturbances are close to linear (both the disturbance distribution and growth rate). In this weakly non-linear region the changes in the mean flow and temperature distributions could be estimated by using the linear eigenfunctions of the disturbances where the amplitude was taken from the measurements. In the fully non-linear (saturated) region the Nusselt number on both the stable and unstable side of the channel was almost twice that at no rotation, however, the skin friction was almost unaffected on the stable side. This shows that the Reynolds analogy between momentum and heat transfer is not valid in this flow situation. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869074
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