A localized 3 - D roughness causes boundary - layer separation and ( weak ) shocks . Most importantly , streamwise vortices occur which induce streamwise ( low U , high T ) streaks . Immersed boundary method ( volume force ) suitable to represent roughness element in DNS . Favorable comparison between bi - global stability theory and DNS for a " y - mode " Outlook : Understand the flow physics ( investigate " z - modes " in DNS through sinuous spanwise forcing , study origin of the beat in DNS ) .