Thursday, March 28, 2013

1303.6852 (R. Hänninen)

Dissipation enhancement due to a single vortex reconnection in
superfluid helium

R. Hänninen
We investigate a single vortex reconnection event in superfluid helium at finite temperatures by using the vortex filament model. After the reconnection Kelvin waves are induced on vortices which strongly increase the energy dissipation. We evaluate this extra mutual friction dissipation and show that the dissipation power has a universal form which can be obtained when scaling the time (measured from the reconnection event) and the power by the mutual friction parameter alpha. This observation indicates that the Kelvin-wave cascade is not important in the energy dissipation process, at least within the parameter range used here, e.g. alpha > 0.001. More importantly, even if the Kelvin waves greatly enhance the energy dissipation after reconnection, no change is seen in the angular momentum dissipation. This would explain why, at low temperatures, we need two effective dissipation coefficients: one for the energy and one for the momentum. This is observed in experiments on vortex front where the rotational motion of the front decouples from the external rotation reference frame. Our results also confirm the previous observations that the minimum distance between vortices scales approximately like d ~ sqrt{|t-trec|}, both before and after the reconnection event. The prefactor is somewhat larger after the reconnection, simply due to larger curvatures appearing on the vortex.
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