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化学非平衡对钝体绕流激波脱体距离影响的数值分析
英文题名Numerical Analysis of Chemical Nonequilibrium on Shock Stand-Off Distance of Supersonic Flow over Blunt Body
桂志宏
导师孙泉华
2018-05-27
学位授予单位中国科学院大学
学位授予地点北京
学位类别硕士
学位专业流体力学
关键词钝体绕流 热化学非平衡 激波脱体距离 Cfd模拟
摘要

高超声速流动具有高温真实气体效应,其中的热化学非平衡特性至今仍是研究的重点。一方面,气体分子在来流激波压缩下发生热化学变化,影响流场特性和飞行器气动性能。能够精确预测宽速域、宽空域条件的热化学模型是高温真实气体效应精确表征的基础。另一方面,根据流场关键特征评估热化学模型,是发展热化学模型的重要环节。因此,建立流动热化学状态与流动关键特征之间的联系对于研究高超声速流动和热化学模型都有帮助。

本文针对高超声速钝体绕流问题,采用数值求解Navier-Stokes方程组,开展不同热化学状态下的无黏和黏性计算,分析热化学冻结、热平衡化学冻结、热平衡化学非平衡状态对超声速流动特别是激波脱体距离的影响及其规律,然后基于数值模拟结果,发展一般条件下的激波脱体距离的预测公式。

本文的主要工作包括数值模拟与理论分析两个方面。

数值模拟方面,利用CFD软件对不同来流条件的无黏和黏性流场进行了相对系统而详细的数值计算,数值迭代采用定常方法计算,即计算物理量密度、动量、能量和组分质量分数等的L2范数,并定义为该物理量的残差,当残差小于设定数值时并认为计算收敛,默认值为0.00001,最后对数值计算结果进行了初步的网格独立性验证。本文选取了圆柱绕流和圆球绕流两个基本问题,其半径均为0.025m,来流马赫数变化范围为2~30,流动状态选取为热化学冻结、热平衡化学冻结和热平衡化学非平衡这三种流动状态。计算发现,热平衡效应使得激波层内流场的温度减小,密度增大,激波脱体距离减小,而化学非平衡效应对流场的影响和流场有无黏性相关。对于无黏流动,由于激波层的离解反应吸收大量的热,因此激波层内的温度整体大幅降低,导致激波脱体距离进一步减小;而对于黏性流动,由于来流温度保持不变,在相同来流马赫数条件下,通过改变来流密度来改变雷诺数的大小。具体来说,来流密度减小时,雷诺数减小,激波后的密度也会相应减小,进一步导致激波层内的化学反应速率减小,在流体微团经过相同的流动尺度时,离解反应吸收的热量减小,因此流场的温度下降速度变小;考虑到黏性流场边界层内会发生复合反应,而复合反应放热使得边界层内的温度增大。因此对于黏性流动,化学非平衡效应对流场的影响不仅取决于激波层的空间尺度,还与雷诺数的大小有关。如对于二维高马赫数流动,流体经过激波后,离解反应在到达驻点前就已经充分进行,在进入边界层内,复合反应使得温度升高,因此,减小雷诺数使得边界层的厚度增大,复合反应放热升高,流场温度增大,激波脱体距离增大。而对于轴对称高马赫数流动,由于流动特征尺度较小,离解反应在到达驻点时还没完全进行,因此在边界层内不会存在二维流动情形的复合反应问题,激波脱体距离主要受离解反应的吸热量的影响。

理论分析方面,通过查阅文献中的理论分析方法,严格推导得到了激波脱体距离与驻点线上径向质量通量梯度的关系式,基于严格的理论分析,进一步通过引入驻点线质量通量线性变化假设和钝头体壁面压强的修正牛顿公式推出了激波脱体距离的近似表达式,其表述形式更为简洁。根据推导结果,激波脱体距离和来流密度与驻点密度比值正相关、和来流马赫数负相关。另一方面,文献中得到的激波脱体距离工程公式也显示出相似的规律。注意到上述关联特点,根据CFD计算数据拟合得到了激波脱体距离的工程预测公式。相比文献中的公式,本文得到的公式的计算结果和CFD计算结果吻合更好,尤其在低马赫数和黏性流动情形,本文计算公式的优势较为明显。

英文摘要

Hypersonic flow has high temperature gas effects, and the thermochemical non-equilibrium characteristic is still the current research focus. On one hand, gas molecules undergo thermochemical changes due to the bow shock compress, which affects the flow field and the aerodynamics of the aircraft. The thermochemical models capable of accurately predicting wide speed and wide altitude conditions are the basis for accurate characterization of high temperature gas effects. On the other hand, evaluation of thermochemical models based on the key characteristics of the flow field is an important step in developing thermochemical models. Therefore, to establish the relationship between the thermochemical state and the key characteristics of flow is helpful to the study of both hypersonic flow and thermochemical models.

In this thesis, the problem of flow over hypersonic blunt body is studied by solving numerically the Navier-Stokes equations w/o viscosity under several thermochemical states (thermochemical frozen, thermal equilibrium chemical frozen, and thermal equilibrium chemical non-equilibrium state), and the shock stand-off distance is analyzed based on CFD results with an aim to develop an engineering formula for general conditions.

The main work of this thesis includes two aspects: numerical simulation and theoretical analysis.

In numerical simulation, a CFD software is used to carry out systematic and detailed numerical calculations for inviscid and viscous flows with different incoming flow conditions. Two basic problems of flow over cylinder and flow over sphere are studied. The incoming flow Mach number is ranged from 2~30, and the flow state includes thermochemical frozen, thermal equilibrium chemical frozen, and thermal equilibrium chemical non-equilibrium state. It is found that the excitation of vibrational energy makes the flow temperature in the shock layer decreases, the density increases, the shock stand-off distance decreases, and the chemical non- equilibrium effect depends on flow viscosity. For inviscid flow, due to the absorption of heat in the dissociation reaction within the shock layer, the flow temperature is greatly reduced, which results in a further decrease of the shock stand-off distance. For viscous flow, the flow density changes with the Reynolds number as defined. Specifically, when the Reynolds number is reduced, the flow density decreases, and the chemical reaction rate decreases. When the fluid element passes through the same flow length, the absorption heat of the dissociation reaction decreases, thus the temperature drop in the flow field decreases. As combine reaction will take place in the boundary layer of the viscous flow, the flow temperature increases with the combine reaction. So for viscous flow, the chemical non-equilibrium effect depends not only on the flow length scale, but also on the Reynolds number. For two dimensional high Mach number flow, the dissociation reaction takes place after the shock wave, and will be completed before reaching the stagnant point. The combine reaction makes the temperature rise in the boundary layer. Therefore, the reduction of the Reynolds number makes the boundary layer increases, the heat release due to combine reactions increase, the flow temperature increases, and the shock stand-off distance increases.

In theoretical analysis, the relationship between the shock stand-off distance and the radial mass flux gradient along the stagnant line is derived, and the expression for the shock stand-off distance is derived by assuming linear distribution of the mass flux and adopting the modified Newton formula for the surface pressure on the blunt body. According to the derivation result, the shock stand-off distance is positively related to the ratio of incoming density over the density at the stagnant point, and is negatively correlated with the incoming Mach number. According to the above correlation, an engineering prediction formula is obtained based on the CFD data fitting for the shock stand-off distance. Compared with the formula in the literature, the current formula is better in the accuracy, especially in the case of low Mach number flow and viscous flow.

语种中文
文献类型学位论文
条目标识符http://dspace.imech.ac.cn/handle/311007/73164
专题高温气体动力学国家重点实验室
作者单位中国科学院力学研究所
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桂志宏. 化学非平衡对钝体绕流激波脱体距离影响的数值分析[D]. 北京. 中国科学院大学,2018.
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