Matrix-relative Brinkman velocity
= Matrix-relative Brinkman velocity
Porous-matrix drag depends on fluid <velocity> relative to the matrix, so it is not invariant under adding a uniform fluid <velocity> while leaving the matrix fixed. In a swimmer frame with stationary laboratory matrix, the drag is proportional to $\mathbf u-U\mathbf e_x$. A constant shift can alternatively be absorbed into a linear <pressure> term, whose physical interpretation must then be retained.