Transition in Hypersonic Boundary Layers: Role of Dilatational Waves

Transition and turbulence production in a hypersonic boundary layer is investigated in a Mach 6 quiet wind tunnel using Rayleigh-scattering visualization, fast-response pressure measurements, and particle image velocimetry. A previously undiscovered unusual behavior of the second instability mode is noticed. Very high frequency dilatational waves are observed to grow rapidly followed by very fast annihilation. The second instability mode is a key modulator of the hypersonic laminar-to-turbulence transition, and the bulk viscosity plays an important role in that dynamical process. At its peak, the second mode strongly interacts with the first instability mode to directly promote a rapid growth of the latter and immediate transition to turbulence. This interaction can be explained by a nonlinear coupling of vorticity and dilatation in the interior of the boundary layer, combined with a viscous linear coupling at the wall. Our study of transition in hypersonic flows suggests that more attention should be given to the inviscid dilatational waves and their coupling with transverse vortical structures.