Simple rectangular mesh with one grain (circle) in the middle of the mesh
 

Hi everyone,

I'm trying to model a rectangular 2D soil sample with flow from left to right in OpenFOAM. For that, I've modified the cavity tutorial in the icoFoam folder and it seems to work.
Secondly, I want to put one grain in the middle of this soil sample, in which the grain is represented by a circle. I've tried a lot and used Google and this forum to look for answers online. Unfortunately, I haven't been able to make it work. Therefore, I wanted to ask you guys I've you could help me out.
How can you put a circle in the soil sample and let it work?