Measuring the Wall-shear Stress for Turbulent Flows Using the Micro-Pillar Shear-Stress Sensor MPS3

For wall-bounded turbulent flows, knowledge of the mean and the fluctuating components of the wall-shear stress is of vital importance for the investigation of near-wall flow structures, which are responsible for the turbulent energy and momentum transfer. In this study, the wall-shear stress distribution in wall-bounded turbulent flows is experimentally investigated using the micro-pillar shear-stress sensor (MPS3). Firstly, the impact of a riblet-structured surface in a turbulent boundary layer is investigated. Secondly, extreme events occurring at the wall, including the wall-normal velocity spikes and backflow events, are detected and analyzed based on the measured wall-shear stress patterns in a turbulent channel flow. Thirdly, the wall-shear stress pattern at different streamwise positions along a sinusoidal side-wall of a turbulent channel flow is analyzed. The flow field is distinguished from that over a flat wall by a separation region at the downslope and a free-shear layer over the trough, whose effects become weaker at higher Reynolds numbers. In conclusion, this thesis proves the capability of the sensors to measure the wall-shear stress distribution for more complicated flow cases and for surfaces with slight curvatures. The results substantiate results from previous numerical studies and may facilitate a further understanding of the near-wall structures in turbulent flows.