Poisson equation for pressure homogeneous boundary condition
 

In solving the Poisson equation for pressure some authors advice the use of "homogeneous boundary conditions".

I'm not sure how to interpret this:

1. dp/dn=0 the derivative in the normal direction at a frontier is zero ?

or

2. P_frontier = 0 ?

I use this Poisson equation in a MAC grid type (staggered grid in which the pressure is located at the centers of the cells and the velocities on the cell's sides) for solving the incompressible Navier-Stokes equation.

Any help will be appreciated.

Thanks

Homogeneous Neumann dp/dn=0 on MAC type grids :).