On the outflow boundary condition for external incompressible flows: A new approach

This work presents a new procedure for extrapolating velocities at the outflow boundary in the computations of incompressible flows around rigid bodies. The extrapolation procedure is based on the radial variation of the velocity field at large distances from the rigid body, which can be inferred from mass conservation and vorticity considerations. Since the extrapolation is based on these physical considerations, the proposed boundary condition is considered to be physically consistent. It has been demonstrated, via numerical simulations of 2D, laminar, incompressible, viscous flow past a circular and a square cylinder at Re = 100, that the application of the proposed boundary condition allows one to limit the unbounded domain to a small size (6-8 times the characteristic size of the body) without any significant change in the flow characteristics like the lift coefficient (CL) and the Strouhal number (St). Thus, the proposed boundary conditions can enhance the computational efficiency of this class of flows.