you can use a const analytical functor. They are used in the some example to fix the outflow pressure. Please, also have a look in our documentation on functors. Last but not least, consider to come to our next spring school which will help you to get started!
Thank you for your answer, I managed to get what I wanted!
Then, I would have another question: I would like to have a slip condition on the top and on the bottom of my domain, so I added this line where the boundary conditions are defined (velocity and pressure):