Superfluid Drain Vortex

Drain vortices are among the most common observed in everyday life, yet their physics is complex due to competition of vorticity's transport and diffusion, presence viscous layers a free surface. Recently, it has become possible study experimentally drain superfluid liquid helium, fluid which simplified by absence viscosity quantisation circulation. Using Gross-Pitaevskii equation, we make simple model problem captures essential ingredients, showing that vortex consists bundle lines twist, thus strengthening axial flow into drain.