Hi everyone,

I have a simple question. I would like to know if there are ways to switch between fixed DirichletBC and free Neumann on a certain boundary in case of time-dependent problems.

For example in a 3D problem I want to apply a fixed boundary u=0 for 20 seconds and then release this boundary:

```
if time <= 20
bc = DirichletBC(V, Constant((0.0,0.0,0.0)), left) # fixed condition for certain time
else #free boundary condition
```

Thank you in advance

Ali