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

在气体流过固体表面时，薄的近壁区域会变得混乱和紊流；这些湍流运动可以产生强烈的、向外传播的声波，特别是当气流是超音速的时候。该项目旨在了解当所讨论的气体（例如二氧化碳或高温空气）吸收特定频率的声波时产生的噪音。特别是，人们认为，通过将这种气体引入近地面的空气流动中，可以减少高速风洞等配置中的噪音产生，在这些配置中，这种噪音会严重污染测量结果。与此同时，将开展各种教育活动，向各级学生介绍高速气流，包括创建一个YouTube频道，学生可以在该频道中建议在8马赫风洞中飞行的物体。我们将进行系统的实验研究，以研究在振动松弛气体存在下湍流产生的噪声。声学衰减测量将在静态纯气体（如CO2、N2O）和气体混合物中进行，以确定在与高速流动相关的压力和温度下的强吸声组合；结果将与理论衰减模型进行比较。在超声速和高超声速风洞中进行的实验将检验高速湍流边界层产生的声学噪声：首先，将在早期静态实验中确定的物种作为测试气体；随后，这些物种被引入气流的边界层。非平衡对表面压力和湍流速度波动的影响也将被确定。这项工作将最终提供对高速湍流的热非平衡效应的基本理解，并可能为降低高超声速风洞中的噪声提供控制机制。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.