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Pulsating jets are very common and sometimes useful in industrial fields, due to some differences in basic characteristics from steady jets. In this research, the authors focus upon the mechanism of the frequency effect on a finite-amplitude pulsating jet. Experiments are conducted at a Reynolds number of 5,000, Strouhal numbers of 0.13, 0.20 and 0.27, and a velocity-amplitude ratio of 0.5. Using olive-oil smoke, the authors visualise the flow from a nozzle exit with a circular cross section, and get quantitative information by a PIV technique. As a result, the authors have succeeded in defining the locations of main and subsidiary ring vortices. And, the authors have revealed the vortices' convection manners at three frequencies. Besides, the instantaneous measurements on entraining flow with a conditional-sampling technique have shown the complexity of the frequency effect, which is affected by two factors in a trade-off relation.

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