On the calibration-free two-component wall-shear-stress measurement technique using dual-layer hot-films

The wall shear stress vector is an important quantity in fluid mechanics and is difficult to be measured. In this work, we first demonstrate that the directional sensitivity (sensitivity to yaw angle α) of a flush-mount hot-film sensor is cos^1/3α using theoretical and experimental methods. Based on the directional sensitivity, a local two-component wall-shear-stress measurement technique is proposed using a pair of un-calibrated dual-layer hot-film sensors positioned perpendicular to each other. This technique use the heat fluxes transferred from the sensors to the fluid to determine both the magnitude and the direction of the wall shear stress so that a calibration is not required. Experimental results demonstrate that this technique is feasible when the angle between the stress and the centerline of the sensor is within ±15°. This valid angle range can be potentially increased if the two sensors are positioned with an angle larger than 90°.