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为了研究纳秒脉冲激光能量沉积减小高超声速飞行器波阻的机理和规律,首先要研究纳秒脉冲激光能量在静止空气中的沉积现象。提出一种新方法测量了激光能量吸收率。并采用高分辨率纹影系统,对纳秒脉冲Nd:YAG固体激光器(波长532nm,最大激光能量368m J/pulse)击穿静止空气后所形成的等离子体热核进行观测。基于FLUENT软件并编写UDF,结合非对称能量沉积模型和空气等离子体参数,采用层流模型、Roe-FDS通量格式对激光能量沉积后的流动现象进行了数值模拟。结果表明,激光能量吸收率随着入射激光能量的增大而不断增大,并最终稳定在0.45左右。纳秒脉冲激光能量沉积后的流场纹影序列图像很好地呈现了爆炸波的传播、等离子体热核的演变和涡环的形成。激光能量沉积后60~120μs,涡环的涡核平均直径基本不变,且与入射激光能量大小呈二次函数关系。爆炸波约在t=60μs之后衰减至近似声波,此后其波速受入射激光能量大小的影响较小。数值模拟结果表明,Richtmyer-Meshkov不稳定性和激光能量的非对称沉积,是等离子体演化出尖刺的原因。
In order to study the mechanism and rule of nanosecond pulse laser energy deposition to reduce the acoustic impedance of hypersonic vehicles, we first study the deposition of nanosecond laser energy in still air. A new method is proposed to measure the laser energy absorption rate. The thermal nuclei of plasma generated by the nanosecond pulsed Nd: YAG solid state laser (wavelength 532nm, the maximum laser energy 368m J / pulse) penetrating still air were observed by using high resolution grazing system. Based on the FLUENT software and the preparation of UDF, the flow phenomenon after laser energy deposition was numerically simulated by laminar flow model and Roe-FDS flux format based on asymmetric energy deposition model and air plasma parameters. The results show that the laser energy absorption rate increases with the increase of incident laser energy, and finally stabilizes at about 0.45. The flow field image after nanosecond pulse laser deposition shows the propagation of blast wave, the evolution of plasma thermonuclear and the formation of vortex ring. After laser energy deposition for 60 ~ 120μs, the average diameter of the vortex nuclei in the vortex ring is basically unchanged, and it is quadratic with the incident laser energy. The explosion wave is attenuated to the approximate sound wave after t = 60μs, and thereafter the wave velocity is less affected by the incident laser energy. Numerical simulations show that the asymmetric deposition of Richtmyer-Meshkov instability and laser energy is the reason for the spike evolution in plasma.