Experimental Investigation On Aero-optical Effects Induced By Hyper/super-sonic Optical Dome

When the high-speed（Mach number>3）optical dome flies in atmosphere,the serious aerodynamics heating near the imaging window will not only make the infrared signal saturation,but also may damage the window.As a common heat insulation method,supersonic film cooling can effectively isolate the optical window from the high temperature mainstream.However,the interaction between the cooling film and the main stream around the optical window forms the flow field structure including shock wave,boundary layer,mixing layer,coolant layer and their mutual interference,which affects the imaging quality of the detector and causes the target image to shift,shake,blur and energy reduction.This phenomenon is collectively referred to as the aero-optical effects.The existence of aero-optical effects has seriously affected the accuracy of imaging guidance,which has become one of the key technologies to be solved in the development of high-speed infrared imaging optical dome.As one of the typical structures in the flow around the high-speed optical dome,supersonic turbulent boundary layer has become an important part of the research of aero-optical effects.Based on nano-particles,the Nano-tracer-based Planar Laser Scattering（NPLS）technique has the characteristics of high spatial-temporal resolution,which can effectively capture the fine structure and time evolution process of high-speed flow.Based on the flow visualization results of supersonic（Mach 3）turbulent boundary layer obtained by NPLS technique,the influences of different layers in the boundary layer,turbulence scales and light incident angle on aero-optics were examined and the underlying flow physics were analyzed.The large-scale structure in the supersonic turbulent boundary layer played a leading role in the aero-optical effect.Based on the general aero-optical linking equation and the spatial two-point cross-correlation method,the effect of turbulence anisotropy on the aero-optical effect at different incident angles was verified.Based on the flow visualization results of hypersonic（Mach 6）turbulent boundary layer obtained by NPLS technique,the Scale Invariant Feature Transformation（SIFT）algorithm was used to obtain velocity information of hypersonic turbulent boundary layer.Based on the results of mean velocity and fluctuating velocity distribution,the feasibility of this method was verified.The spatial two-point cross-correlation method was used to study the distribution characteristics of coherent structure in hypersonic turbulent boundary layer and its relationship with Reynolds number.The far-field distribution of light passing through the hypersonic turbulent boundary layer at different Reynolds numbers was studied.The results showed that with the increase of Reynolds number,the increase of jitter component was not significant,and the increase of spread component was obvious.With the consideration of the thickness change of refractive index field,introduction of the near-field correction and construction of the double telecentric configuration,wavefront measurement accuracy of Background Oriented Schlieren（BOS）was improved.A standard plano-convex lens was utilized to quantitatively evaluate the improved results,and the effectiveness of the improvement was verified.Determination methods of spatial resolution,sensitivity and dynamic measurement range of wavefront measurement based on BOS were studied.The influences of cross-correlation interrogation window size and step length on the wavefront reconstruction accuracy were investigated.The wavefront results of supersonic film at different positions and cases were obtained.The fitting results showed that there was a good linear relationship between OPD_rmsandρ₂/ρ_SLat different locations.Under different experimental cases,the aero-optical distortion tended to become stronger first and then weaker along the flow direction.The Large Aperture Approximation（LAA）could be used to predict Strehl Ratio（SR）values effectively in a considerable range by OPD_rms.Based on the KD-01 hypersonic gun wind tunnel,the aero-optical effect measurement platform for hypersonic（Mach 6）optical dome was built to measure the wavefront from transient exposure to long exposure.With the increase of exposure time,the accuracy of OPD_high-orderreconstruction by low-order Zernike polynomials increased from 62.2%to 88.6%.The increase of exposure time was helpful to reduce the complexity of wavefront spatial distribution structure.In principle,it could reduce the difficulty of wavefront adaptive correction systems.With the increase of exposure time,the OPD_rmscorresponding to OPD_high-orderincreased gradually,the amplitude decreased gradually,and the difference of OPD_rmsdecreased gradually at different times.Under different exposure times,LAA principle could achieve a better prediction of SR values.With the increase of exposure time,the imaging integral resolution,R,decreased obviously and it was stable at about 1.43R₀.Compared with that,R was improved by about 30%when the exposure time was 6ns.When the exposure time was 20μs,Bore Sight Error（BSE）is small at PRJ=0,with the increase of PRJ,BSE tend to increase.BSE has a local small value at PRJ=1.When PRJ>1,BSE tend to increase with the increase of PRJ.With the introduction of Micro Vortex Generators（MVGs）,the mitigation of OPD_high-orderwas well realized at different PRJ conditions.MVGs played an obvious role in improving the wavefront stability as well.