Features of laminar-turbulent transition of adverse pressure gradient flows at low and high Reynolds numbers

Recent experimental studies of laminar-turbulent transition are presented withfocus on adverse pressure gradient flows. New developments in secondary instabilityof streaks and separation control are reported. In the first part of the presentation thestudies of laminar-turbulent transition in non-swept and swept wing boundary layersare discussed, and in the second part the investigations of separated flows on wingswith ordinary (plain) and modified (wavy-shaped) surfaces are considered. The role ofthree-dimensional velocity perturbations such as streamwise vortices and streaks in thelaminar-turbulence transition process is demonstrated, and the importance ofsecondary instability mechanisms is outlined. For a straight wing, the experimentalstudies of the nonlinear varicose secondary instability are compared at zero andadverse streamwise pressure gradients. Results obtained testify to the strong influenceof the pressure gradient upon the breakdown of streaks with developed instability [1].For the swept wings, the stationary vortex packets are most likely to be generatedunder natural flight conditions and transition to turbulence is known to be thequickest within these disturbances. Two modes of the secondary instability are foundto develop and the preferred mode is dependent on the properties of the primarystationary disturbance. The instability modes studied are the \u27y\u27 and \u27z\u27 high-frequencysecondary instability modes, which are investigated separately and also theirinteraction under fully controlled experimental conditions [2]. Furthermore, theinfluence of wing surface modifications on the wing performance and drag is studied.The results testify that for the low-Reynolds-number flow conditions the critical angleof attack of a wing with a wavy-shaped surface can be up to 1.5 times or more thanthat of a similar plane-surface-wing, and for the attack angles in the range between 5and 20 degrees the wing with a modified surface reveal higher aerodynamic qualityowing to decreased size of the separated zones [3].1 Litvinenko et al., Phys. Fluids 17, 118106 (2005).2 Chernoray et al., J. Fluid Mech. 534, 295 (2005).3 Kozlov et al., Proc. EUCASS Conf. Moscow, Russia, Paper 2.11.07 (2005)