Hello,

Consider the boundary term `dot(p_n*n, v)*ds`

on line 80 in this Fenics Navier Stokes Example. I wonder whether this term is identically zero, and thus can be omitted from `F1`

.

In fact, at each time step one solves for `p_`

with the boundary condition (BC) that `p_`

is zero at the `outflow`

:

```
bcp_outflow = DirichletBC(Q, Constant(0), outflow)
bcp = [bcp_outflow]
for n in range(num_steps):
[...]
# Step 2: Pressure correction step
b2 = assemble(L2)
[bc.apply(b2) for bc in bcp]
solve(A2, p_.vector(), b2, 'bicgstab', 'hypre_amg')
[...]
```

then `p_`

is stored into `p_n`

with `p_n.assign(p_)`

.

At the later steps, `p_n`

enters into the term `dot(p_n*n, v)*ds`

of `F1`

: given that `p_n`

vanishes on the outflow, `dot(p_n*n, v)*ds`

vanishes also on the `outflow`

. Also, given that we impose Dirichlet BCs on the velocity on the `inflow`

and on the `walls`

, `v`

vanishes on the `inflow`

and on the `walls`

, i.e., everywhere.

Is this right?