Elimination of PIV light reflections via a temporal high pass filter

A novel approach for PIV image pre-processing is proposed to deal with the undesired effect of laser light reflections from solid walls in wind tunnel experiments. The method can be applied for both stationary interfaces as well as when the image of the interface is moving due to vibration of either the model or the imaging system. The working hypothesis is that the motion of the interface is resolved temporally, which is typically the case when employing high-speed PIV systems. The method is based on the decomposition of the pixel intensity in the frequency domain. The high-frequency content of the signal is due to the transit of seeding particles, whereas undesired reflections will appear in the low-frequency range. Applying a high-pass filter on the light intensity time history retains only the contribution of the seeding particles and rejects the undesired light reflections. Two experiments show the application of the method. In the low-speed flow regime around a pitching airfoil, the trace of the laser impinging on the moving surface can be mostly eliminated, enabling cross correlation analysis of the flow closer to the wall. In the transonic regime, experiments are performed in an industrial wind tunnel around the base region of the ARIANE V launcher model. Here, the high-pass filter eliminates all secondary reflections and enhances the particles peak intensity relative to the reflections and the background light.