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The control of circular jets has been studied extensively in the fluid dynamics literature. Most studies introduce an entrance pipe upstream of the nozzle outlet to straighten the flow. In this study, we studied the free jet emitted from a circular nozzle that converges with the entrance pipe. We identified the cause of the naturally occurring vortex ring at the beginning of the jet, and we designed a mechanical intervention that suppresses the vortex. This experiment was performed at Reynolds number Re = 1400–5600. In the experiment, the visualized jet cross section and the flow velocity measured by the hot-wire anemometer were processed with a fast Fourier transform to determine the frequency at which the vortex ring arises. The frequency of the naturally occurring vortex ring depends on acoustic resonance inside the nozzle and is affected by the length of the entrance pipe. Also, the generation frequency of the vortex ring did not increase linearly with Re, but instead increased in discrete steps. A nozzle modification was also tested, with an acoustic material installed inside the nozzle to suppress pressure fluctuations caused by resonance inside the nozzle. This test was performed at Re = 2000, 5000, and 8000. The acoustic material suppressed the naturally occurring vortex ring, which reduced jet diffusion.

Cause of jet of vortex ring and its suppression in circular converging nozzle with entrance pipe