Centrifugal oblique waves in hypersonic separated flows*

Experiments on high-speed separated flows show unsteady fluctuations in the separation zone before the transition to turbulence. We investigate the origin of these unsteady perturbations by evaluating the amplification of external disturbances in hypersonic compression ramp boundary layer using input-output analysis. The dominant unsteady flow response appears in the form of oblique waves in the separated shear layer. We show that the growth of the perturbation energy associated with these oblique waves arises from the streamline curvature of the laminar separated flow, in contrast to the attached boundary layers where no such mechanism exists. This newly identified physical amplification process causes large growth of oblique waves in shock-wave-boundary-layer-interactions (SWBLI). The role of unsteady centrifugal waves in transition to turbulence in compression ramp SWBLI is evaluated using direct numerical simulations (DNS). The dependence of the centrifugal growth on the Reynolds number and the ramp turn angle is also discussed.