The simplest boundary conditions for the Navier system are of the free type, i.e., all stresses normal to the boundary are zero . For a ``bottom'' boundary , these conditions can be written as
Considering the DWN shown at bottom in Figure 5.26, and the associated computational grid shown in Figure 5.27, it is easy to see that in this case, it best to arrange the grid such that parallel junctions (at which approximations to and are calculated) lie on this bottom boundary. The first two of conditions (5.55) can be ensured by short-circuiting the parallel junctions. As a result, the remaining series junctions on the boundary (at which approximations to are calculated) are decoupled from the parallel junctions, and it remains only to set a self-loop impedance at these junctions so as to approximate the condition . We leave the determination of these self-loop impedances as an exercise to the reader.