Suppression Of Wall Reflection In PIV Images Over Rough Walls

Abstract

Wall reflections in particle image velocimetry (PIV) measurements is one of the limiting factors in obtaining velocity information in the vicinity of rough walls. This work tests two different techniques to suppress wall reflections for time-resolved, two-dimensional particle tracking velocimetry (2D-PTV) and stereoscopic-PIV (stereo-PIV). The 2D-PTV was performed for a turbulent boundary layer developed on a rough wall in a water towing tank facility. To measure velocity in the vicinity of the rough wall, the near-wall region on the viewing window was covered with tinted papers to mitigate the wall reflection and maintain a visible reference plane of the towed wall. Using this technique allowed detection of particles at a wall-normal distance of approximately 300 μm (10 wall units) from the roughness valleys for an imaging system with 1 m standoff distance. To suppress the wall refection for the stereo-PIV over a rough wall performed in a wind tunnel, the roughness is coated with fluorescent paint to shift the reflected light to a higher wavelength. A narrow band-pass filter was then used on the imaging lenses, allowing only particle images to be recorded while extinguishing the majority of the reflected laser light from the surface. Reliable velocity measurements with this technique were obtained up to 500 μm (5.0 wall units) from the roughness valleys.