Optical MEMS sensors for wall-shear stress measurements

Abstract

This research reports on the development and experimental characterisation of optical sensors based on Micro-Electro-Mechanical-Systems (MEMS) technologies for walls hear stress quantification in turbulent boundary-layer flows. The MEMS sensors are developed to measure the instantaneous wall-shear stress directly via a miniature flush-mounted floating element, which is on the order of hundreds of microns square. The floating element is suspended flush to the wall by up to four specially designed micro-springs. As the flow passes over the wall, the sensor’s floating element moves, allowing direct measurement of the local forces exerted by the flow on the wall. A new optical transduction scheme based on the Moiré fringe pattern is developed alongside with an optical pathway to measure the instantaneous wall-shear stress using a single photodetector. Using this new optical technique consists of a lens array and fibre optics that provides the ability to detect the wall-shear stress using different sensing element sizes, leads to miniaturisation of sensors. Utilising the lens array, the focused light spot size is controlled, providing the opportunity of scanning the Moiré fringe pattern area on the sensors with different sensing element sizes. The microfabrication process of the devices are carried out by using a four mask bulk Silicon-on-Insulator (SOI) process and a BF33 wafer, where each device is placed at the center of a 5 mm × 5mm chip. Two generations of sensor packaging are developed to accommodate the sensors’ dies as well as the sensors’ optoelectronics, whilst the floating element is flush-mounted to the surface. The MEMS sensors calibration is carried out in a laminar flow rig over a wall-shear stress range of 0 to 5.32 Pa, where the results indicate a sensitivity range of 38 to 740 nm/Pa, an accuracy range of 1.4 to 2.36% and a repeatability range of 0.68 to 1.96%. The value of the of minimum detectable wall-shear stress for the developed MEMS wall-shear stress sensors varies in a range of 17 to 593 µPa, resulting in a minimum and maximum dynamic range value of 79 dB and 109 dB, respectively. The results from the dynamic characterisation indicate a resonant frequency range of 1 to 8.3 kHz. In a series of wind tunnel experiments over a range of Reτ = 560 to 1320, the instantaneous wall-shear stress within the turbulent boundary-layer flow is measured simultaneously by the MEMS sensors and an by either hot-wire anemometry or laser Doppler velocimetry using the near-wall velocity gradient technique. Excellent agreement is observed in the time series and statistics across these three independant measurement techniques.