Dynamic Calibration Technique for the Micro-Pillar Shear-Stress Sensor MPS$^{3}$

Based on magnetic excitation a dynamic calibration technique for the micro-pillar shear-stress sensor MPS$^{3}$, which allows to determine the local wall-shear stress in turbulent flows by optically measuring the velocity gradient within the viscous sublayer of turbulent flows, is described. The proposed dynamic calibration technique allows to assess the micro-pillar dynamic response for different flow media up to approximately 10kHz. The results do convincingly agree with the findings of a second-order analytical approximation based on experimentally determined damped eigenfrequencies and damping coefficients. Measurements for different sensor geometries and in various fluids show the sensor to possess transfer functions ranging from a flat low-pass filtered response to a strong resonant behavior. The results further indicate the pillar to possess a very constant transfer function amplitude at frequencies reasonably below the resonance making it ideal for the measurement of fluctuating wall-shear stress.