基于小波包方法的超声速气膜气动光学效应相干结构

Investigation on coherent structure of supersonic film aero-optics based on wavelet packet

当高速成像制导导弹在大气中飞行时,其光学窗口承受着严重的气动加热。超声速气膜冷却方法可以有效地隔离外部加热,但是超声速气膜流动会引起光束退化,降低图像质量。为了研究超声速气膜气动光学效应,本文构建了主流马赫数为3.4,设计喷流马赫数为2.5,实际测得喷流马赫数为2.45的超声速气膜实验装置。利用基于纳米粒子的平面激光散射技术获得了高时空分辨率流场图像,并对气膜冷却流动的密度场进行重构,利用光线追迹法获取了对应密度场的光程差。通过将光程差分布和K-H涡对比后发现,光程差的波谷位置对应于涡卷的中心,而光程差的波峰对应于涡卷中心之间的连接部分。但是,随着涡结构的发展破碎,对应关系不再成立。根据超声速气膜NPLS流场图像结果,利用分形原理获取的分形维数结果,将其沿流向划分为三个区域,其对应平坦度分别为3.4,2.9,3.6,验证了区域2更适合进行相干结构提取。

While a high-speed aircraft is flying in the atmosphere, its optical hood is subjected to severe aerodynamic heating. Supersonic film cooling method can effectively isolate external heating, but the flow structures formed by the supersonic film cooling can cause beam degradation and affect the imaging quality. To investigate the aero-optics of supersonic film cooling, an experimental model was adopted in this paper, with its mainstream Mach number 3.4, designed jet Mach number 2.5, and measured jet Mach number 2.45. High-resolution images of flow were acquired by the nano-based planar laser scattering （NPLS） technique, by reconstructing the density field of supersonic film cooling, and then, the optical path difference （OPD） was acquired by the ray-tracing method. Depending on the comparison between the K-H vortex and OPD distribution, the valleys of OPD correspond to the vortex ‘rollers’ and the peaks to the ‘braids’. However, the corresponding relationship becomes quite irregular for the flow field with developed vortices, and cannot be summarized in this manner. According to the NPLS result of the supersonic film flow field, based on the fractal dimension results obtained by the fractal principle, the flow field is divided into three regions along the direction of flow, and the coherent structures of the corresponding OPD are calculated respectively. The corresponding flatness is 3.4, 2.9, and 3.6, which verifies that Zone 2 is more suitable for coherent structure extraction.