Fluctuation-Dissipation Relations for Motions of Center of Mass in Driven Granular Fluids under Gravity

TL;DR

This study tests the fluctuation-dissipation relation in driven granular fluids under gravity using theory and simulations, finding it holds at high frequencies but deviates at low frequencies due to system dynamics.

Contribution

The paper extends a Langevin-type theory to higher dimensions and verifies the fluctuation-dissipation relation in granular media through detailed simulations.

Findings

Fluctuation-dissipation relation holds at high frequencies.

Deviations occur at low frequencies linked to system time scales.

Effective temperature is defined by the center of mass kinetic energy.

Abstract

We investigated the validity of fluctuation-dissipation relations in the nonequilibrium stationary state of fluidized granular media under gravity by two independent approaches, based on theory and numerical simulations. A phenomenological Langevin-type theory describing the fluctuation of center of mass height, which was originally constructed for a one-dimensional granular gas on a vibrating bottom plate, was generalized to any dimensionality, even for the case in which the vibrating bottom plate is replaced by a thermal wall. The theory predicts a fluctuation-dissipation relation known to be satisfied at equilibrium, with a modification that replaces the equilibrium temperature by an effective temperature defined by the center of mass kinetic energy. To test the validity of the fluctuation-dissipation relation, we performed extensive and accurate event-driven molecular dynamics simulations for the model system with a thermal wall at the bottom. The power spectrum and response function of the center of mass height were measured and closely compared with theoretical predictions. It is shown that the fluctuation-dissipation relation for the granular system is satisfied, especially in the high-frequency (short time) region, for a wide range of system parameters. Finally, we describe the relationship between systematic deviations in the low-frequency (long time) region and the time scales of the driven granular system.