I appreciate your very very helpful comment.nYes, but my project is about showing the better distribution of the particle on that larger tube (31m) after it comes out from the nozzle but I don't want all of them to get trapped in the very first section. If I change the setting of the larger tube wall to reflect, I am afraid of having almost all lot of particles escaping out from the outlet. I was hoping to see a few of the particles escaping from the outlet after it completing the path and having a better distribution of the particle in every divided section of the larger tube.nn