【作者】 鄢昌渝；

【导师】 吴建军；

【作者基本信息】 国防科学技术大学 ， 航空宇航科学与技术， 2008， 博士

【摘要】 本文以大气吸气式激光推进光船为研究对象,采用数值计算方法对激光作用下等离子体吸收波的形成和演化机制与光船工作过程进行了研究。建立了不同条件下的控制方程组,计算了各空气组元的初始密度。对激光推进中热化学非平衡/平衡流的物理化学模型进行比较分析,在热化学非平衡流中选择了Park建立的三温度11组元空气化学反应模型,在热化学平衡流中选择了Gupta建立的高温平衡空气模型,并分析了相应的能量交换模型、输运模型等。分析了有限差分NND格式和有限体积NND格式,讨论了热化学非平衡流计算中的耦合法和解耦法,研究了求解化学反应源项和能量源项的点隐式方法,并给出了求解无粘通量项的LU-SGS隐式方法。探讨了数值计算中的等离子体点火模型。采用三温度11组元热化学非平衡空气模型,考虑了激光在等离子体中的逆韧致吸收和共振吸收机制,用有限差分NND格式对控制方程组进行数值离散,编制完成了相应的一维计算程序,耦合计算了一维平面波、一维柱面波和一维球面波条件下等离子体吸收波的形成和演化机制。详细研究了等离子体吸收波中各种参数的变化情况。研究中发现一有趣现象,在聚焦条件下,随着等离子体吸收波向前推进,最大电子数密度将减少而最大电子温度将增加。根据等离子体吸收波的结构特征,提出了两种计算激光能量沉积过程的简化模型:1)电子数密度限制器模型;2)等离子体吸收波波速模型。基于电子数密度限制器模型,数值计算了激光推进光船工作过程。为了提高计算效率,采用三温度11组元热化学非平衡空气模型计算激光能量在等离子体中的沉积过程,并用平衡空气模型求解激光结束后的后续流场。将组分生成项和能量源项与流动控制方程组进行解耦计算,对控制方程组采用有限体积NND格式进行数值求解,编制完成了相应的二维计算程序。研究了热化学非平衡/平衡空气模型的转换时刻,发现热化学非平衡效应对光船推进性能有重要影响;对Schall的光船实验进行数值模拟,所得数值计算结果与实验结果吻合;研究了单脉冲能量、脉冲宽度等激光参数对光船推进性能的影响。将等离子体吸收波波速模型推广到球冠等离子体吸收波波速模型和二维等离子体吸收波波速模型,结合高温平衡空气模型,实现了不同激光入射条件下激光推进光船工作过程数值模拟,编制完成了相应的二维计算程序。研究了速度系数对流场参数和光船推进性能的影响,分析了聚焦后激光具有二维分布特征时的流场特性,发现激光空间分布可能对流场具有重要影响。更多还原

【Abstract】 For an air-breathing laser propulsion lightcraft, the mechanism of formation and evolvement of plasma absorptive wave in lightcraft due to laser and air interaction, and the operating process were investigated by means of numerical computation and simulation in this dissertation.The governing equations were established under different conditions and cases, and the initializing conditions were also given. Physical and chemical models of thermo-chemical nonequilibrium flow and equilibrium flow in laser propulsion were analyzed and discussed. Park’s reaction model with three-temperature and 11 air species was chosen to solve thermo-chemical nonequilibrium flow. Gupta’s high temperature equilibrium air model was chosen to solve thermo-chemical equilibrium flow. Then, some corresponding energy exchange mechanisms and transport models were discussed.Some numerical methods such as the finite difference NND scheme and finite volume NND scheme were analyzed in detail. The uncoupled method and coupled one used to solve thermo-chemical nonequilibrium flow were discussed. The dot implicit method used to solve source terms of species and energy in chemical reaction was studied. Then, the LU-SGS implicit method was given to solve the non-viscous flux.The ignition model of plasma in numerical computation was discussed. The governing equations based on thermo-chemical non-equilibrium model with three-temperature and 11 air species were discretized numerically by finite difference NND scheme. The inverse bremsstrahlung absorption and plasma resonance absorption were considered when laser is transmitted in air plasma. The corresponding computing codes were implemented to simulate the formation and evolvement of plasma absorptive wave at one dimension plane/ cylindrical/ spherical wave. The variations of some key parameters in plasma absorptive wave were studied in detail. An interesting phenomenon was found, that is, when laser is focused, the maximum electron density decreases with the progress of plasma absorptive wave, but the maximum electron temperature increases. Two simplified models based on the characteristics of plasma absorptive wave were proposed to solve the absorptive process and aggradation of laser energy: 1) the limited electron density model. 2) the velocity model of plasma absorptive wave.The operating process of laser propulsion lightcraft was computed based on the limited electron density model. In order to improve the computational efficiency, the absorptive process and aggradation of laser energy was solved by the thermo-chemical non-equilibrium model with three-temperature and 11 air species, and the followed flow field was solved by the high temperature equilibrium air model. The governing equations uncoupled with the source terms of species and energy were discretized numerically by finite volume NND scheme. The corresponding two dimensional programs were implemented. The time when the thermo-chemical non-equilibrium model was switched to equilibrium air model was studied. The results show that thermo-chemical non-equilibrium effect has significant influence on the propulsion performance of lightcraft. The predicted coupling coefficients for the lightcraft by simulation are coincident reasonably well with those of tests done by Schall. In addition, the influence of some laser parameters such as single pulse energy and pulse width on the propulsion performance of lightcraft was also discussed.The velocity model of plasma absorptive wave was extended to spheral and two dimensional cases. The operating process of laser propulsion lightcraft in different incident laser was simulated based on high temperature equilibrium air model. The corresponding two dimensional programs were also implemented. The influence by velocity coefficient on the flow field parameters and the propulsion performance of lightcraft was studied. The flow field characteristic was analyzed when the focused laser has two- dimensional distribution feature. The results show that the space distribution of laser may have significant influence on the flow field.更多还原