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时，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解释了整个流动历史过程中涉及的许多自由度，例如各种气体物种及其激发状态。此外，现有的热力学和化学数据没有得到充分的描述，现有的非平衡模型仍然存在不确定性。然而，非平衡现象对高速工程应用具有重要影响，特别是在大气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飞行器的空气动力学、热负荷和推进系统效率。一个明显的例子是航天器进入行星大气层时所经历的加热。这种加热主要来自飞行器外缘周围等离子体的膨胀，产生来自远离热力学平衡的激发态气体的辐射。人们通常认为，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低读数所表明的那样。然而，随后的发现显示，辐射计的校准波长错误[1]，令人怀疑现有的双温度模型在飞行器进入地球大气层后非平衡流动的有效性。对于含有碳物种的大气，如火星或金星，人们对非平衡现象的理解甚至更加有限，因为在这些大气中，非平衡尾迹对后体加热有很大贡献。因此，为此类行星设计的飞行器的尾部加热分析通常依赖于数值模型，其有效性仍未得到验证[2]。此外，随着NASA和ESA等空间机构计划在本十年内前往冰川巨人[3]，显然迫切需要加强对膨胀构型中非平衡流动的理解。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了解，并将这些知识封装在工程计算中常用的双温度方程数值模型中。具体地说，围绕航天器边缘产生的膨胀流可以通过膨胀管中的非稳定膨胀来复制。在膨胀过程中，密度和平移温度的降低有效地减慢了非平衡热化学过程的速度，通过利用光谱学，可以在短时间内观察到这些现象。因此，该项目的主要目标是开发一种稳定扩张流动的数值模型，该模型能够表示任意细节的内部自由度。该模型将有助于分析在膨胀管测试活动中收集的光谱数据。创新的数值模型将植根于激发和电离物种的非平衡混合物的Navier-Stokes方程，包括表示测试装置中心线流线发散的项，该测试装置旨在产生稳定的膨胀波，并将基于轴对称的Navier-Stokes解算器，称为激波管重叠模拟框架(Frosst)和拉格朗日激波管分析(LASTA)代码[4]。“进入地球的后体辐射加热特征。”《航天器与火箭杂志》52.1(2015)：105-119。火星科学实验室HEA的气动热力学设计

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