Numerical investigation on image degradation caused by aero-optical effects of the supersonic mixing layer

The supersonic mixing layer over the optic window of a high-speed imaging guided vehicle is the primary turbulent flow causing aero-optical effects such as the target image blurring, jittering, and offsetting. Employing the large eddy simulation and ray tracing methods, this paper presents a numerical investigation on target image degradation of a beam passing through the supersonic mixing layer under different conditions. The distorted wavefront, degraded target image, and peak signal-to-noise ratio of the degraded image are proposed to quantitatively evaluate the target image degradation. On the whole, the degraded target image is mainly shown as image blurring, and the larger the convective Mach number of the flow field, the more severe the image degradation. It is found that the beam incident position should be away from transition area of the supersonic mixing layer for a clearer image, the larger inlet fluid density difference results in a more severe target image degradation due to the stronger density fluctuation of turbulent flow structures, and the optimal beam incident angle is about 30° for the current calculation conditions.