Testing Block Probes for Wall Shear Stress Measurement in Water Flows

The authors developed and tested two types of sublayer pressure probe or blocks, to indirectly measure the average modulus and direction of the wall parallel component of the shear stress tensor, usually referred to as wall shear stress, or boundary shear, in water flows. The local shear stress has been investigated by means of a Preston tube for different Reynolds numbers, in a smooth rectangular section duct. The observed data have been used to calibrate the blocks. The blocksʹ design is a direct outcome of the Dexter yaw meter, developed to function in an air flow. The observed pressure field associated with a given shear stress is compared to its theoretical counterpart, based on the assumption that the velocity profile satisfies a twodimensional law of the wall, for smooth boundary.