This phenomena could be observed if a particle histogram is evaluated at interface boundaries. Here it is important to distinguish between 1:1 interface and GGI. For 1:1 interfaces, there will be no data available on the interface sides and only zero values will be written into the .csv file. This is because the particle solver doesn’t see any boundary, the mesh is continuous and no data could be saved. Since R19.0 a warning message pointing to the usage of the 1:1 interface will be printed out. In the case of GGI, there will be data in the histogram .csv file available, but only for the outgoing particles at interface and not for the incoming particles. Since ANSYS CFX R18.2, a development fix is available and the .csv file will be filled with data at each interface side.