A numerical investigation of mixing and spreading across an angled discontinuity

We study mixing and spreading in a very simple heterogeneous system: a numerical flow cell comprised of two homogeneous porous media separated by a sharp angled interface. The two media have identical porosity, but differing permeability and dispersivity characteristics representing a fine and a coarse medium. We focus on transport in advection-dominated systems as the primary role of the angled interface is to distort the velocity field. We demonstrate that the angle of the interface influences spreading in the system as observed by depth averaged breakthrough curves, but that the direction of flow does not appear to have a significant role on this. We also study evolution of mixing in the system as quantified by the dilution index. We demonstrate that when transverse dispersion is neglected global rates of mixing appear very similar for all considered interface angles, despite discernible differences in spreading. As a consequence this paper demonstrates that systems with quite different spreading characteristics can have similar mixing characteristics, and likewise systems with identical spreading characteristics can have quite different mixing. This work reinforces the limited applicability of using information from depth averaged breakthrough curves in assessing mixing and mixing driven phenomena.