We study physical mechanisms that trigger transient growth in a high-speed spatially developing laminar boundary layer that interacts with an oblique shock wave. We utilize an approach based on power iteration, with the global forward and adjoint linearized equations, to quantify the transient growth in compressible boundary layers with flow separation. For a Mach 5.92 boundary layer with no oblique shock wave, we show that the dominant transient response consists of oblique waves, which arise from the inviscid Orr mechanism, the lift-up effect, and the first-mode instability. We also demonstrate that the presence of the oblique shock wave significantly increases transient growth over short time intervals through a mechanism that is not related to a slowly growing global instability. The resulting response takes the form of spanwise periodic streamwise elongated streaks, and our analysis of the linearized inviscid transport equations shows that base-flow deceleration near the reattachment location contributes to their amplification. The large transient growth of streamwise streaks demonstrates the importance of nonmodal effects in the amplification of flow perturbations and identifies a route for the emergence of similar spatial structures in transitional hypersonic flows with shock-wave/boundary-layer interactions.
Bibliographical noteFunding Information:
Financial support from the Office of Naval Research (ONR) under Awards N00014-19-1-2037 and N00014-17-1-2496 and from the Air Force Office of Scientific Research (AFOSR) under Award FA9550-18-1-0422 is gratefully acknowledged. The views and conclusions contained herein are those of the authors and should not be interpreted as representing the official policies or endorsements, either expressed or implied, of the ONR, the AFOSR, or the U.S. Government.
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