Turbulent Mass Inhomogeneities induced by a point-source

Abstract

We describe how turbulence distributes tracers away from a localized source of injection, and analyse how the spatial inhomogeneities of the concentration field depend on the amount of randomness in the injection mechanism. For that purpose, we contrast the mass correlations induced by purely random injections with those induced by continuous injections in the environment. Using the Kraichnan model of turbulent advection, whereby the underlying velocity field is assumed to be shortly correlated in time, we explicitly identify scaling regions for the statistics of the mass contained within a shell of radius $r$ and located at a distance $\rho$ away from the source. The two key parameters are found to be (i) the ratio $s^2$ between the absolute and the relative timescales of dispersion and (ii) the ratio $\Lambda$ between the size of the cloud and its distance away from the source. When the injection is random, only the former is relevant, as previously shown by Celani, Martins-Afonso $\&$ Mazzino, $J. Fluid. Mech$, 2007 in the case of an incompressible fluid. It is argued that the space partition in terms of $s^2$ and $\Lambda$ is a robust feature of the injection mechanism itself, which should remain relevant beyond the Kraichnan model. This is for instance the case in a generalised version of the model, where the absolute dispersion is prescribed to be ballistic rather than diffusive.