title: Anatomy of a bathtub vortex.
abstract: We present experiments and theory for the "bathtub vortex," which forms when a fluid drains out of a rotating cylindrical container through a small drain hole. The fast down-flow is found to be confined to a narrow and rapidly rotating "drainpipe" from the free surface down to the drain hole. Surrounding this drainpipe is a region with slow upward flow generated by the Ekman layer at the bottom of the container. This flow structure leads us to a theoretical model similar to one obtained earlier by Lundgren [J. Fluid Mech. 155, 381 (1985)]], but here including surface tension and Ekman upwelling, comparing favorably with our measurements. At the tip of the needlelike surface depression, we observe a bubble-forming instability at high rotation rates.
authors.affiliation: The Technical University of Denmark, Department of Physics, DK-2800 Kgs. Lyngby, Denmark.
authors.forename: A
authors.initials: A
authors.lastname: Andersen
authors.forename: T
authors.initials: T
authors.lastname: Bohr
authors.forename: B
authors.initials: B
authors.lastname: Stenum
authors.forename: J Juul
authors.initials: JJ
authors.lastname: Rasmussen
authors.forename: B
authors.initials: B
authors.lastname: Lautrup