MEMS sensors for wall shear stress and flow vector measurement

Abstract

The accurate measurement of airflows is an important area of experimental aerodynamics. MEMS technology has been applied to the measurement of wall shear stress and freestream velocity vectors. Existing methods of measuring wall shear stress vary greatly and have different strengths and weaknesses, making them each applicable to specific situations. Probes designed for measuring 3D velocity components are relatively large in diameter, introducing significant disturbances into the airflow. The tip diameters of such probes are typically of the order of several millimetres and the minimum diameter is around 1 mm. A sensor for measuring wall shear stress, consisting of a surface fence structure 5 mm long, 750 μm high and 20 μm thick was developed. The fence, and main body on which it was mounted, were fabricated from the photo-definable polymer SU8 with an integrated gold resistive strain gauge to measure the pressure-induced deflection. Wind tunnel testing gave a voltage output of 0.18 mV for a shear stress of approximately 0.35 Pa. This concept was then adapted and an in-plane cantilever sensor was developed. The cantilever sensor was manufactured from SU-8 with an integrated resistive strain gauge of NiCr. The pressure-induced deflection of the cantilever, calibrated by the integrated strain gauge, could be related to the wall shear stress on the surface. The sensor gave a response of 9.6x10(^-4) (mV/V) μm under mechanical deflection. For a 2 mm long, 400 μm wide cantilever when tested on a flat plate in a wind tunnel, a response of 1 mV for a shear stress of 0.35 Pa was seen. Four cantilever sensors were arranged orthogonally to create a new type of probe for measuring flow direction and velocity, which could also measure total pressure. The probe was shown to be able to measure these variables and with further development had the potential to allow the fabrication of a smaller probe tip than that possible by conventional methods.