We describe a simple classroom demonstration of a fluid-dynamic instability. The demonstration requires only a bucket of water, a piece of string, and some used tealeaves or coffee grounds. We argue that the mechanism for the instability, at least in its later stages, is two-dimensional barotropic (shear-flow) instability and we present evidence in support of this. We show results of an equivalent basic two-dimensional numerical non-linear model, which simulates behavior comparable to that observed in the bucket demonstration. Modified simulations show that the instability does not depend on the curvature of the domain, but rather on the velocity profile.

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See supplementary material at https://www.scitation.org/doi/suppl/10.1119/10.0002438 for Video clip of demonstration: VideoS1; Video clip of numerical simulation (slow motion): VideoS2; Video clip of numerical simulation (real time): VideoS3; Video clip of taller, narrower configuration (120 mm diameter by 100 mm deep): VideoS4; and Video clip of configuration with rigid lid: VideoS5.

Supplementary Material

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