The characteristics of the free jet flow, which were issued from a circular pipe and affecting by the reciprocal oscillating flow, were studied in this paper. The reciprocal oscillating flows issue from two circular pipes placed at two sides of the free jet, and have the contrary phase each other. Moreover, the net flow rate of the two reciprocal flows was set to zero. The flow visualization and the PIV techniques were used to investigate the jet diffusion. As a result, it has been found that

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... e jet diffusion is largely affected by the reciprocal oscillating flows of higher frequency. The jet diffusion is considerably promoted under the influence of the reciprocal oscillating flows. Furthermore, it has been confirmed that the jet issues with a large swing.