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Effects of upstream boundary layer on the unsteadiness of shock-induced separation

Published online by Cambridge University Press:  07 August 2007

B. GANAPATHISUBRAMANI
Affiliation:
Center for Aeromechanics Research, The University of Texas at Austin, Austin, TX 78712, USA
N. T. CLEMENS
Affiliation:
Center for Aeromechanics Research, The University of Texas at Austin, Austin, TX 78712, USA
D. S. DOLLING
Affiliation:
Center for Aeromechanics Research, The University of Texas at Austin, Austin, TX 78712, USA

Abstract

The relationship between the upstream boundary layer and the low-frequency, large-scale unsteadiness of the separated flow in a Mach 2 compression ramp interaction is investigated by performing wide-field particle image velocimetry (PIV) and planar laser scattering (PLS) measurements in streamwise–spanwise planes. Planar laser scattering measurements in the upstream boundary layer indicate the presence of spanwise strips of elongated regions of uniform momentum with lengths greater than 40δ. These long coherent structures have been observed in a Mach 2 supersonic boundary layer (Ganapathisubramani, Clemens & Dolling 2006) and they exhibit strong similarities to those that have been found in incompressible boundary layers (Tomkins & Adrian 2003; Ganapathisubramani, Longmire & Marusic 2003). At a wall-normal location of y/δ=0.2, the inferred instantaneous separation line of the separation region is found to oscillate between x/δ=−3 and −1 (where x/δ=0 is the ramp corner). The instantaneous spanwise separation line is found to respond to the elongated regions of uniform momentum. It is shown that high- and low-momentum regions are correlated with smaller and larger size of the separation region, respectively. Furthermore, the instantaneous separation line exhibits large-scale undulations that conform to the low- and high-speed regions in the upstream boundary layer. The low-frequency unsteadiness of the separation region/shock foot observed in numerous previous studies can be explained by a turbulent mechanism that includes these elongated regions of uniform momentum.

Type
Papers
Copyright
Copyright © Cambridge University Press 2007

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