Study on the Relaminarization of Transitional and Turbulent Boundary Layers

首先研究了边界层再层化和再转捩过程中的扰动发展。边界层抽吸局部应用通过一个短的穿孔壁带的湍流边界层，检查近壁低速条纹的发展在回流区下游的抽吸带。低频速度波动，这是显着抑制附近的吸力壁，开始增长代数下游的吸力带，导致再过渡到壁湍流。近壁低频波动的代数增长对应于低速条纹的发展，并且是由幸存但衰减的湍流涡通过提升机制引起的，由于相关的流向涡分量。新发展的低速条纹的展向间距即使在条纹发展的早期也与无吸力的湍流边界层中的展向间距基本相同 ...更多信息 我是说。在条纹发展达到饱和后，低速条纹发生二次失稳并最终破裂，同时伴有壁面破裂。随着条纹破裂的进行，平均速度剖面开始出现对数律区域。其次，通过在平板层流边界层中人工产生展向周期性低速条纹，对条纹不稳定性进行了实验研究。基本和次谐波模式的激励为每一个曲折和静脉曲张的不稳定性。次谐波弯曲模的发展对条纹间距的依赖性不强，其增长率与单条纹几乎相同。相比之下，基本弯曲模式的发展是非常敏感的条纹间距，并完全抑制时，条纹间距小于一个临界值，约2.5倍的条纹宽度为低速条纹检查。在曲张不稳定性中，基模的放大作用小于次谐波模，但两种模的增长都比单条纹的情况弱。

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