Bypass transition, i.e. transition of a boundary layer at subcritical Reynolds numbers, has been studied. Fundamental studies of the phenomenon as such have been performed side by side with experiments aimed at controlling, i.e. delaying, transition. The experiments have been performed in three different flow facilities, two with air as the working fluid (a plane channel flow and a wind-tunnel) and one with water (a water channel).
From the water channel data the well known low-speed streaks appearing in a boundary layer under a turbulent free stream are found to be correlated with upward motion in the boundary layer.
The streaks are found to scale in proportion to the boundary-layer thickness in both the streamwise and wall-normal directions. The streamwise length is around hundred boundary-layer thicknesses.
It is found that the secondary instability of the streaks grows slower for disturbances consisting of less than four wavelengths, as compared to continuous wavetrains.
Elongated low-speed structures are controlled, first in the plane channel flow and then by a reactive system in the wind-tunnel. In the channel, the breakdown of generated streaks is delayed by applying localized suction under the regions of low velocity. Measurements of the disturbance environment withand without control applied show that both the growth of the secondary instability and its spreading in the spanwise direction are reduced when applying the control. In order to be successful, the control has to be applied to a narrow region (about 1/10th of a streak width) around the position of minimum velocity.
The reactive system in the windtunnel, comprising four upstream sensors and four suction ports downstream, inhibits the growth of the amplitude of the streaks for a certain distance downstream of the suction ports. After the inhibited growth the disturbances start to grow again and far downstream the streak amplitude returns to close to the uncontrolled values.
Stockholm: KTH , 2003. , x, 68 p.
fluid mechanics, active control, Laminar-turbulent transition, free-stream turbulence