A time-space Hausdorff fractal model for non-Fickian transport in porous media

TL;DR

This paper introduces a fractal PDE model using Hausdorff derivatives to better describe non-Fickian solute transport in heterogeneous porous media, capturing complex early arrivals and tailing in breakthrough curves.

Contribution

It develops a novel time-space Hausdorff derivative model on fractal metrics, offering improved physical insight and fitting accuracy over traditional models for porous media transport.

Findings

Fractal PDE captures early arrival and heavy tail in breakthrough curves.

Model parameters relate linearly to transport distance.

Compared to scale-dependent models, it better fits experimental data.

Abstract

This paper presents a time-space Hausdorff derivative model for depicting solute transport in aquifers or water flow in heterogeneous porous media. In this model, the time and space Hausdorff derivatives are defined on non-Euclidean fractal metrics with power law scaling transform which, respectively, connect the temporal and spatial complexity during transport. The Hausdorff derivative model can be transformed to an advection-dispersion equation with time- and space-dependent dispersion and convection coefficients. This model is a fractal partial differential equation (PDE) defined on a fractal space and differs from the fractional PDE which is derived for non-local transport of particles on a non-fractal Euclidean space. As an example of applications of this model, an explicit solution with a constant diffusion coefficient and flow velocity subject to an instantaneous source is derived and fitted to the breakthrough curves of tritium as a tracer in porous media. These results are compared with those of a scale-dependent dispersion model and a time-scale dependent dispersion model. Overall, it is found that the fractal PDE based on the Hausdorff derivatives better captures the early arrival and heavy tail in the scaled breakthrough curves for variable transport distances. The estimated parameters in the fractal Hausrdorff model represent clear physical mechanisms such as linear relationships between the orders of Hausdorff derivatives and the transport distance. The mathematical formulation is applicable to both solute transport and water flow in porous media.