Recent Innovations in Wall Shear Stress Sensor Technologies

Abstract

Over the past decade significant improvements have been made in wall shear stress (WSS) sensor technologies. Due to the need to resolve flow structures on the order of 100 μm at frequencies up to 10 kHz, classes of microelectromechanical (MEMS) sensors have been developed to overcome these limitations. Three main classes exist, including floating element, thermal, and optical MEMS flow sensors. A handful of new patents have been issued in all three of these classes. For floating element MEMS flow sensors recent US patents. For thermal MEMS flow sensor. Floating element sensors have the advantage that they provide a direct measurement, while thermal sensors use an empirical formulation based upon the heat transfer from the sensor to the flow. Floating element sensors suffer from error associated with pressure gradients, cross-axis sensitivity to acceleration and vibration inputs, and fabrication issues including misalignment between the floating element and gap, debris becoming trapped in the gap between the floating element and the sensor mount and a compromise between durability and sensitivity. Thermal sensors are more mature as they are based on well established methods; however, the major flaw with this technique is the difficulty associated with minimizing heat transfer between the sensing element and the substrate. The third class of sensors, optical MEMS flow sensors, are quickly emerging as perhaps the most accurate and reliable of the three techniques.