Kitchen sink systems give rise to interesting instabilities which are curently not well understood, but again closely related with separation. When the outer height in the hydraulic jump becomes too large, there is a transition in which a new separation vortex occurs at the surface - like for a breaking wave (T. Bohr, C. Ellegaard, A. E. Hansen and A. Haaning, Physica B, 228, 1 (1996)).
This “Type II” jump can then be unstable agianst breaking of the axial symmetry and give rise to beautiful “polygons”.
C. Ellegaard, A. E. Hansen, A. Haaning, K. Hansen, A. Marcussen, T. Bohr, J. L. Hansen and S. Watanabe, Nature 392, 767 (1998)
For sand-ripples, the secondary transitions leading from one ordered state to another are also extremely interesting and pose many fundamental questions, again related to the creation of separation vortices and their interactions and instabilities.
J. L. Hansen, M. van Hecke, A. Haaning, C. Ellegaard, K. H. Andersen, T. Bohr, and T. Sams, Nature (London) 410, 324 (2001)
J. L. Hansen, M. van Hecke, C. Ellegaard, K. H. Andersen, T. Bohr, A. Haaning, and T. Sams: Physical Review Letters 87, 204301 (2001)
The difference is that we now have to do with a periodically forced flow, and the problems here are closely related to those of insect or bird flight, where the wings are flapping periodically and leading edge vortices - rather than leading to stall as for an airplane - strongly enhance the lift. This connection will be a main point in our planned investigation of insect flight.