Numerical Simulation of Riblet Controlled Spatial Transition

To analyze the fundamental physical mechanism which determines the damping effect of a riblet surface on three-dimensional transition several numerical simulations of spatial transition in a flat plate zero-pressure gradient boundary layer above a riblet wall are performed in this study. Two types of transition are investigated. The first type of transition, namely K-type transition, is induced by a dominant two-dimensional Tollmien-Schlichting (TS) wave and a weak spanwise disturbance. The second type of transition is purely excited by two oblique waves. The two-dimensional TS waves are found to be amplified by riblets, whereas three-dimensional structures, i.e., Λ-, hairpin, and streamwisely aligned vortices, are damped and their breakdown to turbulence is delayed compared to transition on a clean surface. The investigation of the near wall flow structure reveals secondary flows induced by the riblets and reduced wall normal ejections as well as a reduced downwash. Overall, especially the oblique transition is delayed by the riblets.