Research On Modeling And Simulation Method Of Aero-Optical Effect On Image Quality Of High-Speed Aircraft

In the aerodynamic thermal environment,the temperature of an optical dome rises rapidly when an aircraft with infrared guidance device is flying at high speed.Due to the uneven heating,the temperature and refractive index of the optical dome is unevenly.When the parallel beam passes through the flow field and optical dome with the non-uniform refractive index distribution,its propagation direction will change and additional phase will be added.The target image received by the detector appears blur,jitter and image offset.This phenomenon is known as aero-optical transmission effect.Meanwhile,infrared radiation rays will be emitted from the aerodynamic heated optical dome.The infrared radiation rays are received by the detector behind the optical dome,causing background noise interference to the target signal on the detector.When the infrared radiation from the optical dome is strong,the detector will reach saturation and the target image will be submerged by infrared noise.This phenomenon is known as aero-thermal radiation effect.Aero-optical transmission effect and aero-thermal radiation effect are collectively referred to as aero-optical effect.The aero-optical effect affects the ability of the infrared seeker to detect,identify and track the target,and restricts the guidance accuracy of the seeker seriously.The thesis focuses on the theory and simulation method of aero-optical transmission effect and aero-thermal radiation effect of optical dome.The comprehensive performance evaluation function of the optical dome is established to optimize its shape.Under the film cooling condition,the combined optical transmission model of the flow field and the optical window is established to correct the aero-optical effect,the main research contents include:Research on three dimensional refractive index field of optical dome in aero-thermal environment.According to the theory of aerodynamics,the finite element model of the flow field is established by adopting the finite element method.The real aerodynamic thermal environment of the flow field outside the optical window is simulated.The temperature field,pressure field and heat flux field distribution of the flow field is calculated.According to the theory of heat transfer and structural mechanics,the thermal and structure coupling finite element model of optical dome is established.The three-dimensional temperature field distribution and deformation distribution of the optical dome in the aero-thermal environment is calculated.According to the principle of thermo-optic effect,the refractive index distribution can be calculated from the temperature distribution of an optical dome.Research on the aero-optical transmission effect and the aero-thermal radiation effect of the optical dome.The tracking accuracy of three ray tracing algorithms(Taylor series expansion method,three order Runge-Kutta method and four order Runge-Kutta method)is compared in optical media with non-uniform refractive index field distribution.The mathematical models of aero-optical transmission effect and the aero-thermal radiation effect of an optical dome is established respectively.The evaluation indexes of image quality degradation under the combined influence of the aero-optical transmission effect and the aero-thermal radiation effect are obtained.Research on the influence of flight parameters and structural parameters of an optical dome on aero-optical effect.Taking the conformal dome as the research object,and taking aerodynamic force,aerodynamic heat,optical image quality evaluation index and irradiance distribution as evaluation criteria,the influence of structural parameters of an optical dome and flight conditions on aero-optical effect is calculated.Compared with the traditional hemispherical optical dome,the conformal dome has better aerodynamic performance under the same flight conditions.However,the distortion image of the conformal dome is more blurred due to the influence of aero-optical effect.By increasing the thickness of the optical dome,its temperature can be reduced.Thus the imaging quality of the high-speed aircraft under the comprehensive influence of the aero-optical transmission effect and the aero-thermal radiation effect is improved.It provides a new kind of method to correct aero-optical effects.Based on the variation of normalized drag coefficient,wavefront aberration and irradiance with the quadratic term constant of an optical dome,the comprehensive performance evaluation function of the optical dome is established,which provides a new method for the design and optimization of optical dome.The influence law of the flight parameters on aero-optical effect is obtained,which provides the basis for reasonably planning imaging guidance path of optical dome.Research on combined aero-optical effect of the flow field and the optical window.Tangential film cooling is one of frequently-used methods to reduce the temperature of optical window.When the cooling gas is ejected,the structure of the flow field and the temperature distribution of the optical window will become more complex.The mathematical model of optical transmission effect combining flow field with optical side window is established,and the accuracy of the model is verified.This mathematical model is suitable for optical side window of arbitrary shape.The influence of aero-optical effect on image quality is calculated.The results show that the aerodynamic thermal radiation effect is related to the temperature of the optical window,and film cooling can effectively reduce the temperature of the optical window,so as to weaken the influence of aerodynamic thermal radiation effect on the image quality.The aero-optical transmission effect is related to the optical path of rays propagation.The ejected cooling gas may increase the refractive index gradient of the flow field and the optical window,and increase the optical path difference when the rays passes through the flow field and optical window,resulting in the influence of aero-optical transmission effect on the image quality enhancing,thus the image becoming more blurred.