26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

A Universal Description of the Critical Vortex Nucleation Velocity in Superfluid Flow

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral AIP | Quantum Science and Technology (QST)

Speaker

Maarten Christenhusz (University of Queensland)

Description

Superfluids exhibit frictionless flow past an obstacle below a critical velocity. Beyond this threshold, dissipation emerges through the nucleation of quantised vortices, opening the pathway to quantum turbulence. Although the critical velocity has been studied extensively, its dependence on obstacle shape is still not fully understood. We investigate whether a universal geometric description can describe the critical velocity across different obstacle shapes.

We address this using analytical modelling based on ideal inviscid (Euler) flow, Gross–Pitaevskii simulations, and experiments on ultracold atomic Bose–Einstein condensates. This provides a framework in which geometric predictions for the critical velocity are derived from ideal flow theory, tested in a microscopic Gross–Pitaevskii model of a bulk superfluid, and validated against experimental observations. Together, they provide a systematic comparison of the onset of dissipation across a range of obstacle geometries.

We find that the dependence of the critical velocity $U_c$ on obstacle size $D$ is strongly geometry-dependent. For smooth obstacles, $U_c$ becomes independent of size once the obstacle exceeds a few healing lengths, consistent with a regime where quantum pressure effects are negligible and the flow is effectively scale-invariant. In contrast, we show that a power-law decrease $U_c \propto D^{-1/2}$, previously observed for flat plate geometries, is in fact universal across all obstacles with sharp boundaries. This scaling arises from quantum pressure regularising the flow near corners on the healing-length scale. These results demonstrate that the scaling behaviour of $U_c(D)$ is controlled by whether quantum pressure contributes to setting the local flow near the obstacle.

These results provide a connection between microscopic vortex nucleation and macroscopic flow behaviour through the geometry dependence of the critical velocity. This may be relevant for high-Reynolds-number superfluid flows, where vortex-driven motion begins to resemble classical fluid behaviour.