Investigation of non-equilibrium thermochemistry in expanding flows

该项目属于EPSRC：流体动力学和空气动力学在高超声速下，气体的行为表现出非平衡现象，代表了理解和预测高超声速流动的最复杂的方面。模拟这些现象的主要困难在于考虑整个流动历史中涉及的众多自由度，例如各种气体种类及其激发态。此外，现有的热力学和化学数据特征不充分，现有的非平衡模型仍然具有不确定性。然而，非平衡现象对高速工程应用具有重要意义，特别是在大气层进入飞行器的设计和安全方面。事实上，这些现象会显著影响车辆的空气动力学、热负荷和推进系统效率。一个显著的例子是航天器进入行星大气层时所经历的加热。这种加热主要是由车辆外缘周围等离子体的膨胀引起的，这些等离子体产生的辐射来自激发态的气体，远离热力学平衡。一般认为，与对流加热相比，进入地球的后体辐射可以忽略不计，正如阿波罗后体辐射计的最小读数所表明的那样。然而，随后的研究结果显示，辐射计的校准波长错误，这使人们对现有的双温度模型在飞行器进入地球大气层后的非平衡流的有效性产生了怀疑。对于含有碳的大气，如火星或金星，对非平衡现象的理解甚至更为有限，在这些大气中，非平衡尾流在很大程度上有助于后体加热。因此，针对此类行星设计的车辆的后体加热分析往往依赖于其有效性尚未得到验证的数值模型[1]。此外，随着美国国家航空航天局（NASA）和欧洲航天局（ESA）等太空机构计划在这十年里前往冰巨星的任务，加强对膨胀构型中非平衡流动的理解的迫切需要变得显而易见。该项目的目标是研究非平衡流动，以获得对气体微观状态的全面理解，并将这些知识包含在工程计算中常用的双温方程数值模型中。具体来说，在航天器边缘周围产生的膨胀流可以通过膨胀管中的非定常膨胀来复制。在膨胀过程中，密度和平移温度的降低有效地减缓了非平衡热化学过程，允许在短时间内通过利用光谱学在短时间内可用的设施中观察这些现象。因此，该项目的主要目标是为稳定扩展的流动开发一个能够表示任意细节的内部自由度的数值模型。该模型将有助于分析膨胀管测试活动中收集的光谱数据。创新的数值模型将扎根于激发态和电离态非平衡混合物的Navier-Stokes方程，结合用于表示旨在产生稳定膨胀波的测试装置中线流线发散的术语，并将基于轴对称的Navier-Stokes解算器，即激波管超集模拟框架（FROSST）和拉格朗日激波管分析（LASTA）代码[4].[1]约翰斯顿，克里斯托弗O，和亚伦M.布兰迪斯。进入地球后体辐射加热的特点航天器与火箭学报，52.1 (2015):105-119卡尔·埃德奎斯特等人。火星科学实验室的空气热力学设计

This project falls within the EPSRC: Fluid dynamics and aerodynamicsAt hypersonic velocities, the behaviour of gases manifests non-equilibrium phenomena, representing the most intricate aspect of understanding and predicting hypersonic flows. The major difficulty in modelling these phenomena lies in accounting for the multitude of degrees of freedom involved throughout the flow history, such as the various gas species and their excitation states. Furthermore, the available thermodynamic and chemical data are inadequately characterized, and existing non-equilibrium models remain marked by uncertainties. However, non-equilibrium phenomena hold critical implications for high-speed engineering applications, particularly in the design and safety of atmospheric entry vehicles. In fact, these phenomena significantly affect vehicle aerodynamics, thermal loads, and propulsion-system efficiency. A notable example is the heating experienced by spacecraft upon entry into planetary atmospheres. This heating predominantly arises from the expansion of plasma around the vehicle's outer edge generating radiation from gases in excited states, far from thermodynamic equilibrium.It was generally assumed that afterbody radiation for Earth entry was negligible in comparison to convective heating, as indicated by the minimal readings from Apollo's afterbody radiometers. However, subsequent findings revealed that the radiometers were calibrated for the wrong wavelength [1], casting doubt on the efficacy of the existing two-temperature model for non-equilibrium flows in the wake of vehicles entering Earth's atmosphere. The understanding of non-equilibrium phenomena is even more limited for atmospheres containing carbon species, such as Mars or Venus, where the non-equilibrium wake largely contributes to afterbody heating. Consequently, the analysis of afterbody heating for vehicles designed for such planets often relies on numerical models whose validity remains unverified [2]. Moreover, with planned missions to ice giants in this decade by space agencies like NASA and ESA [3], the pressing need for an enhanced comprehension of non-equilibrium flow in expanding configurations becomes evident.The objective of this project is to investigate non-equilibrium flows to attain a comprehensive understanding of the microscopic state of the gas and encapsulate this knowledge within commonly used two-temperature equation numerical models in engineering computations. Specifically, the expanding flow, generated around the edge of a spacecraft, can be replicated through unsteady expansions in expansion tubes. During the expansion process, a reduction in density and translational temperature effectively decelerates non-equilibrium thermochemical processes, allowing the observation of these phenomena within the short time frame available in short-duration facilities through the utilization of spectroscopy. The project's primary objective is thus to develop a numerical model for steady expanding flows that is capable of representing the internal degrees of freedom to an arbitrary detail. This model will be instrumental in analysing spectroscopic data gathered in expansion tube test campaigns. The innovative numerical model will be rooted in the Navier-Stokes equations for a non-equilibrium mixture of excited and ionized species, incorporating terms to represent streamline divergence at the centreline of a test setup designed to produce steady expansion waves and will be based on the axisymmetric Navier-Stokes solver known as FRamework for Overset Simulation of Shock Tubes (FROSST) and the LAgrangian Shock Tube Analysis (LASTA) code [4].[1] Johnston, Christopher O., and Aaron M. Brandis. "Features of afterbody radiative heating for earth entry." Journal of Spacecraft and Rockets 52.1 (2015): 105-119.[2] Edquist, Karl, et al. "Aerothermodynamic design of the Mars Science Laboratory hea