Roughness-Induced Transient Growth on a Hypersonic Blunt Cone

The effects of surface roughness on boundary-layer disturbance growth and laminar-to-turbulent transition are not well understood, especially in hypersonic boundary layers. The transient growth mechanism that produces algebraic growth of streamwise streaks via decaying streamwise vortices may play a key role in roughness-induced transition but has not previously been deliberately observed in hypersonic flow. To make such measurements, the present work studies the boundary layer of a 5 half-angle smooth cone paired with a slightly blunted nosetip and a ring of periodically-spaced discrete roughness elements. Measurements are made in the low-disturbance Texas A&M Mach 6 Quiet Tunnel. The roughness element height is approximately equal to the boundary-layer thickness, yet no transition to turbulence is observed for freestream unit Reynolds numbers between 8.7 106m-1and 9.8 106m-1. Pitot measurements reveal azimuthally-alternating highand low-speed streaks growing downstream of the roughness. Large pressure fluctuations are measured in the roughness wake but do not grow downstream. This experiment is the first to deliberately observe the detailed structure of roughness-induced transient growth in a hypersonic boundary layer.

Roughness-Induced Transient Growth on a Hypersonic Blunt Cone Conference Paper