Research On The Estimation Method Of Fluorescence Oil Film Velocity Based On Optical Flow Method

In wind tunnel experiments,analyzing the temporal images of fluorescent oil film flow on the surface of the aircraft test model enables the calculation and visualization of the surface flow velocity field using the optical flow method.This allows for the identification of the location,mode,and characteristics of flow separation on the model surface,as well as understanding the formation mechanisms of vortices and other phenomena.The research data obtained contributes to the study of drag reduction for aircraft and provides a foundation for aerodynamic research.Since the introduction of the optical flow concept,the algorithms based on optical flow theory have been significantly improved and refined,resulting in increased accuracy.However,in practical wind tunnel experiments,the estimation of fluorescent oil film velocity using the optical flow method still faces significant challenges due to complex test conditions.To address the issues encountered with the optical flow method in current wind tunnel experiments with a fluorescent oil film,and to improve the accuracy of velocity estimation based on the optical flow method,this paper focuses on studying the effects of illumination variation in fluorescent oil film images and model vibrations in wind tunnel tests.The main research contributions are as follows:(1)Correction of illumination variation in oil film.The fluorescent oil film material is improved by replacing the inorganic fluorescent powder with organic anti-counterfeiting fluorescent powder,increasing the luminescent efficiency of the fluorescent material.Centrifugal dispersion grinding technology is employed to reduce the proportion of large particles of fluorescent powder,thereby minimizing the illumination variation caused by the oil film itself.Additionally,a nanosecond-level ultraviolet lamp is used as the illumination source to reduce environmental lighting changes.Finally,global and local intensity correction is performed to adjust the brightness of the two frames of images,ensuring consistency of the total grayscale value with the initial frame,and maintaining brightness invariance.The corrected streamlined maps conform to the flow phenomena.(2)Correction of model vibration errors.A model vibration elimination method based on the superposition of minimum common multiple periods is proposed,taking into account the first-order natural frequency of the model itself.Multiple vibration periods are superimposed to mitigate errors caused by single-frame data loss.Simulation experiments demonstrate that by superimposing data from the least common multiple period,the average endpoint error is1.7316 pixels/second,compared to 92.16 pixels/second for single-image pairs.Ground vibration simulation experiments show that the velocity field maps conform to the flow phenomena,and the errors caused by model vibration are eliminated.(3)Velocity measurement calibration.Velocity measurement calibration of the optical flow method is performed using the image correlation method.A calibration model is derived based on the velocity data obtained from both methods.The root mean square error(RMSE)after correction is 0.1982pixels/frame,significantly improved compared to the pre-correction RMSE of 0.4802pixels/frame.The accuracy of the optical flow method is noticeably enhanced after correction.(4)Development of a prototype system for fluorescent oil film velocity measurement based on optical flow method.A CPU-GPU heterogeneous framework and corresponding hardware experimental platform are utilized to build the system.Interactive software is developed,with the CPU responsible for functions such as image preview,data transmission,and map display,while the GPU handles image compression,storage,and optical flow computation.Ground experiments and wind tunnel tests are conducted using the system,and the oil flow motion maps conform to the actual motion phenomena,demonstrating consistency with the flow mechanism.