It is known that a swirling flow in a diverging pipe breaks down more easily than a flow in a converging one. In this report, experiments are made using a divergent pipe, a convergent pipe and a straight pipe, in order to clarify the relation between the shape of pipes and the occurrence of the breakdown. A swirling flow field, which is induced in a rotating conical-pipe, is measured by a laser-Doppler-velocimeter. When each pipe rotates faster than a certain critical value, a stationary internal wave occurs. The vortex breakdown phenomenon is ascribed to the internal waves in swirling flows, as pointed out in preceding reports.
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