Hypersonic Measurements of Roughness-Induced Transient Growth

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 stream wise streaks 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 nose tip and a ring of 18 periodically-spaced cube-like discrete roughness elements 1-mm tall by 1.78-mm wide by 1.78-mm long. The roughness element height is approximately equal to the boundary-layer thickness. Measurements are made in the low-disturbance Texas A&M Mach 6 Quiet Tunnel. No transition to turbulence is observed for freestream unit Reynolds numbers between 7.5 ? 10^(6) m^(-1) and 9.8 ? 10^(6) m^(-1). Pitot measurements reveal azimuthally-alternating high- and low-speed streaks growing downstream of the roughness. Large unsteadiness is measured in the roughness wake but decays downstream. The stream wise evolution of the steady and unsteady disturbance energy is consistent with low-speed observations of transient growth in the mid-wake region behind periodically-spaced cylindrical roughness elements. This experiment contains the first quantitative measurements of roughness-induced transient growth in a high-speed boundary layer.