CAREER: Effects of thermal nonequilibrium on the acoustic noise radiated by a compressible turbulent boundary layer

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In gaseous flows over solid surfaces, the thin near-wall region can become chaotic and turbulent; these turbulent motions can then cause the generation of intense, outward-propagating sound waves, especially when the flow is supersonic. This project seeks to understand such noise generation when the gas in question (e.g., carbon dioxide, or high-temperature air) absorbs sound waves at certain frequencies. In particular, it is thought that by introducing such a gas into a near-surface flow of air, the noise generation can be reduced in configurations such as high-speed wind tunnels, where this noise can severely contaminate measurements. In conjunction, a variety of educational activities will be undertaken to introduce students at all levels to high-speed flows, including the creation of a YouTube channel where students can suggest objects to be flown in a Mach-8 wind tunnel.A systematic experimental study will be conducted to investigate turbulence-generated noise in the presence of a vibrationally relaxing gas. Acoustic attenuation measurements will be performed in quiescent pure gases (e.g., CO2, N2O) and gas mixtures to identify strong acoustically absorbing combinations at pressures and temperatures relevant to high-speed flows; results will be compared with theoretical attenuation models. Experiments in supersonic and hypersonic wind tunnels will examine the acoustic noise generated by a high-speed turbulent boundary layer: first, with the species identified in the earlier quiescent experiments as test gases; and subsequently with these species introduced locally into the boundary layer of an air flow. Effects of nonequilibrium on surface-pressure and turbulent velocity fluctuations will also be determined. This work will ultimately provide a fundamental understanding of thermal nonequilibrium effects on high-speed turbulence, as well as potentially provide a control mechanism for the reduction of acoustic noise in hypersonic wind tunnels.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.