Clustering of advected passive sliders on a fluctuating surface

We study the clustering properties of advected, noninteracting, passive scalar particles in a Burgers fluid with noise, a problem which maps to that of passive sliding particles moving under gravity on a surface evolving through the Kardar-Parisi-Zhang equation. Numerical simulations show that both the density-density correlation function and the single-site mass distribution scale with system size. The scaling functions diverge at small argument, indicating strong clustering of particles. We analytically evaluate the scaling functions for the two-point correlation and mass distribution of noninteracting particles in thermal equilibrium in a random landscape, and find that the results are remarkably similar to those for nonequilibrium advection. Keywords—Passive scalar, clustering, advection, Burgers equation, KPZ equation