Modeling of a MEMS Floating Element Shear Sensor
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Modeling of a MEMS Floating Element Shear Sensor Nikolas Kastor 1, Zhengxin Zhao1 and Robert D. White1 1 Mechanical Engineering Department, Tufts University, Medford, MA, United States. ABSTRACT A MEMS floating element shear stress sensor has been developed for flow testing applications, targeted primarily in ground and flight testing of aerospace vehicle and components. 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 numerical model of the sensor has been constructed. In this paper, a computational fluid dynamics (CFD) model is described. The CFD model directly models a laminar flow cell experiment that is used to calibrate the shear sensor. The computational model allows us to quantify 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. The results are compared to experimental data, validating the model and resulting in the following: Surface shear stress contributes approximately 40% of the total flow direction force; pressure gradient effects contribute nearly 45% for the textured shuttle described here; lift forces and pitching moments are non-zero. Thus, it is found that flow interactions are complex and that it is insufficient to simply assume that flow forces on the sensor are the top area multiplied by wall shear, as is sometimes done. Pressure gradient effects, at least, must be included for accurate calibration. INTRODUCTION Wall shear stress and skin friction are important measurement values in flow testing of vehicles and devices in aerospace applications. Existing techniques, such as oil film interferometry, boundary layer profile surveys, or thermal methods, can be used to determine these values, but the measurements can be difficult to apply, are indirect and may not provide real time data [1, 2]. 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 in multiple directions. The shear sensor modeled here, shown in Figure 1, has been described previously by our group [3, 4]. The device is fabricated using surface micromachining on a glass substrate. The structure itself is 8 micron thick electroplated nickel, with 12 micron tall raised posts on the top surface. The structure is separated from the glass substrate by a 5 micron high air gap. The comb fingers are 5.2 microns wide with 2.8 micron air gaps. Additional details regarding geometry and fabrication can be found in the reference [3, 4]. As flow passes over the device, hydrodynamic forces cause the sensor to deflect, creating a differential capacitance chang
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