Study on the Relaminarization of Transitional and Turbulent Boundary Layers

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First examined are disturbance developments in the relaminarization and retransition process of boundary layer. Boundary layer suction is locally applied through a short perforated wall strip to a turbulent boundary layer to examine the development of near-wall low-speed streaks in the retransition region downstream of the suction strip. Low-frequency velocity fluctuations, which are markedly suppressed near the wall by the suction, start to grow algebraically downstream of the suction strip, leading to the retransition to wall turbulence. The algebraic growth of near-wall low-frequency fluctuations corresponds to the development of low-speed streaks and is caused by survived but decaying turbulent vortices through lift-up mechanism due to the associated streamwise vorticity component. The spanwise spacing of the newly developed low-speed streaks is fund to be almost the same as that in the turbulent boundary layer in the absence of suction even in the early stage of the streak develop … More ment. Immediately after the saturation of streak development, the secondary instability and eventual breakdown of low-speed streaks occurs being accompanied by wall burst. As the streak breakdown proceeds, the log-law region starts to appear in the mean velocity profile.Second the streak instability is examined experimentally by artificially generating spanwise-periodic low-speed streaks in a laminar boundary layer on a flat plate. Fundamental and subharmonic modes are excited for each of the sinuous and varicose instabilities. The development of subharmonic sinuous mode does not strongly depend on the streak spacing and it grows with almost the same growth rate as that for the single streak. By contrast, the development of fundamental sinuous mode is very sensitive to the streak spacing and is completely suppressed when the streak spacing is smaller than a critical value, about 2.5 times the streak width for the low-speed streaks examined. On the varicose instability, the fundamental mode is less amplified than the subharmonic mode, but the growth of both modes is weak compared with the case of the single streak Less