When particles rebound, what's the wall velocity?
 

Hello everyone,
I want to use DPMFoam and I investigated how a particle rebounds when encountering walls:
U = U(particle) - U(wall)
U -=(1+e)*(U&nw)

However, I 'm confused about U(wall), U(wall) is from KinematicCloud.C, line 726, the comment there says "Only wall patches may have a non-zero wall velocity from the velocity field when the mesh is not moving." Accoring to the codes, if a patch is wall, U(wall) shall be U.ari[boundary], else U(wall) shall be zero.

What does the comment mean?If the bottom of a pipe is wall, U(wall) shall be the velocity of fluid at bottom, though the bottom is not moving!I'm quite confused.

I think this would be the case if you have a partial or full slip condition at a wall.