Cover Illustration: Polygonal hydraulic jumps

This year's cover illustration shows an unexpected singular structure occurring in a flow seen in an ordinary household kitchen sink. When a vertical jet of liquid impinges on a flat surface it creates a ring discontinuity, the circular hydraulic jump, at a well-defined distance from the jet, where the depth of the fluid layer changes by an order of magnitude. By using a liquid more viscous than water, we have noticed that the circular hydraulic jump may spontaneously deform to a polygonal structure breaking the axial symmetry. The corners can be very sharp and carry a large radial flux while the edges (sides) generate resistance on the stream. We describe this experiment in detail, and present a heuristic explanation of the rather aesthetically pleasing states. The explanation is admittedly not systematic, but illuminates the role of the internal eddy structure in the flow, and qualitatively agrees with the observed parameter dependence.