Flow structure, performance and scaling of acoustic jets.

Abstract

Acoustic jets are studied, with an emphasis on their flow structure, performance, and scaling. The ultimate goal is the development of a micromachined acoustic jet for propulsion of a micromachined airborne platform, as well as integrated cooling and pumping applications. Scaling suggests an increase in performance with decreasing size, motivating the use of micro-technology. Experimental studies are conducted at three different orders of magnitude in size, each closely following analytic expectations. The jet creates a periodic vortical structure, the details of which are a function of amplitude. At small actuation amplitude, but still well above the linear acoustic regime, the flow structure consists of individual vortex rings, propagating away from the nozzle, formed during the outstroke of the acoustic cavity. At large amplitude, a trail of vorticity forms between the periodic vortex rings. Approximately corresponding to these two flow regions are two performance regimes. At low amplitude, the jet thrust increases with the fourth power of the amplitude; and at large amplitude, the thrust equals the momentum flux ejected during the output stroke, and increases as the square of the amplitude. Resonance of the cavity, at Reynolds numbers greater than approximately 10, enhances the jet performance beyond the incompressible behavior. Gains of an order of magnitude in the jet velocity occur at Reynolds numbers of approximately 100, and the data suggest further gains with increasing Reynolds number. The smallest geometries tested are micromachined acoustic jets, manufactured using MEMS technology. The throat dimensions are 50 by 200 mu<italic>m</italic>, and the overall device size is approximately 1 <italic>mm</italic><super> 2</super>, with eight throats per device. Several jets are manufactured in an array, to suit any given application. The performance is very dependent on frequency, with a sharp peak at the system resonance, occurring at approximately 70 <italic>kHz</italic> (inaudible). The mean jet velocity of these devices is about 1 <italic>m</italic>/<italic>s</italic>, and the thrust is about 1 mu<italic>N</italic> per resonator. The resulting thrust per unit area is about 1 <italic>N</italic>/<italic>m</italic><super>2</super>, with an estimated power to thrust ratio of about 20 <italic>W</italic>/<italic>N</italic>.