Self-Induced Vibration in Whirlpool
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Shakerin, Said; Sweet, Jonathan
This short video shows a curious cyclic phenomenon that occurs when a PVC pipe interacts with an air-core vortex, created in a cylinder with a tangential inflow and central outflow at the bottom.
The vortex is created in a cylinder (5-in diameter, 15-in high) where water flows in tangentially near the bottom and flows out at the bottom center through a drain hole into a bucket. A submersible pump circulates the water. A PVC pipe is inserted in the vortex and its bottom end is positioned freely at the drain hole. This insertion breaks down the air-core vortex. As a result, more water drains out causing the water level to drop below the pipe's mid-point. The incoming flow then drags the pipe with it around the cylinder, while the pipe's bottom end remains at the drain hole. Eventually, the pipe is oriented vertically along the central axis due to radial pressure gradients (i.e., higher pressure near the cylinder wall, lower pressure near the central axis). As soon as the water level rises above the pipe, the air-core vortex is re-formed above and through the pipe. The water level keeps increasing to about 2-3 inches above the pipe. At this point, air-core instabilities become large enough to induce oscillations which throw the pipe's top end off center. This action destroys the air-core vortex which again increases the out flow rate, lowering the water level. Then, the above sequence is repeated; it continues as long as the incoming flow is maintained. We observed the phenomenon quite accidentally while experimenting with creation of water fountains with special effects. It appears the phenomenon has not been reported in the literature, and we would like to investigate it further. One possible application might be a source of whirling motion in kinetic sculptures. The video was presented in the APS-DFD Gallery of Fluid Motion in Salt Lake City, November 2007.
Air-core vortex; Self-induced vibration; Fluid Mechanics Video; Kinetic Sculpture