The boundary conditions of flow are related to the momentum conservation equations in x and y directions. And in the P’ equation the boundary conditions are directly given by its source term, i. e., its boundary conditions are the obtained value from the motion equations (u and v).

Notice that pressure for all fictitious volumes is equal to zero, and for that reason we should use lower order approximations in the real volumes at the boundary, we can see this in Eq. (4.16) to (4.23) and Eq. (7.4.10, 11, 15, 16, 20, 21, 25, 26).

We must also remark that the contravariant velocities- asterisk velocities, Eq. (7.4.27) to (7.4.30) – at boundaries of the domain we obtain the velocities from boundary conditions, so in the following sections we will explain how this calculation is done, according to boundary condition.