ERI: Experimental Investigation of Compressibility Effects on Turbulent Kinetic Energy Production in Supersonic Flows

高超声速飞行有可能给全球旅行和太空探索带来革命性的变化。实现这一目标的一项关键技术是为吸气式高超声速发动机开发高效超音速燃烧室。这些燃烧器与传统燃烧器的不同之处在于它们的停留时间极短，通常约为1毫秒。在如此短的时间内，燃料必须被注入、汽化(对于液体燃料)、分散，并与进入的空气混合到分子水平，然后才能发生燃烧。湍流有助于分散和混合，在这一过程中至关重要。然而，对可压缩性如何影响湍流生产的了解仍然不完整，从而减缓了高超声速飞行的发展。该项目旨在通过在密苏里科技大学的3马赫风洞中直接测量湍流来研究可压缩性对湍流的影响。由于湍流在混合过程中起着关键作用，了解湍流的产生及其受可压缩性的影响将有助于更好地理解超音速流动中的混合过程和超音速燃烧室的设计。虽然最近人们对超音速流动中的湍流产生进行了研究，但在与混合应用相关的非壁面流动中，可压缩性的影响，特别是对密度涨落的影响，在很大程度上被忽略了。最近的研究表明，即使超燃冲压发动机燃烧室的超音速马赫数相对较低(飞行马赫数的30%-40%)，可压缩性对湍流产生的影响也可以与不可压缩湍流产生相媲美。这一分析基于理论论证和现有文献中有限的实验/数值数据，表明最初针对边界层发展的强雷诺模拟(速度和密度起伏之间的关联)对于无边界超音速混合流也是有效的。本项目提出利用两点聚焦激光微分干涉术(2-FLDI)直接测量密度和速度起伏，以实验验证强雷诺类比在超音速混合无界流动中的有效性。2-FLDI是一种经过验证的低成本技术，允许对密度和速度波动进行高频、同时、非侵入性测量。流动将包括一个简单的平面喷流从塔式喷射器在3马赫的自由气流。这项工作还将通过发展夏令营来支持STEM的外展，并通过支持研究生和本科生来支持高超声速劳动力的发展。这项工作的发现也将被纳入关于湍流的研究生班。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Hypersonic flight offers the potential to revolutionize global travel and space exploration. A crucial technology for making this a reality is the development of efficient supersonic combustors for air-breathing hypersonic engines. These combustors differ from traditional ones in their extremely short residence times, typically around 1 millisecond. In such a short time, the fuel must be injected, vaporized (for liquid fuels), dispersed, and mixed to the molecular level with the incoming air before combustion can occur. Turbulence, which aids dispersion and mixing, is vital in this process. However, understanding how compressibility affects turbulence production is still incomplete, slowing down hypersonic flight development. This project aims to investigate compressibility effects on turbulence by directly measuring turbulence in the Mach 3 wind tunnel at Missouri University of Science and Technology. As turbulence plays a key role in the mixing process, understanding its production and how it is affected by compressibility would facilitate a better understanding of the mixing process in supersonic flows and the design of supersonic combustors.While turbulence production in supersonic flows has been recently investigated, compressibility effects – particularly on density fluctuations – have been largely neglected in non-wall-bounded flows such as jets and shear, which are relevant for mixing applications. Recent research indicates that even for the relatively low supersonic Mach number of a scramjet combustor (30-40% of the flight Mach), compressibility effects on turbulence production can be comparable to incompressible turbulence production. This analysis, based on theoretical arguments and limited experimental/numerical data from the available literature, indicates that the Strong Reynolds Analogy (a correlation between velocity and density fluctuations), initially developed for boundary layers, is also valid for unbounded supersonic mixing flows. This project proposes to experimentally verify the validity of the Strong Reynolds Analogy in a supersonic mixing unbounded flow by directly measuring density and velocity fluctuations utilizing two-point Focused Laser Differential Interferometry (2-FLDI). 2-FLDI is a proven low-cost technique that allows high-frequency, simultaneous, non-intrusive measurements of density and velocity fluctuations. The flow will consist of a simple planar jet from a pylon injector in a Mach 3 free stream. This work will also support STEM outreach through the development of a summer camp, and workforce development in hypersonics by supporting both graduate and undergraduate students. Findings from this work will also be incorporated into a graduate class on turbulent flows.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.