Department of Mechanical and Aerospace Engineering, Royal Military College of Canada, Kingston, ON, Canada K7K 7B4.
Center for Turbulence Research, Stanford University, Stanford, CA 94305-3035;
Proc Natl Acad Sci U S A. 2017 Jul 3;114(27):E5292-E5299. doi: 10.1073/pnas.1704671114. Epub 2017 Jun 19.
Two observations drawn from a thoroughly validated direct numerical simulation of the canonical spatially developing, zero-pressure gradient, smooth, flat-plate boundary layer are presented here. The first is that, for bypass transition in the narrow sense defined herein, we found that the transitional-turbulent spot inception mechanism is analogous to the secondary instability of boundary-layer natural transition, namely a spanwise vortex filament becomes a [Formula: see text] vortex and then, a hairpin packet. Long streak meandering does occur but usually when a streak is infected by a nearby existing transitional-turbulent spot. Streak waviness and breakdown are, therefore, not the mechanisms for the inception of transitional-turbulent spots found here. Rather, they only facilitate the growth and spreading of existing transitional-turbulent spots. The second observation is the discovery, in the inner layer of the developed turbulent boundary layer, of what we call turbulent-turbulent spots. These turbulent-turbulent spots are dense concentrations of small-scale vortices with high swirling strength originating from hairpin packets. Although structurally quite similar to the transitional-turbulent spots, these turbulent-turbulent spots are generated locally in the fully turbulent environment, and they are persistent with a systematic variation of detection threshold level. They exert indentation, segmentation, and termination on the viscous sublayer streaks, and they coincide with local concentrations of high levels of Reynolds shear stress, enstrophy, and temperature fluctuations. The sublayer streaks seem to be passive and are often simply the rims of the indentation pockets arising from the turbulent-turbulent spots.
本文提出了从经过彻底验证的典型空间发展、零压力梯度、光滑平板边界层的直接数值模拟中得出的两个观察结果。第一个是，对于本文定义的狭义旁路转捩，我们发现，过渡-湍流斑起始机制类似于边界层自然转捩的二次不稳定，即一个展向涡丝变成一个[公式：见文本]涡，然后是一个发夹包。长条纹蜿蜒确实会发生，但通常是当一个条纹被附近存在的过渡-湍流斑感染时。条纹的波纹和破裂不是这里发现的过渡-湍流斑起始的机制。相反，它们只是促进了现有过渡-湍流斑的生长和扩散。第二个观察结果是，在发展湍流边界层的内层中发现了我们所谓的湍流-湍流斑。这些湍流-湍流斑是由发夹包产生的、具有高旋转强度的小尺度涡的密集浓度。尽管在结构上与过渡-湍流斑非常相似，但这些湍流-湍流斑是在完全湍流环境中局部产生的，并且随着检测阈值水平的系统变化而持续存在。它们对粘性底层条纹施加凹陷、分段和终止作用，并且与高雷诺剪切应力、涡度和温度波动的局部浓度相吻合。底层条纹似乎是被动的，通常只是由湍流-湍流斑产生的凹陷口袋的边缘。
