Browsing by Subject "hypersonic flow"
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Item Hypersonic Measurements of Roughness-Induced Transient Growth(2014-04-17) Sharp, Nicole SusanneThe 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.Item Near-Region Modification of Total Pressure Fluctuations by a Normal Shock Wave in a Low-Density Hypersonic Wind Tunnel(2014-05-01) Mai, Chi Luong NhatScientific understanding of the modifications to turbulence due to a normal shock wave at hypersonic speeds is lacking. The overarching research objective of this study was to characterize the effects of a hypersonic shock wave on the structure of locally homogeneous turbulence. The current study, believed to be the first hypersonic shock-turbulence interaction experiments conducted, examined in the near-region of a normal shock wave the effect on the total pressure fluctuations in a low-density hypersonic wind tunnel. Measurements were obtained with a fast-response Pitot pressure probe traversing in the freestream direction. The tunnel freestream noise level was characterized and served as the inflow/upstream condition to the interaction with the normal shock, which was a Mach stem created by the prescribed Mach reflection of two oblique shock waves. Measurements were made downstream of the Mach stem and results (noise values, auto correlation coefficient functions, integral scales, and power spectral density estimates) were compared with the freestream measurements. Overall, it was observed that amplification factors for the noise, time scales, and power spectral density estimates content were higher for the lower Re/m condition (i.e., lower freestream noise) than for the higher Re/m condition (i.e., higher freestream noise). In addition, the amplification factors across the range of unit Reynolds numbers were higher at 4.4 mm downstream from the Mach stem than for 2.4 mm downstream, indicating that the turbulent structures perhaps took time to grow after crossing the shock wave. Amplification was observed to be greater for higher frequencies.