Development of the micro-pillar shear stress sensor MPS3 for turbulent flows

Wall-shear stress results from the relative motion of a fluid over a body surface as a consequence of the no-slip condition of the fluid in the vicinity of the wall. To determine the two-dimensional wall-shear stress is of utter importance in theoretical and applied turbulence research. However, despite the variety of different techniques that have been proposed, it can be stated that the determination of the planar distribution of wall-shear stress is still an open issue in the field of experimental fluid mechanics. In this thesis the development of the Micro-Pillar Shear-Stress Sensor MPS 3 , which offers the potential to measure the two-dimensional dynamic wall-shear stress distribution in turbulent flows, will be reported. The sensor is based on flexible micro-pillars protruding into the near-wall region of turbulent flows and bending in reaction to the exerted drag forces. The deflection of the pillars is detected by optical means and is a representative of the local wall-shear stress. The pillar technique allows extremely high spatial resolutions and the measurement of the wall-shear stress distribution with up to 1000 pillars has already successfully been approved. The sensor possesses the advantage of very low flow interference and is characterized by an relatively easy mounting possibility. Depending on the geometry and material characteristics of the sensor turbulent scales down to less than 50 μm and time scales in the order of a few kHz can be resolved. These features make the novel technology a simple technique to visualize and measure the planar turbulent wall-shear stress distribution in its two wall-parallel components.