Observation of von K\'arm\'an Vortex Street in an Atomic Superfluid Gas

TL;DR

This study experimentally observes vortex shedding patterns in an atomic superfluid, revealing a transition from regular to turbulent vortex clusters and drawing parallels with classical fluid dynamics.

Contribution

It reports the first observation of von Kármán vortex street formation in an atomic superfluid Bose-Einstein condensate, highlighting superfluid turbulence characteristics.

Findings

Vortex clusters form a von Kármán street at low obstacle velocities.

Shedding pattern becomes irregular with increased velocity.

Stouhal number saturates as velocity increases.

Abstract

We report on the experimental observation of vortex cluster shedding from a moving obstacle in an oblate atomic Bose-Einstein condensate. At low obstacle velocities $v$ above a critical value, vortex clusters consisting of two like-sign vortices are generated to form a regular configuration like a von K\'arm\'an street, and as $v$ is increased, the shedding pattern becomes irregular with many different kinds of vortex clusters. In particular, we observe that the Stouhal number associated with the shedding frequency exhibits saturation behavior with increasing $v$. The regular-to-turbulent transition of the vortex cluster shedding reveals remarkable similarities between a superfluid and a classical viscous fluid. Our work opens a new direction for experimental investigations of the superfluid Reynolds number characterizing universal superfluid hydrodynamics.