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Low-frequency dynamics of shock-induced separation in a compression ramp interaction

Published online by Cambridge University Press:  25 September 2009

B. GANAPATHISUBRAMANI ,
N. T. CLEMENS  and
D. S. DOLLING
B. GANAPATHISUBRAMANI*
Affiliation:
Department of Aeronautics, Imperial College London, Prince Consort Road, London SW7 2AZ, UK
N. T. CLEMENS
Affiliation:
Centre for Aeromechanics Research, The University of Texas at Austin, 210 E. 24th Street, WRW220, Mail Code C0604, Austin TX 78712, USA
D. S. DOLLING
Affiliation:
Centre for Aeromechanics Research, The University of Texas at Austin, 210 E. 24th Street, WRW220, Mail Code C0604, Austin TX 78712, USA
*
†Email address for correspondence: g.bharath@imperial.ac.uk
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Abstract

The low-frequency dynamics of the shock-induced separation region in a Mach 2 compression ramp interaction is investigated by performing high-speed particle image velocimetry (HSPIV) measurements, at a rate of 6kHz, in a streamwise–spanwise plane. The HSPIV measurements made in the upstream turbulent boundary layer indicate the presence of spanwise strips of elongated regions of uniform streamwise velocity that extend to lengths greater than 30δ, validating previous results based on planar laser scattering measurements obtained by Ganapathisubramani, Clemens & Dolling (J. Fluid Mech., vol. 585, 2007, p. 369). At a wall normal-location of y/δ=0.2, a surrogate for separation based on a velocity threshold is found to fluctuate over a streamwise range of ±1.2δ, consistent with previous studies. The amplitude of unsteadiness has contributions from at least two sources that are related to the incoming boundary layer. First, the velocity threshold based surrogate separation line exhibits large-scale undulations along the spanwise direction that conform to the passage of elongated low- and high-speed regions in the upstream boundary layer. This motion is classified as the local influence of the upstream boundary layer. Second, the spanwise-averaged surrogate separation is found to respond to the overall change in streamwise velocity in the incoming boundary layer and is classified as the global influence of the upstream boundary layer. However, this global influence includes the contributions from the elongated low- and high-speed regions. Preliminary findings based on statistical analysis suggest that the local influence contributes nearly 50% more than the global influence. Regardless, the low-frequency unsteadiness of the separation-region can be attributed to the local and global influences of the incoming boundary layer.

JFM classification

Compressible Flows: Compressible Flows Turbulent Flows: Turbulent boundary layers Wakes/Jets: Separated flows
Type
Papers
Information
Journal of Fluid Mechanics , Volume 636 , 10 October 2009 , pp. 397 - 425
Copyright
Copyright © Cambridge University Press 2009

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References

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Ganapathisubramani et al. supplementary movie

Movie 1.This movie shows the time variation of streamwise velocity contours in a streamwise-spanwise plane of y/δ = 0.2. The contours are normalised by the freestream velocity and the coordinates are normalised by the boundary layer thickness. The 'surrogate' separation point that was identified using a velocity threshold is marked at every spanwise location using a square symbol. The time interval between successive frames in the movie is 166 μs (6.8δ/U∞). The movie depicts the response of the upstream separation envelope to the passage of the elongated regions of uniform momentum that appear in the upstream boundary layer.

Download Ganapathisubramani et al. supplementary movie(Video)
Video 2.8 MB

Ganapathisubramani et al. supplementary movie

Movie 2. This movie comprises of three different plots. The first plot (upper left) shows streamwise velocity contours in a streamwise-spanwise plane and is identical to movie 1. However, the surrogate separation point at z/δ = 0.85 is marked with a green square. The second plot (upper right) shows the time history of the separation point marked with the green square. The third plot shows the time variation of Ul that is computed upstream of the point marked with the green square. The separation between successive points in the time history is approximately 6.8δ/U∞. The movie reveals a similarity between the motion of the separation location and Ul.

Download Ganapathisubramani et al. supplementary movie(Video)
Video 900.8 KB

Ganapathisubramani et al. supplementary movie

Movie 3. This movie shows low-pass filtered time variation of streamwise velocity contours (normalised by the freestream velocity) in a streamwise-spanwise plane. The high-speed PIV data shown in movie 1 is smoothed by computing a 6 frame running average to remove small-scale temporal fluctuations. The filtering effectively removes temporal frequencies higher than 1 kHz. The upstream envelope of the separation region is marked with squares. The separation between successive frames is 166 μs. The low-frequency dynamics of the separation envelope can be observed in this filtered data. The motion of the separation location is correlated to the appearance of elongated low- and high-speed regions in the upstream boundary layer.

Download Ganapathisubramani et al. supplementary movie(Video)
Video 1.5 MB

Ganapathisubramani et al. supplementary movie

Movie 4. This movie utilises the running-average filtered data to depict the unsteady motion of the separation location. The data is smoothed in time by computing a six-frame running average. Therefore, temporal frequencies greater than 1kHz are removed (the data was acquired at 6 kHz). The movie consists of three plots. The first plot (upper left) shows time variation of 6 frame running-averaged streamwise velocity contours at y/δ = 0.2. A green square marks the separation location at z/δ = 0.85. The second plot (upper right) shows the time history of the separation location at z/δ = 0.85 (marked with a green square in the first plot). The third plot shows the temporal variation of Ul that is computed upstream of the separation location at z/δ = 0.85. Note that all quantities are computed based on the low-pass filtered velocity information. The time separation between successive frames in the movie is 166 μs. The movie shows that running-average filtering of data highlights the similarity between xsep and Ul.

Download Ganapathisubramani et al. supplementary movie(Video)
Video 569.2 KB