## Preprint 2023-26

## Sergio Caucao, Gabriel N. Gatica, Juan P. Ortega:

### A three-field mixed finite element method for the convective Brinkman--Forchheimer problem with varying porosity

### Abstract:

In this paper we present and analyze a new mixed finite element method for the nonlinear problem given by the stationary convective Brinkman--Forchheimer equations with varying porosity. Our approach is based on the introduction of the pseudostress and the gradient of the porosity times the velocity, as further unknowns. As a consequence, we obtain a mixed variational formulation within a Banach spaces framework, with the velocity and the aforementioned tensors as the only unknowns. The pressure, the velocity gradient, the vorticity, and the shear stress can be computed afterwards via postprocessing formulae. A fixed-point strategy, along with monotone operators theory and the classical Banach theorem, are employed to prove the well-posedness of the continuous and discrete systems. Specific finite element subspaces satisfying the required discrete stability condition are defined, and optimal a priori error estimates are derived. Finally, several numerical examples illustrating the performance and flexibility of the method and confirming the theoretical rates of convergence, are reported.