  We introduce and analyze a partially augmented fully-mixed formulation and a
mixed finite element method for the coupled problem arising in the interaction
between a free fluid and a poroelastic medium. The flows in the free fluid and
poroelastic regions are governed by the Navier-Stokes and Biot equations,
respectively, and the transmission conditions are given by mass conservation,
balance of fluid force, conservation of momentum, and the
Beavers-Joseph-Saffman condition. We apply dual-mixed formulations in both
domains, where the symmetry of the Navier-Stokes and poroelastic stress tensors
is imposed in an ultra-weak and weak sense. In turn, since the transmission
conditions are essential in the fully mixed formulation, they are imposed
weakly by introducing the traces of the structure velocity and the poroelastic
medium pressure on the interface as the associated Lagrange multipliers.
Furthermore, since the fluid convective term requires the velocity to live in a
smaller space than usual, we augment the variational formulation with suitable
Galerkin type terms. Existence and uniqueness of a solution are established for
the continuous weak formulation, as well as a semidiscrete continuous-in-time
formulation with non-matching grids, together with the corresponding stability
bounds and error analysis with rates of convergence. Several numerical
experiments are presented to verify the theoretical results and illustrate the
performance of the method for applications to arterial flow and flow through a
filter.
