Growing non-equilibrium length in granular fluids: from experiment to fluctuating hydrodynamics

TL;DR

This study investigates velocity correlations in a 2D granular fluid through experiments and simulations, revealing two energy scales and a growing non-equilibrium coherence length that characterizes order in the velocity field.

Contribution

It introduces a fluctuating hydrodynamics model to define and measure a non-equilibrium coherence length in granular fluids, linking it to density.

Findings

Identification of two energy scales in velocity correlations.

Observation of a growing non-equilibrium coherence length with density.

Validation of the fluctuating hydrodynamics model for granular fluids.

Abstract

Velocity correlations in a 2D granular fluid are studied in experiments and numerical simulations. The transverse component of the velocity structure factor reveals two well defined energy scales, associated with the external "bath temperature" $T_b$ and with the internal granular one, $T_g<T_b$, relevant at large and small wavelengths respectively. Experimental and numerical data are discussed within a fluctuating hydrodynamics model, which allows one to define and measure a non-equilibrium coherence length $\xi$, growing with density, that characterizes order in the velocity field.