Coupled Brinkman and Kozeny–Carman model for railway ballast washout using the finite element method

This study investigates the use of a nonlinear model based on the penalisation approach to couple fluid flow and porous media flow. The problem is formulated using a unified Brinkman equation on the domain with a nonlinear permeability which is given a function of porosity, which in turn is governed by an advection equation. The permeability is assumed to be governed by the Kozeny–Carman equation which relates the permeability with the average grain size and porosity. The model is solved using an adaptive finite element method in space and the method of characteristics in time. Finally, numerical examples are provided to illustrate the model and discuss possible extensions.