Vortex line density of superfluid suction vortex

Although a suction vortex is the one of the most common vortices appearing in our everyday life, its internal structure has not been fully understood yet. One of the few things that we understand is that the radius of the core results from the competition between the diffusion of the vorticity due to the kinematic viscosity and the transportation of the vorticity due to the inward flow. To understand the structure of the suction vortex, we investigated a suction vortex produced in liquid helium. In the normal fluid phase, liquid helium behaves as a viscous fluid. In contrast, in the superfluid phase, a quantum vortex line carries all the microscopic circulation, and the suction vortex can be understood as a complex of quantized vortex lines. We generated a superfluid suction vortex with a cryogenic turbine and measured its circulation with a pulsed first sound circulation meter, and also measured the vorticity by the second sound attenuation technique. The experimental results indicate that the quantized vortex lines accumulated in the narrow region around the axis of symmetry due to an inward flow, the core structure of which cannot be described by a simple bundle of fully polarized quantized vortex lines.