With semi-natural outflow condition we mean tangential velocity and normal stress prescribed at the boundary, i.e.
Although 
prescribed does in general not imply
prescribed we assume that instead of (7.40) the following
boundary condition is given:
where 
is related to 
by (7.9).
Boundary condition (7.41) influences both the tangential velocity
cell as sketched in Figure 7.1, as the normal velocity half-cell
sketched in Figure 7.3.
With respect to the tangential cell, the molecule is built in the same way
as for the inner cells. The only difference is that virtual velocity
components and virtual pressures are eliminated by linear extrapolation,
i.e. by applying formulae (7.3) and (7.4).
The normal velocity half cell gives rise to the following discretization of
the stress tensor (see [34], formula (6.19)):
where 
is given by (7.9). Virtual velocities
are eliminated by linear extrapolation using formula (7.3). It must
be remarked that the first term with respect to the pressure is evaluated
in the points (1,1) and (-1,1) instead of (1,0) resp. (-1,0).
The convective terms are evaluated by expanding
using the standard inter- and extrapolations.
