Wall shear stress imaging using micro-structured surfaces with flexible micro-pillars
[ jeudi 25-11-2004 12:00
| français ]Wall shear stress imaging using micro-structured surfaces with flexible micro-pillars
A new optical sensor technique based on flexible micro-pillars is presented herein to determine the temporal and spatial wall shear field in boundary layer flows. The sensor principle uses the pillar deflection in the viscous sublayer as a direct measure of the wall shear. The transparent pillars act as optical micro-fibres which guide the light to the pillar tips on which the camera is focussed. The pillar images appear as a grid of small bright spots on the imaging sensor. All pillars can be measured simultaneous by high-speed imaging of the illuminated sensor-film. The centre positions of the pillar spots are detected by image processing routines with subpixel accuracy. With a typical displacement resolution of the order of 1mm which is similar to the resolution of other optical measurement techniques relying on the detection of small spots (like Particle-Image-Velocimetry) the minimum resolvable wall friction value results to tw ? 10 mPa. The equivalent point force exerted on the pillar head can be detected down to 50 pN. With smaller structures than that obtained herein one can expect even smaller resolution limits.