Modeling of a MEMS Floating Element Shear Stress Sensor

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. Shear stress, known also as skin friction, is one of the major components of drag in vehicles, but is difficult to measure using existing techniques such as oil film interferometry, boundary layer profile surveys, or thermal methods. Direct floating element MEMS sensors address these issues by providing real-time momentum transfer based unsteady shear measurements at a surface with the potential for low topology and array sensing. However, concerns remain about the interaction of the flow with the mechanical elements of the structure at the micro-scale. In particular, there are concerns about the validity of laminar flow cell calibration to measurement in turbulent flows, and the extent to which pressure gradients may introduce errors into the shear stress measurement.;In order to address these concerns, a comprehensive numerical model of the Tufts floating element shear stress sensor has been constructed. The model describes the behavior of the mechanical components, fluid interaction and electrostatics of a micro-machined nickel floating element fluid shear stress sensor. Three 3-D numerical simulations were performed on the geometry of this sensor: A finite element model of the static element; a computational fluid dynamics (CFD) model of the floating element, for flat and textured versions of the floating shuttle; and a finite element model of the capacitive sensing combs. The static, small displacement structural element model subjected the sensor element to applied loads of 2Pa to 12Pa. This model shows linear deflection in the flow direction on the order of 10-10 m and deflection in the transverse flow direction on the order of 10-11 m at 12Pa. In the CFD model, the shear sensor was subjected to internal duct flows of 5CFH to 40CFH. The geometry of the floating element was studied to determine which features contributed what percentage of the total applied force and sensitivity. Pressure gradient was determined to contribute to 25% of the sensitivity of the flat sensor and close to 60% for the textured shuttle showing that a textured shuttle surface adds to the pressure gradient sensitivity and non-linearity. The finite element model of the capacitive combs shows the electrostatic coupling and fringe effects and how they contribute to the sensitivity of the sensor. With the combination of these three numeric models, sensor output as a function of steady state fluid flow parameters can be predicted and directly compared to experimental data. The computational model allows for quantifying of the contributions (e.g. pressure gradient vs shear, top surface vs. lateral surfaces) to the sensor output in a manner that is difficult by purely experimental means.