静止空气中单脉冲激光能量非对称沉积实验与数值模拟

Experimental and Numerical Study of Nanosecond Pulsed Laser Energy Asymetric Deposition in Quiescent Air

关键词:  纳秒脉冲  激光能量沉积  激波  等离子体  纹影  数值模拟

Key words:  Nanosecond pulse  Laser energy deposition  Shock wave  Plasma  Schlieren  Numerical simulation

摘要:

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368mJ/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。

Abstract:

In order to study wave drag reduction mechanism of hypersonic vehicle by laser energy deposition，laser-induced plasma hot core decay in quiescent air was studied experimentally and numerically. A new method for measurement of laser energy absorption rate was proposed. High resolution schlieren system was constructed to visualize the flow after nanosecond pulsed laser energy deposition. The hot core was obtained by focusing a Q-switched single pulse Nd:YAG laser (wavelength 532nm) with maximum laser energy of 368 mJ/pulse. Numerical simulation based on commercial software FLUENT with UDF was conducted to simulate the decay of laser-induced plasma，using asymmetric energy deposition model，air plasma properties，laminar model and Roe averaged flux difference splitting. Laser energy absorption rate increases with the increase of incident laser energy and stabilizes at 0.45. Sequential schlieren images show the blast wave propagation，the laser-induced plasma decay and the vortex ring formation. Diameter of vortex core remains nearly the same between 60～120μs after breakdown and has quadratic relationship with incidence laser energy. It shows that the blast wave attenuates rapidly to sound wave and the incident laser energy has little influence on wave speed 60μs after breakdown. Numerical results show that Richtmyer-Meshkov instability and asymmetry energy deposition contribute to the formation of a spike structure from laser-induced plasma.