  Rapid new developments have occurred in superfluid hydrodynamics since the
discovery of a host of unusual phenomena which arise from the diverse structure
and dynamics of quantized vortices in 3He superfluids. These have been studied
in rotating flow with NMR measurements which at best provide an accurate
mapping of the different types of topological defects in the superfluid order
parameter field. Four observations are reviewed here: (1) the interplay of
different vortex structures at the first order interface between the two major
superfluid 3He phases, 3He-A and 3He-B; (2) the shear flow instability of this
phase boundary, which is now known as the superfluid Kelvin-Helmholtz
instability; (3) the hydrodynamic transition from turbulent to regular vortex
dynamics as a function of increasing dissipation in vortex motion; and (4) the
peculiar propagation of vortex lines in a long rotating column which even in
the turbulent regime occurs in the form of a helically twisted vortex state
behind a well-developed vortex front. The consequences and implications of
these observations are discussed, as inferred from measurements, numerical
calculations, and analytical work.
