Casey Barnard

Characterization of a high‐frequency pressure‐field calibration method.

A method for the direct measurement of the pressure sensitivity of a microelectromechanical system (MEMS) microphone is developed for frequencies from 10 to 100 kHz. The use of a simultaneous pressure field measurement reduces the errors in calculating the pressure sensitivity from free field measurements, and it allows calibration of non‐reciprocal and non‐capacitive transduction schemes. An ionophone is used as a calibration source in an anechoic chamber. An aluminum plate is mounted in the anechoic chamber and contains a linear array of pressure field condenser microphones. Treatment of the plate edge is used to minimize diffraction within the pressure field. The microphone data are used to characterize the directivity of the ionophone and localize sources of acoustic diffraction that could impact future MEMS microphone magnitude and phase calibrations.

Phase relationships between velocity, wall pressure, and wall shear stress in a forced turbulent boundary layer

A large scale spatio-temporally periodic disturbance was excited in a turbulent boundary layer via a wall-actuated dynamic roughness. Streamwise velocity, wall pressure, and direct wall shear stress measurements were made with a hot wire, pressure microphone, and a micro-scale differential capacitive sensor, respectively. Phase-averaged fields for the three quantities were calculated and analyzed. A phase calibration between the various sensors was performed with an acoustic plane wave tube over a range of operating conditions to validate a direct phase comparison between the respective quantities. Results suggest encouraging agreement between the phase of the wall shear stress and velocity near the wall; however, more refined velocity measurements are needed to make quantitative comparisons to the wall pressure. Overall, this work highlights the potential for wall-based control with applications towards reducing turbulent drag.

A system for vector measurement of aerodynamic wall shear stress

This paper describes a sensor system designed for vector measurement of aerodynamic wall shear stress. The microelectromechanical systems (MEMS) prototype utilizes a serpentine tether flexure for dual-axis in-plane motion. Capacitive gap pairs coupled to a multi-frequency modulation/demodulation system allow for independent transduction across both axes. Sensor performance includes an average dynamic/mean shear stress sensitivity of 0.16 mV/Pa, corresponding to a minimum detectable signal of 0.3 mPa at 1.128 kHz. Pressure rejection is calibrated at 74 dB, with in-plane cross-axis isolation in excess of 24 dB.

An instrumentation grade wall shear stress sensing system

This paper describes development of a MEMS wall shear stress sensor system to be used in wind tunnel applications. An analog synchronous modulation and demodulation interface circuit allows for both ac and dc measurement of varying capacitance, yielding real-time dynamic and mean flow information with a low noise floor. Directional information of the input flow is retained throughout the system with minimal phase delay. A sensitivity of 3.45mV/Pa at 1.128kHz and a DC stability of 0.2mV indicate minimum resolution of mean shear values of 58mPa. Physical resonance of 3.5kHz and a pressure rejection ratio of 72dB are also observed.

Simultaneous Wall Shear Stress and Velocity Measurements in a Flat Plate Turbulent Boundary Layer

A floating element differential capacitive shear stress sensor (CSSS) was used to study the large scale motions in a zero pressure gradient turbulent boundary layer experiment, in conjunction with single component velocity measurements from a hot-wire anemometer. Boundary layer profiles characterize the fully turbulent boundary layer created by the flat plate model. The limited sensor bandwidth due to its resonant frequency allows only large scale motions and part of the inertial scales to be resolved, and shear stress fluctuation statistics are underpredicted. High levels of correlation between the filtered shear stress and the velocity signal are reported. The correlation peaks are used to track and identify the large scale structures inclined to the wall. Conditional sampling on the shear stress provides a conditional mean velocity profile, and the effects of conditional normalization are discussed to explain the link between large scale structures far from the wall and the skin friction.

Development of a MEMS dual-axis differential capacitance floating element shear stress sensor

A single-axis MEMS wall shear stress sensor with differential capacitive transduction method is produced. Using a synchronous modulation and demodulation interface circuit, the system is capable of making real time measurements of both mean and fluctuating wall shear stress. A sensitivity of 3.44 mV/Pa is achieved, with linearity in response demonstrated up to testing limit of 2 Pa. Minimum detectable signals of 340 μPa at 100 Hz and 120 μPa at 1 kHz are indicated, with a resonance of 3.5 kHz. Multiple full scale wind tunnel tests are performed, producing spectral measurements of turbulent boundary layers in wind speeds ranging up to 0.5 Ma (18 Pa of mean wall shear stress). The compact packaging allows for minimally invasive installation, and has proven relatively robust over multiple testing events. Temperature sensitivity, likely due to poor CTE matching of packaged materials, is an ongoing concern being addressed. These successes are being directly leveraged into a development plan for a dual-axis wall shear stress sensor, capable of producing true vector estimates at the wall.