<!-- This form is for feature requests ONLY! If you're looking for help check out the [readme](/README.md). --> **Feature request** Periodic Boundary Conditions for FreeFlow **What does this feature / why does DuMux need it**: Periodic Boundary Conditions are a common and useful BC and help to overcome the problem of finding appropriate BC for finite domains. **Which issue does this feature fix (if any)** It would enable using Periodic Boundary Conditions in a coupled Darcy-FreeFlow model. Periodic Boundary Conditions are already implemented for Darcy's law but can't be used reasonable in a coupled model with freeflow, because freeflow doesn't allow Periodic Boundary Conditions. **Anything else we need to know?**: Already mentioned in dumux-repositories/dumux#487