# Transient hydrodynamic finite size effects in simulations under periodic boundary conditions

## TL;DR

This paper investigates transient hydrodynamic finite size effects in fluid simulations with periodic boundary conditions using Lattice-Boltzmann and analytical methods, revealing detailed time-dependent behaviors and scaling laws.

## Contribution

It introduces a new approach combining Lattice-Boltzmann simulations and analytical calculations to analyze transient hydrodynamic finite size effects in fluids.

## Key findings

- Long-time cross-over from hydrodynamic tail to oscillatory decay
- Scaling laws for cross-over time, decay rate, and oscillation frequency
- Validation of finite size effects on diffusion coefficient

## Abstract

We use Lattice-Boltzmann and analytical calculations to investigate transient hydrodynamic finite size effects induced by the use of periodic boundary conditions in simulations at the molecular, mesoscopic or continuum levels of description. We analyze the transient response to a local perturbation in the fluid and obtain via linear response theory the local velocity correlation function. This new approach is validated by comparing the finite size effects on the steady-state velocity with the known results for the diffusion coefficient. We next investigate the full time-dependence of the local velocity auto-correlation function. We find at long times a cross-over between the expected $t^{-3/2}$ hydrodynamic tail and an oscillatory exponential decay, and study the scaling with the system size of the cross-over time, exponential rate and amplitude, and oscillation frequency. We interpret these results from the analytic solution of the compressible Navier-Stokes equation for the slowest modes, which are set by the system size. The present work not only provides a comprehensive analysis of hydrodynamic finite size effects in bulk fluids, but also establishes the Lattice-Boltzmann method as a suitable tool to investigate such effects in general.