Mark Sheplak

Modern developments in shear-stress measurement

Abstract This paper reviews three relatively modern categories of skin-friction measurement techniques that are broadly classified as microelectromechanical systems (MEMS)-based sensors, oil-film interferometry, and liquid crystal coatings. The theory, development, limitations, uncertainties, and misconceptions of each of these techniques are presented. Current and future uses of the techniques are also discussed. From this review, it is evident that MEMS-based techniques possess great promise, but require further development to become reliable measurement tools. Oil-film techniques have enhanced capabilities and greater accuracy compared to conventional shear-stress measurement techniques (i.e., Preston tube, Clauser plot, etc.) and, as a result, are being employed with increasing frequency. Liquid crystal coatings are capable of making measurements of mean shear-stress vector distributions over a region of a model, but complex calibration and testing requirements limit their usefulness.

Lumped Element Modeling of Piezoelectric-Driven Synthetic Jet Actuators

Abstract : This paper presents a lumped element model of a piezoelectric-driven synthetic jet actuator. A synthetic jet, also known as a zero net mass-flux device, uses a vibrating diaphragm to generate an oscillatory flow through a small orifice or slot. In lumped element modeling (LEM), the individual components of a synthetic jet are modeled as elements of an equivalent electrical circuit using conjugate power variables. The frequency response function of the circuit is derived to obtain an expression for Q(sub out)/V(sub AC), the volume flow rate per applied voltage. The circuit is analyzed to provide physical insight into the dependence of the device behavior on geometry and material properties. Methods to estimate the model parameters are discussed, and experimental verification is presented. In addition, the model is used to estimate the performance of two prototypical synthetic jets, and the results are compared with experiment.

A MEMS acoustic energy harvester

This paper presents the development of a micromachined acoustic energy harvester for aeroacoustic applications. The acoustic energy harvester employs a silicon-micromachined circular, piezoelectric composite diaphragm for electroacoustic transduction. Lumped element modeling, design, fabrication and characterization of a micromachined acoustic energy harvester prototype are presented. Experimental results indicate a maximum output power density of 0.34 µW cm−2 at 149 dB (ref. 20 µPa) and suggest a potential output power density, for this design, of 250 µW cm−2 with an improved fabrication process.

Process compatible polysilicon-based electrical through-wafer interconnects in silicon substrates

Electrical through-wafer interconnects (ETWI) which connect devices between both sides of a substrate are critical components for microelectromechanical systems (MEMS) and integrated circuits (IC), as they enable three-dimensional (3-D) structures and permit new packaging and integration geometries. Previously demonstrated ETWI are very difficult to integrate with standard semiconductor fabrication processes, not compatible with released sensors, do not permit extensive processing on both sides of the wafer, and are in general very application specific. This work describes the design, fabrication, and characterization of an ETWI technology for silicon substrates that can be broadly integrated with MEMS and IC processes. This interconnect is a passively isolated electrical through-wafer polysilicon plug, with a 20 /spl mu/m diameter, 10-14 /spl Omega/ resistance, and less than 1 pF capacitance. Plasma etching from both sides of the wafer is used to achieve a high-aspect ratio via (20:1 through 400 /spl mu/m). The process is compatible with standard lithography, standard wafer handling, subsequent high-temperature processing, and released sensors integration. N-type and p-type versions are demonstrated, and isolated ground planes are added to provide shielding against substrate noise. Electrical properties of these ETWI are measured and analytically modeled. These ETWI are appropriate for integration with devices with impedances much greater than the ETWI, such as piezoresistive and capacitive sensor arrays.

Acoustic energy harvesting using an electromechanical Helmholtz resonator.

This paper presents the development of an acoustic energy harvester using an electromechanical Helmholtz resonator (EMHR). The EMHR consists of an orifice, cavity, and a piezoelectric diaphragm. Acoustic energy is converted to mechanical energy when sound incident on the orifice generates an oscillatory pressure in the cavity, which in turns causes the vibration of the diaphragm. The conversion of acoustic energy to electrical energy is achieved via piezoelectric transduction in the diaphragm of the EMHR. Moreover, the diaphragm is coupled with energy reclamation circuitry to increase the efficiency of the energy conversion. Lumped element modeling of the EMHR is used to provide physical insight into the coupled energy domain dynamics governing the energy reclamation process. The feasibility of acoustic energy reclamation using an EMHR is demonstrated in a plane wave tube for two power converter topologies. The first is comprised of only a rectifier, and the second uses a rectifier connected to a flyback converter to improve load matching. Experimental results indicate that approximately 30 mW of output power is harvested for an incident sound pressure level of 160 dB with a flyback converter. Such power level is sufficient to power a variety of low power electronic devices.

Analytical Electroacoustic Model of a Piezoelectric Composite Circular Plate

This paper presents an analytical two-port, lumped-element model of a piezoelectric composite circular plate. In particular, the individual components of a piezoelectric unimorph transducer are modeled as lumped elements of an equivalent electrical circuit using conjugate power variables. The transverse static deflection field as a function of pressure and voltage loading is determined to synthesize the two-port dynamic model. Classical laminated plate theory is used to derive the equations of equilibrium for clamped circular laminated plates containing one or more piezoelectric layers. A closed-form solution is obtained for a unimorph device in which the diameter of the piezoelectric layer is less than that of the shim. Methods to estimate the model parameters are discussed, and model verification via finite-element analyses and experiments is presented. The results indicate that the resulting lumped-element model provides a reasonable prediction (within 3%) of the measured response to voltage loading and the natural frequency, thus enabling design optimization of unimorph piezoelectric transducers.

A Jet Formation Criterion for Synthetic Jet Actuators

This paper proposes and validates a jet formation criterion for synthetic jet actuators. The synthetic jet is a zero net mass flux device, adding additional momentum but no mass to its surroundings. Jet formation is defined as a mean outward velocity along the jet axis and corresponds to the clear formation of shed vortices. It is shown that the synthetic jet formation is governed by the Strouhal number (or Reynolds number and Stokes number). Numerical simulations and experiments are performed to supplement available two-dimensional and axisymmetric jet formation data in the literature. The data support the jet formation criterion , where the constant 2 Re/ S K > K is approximately 2 and 0.16 for two dimensional and axisymmetric synthetic jets, respectively. This criterion is valid for relatively thick orifice plates with thickness-to-width ratios greater than approximately 2. This result is expected to be useful for the design of flow-control actuators and engine nacelle acoustic liners.

MEMS Shear Stress Sensors: Promise and Progress

Abstract : This paper reviews existing microelectromechanical systems (MEMS)-based shear stress sensors. The promise and progress of MEMS scaling advantages to improve the spatial and temporal resolution and accuracy of shear stress measurement is critically reviewed. The advantages and limitations of existing devices are discussed. Finally, unresolved technical issues are summarized for future sensor development.

Dynamic Calibration of a Shear-Stress Sensor Using Stokes-Layer Excitation

The design and implementation of a novel dynamic calibration technique for shear-stress sensors is presented. This technique uses the oscillating wall shear stress generated by a traveling acoustic wave as a known input to the shear-stresssensor.Asilicon-micromachined, eoating-element shear-stresssensorhasbeen dynamicallycalibrated up to 4 kHz using this method. These data represent the e rst broadband, experimental verie cation of the dynamic response of a shear-stress sensor.

Micromachined sensors for static and dynamic shear-stress measurements in aerodynamic flows

We present in this paper the design and test results of micromachined individual sensors and sensor arrays for shear stress measurements in air flows. The authors previously (1996) reported on the sensing principle and fabrication process. The floating-element sensors have since been extensively tested in laminar flows. Static calibrations have shown this sensor to have a maximum nonlinearity of 1% over four orders of wall shear stress (0.0014-10 Pa). In addition, for the first time, the dynamic response of this sensor has also been experimentally verified to 4 kHz. An improved sensing scheme has been developed and has been successfully implemented in a second generation of sensor arrays.