i isplit the cylinder wall and merge to hava 6 wall boundary conditions and finally i hava virtual faces and volume did it affect(virtual) the meshing and then the final results for example i achieve reverse flow thanks in advance

I always try to prevent using virtual faces when splitting the geometry. If anyone does use them and in fact finds them useful then please let me know and I'll spend some time learning them.

All the splitting can be done using real faces volumes edges and vertices. This way gives good results even if taking slightly longer. Just use a combination of moving and copying entities until you get what you need.

I always split volumes with volumes and faces, and faces with faces. Instead of splitting a face with a virtual edge - make the 'virtual' edges into a real face and split with that!

If you have any problems doing this then let me know and I'll see if I can help.

Phil