Coarse-grained pressure dynamics in superfluid turbulence

Sergey K. Nemirovskii, Vladimir V. Lebedev

Jason Laurie, Andrew William Baggaley

Coarse-grained superfluid dynamics are investigated through detailed numerical simulation at the level of individual quantized vortex lines. Simulations show a strong correlation between the formation of quantized vortex bundles and the production of extreme negative pressure regions in excess of 3 standard deviations. This motivates the use of experimental pressure measurements for probing superfluid turbulence at low temperatures.

Jason Laurie, Andrew William Baggaley

Thanks to the recent unprecedented leap in computational power, we can now delve into the fascinating realm of finite-temperature superfluid turbulence and study the intricate interplay of two-fluid dynamics. Our analysis focuses on two-fluid vorticity locking where turbulent normal and superfluid flows interact and create complex pressure dynamics. Through careful examination, we showcase how the flow statistics seamlessly transfer and manifest in the mutual pressure field, unravelling new insights into the fundamental laws of fluid mechanics.

C. P. Lorenson

Seiki Saito

Understanding how macroscopic irreversible hydrodynamics emerges from the reversible unitary dynamics of isolated quantum many-body systems remains a central challenge. Conventional approaches force spin density dynamics into purely diffusive models, obscuring the interplay of pressure, spin current, and local friction, and leave open the role of the observer's temporal resolution. We introduce a fully data-driven framework based on generalized Extended Dynamic Mode Decomposition (gEDMD) integrated with the Mori-Zwanzig projection. By expanding the observable dictionary to include spin currents, we extract the Navier-Stokes hydrodynamic coefficients from a chaotic XXZ spin chain across temporal coarse-graining scales. Our unconstrained extraction reveals a dichotomy: the mechanical elasticity ($c^2$) derives from the exact unitary dynamics, preserving microscopic reversibility, whereas the macroscopic friction ($γ$) and kinematic viscosity ($ν$) show zero net dissipation, oscillating rapidly around zero in the exact-derivative limit. Genuine macroscopic transport therefore requires finite temporal coarse-graining: at a finite observation timescale ($Δt_{cg} > 0$) the system passes through a crossover where these fluctuations average out, establishing an intermediate functional regime with strictly positive friction and viscosity. We further show that the coarse-grained generator is a predictive reduced model requiring only $O(N)$ macroscopic observables, and that the sign of the emergent friction is inherited from the causal, forward-in-time direction of the inference: time-symmetric estimators yield zero net dissipation at any coarse-graining scale. Macroscopic friction in isolated quantum systems is thus not an absolute property but an emergent phenomenon dictated by the temporal resolution and causal direction of the observer's description.

Luca Galantucci, Michele Sciacca, Carlo F. Barenghi

We perform fully coupled two-dimensional numerical simulations of plane channel helium II counterflows with vortex-line density typical of experiments. The main features of our approach are the inclusion of the back reaction of the superfluid vortices on the normal fluid and the presence of solid boundaries. Despite the reduced dimensionality, our model is realistic enough to reproduce vortex density distributions across the channel recently calculated in three dimensions. We focus on the coarse-grained superfluid and normal fluid velocity profiles, recovering the normal fluid profile recently observed employing a technique based on laser-induced fluorescence of metastable helium molecules.