部分电离稀薄高超声速流动中双极扩散近似的评估
Evaluation of the Ambipolar Diffusion Approximation in Partially Ionized Rarefied Hypersonic Flows
AI总结：
本文通过氩气的高超声速流动动力学模拟，评估双极扩散近似在稀薄高超声速流动中的有效性，提出了等离子体扩散区域识别及该近似应用的新准则。
AI中文摘要：
稀薄高超声速等离子体的精确数值模拟对于优化再入航天器设计和开发先进航空航天技术日益重要。对于动力学模拟方法，通常会强制离子和电子以相同速率扩散，这一做法被称为双极扩散近似。该方法避免了对快速电子运动的高成本求解，但忽略了离子和电子的复杂等离子体动力学。几乎所有研究双极扩散近似在高超声速领域有效性的报告都指出，当使用静电建模时，流场特性存在显著差异，包括飞行器表面热通量增加和电子温度降低。然而，目前尚不清楚这些报告的差异是直接源于带电粒子通过电场的加速与减速，以及带电粒子与中性粒子之间的动量交换碰撞（即一阶效应），还是源于受一阶效应影响的粒子之间的后续相互作用（即二阶效应）。采用氩气开展带静电建模的高超声速流动动力学模拟，以量化双极扩散近似在捕捉一维驻点流线上一阶等离子体效应方面的有效性。在两组稀薄来流条件下研究三种不同的等离子体扩散 regimes（区域），并从预测等离子体密度分布、电子温度和驻点热通量的角度对该近似进行评估。本文提出了用于识别高超声速流动中等离子体扩散区域以及应用双极扩散近似的新准则。
英文摘要：
Accurate numerical simulation of rarefied hypersonic plasmas is increasingly important for optimization of re-entry spacecraft design and the development of advanced aerospace technologies. For kinetic simulation methods, it is convention to enforce ions and electrons to diffuse at the same rate, known as the ambipolar diffusion approximation. This approach circumvents costly resolution of fast electron motion, but neglects the complex plasma dynamics of ions and electrons. Almost all studies that investigated the efficacy of the ambipolar diffusion approximation in hypersonics report noticeable differences in flowfield properties when electrostatic modeling is used, including increases in vehicle surface heat flux and decreases in electron temperature. However, it is unknown whether these reported differences originate directly from acceleration and deceleration of charged species through the electric fields and momentum-exchange collisions between charged and neutral species, defined as first-order effects, or from subsequent interactions with particles experiencing first-order effects, defined as second-order effects. Kinetic hypersonic flow simulations with electrostatic modeling are performed with argon to quantify the validity of the ambipolar diffusion approximation in terms of capturing first-order plasma effects along a one-dimensional stagnation streamline. Three different plasma diffusion regimes are studied under two sets of rarefied freestream flow conditions. The approximation is evaluated in terms of predicting plasma density distributions, electron temperature, and stagnation point heat flux. New criteria are proposed for identification of plasma diffusion regimes in hypersonic flows and use of the ambipolar diffusion approximation.
