A turbofan engine has a fan portion in fluid communication with a core stream and a bypass stream of air separated by splitters disposed both upstream and downstream of the fan portion. A fluid passage is defined between the splitters. The turbofan engine has a plurality of surface interruptions on a core side surface of the upstream splitter and a plurality of vortices originating from the surface interruptions, thereby restricting the migration of the core stream into the bypass stream through the fluid passage.