  Many hydrodynamic instability patterns can be put into correspondence with a
subset of characteristic surfaces of tangential discontinuities. These
topological limits sets to systems of hyperbolic PDE's are locally unstable,
but a certain subset associated with minimal surfaces are globally stabilized,
persistent and non-dissipative. Sections of these surfaces are the spiral
scrolls so often observed in hydrodynamic wakes. This method of wake production
does not depend explicitly upon viscosity.
