Experiments on waves induced in the hollow core of vortices

Abstract

Recent experimental findings with respect to wave activity in the core of a hollow disk-shaped vortex generated in a cylindrical container with a flat disk rotating at the bottom are reported herein. For relatively low viscosity fluid, several stationary states of the core exist within a certain range of disk speeds. This range becomes narrow as the wave number grows. Between the stationary states, mixed, time dependent states were found to occur. Their interval of endurance decrease with the wave number. The disk speed at which the static state first appears and ends increases linearly with the original height. The phase velocity of the stationary waves rises with the angular velocity of the disk. For a liquid with an intermediate value of viscosity, abrupt transitions from one equilibrium state to the other is taking place. Steady vortex core patterns with wave numbers from one to eleven are observed. States were the basic pattern is subharmonically modulated and states where a wave packet encircles periodically the core are also encountered. Hysterisis is clearly evident. The core of a highly viscous liquid is stable. A suppressed form of instability appears at very large liquid heights and disk speeds.