Global Well-Posedness and Numerical Approximation of a Coupled Darcy-Convection-Diffusion System with Exponential Nonlinearity

TL;DR

This paper establishes the existence, uniqueness, and long-term decay of solutions for a coupled Darcy-convection-diffusion system with exponential nonlinearity, supported by numerical simulations analyzing instability and mixing.

Contribution

It provides the first rigorous proof of well-posedness and exponential decay for this nonlinear coupled system, along with numerical insights into instability and mixing behavior.

Findings

Increasing density contrast amplifies kinetic energy but with diminishing returns.

Adsorption suppresses mixing, with efficiency saturating at higher levels.

Numerical simulations confirm theoretical predictions on system behavior.

Abstract

This paper investigates density driven flow in porous media, focusing on the roles of viscosity contrast, density contrast, and linear adsorption. In this setup, the fluid on top is heavier and more viscous than the fluid below. Under the effect of gravity, this system becomes unstable, and finger-like structures appear. The phenomenon is described mathematically by coupling Darcy's law with a convection-diffusion reaction equation. The nonlinearity in this model arises mainly from the concentration dependence of viscosity and the convective transport term. The existence of a unique pair of weak solutions is shown in both two and three dimensions using the Galerkin approximation method and truncation technique. Moreover, an application of the maximum principle shows non-negativity of the concentration. Additionally, we analyze the long-time behavior of the solution and prove that the concentration converges exponentially to zero in the $L^p$-norm for all $1 \le p \le \infty$ as $t \to \infty.$ To complement the theoretical analysis, we perform numerical simulations based on a pressure formulation. By tracking total kinetic energy and mixing measures over time, we discuss the instability and the mixing efficiency, respectively. The present study reveals that although increasing the density contrast amplifies the total kinetic energy, the marginal impact diminishes with successive increments of density contrast. Similarly, while adsorption acts to suppress mixing, its efficiency in doing so tends to saturate with further increases. These non-linear sensitivities are predicted by our theoretical estimates and confirmed by the numerical simulations.