Dynamics of Quantized Vortices near a Boundary Surface under an Applied Field in Superfluid Helium.

The flow of superfluid helium (helium II) above a critical velocity enters a turbulent state with a dense tangle of quantized vortices. Although very little is known about the mechanism of the transition, it is widely accepted that the transition is influenced by the roughness of the boundary surface. Thus in this paper, we numerically study three-dimensional dynamics of interactive quantized vortices near a pinning site under an applied flow in order to investigate the effect of the surface roughness. First, the conditions under which vortices are attracted to a pinning site are discussed. Secondly, we discuss the finite capacity of a pinning site, and the critical depinning velocity for pinned vortices. Thirdly, the critical depinning velocity under thermal counterflow is compared with some experimental results, and the agreement is found to be very good.