Vortex shedding frequency of a moving obstacle in a Bose-Einstein condensate

TL;DR

This paper experimentally studies vortex shedding in a Bose-Einstein condensate with a moving obstacle, revealing linear relationships between shedding frequency, obstacle velocity, and size, and exploring vortex generation mechanisms.

Contribution

It provides the first detailed measurement of vortex shedding frequency dependence on obstacle velocity and size in a BEC, and investigates vortex generation beyond critical velocity.

Findings

Vortex shedding frequency $f_v$ scales linearly with obstacle velocity $v$ above a critical velocity $v_c$.

The proportionality constant decreases with obstacle strength and size.

Vortices can be generated at the condensate boundary due to phonon emission when oscillating obstacle velocity exceeds $v_c$.

Abstract

We experimentally investigate the periodic vortex shedding dynamics in a highly oblate Bose-Einstein condensate using a moving penetrable Gaussian obstacle. The shedding frequency $f_v$ is measured as a function of the obstacle velocity $v$ and characterized by a linear relationship of $f_v=a(v-v_c)$ with $v_c$ being the critical velocity. The proportionality constant $a$ is linearly decreased with a decrease in the obstacle strength, whereas $v_c$ approaches the speed of sound. When the obstacle size increases, both $a$ and $v_c$ are decreased. The critical vortex shedding is further investigated for an oscillating obstacle and found to be consistent with the measured $f_v$. When the obstacle's maximum velocity exceeds $v_c$ but its oscillation amplitude is not large enough to create a vortex dipole, we observe that vortices are generated in the low-density boundary region of the trapped condensate, which is attributed to the phonon emission from the oscillating obstacle. Finally, we discuss a possible asymptotic association of $a$ with the Strouhal number in the context of universal shedding dynamics of a superfluid.