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  • AERODYNAMICS  (8)
  • MECHANICS  (1)
  • 1
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    Asymptotic behavior of a flat plate wake (1989)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: An experimental study has been conducted to investigate the far-field, self-similar properties of a flat plate wake. A plane turbulent wake was generated at the trailing edge of a smooth splitter plate separating two legs of a Mixing Layer Wind Tunnel, with both initial boundary layers tripped. For the present study, both legs were operated at a free-steam velocity in the test section of 15 m/s, giving a Reynolds number based on wake momentum thickness of about 1750. Single profile measurements were obtained at five streamwise locations using a Pitot probe for the mean velocity measurements and a single cross-wire probe for the turbulence data, which included statistics up to third order. The mean flow data indicated a self-similar behavior beyond a streamwise distance equivalent to about 350 wake momentum thicknesses. However, the turbulence data show better collapse beyond a distance equivalent to about 500 momentum thicknesses, with all the measured peak Reynolds stresses achieving constant, asymptotic levels. The asymptotic mean flow behavior and peak primary stress levels agree well with theoretical predictions based on a constant eddy viscosity model. The present data also agree reasonably well with previous measurements, of which only one set extends into the self-similar region. Detailed comparisons with previous data are presented and discussed in this report.
    Keywords: AERODYNAMICS
    Type: NASA-CR-185917 , NAS 1.26:185917 , JIAA-TR-95
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  • 2
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    Evolution of three-dimensionality in stable and unstable curved mixing layers (1991)
    In:  Other Sources
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    Publication Date: 2019-07-27
    Description: Results are presented of an experimental study tracing the effects of mild streamwise curvature (delta/R is less than 5 percent) on the 3D structure of a two-stream mixing layer at high Reynolds number (about 27,000). Measurements of the mean and fluctuating velocities were obtained on fine cross-plane grids at 11 streamwise locations using a rotatable cross-wide probe. Well-organized streamwise vorticity was generated in both cases through the braid instability. Although the vortex structures decayed with streamwise distance in both cases, the rate of decay for the unstable case was lower. As a result, the unstable layer exhibited noticeable spanwise variations in the mean velocity and Reynolds stresses. Both cases achieved linear growth, but the rate of growth for the unstable case was higher. The far-field spanwise-averaged peak Reynolds stresses were significantly higher for the destabilized case compared to the stabilized case, which exhibited levels comparable to those of a straight case.
    Keywords: AERODYNAMICS
    Type: Symposium on Turbulent Shear Flows; Sept. 9-11, 1991; Munich; Germany
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  • 3
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    Effects of streamwise vorticity injection on turbulent mixing layer development (1990)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: This paper investigates the effect of injected strong streamwise vorticity on the structure and development of a plane mixing layer originating from tripped boundary layers. The experiments were conducted in a mixing layer wind tunnel consisting of two separate legs which were driven individually by centrifugal blowers. It was found that, while the vorticity injection increased the growth rate in the near-field, the asymptotic growth rate was reduced by a factor of about two, together with the peak Reynolds stress levels. The result is attributed to the effect of the relatively strong and short wavelength streamwise vorticity in making the spanwise structures more three-dimensional and slowing down their pairing process, thus reducing entrainment, and hence growth.
    Keywords: AERODYNAMICS
    Type: AIAA PAPER 90-1459
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  • 4
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    Interaction of an oscillating vortex with a turbulent boundary layer (1987)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The effects on mean flow and turbulence caused by meander of a vortex embedded in a two-dimensional boundary layer were investigated experimentally by driving a forced lateral oscillation of the vortex generator. Upstream, the vorticity contours without forcing were found to be round. The forced vortex generator oscillation caused a flattening of the time-averaged vorticity contours and changes in some of the Reynolds stresses. The results indicate that the unforced vortex did not meander significantly upstream, and that the effects of meander can be understood qualitatively from production of Reynolds stresses by the forced motion acting with the exiting three-dimensional velocity field. Farther downstream, the observed differences in the mean vorticity and Reynolds stresses caused by forcing were smaller, mainly because the vortex was substantially diffused at this station, resulting in smaller mean velocity gradients.
    Keywords: AERODYNAMICS
    Type: AIAA PAPER 87-1246
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  • 5
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    Properties of a half-delta wing vortex (1987)
    In:  Other Sources
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    Publication Date: 2019-07-27
    Description: The mean flow and turbulence structure of a single longitudinal vortex generated by a half-delta wing placed at a small angle of attack were investigated. Particular consideration was given to the near-field properties of the generator in order to establish the role of the generator wake in the initial rolling-up of the vortex, as well as to the far-field properties so that the approach to equilibrium could be studied. Measurements were made on fine cross-plane grids at seven streamwise locations using hot cross wires. The results show that the point of maximum vorticity and the generator wake do not merge until a streamwise distance equivalent to three generator heights is reached. Comparison with previous data on vortices produced by double-branched generators confirmed that the present vortex had achieved a fully developed state, and at a relatively short streamwise distance.
    Keywords: AERODYNAMICS
    Type: Symposium on Turbulent Shear Flows; Sept. 7-9, 1987; Toulouse; France
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  • 6
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    Streamwise vortex meander in a plane mixing layer (1993)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Previous measurements in a two-stream plane mixing layer have shown that the mean streamwise vorticity decays monotonically with downstream distance such that it is absent in the far-field (self-similar) region. It was not clear at that time if this measured decay was a result of actual vortex diffusion or increasing vortex meander within the mixing layer. The present experimental study was conducted in order to determine the existence of streamwise vortex meander in a mixing layer, and if present, its significance on the measured properties. The dependence of the velocity cross-correlation on the fixed probe location is considered a good indicator of the stationarity of the streamwise vortex location. The cross-correlation measurements obtained here indicate that the spanwise meander is negligible while the transverse apparent meander (normal to the plane of the mixing layer) is relatively large.
    Keywords: AERODYNAMICS
    Type: AIAA PAPER 93-0553 , AIAA, Aerospace Sciences Meeting and Exhibit; Jan 11, 1993 - Jan 14, 1993; Reno, NV; United States|; 14 p.
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  • 7
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    Three-Dimensional Structure Of A Mixing Layer (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Report describes experimental study of three-dimensional structure of nominally planar two-stream mixing layer. Purpose of study, to obtain quantitative data on evolution of streamwise vortexes.
    Keywords: MECHANICS
    Type: ARC-12506 , NASA Tech Briefs (ISSN 0145-319X); 15; 11; P. 79
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  • 8
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    Interaction of a streamwise vortex with a turbulent mixing layer (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The interaction of a single streamwise vortex with a plane turbulent mixing layer is experimentally studied. Initially, near the mixing layer origin, the vortex rides just below the layer and only affects the mixing layer properties near the bottom side of the mixing layer. Once the vortex starts to become embedded within the mixing layer and active interaction has begun, the effect of the vortex is to grossly distort the mean velocity and turbulence contours over the whole width of the mixing layer. The induced cross-flow velocities in the mixing layer result in the production of additional normal Reynolds stresses which in turn act with mean flow gradients to generate extra shear stresses. In particular, the secondary shear stress (SSS) achieves an absolute peak value equivalent to about 40 percent of the maximum primary shear stress. The position and decay of the SSS peak are then found to be strongly correlated with those of the streamwise vortex.
    Keywords: AERODYNAMICS
    Type: Physics of Fluids A (ISSN 0899-8213); 2; 2011-202
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  • 9
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    Three-dimensional Structure of Straight and Curved Plane Wakes (1993)
    In:  CASI
    Details
    Publication Date: 2019-08-15
    Description: Although the plane wake is marked by the formation of strong spanwise vortices, the initially two-dimensional Karman-like vortices soon develop a three-dimensional structure in the form of secondary streamwise vortices. So far, this streamwise vortex structure has been studied mostly through flow visualization and at relatively low Reynolds numbers. The primary objective of the present program was to investigate the origin and evolution of the three-dimensional structure of straight and curved plane wakes at relatively high Reynolds numbers (Re(sub b) = 28,000) through detailed measurements of the mean and turbulent properties at several streamwise locations. The experiments were conducted in three phases. In the first phase, the development of a straight plane wake was investigated. In the second phase, the effects of imposed streamwise curvature on the wake development were examined. The streamwise curvature was of constant radius and very mild in terms of the curvature ratio (b/(square root of R) is less than 2 percent). In both the first and second phases, the role of initial conditions was examined in wakes generated from both untripped (laminar) and tripped (turbulent) initial boundany layers. In the third phase, the effects of injecting streamwise vorticity and the effects of increased Reynolds number on the tripped wake structure and development were investigated.
    Keywords: AERODYNAMICS
    Type: NASA-CR-194420 , NAS 1.26:194420 , JIAA-TR-110
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