PIV measurements within roughness grooves with enhanced wall reflection suppression

Abstract Wall reflection of laser light in particle image velocimetry (PIV) measurements is one of the limiting factors in obtaining velocity information in the vicinity of rough walls. In this wind tunnel study, wall reflections for stereoscopic PIV (SPIV) measurements over a rough wall are suppressed by coating the rough surface with fluorescent paint to shift the diffuse-reflected light to a higher wavelength. A narrow band-pass filter employed on the imaging lenses extinguishes the majority of this wavelength-shifted surface-reflected laser light, allowing particle images to be recorded in the near-wall vicinity. Three different fluorescent paints were evaluated. Rhodamine 6G was found to have the best performance for suppressing wall and background reflections. Reliable velocity measurements with this technique were obtained as close as 500 µm ( $$\sim 5.0$$ ∼ 5.0 wall units) from the base of the roughness valleys. The technique was successfully used with SPIV to detect the secondary motions within riblet valleys.