Is there any reason why VtkDataWriter3D uses PointData instead of CellData? As far as I know Lattice Boltzmann works with nodes, where each node represent average value of a cell of dimensions 1x1x1 lattice units. It would therefore make more sense to use CellData than PointData.
For example, if you make a wall of thickness 1 lattice node, it is invisible in Paraview - if you change to CellData, the wall will become visible.
You can do that by changing all occurrences of PointData in VtkDataWriter3D to CellData and by changing ImageData WholeExtent and Piece Extent: “domain.x1” to “domain.x1 + 1”, “domain.y1” to “domain.y1 + 1”, “domain.z1” to “domain.z1 + 1”.