Study On The Aero-Optical Effect Of The Flow Field Of Aircraft Hood By Ray Tracing Method

The hypersonic aircraft will act violently with the airflow when flying at high altitude,causing the target image of the optical system to be offset,blurred,and distorted,which is called aero-optical effect.Studying the flow field characteristics around the aircraft and the aero-optical effects caused by the flow field have important research significance for improving the clarity and positioning accuracy of the target imaging and correcting the aerooptical effects.In this paper,the computational fluid dynamics technique is used to establish the blunt cone model of the aircraft optical hood by means of ICEM software.Then the Fluent software is used to simulate density distribution of the high-speed flow field near the wall of the optical hood at different heights and in different Mach numbers.Last,we try to explain this distribution from an aerodynamic point of view.Also,we preprocesses the flow field grid in order to facilitate the study of the target flow field,which includes the conversion of the flow field coordinate system,the selection of the calculated fluid region,and the solution of the refractive index distribution of the flow field calculating by the G-D equation.In order to accurately calculate the transmission of light in the flow field,the error backward feedback(BP)neural network is used to interpolate and train refractive index value in different states.According to the distribution of the density field,the aero-optical effects of the flow field are analyzed from the two aspects of geometric optics and physical optics.Starting with geometric optics,the tracking method based on fluid computing grid is selected,which greatly reduces the calculation amount and computational complexity.According to the transmission of light in the grid,the refractive index interpolation method is distinguished,and four interpolation calculation methods are proposed to improve the calculation accuracy.Finally,the optical path difference,angular deviation and sighting error are calculated,which is caused by the propagation of light in the flow field.The variation of these physical quantities with height,Mach number and incident angle of light is studied.The results show that by increasing the incident angle of the light and the flying Mach number,the aerodynamic optical effect of the flow field is enhanced.On the contrary,the aero-optical effect is weakened as the height increases.When the height is increased to a certain extent,the aero-optical effect caused by other factors can be significantly weakened.Physical optics methods begins with the calculation of optical path difference.Firstly,the specific method of calculating optical path difference,wave aberration,point spread function and Strehl ratio is given.Next,the effects of height,Mach number and beam incident angle on the point spread function and the Strehl ratio in the imaging plane are studied.The results show that the increase of the incident angle of the beam will enhance the aero-optical effect,increase the blur of the imaging and weaken the brightness of the central imaging,and as the height increases,the Strehl ratio becomes larger and the aero-optical effect gradually weakens.And these conclusions are consistent.In this article,we also start from a new perspective.Refractive index field reconstruction based on refractive index gradient threshold is studied,and the accuracy and reliability of aero-optical effects are studied by reconstructing the refractive index field.First,the refractive index gradient threshold was modified.We reconstructed the refractive index field using the refractive index gradient at different thresholds and compared it with the original refractive index field.It is found that when the number of neglected refractive index gradient elements is within 50% of the total number,the reconstructed refractive index field can be almost equivalent to the original refractive index field.Finally,the relative error of the mean square error of the optical path difference is calculated by the ray tracing method,and this conclusion is verified.This has important reference value for studying the mechanism of aero-optical effect and correcting the aero-optical effect,and provides a new idea for the study of aero-optical effect.