Down the Drain
Department of Mechanical Engineering
University of Colorado, Boulder
Only one 'bathtub vortex' formed in a bucket with two holes in the bottom, viewed from the top with food dye.
Students in a course on Flow Visualization (http://flowvis.colorado.edu/) attempted to create interaction between two vortex lines by putting two holes in the bottom of a round bucket. Adding food dye to the flow showed that only the hole in the center of the bucket created the swirling flow of the common ‘bathtub’ vortex. Flow went straight down the off-center hole without swirl. Even when the off-center hole was made larger than the centered hole the vortex continued to form only over the centered hole. The ‘bathtub’ or ‘drain’ vortex is most famous for swirling counter clockwise in the northern hemisphere, and clockwise south of the equator, but in the 1960s this was shown to be a weak effect (Shapiro 1962), and you can make a drain vortex flow either direction by swishing the flow as you like.
More recently, studies of bathtub vortexes use a rotating bucket to study details of the flow and make mathematical models predicting what it will look like (Andersen et al. 2003; Tyvand and Haugen 2005; Yukimoto et al. 2009). However, all of these studies only have one hole centered in the bucket. This image raises the question of why only one vortex forms when the drains are close together.
Andersen, A., Bohr, T., Stenum, B., Rasmussen, J.J., Lautrup, B.,( 2003). Anatomy of a Bathtub Vortex. Physical Review Letters 91: 1045021-1045024.
SHAPIRO, A.H.,( 1962). Bath-Tub Vortex. Nature 196: 1080-1081.
Tabeling, P., Zocchi, G., Libchaber, A.,( 1987). An Experimental Study of the Saffman-Taylor Instability. Journal of Fluid Mechanics Digital Archive 177: 67-82.
Tyvand, P., Haugen, K.,( 2005). An impulsive bathtub vortex. Physics of Fluids, Phys. Fluids (USA) 17: 62105-1.
Yukimoto, S., Niino, H., Noguchi, T., Kimura, R., Moulin, F.Y.,( 2009). Structure of a bathtub vortex: importance of the bottom boundary layer. Theor. Comput. Fluid Dyn. 24: 323-327.
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