  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. It is found that the second instability acoustic mode is the key
modulator of the transition process. The second mode experiences a rapid growth
and a very fast annihilation due to the effect of bulk viscosity. The second
mode interacts strongly with the first vorticity mode to directly promote a
fast growth of the latter and leads to immediate transition to turbulence.

Fluid Dynamics