TWO KINDS OF VACILLATION IN ROTATING LABORATORY EXPERIMENTS

Abstract

Two types of vacillation are distinguished in a rotating differentially heated annulus of fluid: (1) Tilted-trough vacillation, which has been discussed in a number of previous papers in the literature, is characterized by periodic changes in the wave shapes, in which the waves first tilt ``north-west'' to ``south-east'', and later ``north-east'' to ``south-west'', relative to the rotating annulus. (2) Amplitude vacillation, which was first observed by the authors in a series of student demonstrations conducted at the Lamont Geological Observatory, is characterized by periodic expansions and contractions of the wave pattern with no noticeable change in the tilt of the disturbances. It is suggested that the details of the energy conversions are different in the two cases. Some speculations are also presented concerning the role of viscosity in limiting the number of degrees of freedom available for vacillation.