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.