Study On Flow Mechanism And Aero-optics Of Supersonic Film Cooling

Supersonic film cooling(SSFC)is often used for active thermal protection of optical window of imaging guidance vehicle.However,the flow field structure of supersonic cooling gas film is complex.The interaction between jet and main stream forms shock wave,expansion wave,mixing layer and other flow field structures,which makes the density distribution near the optical window uneven.After the imaging time line passes through the uneven density field in space-time,random refraction occurs,which makes the target imaging shift,blur,jitter and energy attenuation,seriously affecting the optical imaging quality Quantity and guidance accuracy.In this paper,the wind tunnel experiment and numerical simulation of supersonic cooling film flow field are carried out to explore the influence of different flow parameters on the structure of the flow field and the aerodynamic optical effect,so as to provide the basis for solving the aerodynamic optical problems faced by the imaging guidance aircraft.The flow field of supersonic cooling film in the mainstream of Mach 3 and Mach 6was studied experimentally.Using schlieren technique and NPLS(nano-tracer-based planar laser scattering)technique,the structural characteristics of the supersonic cooling film flow field under different pressure ratio of jet(PRJ)were studied,and the distribution of the wall pressure along the flow direction of the experimental model was recorded by pressure sensor.The results of schlieren image show that the increase of jet pressure ratio will increase the induced shock angle at the upper lip position,and the reflected shock position at the lower lip position will move downstream.NPLs test results show that when the jet pressure ratio is close to 1,the K-H unstable vortex structure is easy to appear near the jet outlet.When the jet pressure ratio increases,the growth rate of the downstream boundary layer of the jet outlet increases,and the vortex structure in the flow field is more broken.The pressure test results show that the wall pressure at the downstream of the jet outlet first decreases,then increases,and finally tends to be stable.When the jet pressure ratio increases,the wall pressure at the same flow direction increases correspondingly.RANS numerical simulation method is used to simulate the flow field of supersonic cooling film of three-dimensional optical head cover.The flow field structure under different jet pressure ratio is analyzed.Based on the density field information obtained by simulation,the wave front distortion caused by plane wave front passing through the flow field is calculated by ray tracing algorithm,and the influence law of different jet pressure ratio on the aerodynamic optical effect is studied.The simulation results show that the optical path difference(OPD)of the flow field near the optical window is smaller without jet flow,that is,the aerodynamic optical effect is weaker;the optical path difference of the flow field is larger when the jet flow is in the state of under pressure jet and over pressure jet,and the optical path difference of the jet pressure matching(PRJ=1)is the smallest,that is,the aerodynamic optical effect is weaker,and the imaging quality is better The best quantity.