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LAMINAR-TO-TURBULENT TRANSITION IN A SHOCK-INDUCED SEPARATION BUBBLE

L. Krishnan
Aeronautics and Astronautics, School of Engineering Sciences University of Southampton Southampton, United Kingdom

Neil D. Sandham
Aerodynamics and Flight Mechanics Group Faculty of Engineering and the Environment, University of Southampton Southampton SO17 1BJ, UK

Johan Steelant
The European Space Research and Technology Center, Noordwijk, Netherlands

Abstract

Numerical simulations of the compressible Navier-Stokes equations are used to study laminar-to-turbulent transition in a separation bubble created by impingement of an oblique shock wave on a flat plate boundary layer at Mach 2. In contrast to separation bubbles in subsonic Mach numbers, steady laminar solutions are found, even for a strong pressure ratio (p3/p1 = 1.91) across the shock reflection. Such separation bubbles exhibit a multi-vortex recirculation zone. Using three-dimensional large eddy simulations with no upstream forcing a self-sustained transition process is observed, consistent with a change from convective to absolute instability of the two-dimensional solution. Additional simulations are carried out to study the development of the absolute instability, which initially involves spanwise distortion of vortex structures in the recirculation zone near reattachment. The pressure ratio for the onset of absolute instability is found to be between p3/p1 = 1.50 and 1.56. Bubble lengths are reduced when upstream forcing is applied.