1986 Volume 29 Issue 258 Pages 4122-4129
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. In the diverging pipe, the amplitude of the wave is smaller than that in the converging pipe; but the average flow near the axis is fairly retarded, so that the flow reverses at the trough of the superposed wave component, and a bubble type breakdown appears. A mathematical model is presented to give a qualitative explanation for the effect of a sectional area variation.