  The reciprocal energy and enstrophy transfers between normal fluid and
superfluid components dictate the overall dynamics of superfluid $^4$He
including the generation, evolution and coupling of coherent structures, the
distribution of energy among lengthscales, and the decay of turbulence. To
better understand the essential ingredients of this interaction, we employ a
numerical two-way model which self-consistently accounts for the back-reaction
of the superfluid vortex lines onto the normal fluid. Here we focus on a
prototypical laminar (non-turbulent) vortex configuration which is simple
enough to clearly relate the geometry of the vortex line to energy injection
and dissipation to/from the normal fluid: a Kelvin wave excitation on two
vortex anti-vortex pairs evolving in (a) an initially quiescent normal fluid,
and (b) an imposed counterflow. In (a), the superfluid injects energy and
vorticity in the normal fluid. In (b), the superfluid gains energy from the
normal fluid via the Donnelly-Glaberson instability.
