In experimental fluid mechanics, measuring spatially and temporally resolved wall shear-stress (WSS) has proved a challenging problem. The micro-pillar shear-stress sensor (MPS3) has been developed with the goal of filling this gap in measurement techniques. The MPS3 comprises an array of flexible micro-pillars flush mounted on the wall of a wall-bounded flow field. The deflection of these micro-pillars in the presence of a shear field is a direct measure of the WSS. This paper presents the MPS3 development work carried out by RWTH Aachen University and Purdue University. The sensor concept, static and dynamic characterization and data reduction issues are discussed. Also presented are demonstrative experiments where the MPS3 was used to measure the WSS in both water and air. The salient features of the measurement technique, sensor development issues, current capabilities and areas for improvement are highlighted.
Experiments for the quantitative assessment of the dynamic wall-shear stress distribution are still one of the major tasks in turbulence research. The micro-pillar shear-stress sensor (MPS 3 ) offers the possibility to detect this quantity. However, former investigations unveil a resonance behavior at higher Reynolds numbers. This is due to the frequency response of the sensor which is proven to be a second-order low-pass filter with low damping. The wall-shear stress measurements in a turbulent boundary layer show a frequency response of the sensor which deviates from the expected response. This can be explained by aeroelastic effects by added mass and fluid viscosity since the surrounding fluid has different constraints during the estimate of the characteristic eigenfrequency in the dynamic calibration and in the turbulent boundary layer. Nevertheless, using only a reduced frequency bandwidth out of the measured wall-shear stress-signal, the results obtained by the micro-pillar shear stress-sensor show good agreement with the literature.
The drag reducing effect of polymers in a channel flow is well known and it is assumed that the polymer filaments interfere with the turbulent structures in the very near-wall flow. To analyse their precise effect, a micro-pillar shear stress sensor (MPS³) measurement system is developed which allows the detection of wall shear stress at high spatial and temporal resolutions. Different manufacturing techniques for the required micro-pillars are discussed and their influence on the flow is investigated evidencing the non-intrusive character of the pillars. Subsequently, a complete calibration is presented to relate the recorded deflection to wall shear stress values and to assure the correct detection over the whole expected frequency spectrum. A feasibility study about the ability to visualize the two-dimensional wall shear stress distribution completes the discussion about the validity of MPS³. In the last step, the drag reduction of a polymer filament grafted on a micro-pillar compared to a plain pillar and the application of MPS³ in an ocean-type polymer solution are investigated. The results confirm the expected behaviour found in the literature.
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