directionalPressureGradientExplicitSource hard to use
A mesh generator might generate a faceZone (possibly incorrectly oriented) or a cellZone. The faceZone has to be oriented correctly (e.g. orientFaceZone, topoSet with setsToFaceZone and 'flip' option) and the cellZone constructed (topoSet with faceToCell, setToCellZone). Maybe it could be made such that we only start from a cellZone and the 'faceZone' (i.e. measuring plane) gets derived from velocity and outside faces of the cellZone?