Vortex breakers are commonly used to prevent vortexes from forming. They are typically made out of two crossed sections of plate with the height equal to about 1.5 x the ID of the drain line. If you do a search on this site for vortex, there have been a couple of references posted to help you determining if vortex formation is an issue though if you are draining a tank dry, vortexes will become an issue at some point.