2011 Theses Doctoral

Development of a MEMS Sensor for sub-kPa Shear Stress Measurements

O'Grady, Andrew

This dissertation discusses the development of MEMS sensors for measuring sub-kPa (<1000Pa) wall shear stresses in high-speed turbulent flows. Wall shear stress is an important flow quantity that is used to characterize flows that can be found in aerospace, automotive, and biomedical applications. Sensors that can measure this quantity could have many uses ranging from pure turbulence research to flow control of vehicles. MEMS fabrication techniques allow for the creation of micro-scale sensors that are small enough to accurately measure fluctuating turbulent shear stress. Utilizing a direct-shear stress measurement with a floating element allows the sensors to be calibrated in a well-known shear flow before being installed in an unknown flow environment. The sensors use a differential capacitance measurement scheme combined with non-intrusive backside sensor connections, allowing measurements in recirculating and separating flows. As part of the sensor design process, 36 different sensor designs were created with varying feature sizes and performance ranges. This was done to mitigate the risks inherent in MEMS fabrication processes and to increase the chances of developing working sensors which could operate in the desired shear stress range (1 - 1000Pa). The sensors were fabricated with the floating element in the top layer of an SOI wafer, with thru-wafer electrical interconnects (vias) created to connect the frontside of the sensor to the backside of the chip. Post-fabrication, the sensors were characterized electrically and mechanically under a microscope probe station. Sensors were then installed in a custom-made package which integrated off-the-shelf capacitance measuring circuitry with the MEMS sensor. Using a subsonic duct flow setup, sensors were calibrated in compressible turbulent air flow up to a mean shear stress of 335Pa and a friction velocity Re value of 200,000. After numerical temperature compensation was implemented (required due to temperature-dependent material properties) the sensor gain was calculated as 0.16mV/Pa. The mean shear stress calibration was then used to analyze the turbulence fluctuations inherent in the sensor signal. Turbulence measurements (including intensity, spectral density, and probability density function) indicated that the sensors were responding to shear stress fluctuations, but not detecting the entire turbulent energy spectrum due to low-pass filtering caused by the electrical circuitry. Experimental measurement of wall shear stress in compressible turbulent flows is an area that has not been fully explored due to previous limitations in available measurement technology. Once a better understanding is gained of the operation and limitations of the MEMS wall shear stress sensors, there lies a great potential to increase our understanding of turbulent flows with unprecedented measurements.