Hello Louise,
if you want to define some not zero pressure at the pressure boundary you have to transorm it into the needed lattice density. The functor ist densityFromPressure( latticePressure ) and set that value on the pressure boundary. In LBM the pressure is deviation of the calculated density to the predefined constant fluid density, so you need to work with densities if you want to change pressure.
Greetings
Fedor