Dear jan,
It is true that this 2 refers to the material, since I chose the channel flow, I assume that setting 1 or 2 will give the same value. There are also some problems in my compiled code. There is no movement of particles after they are generated at the entrance. I guess it may be caused by too many settings I changed at one time.I am using stokes drag and periodic boundaries first to verify that periodicity is correct.
Best regards,
Rookie