Transition in Hypersonic Boundary Layers: Role of Dilatational Waves

TL;DR

This study investigates how dilatational waves, especially the second instability acoustic mode, influence the transition to turbulence in hypersonic boundary layers at Mach 6, highlighting the role of bulk viscosity in this process.

Contribution

It reveals the critical role of the second acoustic mode and bulk viscosity in the transition mechanism of hypersonic boundary layers, providing new insights into turbulence onset at high speeds.

Findings

Second acoustic mode rapidly grows and is annihilated by bulk viscosity.

Interaction between acoustic and vorticity modes promotes turbulence transition.

Transition occurs immediately after the second mode's growth phase.

Abstract

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.