DIRECT SIMULATION OF BREAKDOWN TO TURBULENCE FOLLOWING OBLIQUE INSTABILITY WAVES IN A SUPERSONIC BOUNDARY-LAYER

The late stages of transition to turbulence in a Mach two boundary lay er are investigated by direct numerical simulation of the compressible Navier-Stokes equations. The primary instability at this Mach number consists of oblique waves, which are known to form a pattern of quasi- streamwise vortices. It is found that breakdown does not follow immedi ately from these vortices, which decay in intensity. The generation of new vortices is observed by following the evolution of the pressure a nd vorticity in the simulation, and analysed by consideration of vorti city stretching. It is found that the slight inclined and skewed natur e of the quasi-streamwise vortices leads to a production of oppositely signed streamwise vorticity, which serves as a strong localised forci ng of the shear layer alongside the original vortices, formed by conve ction and stretching of spanwise vorticity. The shear layer rolls up i nto many new vortices, and is followed by a sharp increase in the ener gy of higher frequencies and in the skin friction.