Direct wall shear stress and pressure field measurements in a turbulent boundary layer

The wall shear stress is a critical quantity in wall-bounded flows, governing the friction velocity, the inner length scale, and viscous drag. Fluctuations in the wall shear stress depend on the entire flow field, leading to interactions that are difficult to measure. This work aims to quantify these interactions by leveraging recent technological advancements to simultaneously measure the wall shear stress, velocity field, and pressure field. Experiments are performed on a turbulent boundary layer in a low-speed wind tunnel at a friction Reynolds number of 1400. A non-intrusive capacitive MEMS sensor directly measures the wall shear stress concurrently with two PIV experiments. High-speed PIV is used to acquire the time-resolved velocity field. Subsequently, the pressure field is evaluated using two methods. The first method requires the time-resolved velocity field to directly evaluate time derivatives. The second method can sample the velocity field at a lower rate by estimating time derivatives with spatial gradients using the convection velocity. The pressure field results of the second method are then benchmarked against the first method. A second PIV setup with four low-speed cameras provides measurements of the velocity field with a wide field of view capable of capturing large motions. The pressure field is then evaluated using the second method. The simultaneous measurements of the wall shear stress and flow field allow for cross-correlations, giving insight into their interactions.