solid volume fraction
 

Hi!
I used mixture model for simulation in a pipe with an initial solid volume fraction of 0.2, after simulation I wanted to consider solid volume fraction at every cross area:
based on the attached picture, why the concentration of particles near the wall for a layer is low (green) after that increase (red)??
my question is that why I have a thin layer of low concentration of solid near the wall (a thin green layer before the red layer near the wall)?
i would be grateful if you could help me
thanks