Numerical investigation of the error caused by the aero-optical environment around an in-flight wing in optically measuring the wing deformation

A major concern for in-flight wing deformation measurement using optical methods is the influence of the aero-optical environment around the wing because the quality of the images degrades due to variations of the index of refraction and then the deformation ameasurement errors will be introduced. By taking a typical 2D OA309 airfoil as the research object, the error caused by the aero-optical environment is analyzed and numerically evaluated to investigate the effects of Mach number, angle of attack, and the placement of the camera on optical wing deformation measurement. At first, the error caused by the aero-optical environment in stereoscopic camera setup is analyzed, and virtual displacement is proposed as an evaluation parameter. Then, based on CFD computation, the fourth-order Runge-Kutta ray-tracing method in combination with triangular mesh interpolation is presented to determine the virtual displacement under different conditions. The results indicate that the virtual displacement in the streamwise direction has the largest level around the leading edge and the Root-Mean-Square value over the wing surface is dependent linearly on the square of Mach number. The virtual displacement reaches the minimum value at −2° angle of attack; within the range of stall angle of attack, the further away from −2° angle of attack, the larger virtual displacement the corresponding aero-optical environment will produce. The virtual displacement increases as the camera-wing distance increases, but the growth rate is slowed down. The overall virtual displacement reaches the minimum when the camera viewing angle is close to 90°. The proposed computational method and results can be helpful to appropriately position the camera in the actual in-flight wing measurement and minimize errors caused by the aero-optical environment.