Influence of nozzle pressure ratio on aero-optical effects in a free-vortex aerodynamic window

Abstract

Aero-optical effects during high-speed flight can significantly degrade the accuracy of imaging guidance systems. Compared to conventional solid optical windows, aerodynamic windows demonstrate superior performance in thermal radiation resistance and structural integrity without thermal fracture. In this study, a free-vortex aerodynamic window is designed, and the influence of the nozzle pressure ratio (NPR) on its optical performance under supersonic flight conditions is investigated using steady-state computational fluid dynamics simulations, geometric ray-tracing method, and diffraction theory. The results show that the free-vortex aerodynamic window is capable of cooling the high-temperature airflow on the vehicle surface and preventing the intrusion of ambient atmosphere into the optical cavity. Within the NPR range of 37.97–57.87, the overall wave-front root-mean-square of the optical path difference remains relatively stable (ΔOPD_rms<0.05λ). According to the diffraction results, at lower NPR, the point spread function (PSF) distribution of the distorted beam after diffraction in the far field shows a positional shift of the focal spot and slight energy spread. For NPR≥37.97, the PSF pattern exhibits not only positional shift and energy spread but also a speckle pattern, indicating significant high-order aberrations. These findings provide quantitative criteria for optimizing aerodynamic window designs in supersonic imaging systems, particularly in balancing NPR selection with wavefront error tolerance.