The flow-field-caused aero-optical effect is becoming a crucial issue in hypersonic flight. The degraded imaging results can seriously affect the aircraft’s ability to capture targets. Experiments were carried out in a Mach 6-gun tunnel, and the imaging quality of a hypersonic optical dome was investigated under six different values of pressure ratio of jet. Background oriented schlieren was used to obtain the point spread functions (PSFs), which were then used to evaluate the imaging quality of the optical dome. The experimental results showed that cooling film with higher pressure will reduce the support field of the PSF and will have a more severe influence on the imaging quality.
Supersonic cooling film can reduce the influence of aerodynamic heating to a certain extent, but the complex flow field structure formed by its interaction with the mainstream will cause imaging target offset, ambiguity and jitter, that is, aero-optical effect. In this paper, the variation of cooling efficiency and aero-optical effect at the optical window under different jet States is studied numerically, and the validity of the numerical method is verified by comparing the experimental results. The results show that the cooling efficiency at the optical window is higher and the aero-optical effect is smaller because of the proper total temperature of the cooling jet; The higher the supply pressure of cooling gas, the better the cooling efficiency of film cooling. In order to ensure the weakest aero-optical effect, the cooling gas supply is in pressure matching state; With the increase of Mach number of cooling gas injection, the cooling efficiency and aero-optical effect of supersonic film cooling increase; In general, nitrogen or air is chosen as cooling gas.
While high-speed aircraft are 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 the beam degradation and affect the imaging quality. To research the aero-optics of supersonic film cooling, an experimental model was adopted in this paper, its mainstream Mach number 3.4, designed jet Mach number 2.5, 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) were acquired by the ray-tracing method. Depending on the comparison between 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. And then, the OPD were analyzed by correlation function and structure function, show that, there is a relationship between the shape of OPD correlation function and the vortex structure, the correlation function type changed with the development of the vortex. The correctness that the mixing layer makes a main contribution to the aero-optics of supersonic film cooling was verified, and the structure function of aero-optical distortion has a power relationship that is similar to that of atmospheric optics. At last, the power spectrum corresponding to the typical region of supersonic film cooling were acquired by improved periodgram.