Influence of non-uniform airflow on optical deformation measurement in subsonic conditions

Non-uniform airflow over a wing causes distortion in real time optically recorded images, which can lead to the erroneous displacement or strain measurement. This situation is more severe when an aeronautical object is tested in high-speed airflow for wind-tunnel or in-flight deformation measurement. In this paper, the principle of the deformation measurement error caused by non-uniform airflow is presented firstly. Then, wind-tunnel experiments were conducted on a two-dimensional OA309 airfoil over the Mach number range from 0.3 to 0.7 and the angle of attack varied from -7.02° to 12.18°. After this, the effect of non-uniform airflow was validated using virtual displacement and strain obtained by image correlation technique. The results showed that the virtual strain varies linearly with the square of Mach number and reciprocal of the cosine of the angle of attack. The beam propagation of the camera through the upstream region rather than the downstream region behind the wing might cause the virtual strain to fall by approximately 50%. To mitigate the effects of non-uniform airflow on measurements, an optimization of the experimental setup is necessary to enable appropriate placement of the camera.