Experimental investigation on aero-optics of supersonic turbulent boundary layers at different light incident angles

Abstract

The aero-optical distortion caused by the compressibility of high-speed flow field has a great influence on the development of airborne optical detection system of (hypersonic) supersonic vehicles. The turbulent boundary layer is one of the most important aspects in the aero-optical study, and has become one of the hot research points in the field of aero-optical study. The nano-particle-based planar laser scattering technique is used to measure the density distribution of the supersonic (Ma=3.0) turbulent boundary layers, and the optical path difference, which is quite crucial for the aero-optical study, is obtained by ray-tracing method. The experimental result is verified by being compared with the theoretical result computed by the aero-optical scaling method of turbulent boundary layers. Five different light incident angles (α=60°, 75°, 90°, 105°, 120°) are selected and used to examine the influences of light incident angles on the supersonic turbulent layer, and the underlying flow physics is analyzed. Research shows that the light propagation path in the supersonic turbulent boundary layer contributes to the light incident angle dependence of aero-optics. The different propagation paths lead to the difference between the light propagation distance in the flow field and the correlation results of the corresponding density fluctuation. The oblique incidence of light results in the increase of the propagation distance in the flow field, and then the aero-optics turns worse. The greater the angle between the incident direction of light and the vertical direction of the wall, the more significant the aero-optics is, the difference increases at different times, the difficulty in correcting the aero-optics is also increased. In the supersonic turbulent boundary layer, a large number of vortices with a specific orientation lead to the anisotropy of the aero-optics in the turbulent boundary layer. By calculating the spatial two-point correlation of the density fluctuations at the streamwise plane (x-y plane), the cross-correlation result of density fluctuations at any light incidence angle (α=0°-180°) can be obtained. The local coherent structure scale is nearly 0.20 mm, which is basically consistent with the aero-optical effective scale (≈ 0.18 mm) computed from the formula proposed by Mani et al. When the light is inclined downstream, the direction of light propagation is consistent with the vortex structure in the flow field, and in this direction, the correlation coefficient of density fluctuation is larger, so the aero-optics is more serious. When the light beam is tilted upstream, the correlation coefficient is smaller, so the aero-optics is weaker.