Babak A. Parviz

Flow structure and performance of axisymmetric synthetic jets

ABSTRACT Experimental analysis is presented of the flow structure and performance of axisymmetric synthetic jets. We report direct thrust and Particle Image Velocimetry (PIV) measurements at varying amplitude and frequency for Reynolds numbers in the range 200-2200. It is shown that the jet performance is determined by the flow length, L, defined as the length of fluid ejected during the output stroke of the synthetic jet. Two flow regimes are identified. At small flow lengths L/D ~3, the thrust equals the momentum flux ejected during the output stroke, and is proportional to L 2 . The entrainment rate of the synthetic jet was measured and found comparable to that of conventional axisymmetric turbulent jets. The mean and time-resolved flow structure in the near field of the synthetic jet varies with flow length. At small flow lengths, vortex rings form close to the orifice and loose some of the vorticity during the input stroke. At large flow lengths, vortex rings with some trailing vorticity form and move away from the orifice thus avoiding re-ingestion of vorticity during the input stroke. The amount of trailing vorticity does not affect the thrust produced by the synthetic jet.

THRUST PERFORMANCE OF MICROMACHINED SYNTHETIC JETS

ABSTRACT The performance of synthetic jets produced by an electrostatically driven acoustic resonator is discussed. A reduced order model of the coupled membrane motion and acoustic field in the resonator is used to determine thrust and power consumption. The coupled system presents unique features depending on the pressure coupling parameter defined as the ratio of the pressure force on the membrane to its inertia. Thrust output increases as the pressure coupling parameter decreases and for stiffer membranes. Theoretical thrust performance of the order of 50 mN per resonator, andpower to thrust ratio in th e range 20-60 m/s could be achieved. We report also progress in the development of a micro propulsion system based on synthetic jets. Measurements of the membrane deformation and flow at the exit of the synthetic jets are reported. the ejector exit area, and U INTRODUCTION resulting in higher Reynolds number. Also, at The development of high impulse momentum sources for application to micro propulsion and flow control remains a very difficult challenge. Recently zero-mass-flux synthetic jets have been proposed as an actuator for flow control

Micromachined Acoustic Resonators for Microjet Propulsion

The thrust produced by synthetic jets designed for micro jet propulsion is discussed. The proposed propulsion system consists of synthetic wall jets located at the throat of an ejector shroud that are powered by Hehnholtz-type acoustic resonators. The theory of acoustic resonators is described and the thrust produced by the resonators calculated. Theoretical results of the exit velocity and thrust are in good agreement with experimental measurements obtained using particle image velocimetry (PIV) in an axisymmetric synthetic jet. Theoretical results are shown to predict well the resonant frequency of a 1O:l scale model of a micromachined resonator. The thrust produced by the micro scale devices is discussed. It is shown that the micro scale devices could be used as propulsion units for Micro Airborne Platforms (MAP). INTRODUCTION There is a great deal of interest on micro scale jets capable of producing large impulse for application to flow control and propulsion. Streaming produced by a large amplitude acoustic field can be used to produce a synthetic jet stream. A very attractive feature of synthetic jets is that the mean mass flow at the source is zero and therefore the jet stream is produced without the need for an air supply. Several investigations have documented the flow field associated with acoustic streaming.‘.’ Synthetic jets have also been implemented

Performance of ultrasonic electrostatic resonators for use in micro propulsion

Graduate Research Assistant, Department of Aerospace Engineering, Member ALU. l Associate Professor, Department of Aerospace Engmeerlng, Senior Member ALU. t Graduate Research AssIstant, Department of Electrical Engineering and Computer Science. : Professor, Department of Electrical Engmeenng and Computer Science. Copyright Q 2000 LUIS P Bemal, Published by the Amencan lnstltute of Aeronautxs and Astronautics, Inc. with permission Helmcho REXMlat Erector Shroud FCC Flow out Thrust Enhancement

A geometric etch-stop technology for bulk micromachining

High speed micro-jets produced by acoustic streaming can be used for micro propulsion in miniature airborne vehicles. A wafer-level technology was developed to fabricate hundreds of resonators to form these jets on a 4-inch silicon wafer. In this paper, modeling and full characterization of these jets is presented. The performance of electrostatic resonators was tested by laser interferometry, video particle imaging and hot-wire anemometry. The occurrence of non-linear streaming phenomenon and jet formation was verified by particle imaging. The effect of various design parameters such as throat size and perforation geometry on jet performance was investigated and an optimum experimental design was identified. Jet velocities as high as 1 m/s were measured and by spatial investigation of the velocity field, the micro jet stream along and away from the centerline was measured and profiled. A coupled equivalent circuit that models the electrostatic drive and acoustic streaming is developed and shown to closely match experimental results.

Numerical simulation of micromachined acoustic resonators

This paper describes a new fabrication method for the simultaneous creation of multi-level single-crystalline silicon structures, each with a different thickness. The method combines deep dry etching and wet anisotropic etching of silicon in order to avoid multiple back-side alignment steps and timed etches. The levels are defined in a single lithographic step from the front side. The fabrication involves etching of deep trenches from the front side of the wafer followed by a refill and etch back process. The final structure is defined by maskless wet etching of the bulk silicon. The progress of the anisotropic wet etch is impeded by the geometric pattern at the bottom of the trenches, and thus structures with various thickness ranging from ten to a few hundred micrometres can be implemented. The effect of various design parameters, such as trench geometry, refill material and reactive ion etching lag, are discussed and design rules are established. The capabilities of the method are demonstrated by the fabrication of a number of devices, such as 1200 × 1200 × 3.5 µm diaphragms supported by a4 0µm thick rim and (1800 × 10 × 3 µm) embedded hot-wire anemometers suspended by a 0.2 µm thick dielectric bridge.

An integrated approach to a portable and low-cost immunoassay for resource-poor settings.

An acoustic resonator coupled with an ejector shroud is being studied as a suitable propulsion system for Micro Airborne Vehicles (MAV’s). The acoustic resonator uses an electrostatically actuated vibrating diaphragm to create an acoustic wave that is amplified in a resonator cavity and focused in an adjoining nozzle to produce acoustic streaming. Multiple resonators placed along the periphery of an ejector will enhance mass flow and provide thrust to act as the propulsion system for a micro airborne platform (MAP). A special feature of this design is that a large array can be fabricated using inexpensive Micro Electra Mechanical Systems (MEMS) technology. A prototype MEMS propulsion device and a 10: 1 scale device have been constructed. A computational fluid dynamics (CFD) model of the 10: 1 device has been developed. This paper reports on these numerical simulations. The CFD simulation models the diaphragm motion in a frequency range of .5 to 8.5 kHz. The diaphragm shape is assumed to be a half sinusoid in two directions and have a deflection amplitude of 60pm peak to peak at the center. When the model is compared with the 10: 1 device. The numerical model appears to accurately predict the resonant frequencies of the system. INTRODUCTION A significant difficulty of achieving flight with Micro Airborne Vehicles (MAV’S) is the lack of an adequate micro propulsion system that is simultaneously efficient and economical to fabricate. A 0 Graduate Research Assistant, Department of Aerospace Engineering, Member AIAA. l Associate Professor, Dcpartmcnt of Acrospacc Engineering, Senior