Measurement sphere problem in recording velocities

Hello everyone,

I am facing a trouble in the obtained data from the measurement spheres in PFC. I created a soil column with square cross section with periodic boundaries in two directions, it is sitting on a rigid wall at the bottom. My contact model for ball-ball is rrlinear, while for ball-facet is linear contact model. I am introducing a cosine velocity time history to the bottom wall shaking the spherical particles in the x-direction. I am recording the average velocities in the x-direction for balls included in each sphere. However, I am not getting the same motion introduced to the floor. Instead, it is a weird random response with great fluctuations, it is not a smooth as the motion history of the bottom wall. Why this happens?
I used kn, ks = 5e5 for ball-ball and ball-facet, rr_fric =0.2, and fric = 0.5 for balls and facets. Thank you all for your time looking into this issue and appreciate any help.

Hello Ahmed,

It’s difficult to say exactly why the motion of your particles does not follow the same wave motion of the bottom wall. My initial guess is that because this is not a bonded assembly there will be more random motion of the particles. It might be a good idea to send this model to to investigate the matter.