Unfortunately, I can’t provide you with a solution to the above problem because I don’t know your simulation setup. Here are a few things you might check:
– Is the periodic boundary applied correctly?
– Maybe also check the “geometry” in ParaView, in the direction of the periodic boundary the material should most likely be 1
– Compare your implementation with examples that use a periodic setup, e.g. Poiseuille2d, are there any differences?
For extensive help with your setup I suggest the Spring School: https://www.openlb.net/spring-school-2023/