Question regarding UDF's in Coveyer Belt Tutorial

I have a doubt regarding the conveyer belt tutorial. The UDF’s there are defined for pure granular flow. I am working on a case where I have to provide velocities to the walls and I am using the same UDF files. In my case fluid is also present in the system. How am I supposed to include flow of fluid along the walls like particles?

Another question: I have provided particle-wall friction coefficients as 1. From what I understand, UDF provides tangential velocities to particle while the walls stays as it is. What will the implications of setting friction coefficients zero or non zero, what exactly would be happening?

Case: I have the case where solid particles are seeded between two moving walls in opposite direction (cyclic conditions in x and y). Opposed to developing a linear velocity gradient the simulation seems to die out after sometime (The moving wall NSW seems to be working fine initially). I am unable reason it out. I have also attached .mfx file for reference. Any help would be appreciated.

particlebetweenplates.mfx (10.8 KB)
geometry.stl (2.9 KB)
Plates (2).stl (684 Bytes)
data_kf_0003.txt (46 Bytes)
data_kf_0004.txt (46 Bytes)
calc_collision_wall_mod.f (44.8 KB)
usr_init_namelist.f (1.7 KB)
usr_mod.f (11.6 KB)
usr0.f (2.9 KB)
usr1_des.f (2.8 KB)
des_time_march.f (10.0 KB)