New ray tracing method for 3D irregular non-uniform refractive index field

When a flight flies at high speed in the atmospheric, aerodynamic heating, the optical window interacts with the external airflow to form a complex flow field structure. Its refractive index distribution was irregular and non-uniform, so it was difficult to accurately obtain the ray trajectory. In order to obtain accurate ray trajectory, three ray tracing methods with fourth-order accuracy were proposed. Via comparing the results with the analytical solutions of helical rays, it is found that the maximum relative error of the fourth-order Runge-Kutta method is 1.6×10-8, Richardson extrapolation method is 1.2×10-8, and Adams method is 1.2×10-11. The Adams method is a high-precision and high-speed method for ray tracing. An arbitrary point interpolation method based on the polynomial fitting can obtain higher accuracy refractive index field than that computed by distance inverse ratio method. The method was applied to compute the distorted wavefront caused by the flow field around the optical window of a missile, and the calculation results were compared. It is found that Adams linear multi-step method starts with Runge-Kutta method, but Admas method does not neglect the calculation results of the previous step and will not lead to the accumulation of errors, so the results are closer to the real solution, while Richardson extrapolation method calculates the optical path difference significantly different from the other two algorithms. © 2019, Editorial Board of Journal of Infrared and Laser Engineering. All right reserved.