Inverse energy transfer in three-dimensional quantum vortex flows

Vortex reconnections play a fundamental role in fluids.They increase the complexity of flow and develop small-scale motions.In this work, we report that in superfluids, they can also excite large scales. We numerically illustrate that during a superfluid vortex reconnection energy is injected into the thermal (normal) component of helium~II at small length scales, but is transferred nonlinearly to larger length scales, increasing the integral length scale of the normal fluid. We show, by studying about fifty different reconnections, that this inverse energy transfer is triggered by the helical imbalance generated in the normal fluid flow by the mutual friction force coupling the superfluid vortices and the normal component. We finally discuss the relevance of our findings to the problem of superfluid turbulence.