Study On The Supersonic Cooling Film Of The Hypersonic Optical Dome And Its Mechanism Of Aero Optics

At present,the hypersonic optical imaging guiding technique is mainly facing two major problems: the optical window thermal protection and the control of aero optical distortion around the window.Through the comprehensive study of hypersonic speed dome film cooling flow field on the flow mechanism,the cooling efficiency and aero optical effect,to put forward the formation of cooling film which not only satisfy the cooling effective but also meet the guidance demand.First of all,the wind tunnel platforms and the testing techniques for the research of supersonic cooling film on hypersonic optical doom are studied.In this paper,the high speed wind tunnel with low noise and the hypersonic gun tunnel are introduced,and the performance of the wind tunnels are studied,and the calibratioin of the flow field is carried out.At the same time,to verify the effect of film cooling technology in high temperature conditions,high temperature experimental environment is needed,so the existing hypersonic gun tunnel is transformed to permit hydrogen nitrogen driven condition,so that it could run in both low and high temperature conditions.As for the testing techniques,the use of NPLS(Nano Partical based Laser Scattering)system on hypersonic gun tunnel is discussed;the temperature/heat flux measureing system of cooling effect measurement is also studied.Secondly,the BOS-WS(Background-oriented Schlieren based Sensor)technique is studied in detail,which is used to study the aero optical effects of hypersonic optical dome.First,the full investigation of the research progress of the main existing wavefront testing techniques is performed.Secondly,it is analyzed that the differences between the technical principle of BOS-WS sensor and the commonly used Hartmann sensor,Malley probe and small aperture beam imaging technology.Thirdly,the wavefront reconstruction method of the BOS-WS system is studied and the measurement precision and error of the system are analyzed.The results show that,compared to other wavefront testing technology,BOS-WS system is more simple,and the demand on optical device is not high,and two-dimensional wavefront measurement can be achieved with high time and space precision,which is more conducive to the application in the field of optical imaging guidance.Thirdly,base on the NPLS flow visualization techniques,the flow mechanism of the two-dimensional plate model with gas film of 30° injection and tangent supersonic injection are studied.The characteristics of flow field structure are analyzed by changing the upper boundary layer conditon and the injection flux,and the qualitative and quantitative comparison of the flow field characteristics under two different jet configurations is carried out.The results show that the structure of the 30° injcetion flow field is more complex than that of tangential injection.The oblique injection film generates complex disturbance after jets out of the nozzle.It causes the adverse pressure gradient in the upstream boundary layer and the separation of the main flow,which induces several waves.The mixing of the film and the mainstream becomes more intensely,and with the increase of the injection flux,the mixing layer turns to turbulence rapidly,and the downstream turbulent boundary layer thickness is greatly increased.Meanwhile,if the upper boundary layer is laminar,the shear layer between the tangent jet and the mainstream can still maintain laminar morphology,and the shear layer growth along the flow direction is relatively slow,and will not produce complex wave structures.However,when the upstream boundary layer is disturbed by the rough zone,the transition of the shear layer will advance,and a large scale vortex structure will be formed and the shear layer becomes thick in downstream.To sum up,on the aspect of flow structure,the tangent jet flow is more suitable for the cooling of the optical guidance window.Fourthly,based on the BOS-WS technique,the optical effect of the hypersonic optical dome model is studied.In this part the models include a 2D slope model and 3D blunt body model,both of which have the same window angle and structure,but without the jet.The main object of this part is to explore an arrangement of the BOS-WS system on the hypersonic low noise wind tunnel,and to explore the basic flow structure and aero optical characteristics of the window flow field without jet.At first,the flow field is visualized with schlieren method;then,the BOS-WS system arrangement on the hypersonic low noise wind tunnel platform is achieved;finally,a comparative study of the aero optical effect of the two models is carried out with the flow condition of Ma=6.0,total temperature T0=420K,and total pressure P0=1.0~1.6MPa,and the difference of wavefront distortion between long exposure mode and transient exposure mode is studied.The results show that the aero optical distortion of the two model is similar;increasing the total pressure,the distortion is enhanced;increasing the model attack angle will also lead to distortion enhancement;the distortion tested under long exposure time results is close to that under transient exposure time,but the long exposure time distortion is more smooth than that of transient exposure time.Finally,the cooling effect and the aero optical effect of the hypersonic optical dome with supersonic cooling film were studied.The wall temperature/heat flux measurement system based on the resistance temperature detector is used to study the cooling effect on the platform of hypersonic low noise wind tunnel and the hypersonic gun wind tunnel.It is found that the cooling film could cover the surface of the window under the condition of pressure matching.The cooling effect of the gas film on window is mainly reflected in two aspects,one is the heat insulation function,avoiding the direct heating of the main flow on the window,which demands a certain thickness of the film;the other is the cooling effect,which means that the low temperature gas film absorbs heat from the window or the mainstream and carries it to downstream,which requires the gas film momentum is large enough,and the heat can be effectively transported.The experimental results show that the pressure matched gas film can not only achieve the heat insulation effect under the normal total temperature,but also can realize the effective thermal protection under the condition of high total temperature of T0=2600K.The aero optical effects is studied using BOS-WS technique,exploring the influence of with and without jet,different film flow,different nozzle parameters and different flow parameters on wavefront distortion.The results show that the introduction of supersonic gas film will enhance the distortion;similar to the conclusion of the previous part,increasing the total pressure will enhance the aero optical distortion;the distortion under the matched pressure condition is relatively low,and it is higher under the over matched condition,but continued to improve the over matched pressure will not lead to a significant impact on the degree of distortion;the peak distortion exists under the condition of under matched pressure,which is mainly affected by the reattachment of the upper boundary layer on the window.Based on the above results,it is concluded that the pressure matched gas film can improve the aero optical effect caused by the film injection while permiting the effect of coolilng of the high speed optical dome,and this is meaningful to dissolve the problems faced by the hypersonic optical imaging guiding.