Multiphysics model investigating performance of a micromachined floating element shear stress sensor

Abstract

A MEMS floating element shear stress sensor has been developed for flow testing applications, targeted primarily at ground and flight testing of aerospace vehicles and components. A comprehensive numerical model of this sensor is described in this paper, quantifying the behavior of the mechanical components, fluid interaction, and electrostatics in three, non-coupled, 3-D numerical simulations: 1) A finite element model of the static element. 2) A steady state, incompressible, viscous, laminar, Newtonian computational fluid dynamics (CFD) model, for both flat and textured versions of the floating shuttle. 3) A finite element model of the capacitive sensing combs. The distribution of aerodynamic forces over the floating element was studied to determine which features contributed most to the total applied force and sensitivity. Shear stress forces account for 74% of the sensitivity of the flat sensor, with the remainder coming primarily from pressure gradient effects. For a textured sensing element, while the total sensor sensitivity increases between 17% and 27%, only 34% of the output is due to shear forces, and the response is more nonlinear. Thus, a flat sensor with as little surface topology as possible is preferable to reduce pressure gradient sensitivity and nonlinearity, even though it may exhibit lower overall sensitivity to flow forces. In addition, the sensor is shown to not only deflect in the direction of flow due to shear forces, but also to lift away from the substrate and pitch its downstream edge away from the surface. Pitch rotation contributes as much as 37% of the output of the sensor for a textured element, but less than 1% for the flat element. For a perfectly symmetric device, differential measurement completely eliminates the contribution from lift. Overall, the model gives a more complete picture of the sensing mechanisms present in a floating element shear stress sensor, and demonstrates the aerodynamic complexities which motivate careful design and calibration of these types of sensors.