Control of Boundary Layer Transition at High Fst by Localized Suction

Abstract

Recently great attention has been paid to streamwise oriented structures (streaky structures-“puff”-structures, longitudinal localized structures) which appear during laminar-turbulent transition in boundary layer flows subjected to high levels of free stream turbulence (FST). The free stream disturbances give rise to streamwise oriented streaky structures inside the boundary layer. These structures, which appear randomly in the spanwise direction, consists of regions of high and low streamwise velocity. Their amplitude increases in the downstream direction and the resulting rms-level of the streamwise fluctuations grows in linear proportion to the boundary layer thickness and may reach up to 10% of the free stream velocity before transition occurs. The breakdown of the streaks into turbulence seems to originate from the development of a high frequency secondary traveling wave instability which results in the formation of turbulent spots.