[oocon]

The geometry is a random pack of 'pebbles' in a cylinder as shown in the image below:

https://www.dropbox.com/s/8q4pvx5fqstp0xr/pebbles.PNG (don't know why the IMG tag isn't working?)
I imported this into Workbench and did a Boolean cut to remove the geometry of the pebbles from the cylinder:

https://www.dropbox.com/s/bnbod6019vpup32/pebbles2.PNG
Within the meshing program I set the top and the bottom to inlet and outlet respectively and the outside of the cylinder and all its inner faces to walls - but I can't get it to mesh! I must be doing something wrong, because it either hangs or just gives me the error:
"The following surfaces cannot be meshed with acceptable quality. Try using a different element size or virtual topology."
I'm quite new to this so I am not quite sure how to fix this problem.