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

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.