Ansys Employee

The age old problem of y+ and particles. The answer is yes, in theory the particles must be smaller than the cells and you need a y+ to suit the wall function. It's why multiphase remains easy to set up but more difficult to understand, and why I always need to be careful with answers. 

For DPM (in Fluent) the problem is wall collision is currently only checked in the near wall cell. So, if the near wall mesh is very fine particles can get trapped in the viscous sublayer and don't hit the wall. However, as the particle is bigger than the cell it should have hit the wall.... 

In Rocky the particle will hit the wall, but the porous term that's fed back into Fluent gets messy as it's intended for particles that are smaller than the cell. 

So, the engineering bit. Do you care most about particles or the flow? Do you have sufficient particles in Rocky that the odd cell not being quite right is a problem? Show some images as it's easier for me to comment on those without bending rules. 

Numerical stuff. There's a semi-resolved model in Rocky that will allow bit particles and that does some refinement on the Fluent mesh (adaption) to mitigate big particles. It works OK for water etc but read the maths before looking at anything much more viscous. I'd leave it for now unless you REALLY need it, it's fairly new and is seeing a fair bit of work.