Hypersonic Measurements of Roughness-Induced Transient Growth
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Date
2014-04-17
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Abstract
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.
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Keywords
fluid dynamics, boundary layer stability, laminar-to-turbulent transition, hypersonic flow, surface roughness, boundary layers, transient growth, nonmodal growth