Abstract
Low-noise supersonic wind tunnels are required for unambiguous experimental research into high-speed laminar-flow instability and transition. The experience of the successful NASA Langley quiet-tunnel development program has been used to design and construct a new kind of low-cost, short-duration, quiet-flow tunnel. Measurements of the flow quality in the 9.7 x 10.9 cm Mach 4 test section were obtained using fast response pressure transducers mounted in the tip of a pitot tube. When the rms pitot pressure is approximately 0.05-0.10% of the mean pitot pressure, bursts of noise appear in the pitot-pressure signals. These bursts appear to be the radiated signature of turbulent spots in the boundary layers on the nozzle walls. Their appearance confirms the presence of laminar nozzle-wall boundary layers and quiet flow when the rms pitot pressure is about 0.06% or less. Based on this criterion, quiet flow is achieved to Reynolds numbers based on the axial length of the quiet-flow test region of more than 400,000 at unit Reynolds numbers of approximately 40,000 per cm. This performance is sufficient for research into receptivity, roughness, and instability effects at high speeds.
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