A robust sensor for wall-shear-stress and near-wall flow-direction measurement

Current methods for measuring wall shear stress and near-wall flow direction in wall-bounded turbulent liquid flows are imperfect because of complexity, marginal reliability, drift, or calibration difficulties; or because they cause nontrivial flow perturbations, or cannot be readily implemented on curved surfaces. This presentation describes a novel and robust sensor that enables reliable wall-shear-stress and near-wall flow-direction measurements in wall-bounded liquid flows over curved surfaces. The new sensor utilizes small (~0.05 mm diam.) bubbles produced at the sensor’s concentric-electrode central orifice, an array of downstream surface electrodes, and the electrical impedance changes induced by the small bubbles to determine bubble shedding rates and convection directions. Sample measurements from prototype sensors placed in a turbulent channel flow are shown and assessed for accuracy, repeatability, robustness, and uncertainty. For fixed volumetric gas-flow rate, the bubble shedding rate is found to be monotonically related to the wall shear stress, while the locations of the electrodes that record the bubbles’ downstream impedance signatures indicate surface flow direction. The new sensor is implemented with a syringe pump and low-cost custom electronics, and operates at bubble shedding rates up to 10 kHz. This sensing scheme may increase the accuracy and robustness of surface flow measurements while reducing their cost and difficulty.