Jessica Meloy

A Microscale Differential Capacitive Direct Wall-Shear-Stress Sensor

This paper presents the development of a floating-element-based capacitively sensed direct wall-shear-stress sensor intended for measurements in a turbulent boundary layer. The design principle is presented, followed by details of the fabrication, packaging, and characterization process. The sensor is designed with an asymmetric comb finger structure and metalized electrodes. The fabrication process uses deep reactive ion etching on a silicon-on-insulator wafer, resulting in a simple two-mask fabrication process. The sensor is dynamically characterized with acoustically generated Stokes layer excitation. At a bias voltage of 10 V, the sensor exhibits a linear dynamic sensitivity of 7.66 mV/Pa up to the testing limit of 1.9 Pa, a flat frequency response with resonance at 6.2 kHz, and a pressure rejection of 64 dB. The sensor has a noise floor of 14.9 μPa/√(Hz) at 1 kHz and a dynamic range >;102 dB. The sensor outperforms previous sensors by nearly two orders of magnitude in noise floor and an order of magnitude in dynamic range.

A metal-on-silicon differential capacitive shear stress sensor

The paper presents a direct, capacitive shear stress sensor with performance sufficient for time-resolved turbulence measurements. The device employs a bulk-micromachined, metal-plated, differential capacitive floating-element design. A simple, two-mask fabrication process is used with DRIE on an SOI wafer to form a tethered floating element structure with comb fingers for transduction. Experimental results indicate a linear sensitivity of 7.66 mV/Pa up to the testing limit of 1.9 Pa at a bias voltage of 10V , and a bandwidth of 6.2 kHz . The sensor possesses a dynamic range ≫ 102 dB and a noise floor of 14.9 μPa/Hz at 1 kHz , outperforming previously reported sensors by nearly two orders of magnitude in MDS.

Experimental Verification of a MEMS Based Skin Friction Sensor for Quantitative Wall Shear Stress Measurement

This paper presents the preliminary wind tunnel characterization of a microelectromechanical systems (MEMS)-based capacitive floating element shear stress sensor. The floating element structure incorporates interdigitated comb fingers forming differential capacitors, which provide electrical output proportional to the floating element deflection. A compact sensor package combined with a synchronous modulation/demodulation system facilitates mounting in a flat plate model located in an open-loop low-speed wind tunnel. Particle image velocimetry is used to measure the boundary layer velocity profiles for laminar, transitional and turbulent flows. The mean wall shear stress estimated from profile curve fits is in agreement with MEMS sensor output.

A microelectromechanical systems‐based piezoelectric microphone for aeroacoustic measurements.

This paper describes the packaging and characterization of micromachined piezoelectric microphones designed as aircraft fuselage instrumentation for full‐scale noise characterization flight tests. An important requirement for such microphones is a high maximum sound pressure level (SPL) >150 dB coupled with noise floor <45 dB SPL (narrow bin) and flat frequency response in the audio band (20 Hz–20 kHz). In contrast to many past micromachined piezoelectric microphones that use PZT and ZnO for piezoelectric transduction, aluminum nitride (AlN) was used due to inherent advantages in dielectric loss and signal‐to‐noise ratio. The microphone structure includes a 500–900‐μ m diameter circular diaphragm composed of a structural layer and an annular AlN/metal film stack on a silicon substrate. Promising microphone die were selected via laser vibrometer measurements followed by packaging of the microphones and associated electronics in individual 1/4 in. tubular housings. Characterization of the packaged microphon...

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.

An aeroacoustic microelectromechanical systems microphone phased array.

Phased microphone arrays are useful tools for noise source localization using a process known as beam‐forming. In scale‐model wind tunnel experiments, the frequency range of interest can extend as high as 90 kHz. In both open and closed wall wind tunnels, microphones with high dynamic range are required to sense large turbulent pressure fluctuations from the open jet shear layer and the tunnel wall boundary layer. Microphones that meet the frequency and dynamic range demands of such experiments are readily available but expensive. When considering the high‐sensor counts typically needed for phased array measurements, total sensor cost can be a limiting factor. The presentation will discuss a proof‐of‐concept phased array consisting of 25 piezoelectric microelectromechanical systems (MEMS) microphones arranged in a log‐spiral pattern on a single printed circuit board. The microphones were designed in‐house and have a dynamic range from 40–160‐dB SPL and possess a resonant frequency greater than 100 kHz. A ...

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.

Microfabricated silicon-on-Pyrex passive wireless wall shear stress sensor

This paper presents the design, fabrication, and characterization of a passive wireless sensor for the measurement of wall shear stress. A micromachined variable-capacitor shear stress transducer is realized using a silicon-on-Pyrex microfabrication process. The design features a diamond-shaped 2.25 mm2 silicon floating-element to accommodate more comb fingers for improved capacitive transduction in a smaller die area. The variable-capacitor device is connected to a fixed inductor on a printed circuit board to enable passive wireless sensing. The nominal resonant frequency of the device is 168 MHz with a quality factor of 8.6. Calibrations of static shear stress in a flow cell show a linear response to over 4 Pa, with a frequency-shift sensitivity of 474 kHz/Pa (1.1% full scale). Theoretically a minimum detectable shear stress of 4.1 mPa can be resolved giving a 61.7 dB dynamic range.