A round turbulent water jet produced normal to, and at the center of a submerged, resonantly driven diaphragm is investigated experimentally. The jet which is formed without mass injection and is comprised entirely of radially entrained fluid, is present only when the excitation amplitude exceeds a given threshold. Above this excitation level, a small cluster of cavitation bubbles appears near the center of the diaphragm. The bubbles grow, apparently collapse, and then disappear during each oscillation cycle. It is conjectured that the jet is synthesized by timehyphen;periodic coalescence of vortex rings that are produced by secondary flow around the bubbles or by the collapse of the bubbles. It is remarkable that even though the jet results from a strong timehyphen;periodic excitation and its timehyphen;periodic features are detected throughout the present range of measurements, the timehyphen;averaged jet structure is similar to that of a conventional turbulent round jet in that the increase in its width and in the inverse of its centerline velocity are both linear functions of the distance from the actuator. In contrast to conventional jets, the present synthetic jets can be manipulated on relatively short time scales that are comparable to the excitation period. copy;1996 American Institute of Physics.
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