Unsteady distributed wall shear stress measurements in fluid flows

Wall-bounded flows are amongst the most common flows encountered in fluid mechanics. Wall shear stress on the walls of these flow fields is an important engineering quantity as it is responsible for skin friction drag, which is a significant portion of the drag on bodies ranging from airplanes to flow in biological systems. Measuring, understanding and eventually controlling the wall shear stress has implicit financial significance. In general there is limited literature reporting unsteady, distributed wall shear stress measurements, especially in air, due to the lack of sensors to carry out such measurements. This work is a small step in the direction of filling this gap in the literature. A wall shear stress sensor, referred to as the micro-pillar wall shear stress sensor is presented from concept to actual measurements in a wall jet flow field. The micro-pillar shear stress sensor is based on the principle that a micro-pillar on the wall of a wall-bounded flow deflects an amount proportional to the drag force experienced by it. This drag force in turn is proportional to the wall shear stress. Hence, tracking the tip deflection of an array of micro-pillars provides a means to measure the unsteady, distributed wall shear stress. The sensor from design to manufacture along with static and dynamic characterization is presented. It's ability to measure unsteady, distributed wall shear stress is studied using demonstrative experiments. Finally, wall shear stress measurements are carried out on the wall of a three-dimensional turbulent wall jet. The wall jet is subsequently excited and the effect of excitation on the wall shear stress in the near jet exit flow field is studied.