DNS and LES for investigating transition and transition control

Applications of Direct Numerical Simulations (DNS) and Large-Eddy Simulations (LES) for investigation of transition and transition control in boundary layers are discussed. An example of DNS is an investigation of the interaction of Klebanoff modes (which can arise from free-stream turbulence) and Tollmien-Schlichting-type instability waves. This simulation shows that the presence of Klebanoff modes significantly modifies die transition process. Results are also presented for investigations of transition control (delay) by wall suction. Various suction configurations (holes, spanwise/streamwise slots, porous wall) were considered and compared with regard to their effectiveness for attenuating primary and secondary instability mechanisms. Finally, results of a LES for a subharmonic-type breakdown transition process are presented and discussed. These results indicate that LES may become a valuable tool for investigating the later stages of the transition process, in particular the initial stages of the random motion.