Spatial correlations in compressible granular flows

The clustering instability in freely evolving granular fluids manifests itself in the density-density correlation function and structure factor. These functions are calculated from fluctuating hydrodynamics. As time increases, the structure factor of density fluctuations develops a maximum, which shifts to smaller wave numbers (growing correlation length). Furthermore, the inclusion of longitudinal velocity fluctuations changes long-range correlations in the flow field qualitatively and extends the validity of the theory for spatial velocity correlations to higher inelasticities. The theoretical predictions agree well with two-dimensional molecular-dynamics simulations.