基于GCV-FFT方法的超声速压缩拐角流场气动光学效应计算

研究超声速压缩拐角流动,基于Rytov近似,采用广义卷积-快速Fourier变换(GCV-FFT),直接求解非均匀介质中的Helmholtz方程,获得激光光束的散射场和衍射场的复振幅分布.给出包括光强、光程差、斯特列尔比、相对光强、光束质心位置和有效光束宽度等气动光学参量.可以看出,激光光束在湍流场区域内传输,光强度分布逐渐出现起伏和偏离.穿过流场区域后,由于衍射作用,光束产生较大幅度的扭曲、偏离和破碎.压缩拐角区的流场对光束质量的影响明显大于充分发展湍流区.从相对光强的分布形态来看,光束的破碎似乎与流场的小尺度相干结构相关.采用传统的OPD均方差或加权均方差值估算获得的斯特列尔比的误差较大,更为准确的方法应该通过计算PSF来进行评估.

Aero-optical Effects in Supersonic Compression Ramp Flow Fields:GCV-FFT Method

Supersonic compression ramp flow is studied. Based on Rytov approximation, a generalized convolution-fast Fourier transform (GCV-FFT) method is used to solve directly Helmholtz equation in inhomogeneous medium. Scattering field of the laser beam and complex amplitude distribution of diffracted field are obtained. Aero-optical parameters including light intensity, optical path difference, Strehl ratio, relative light intensity, beam centroid position and effective beam width are given. It can be seen that the laser beam is transmitted in the turbulent flow field, and the light intensity distribution appears gradually undulates and deviates. As passing through flow field, the beam produces a large degree of distortion, deviation and breakage due to the effect of diffraction. Effect of flow field in compressed ramp region on beam quality is significantly greater than that in fully developed turbulent region. From distribution of relative light intensity, the beam breakage seems to be related to small-scale coherent structure of flow field. The error of Strehl ratio obtained by traditional OPD mean square error or weighted mean square error estimation is greater. More accurate method should be evaluated with PSF method.