A ROUND TURBULENT JET PRODUCED BY AN OSCILLATING DIAPHRAGM

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 enti rely of radially entrained fluid, is present only when the excitation amplitude exceeds a given threshold. Above this excitation level, a sm all cluster of cavitation bubbles appears near the center of the diaph ragm. The bubbles grow, apparently collapse, and then disappear during each oscillation cycle. It is conjectured that the jet is synthesized by time-periodic coalescence of vortex rings that are produced by sec ondary flow around the bubbles or by the collapse of the bubbles. It i s remarkable that even though the jet results from a strong time-perio dic excitation and its time-periodic features are detected throughout the present range of measurements, the time-averaged jet structure is similar to that of a conventional turbulent round jet in that the incr ease in its width and in the inverse of its centerline velocity are bo th linear functions of the distance from the actuator. In contrast to conventional jets, the present synthetic jets can be manipulated on re latively short time scales that are comparable to the excitation perio d. (C) 1996 American Institute of Physics.