On the development and implementation of MEMS sensors for instantaneous wall-shear stress measurements in wind tunnel applications

In this work a new optical Moiré fringe sensor was designed, developed, and characterised for direct measurements of instantaneous wall-shear stress in turbulent air boundary layers. The sensors in this study consist of a small sensing pad of order 100 microns by 100 microns suspended by micro-springs as part of a 5mm square die achieving a hydraulically smooth sensor due to in-plane microfabrication and the absence of any topside connections. The instantaneous displacement and fluctuations of this device are measured through the scattering of light from a pair of gratings illuminated from the backside. The illumination is achieved using an array of LEDs focused onto the backside of the sensors and rippled at high frequency with the reflections measured using a photodiode. The Moiré fringe pattern created by this pair of closely matched pitched gratings amplifying the sensors motion by at least 50 times, improving the overall sensitivity of the device. The devices first have their sensitivities outlined using an analytical framework prior to characterization and calibration in a laminar flow channel before experimental measurements being made in a zero-pressure-gradient turbulent boundary layer. To validate the outcomes of the boundary layer experiments, laser Doppler velocimetry (LDV) will be run simultaneously to the MEMS sensor being positioned above the MEMS sensor. These two independent techniques can then be compared, as well as the moments of the statistics.