Cells are large relative to the channel. You need good near wall resolution for the y+ checks, but what is never made clear in academic courses is the need to resolve the rest of the flow too! I'd prefer a much more gradual jump in cell size from the inflation. Plot a contour of velocity on the centre plane with node values off - how does it compare to node values on?
DPM particles can only see the wall (for collisions etc) when in the near wall cell so you may be underestimating wall contact.
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