Negative skin friction during transition in a zero-pressure-gradient flat-plate boundary layer and in pipe flows with slip and no-slip boundary conditions
Negative skin friction during transition in a zero-pressure-gradient flat-plate boundary layer and in pipe flows with slip and no-slip boundary conditions
复制标题
零压力梯度平板边界层以及具有滑移和无滑移边界条件的管流中过渡期间的负表面摩擦
DOI：
10.1017/jfm.2020.17
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发表时间：
2020
影响因子：
3.7
通讯作者：
S. Ghaemi
中科院分区：
文献类型：
--
作者：
Xiaohua Wu;Michael C. Cruickshank;S. Ghaemi
在研究边界层旁路转捩的机理时，Schubauer和Klebanoff（NACA-TR-1289，1956）在宽范围的自由流紊流强度水平下，对零压力梯度光滑平板边界层（ZPGSFPBL）中从层流到紊流转捩过程中的负表面摩擦事件进行了广泛的研究。他们的结论是，在过渡区的任何部分都没有发现初期气流分离的证据。尽管人们已经知道，在完全湍流的ZPGSFPBL和完全发展的湍流管流中可能发生极其罕见的回流事件，但Schubauer-Klebanoff关于ZPGSFPBL过渡中完全不存在负表面摩擦的结论在过去的六十年中仍然没有受到质疑。在这里，我们报告我们发现的负表面摩擦事件的旁路过渡期间，在广泛研究的代表ZPGSFPBL的情况下，在管流无滑移和滑移边界条件下的周向模式的进口扰动，分别。这种事件的峰值概率位于流向站之间的过渡后期，在该过渡后期，达到最小和最大平均表面摩擦。峰值概率的大小基本上大于下游完全湍流区域中的相应概率：在无滑动管流的情况下，差异为两个数量级，并且主要是由于回流事件的数量密度增加以及此类事件的壁足迹尺寸的显著增加。对于这里考虑的边界层和管流，以及在过渡区和完全湍流区，通过瞬时流动可视化和条件平均，发现初始负表面摩擦力是由反向发夹涡的头部元素引起的。
In searching for the mechanisms of boundary layer bypass transition, Schubauer & Klebanoff (NACA-TR-1289, 1956) conducted an extensive search for negative skin-friction events during the laminar-to-turbulent transition in a zero-pressure-gradient, smooth flat-plate boundary layer (ZPGSFPBL) under a wide range of free-stream turbulence intensity levels. They concluded that no evidence of incipient flow separation could be found in any part of the transition region. Although it has been known that extremely rare backflow events can occur in a fully turbulent ZPGSFPBL and in fully developed turbulent pipe flow, the Schubauer–Klebanoff conclusion regarding the total absence of negative skin friction in ZPGSFPBL transition has remained unchallenged over the past six decades. Here we report our discovery of negative skin-friction events during the bypass transition in an extensively researched representative ZPGSFPBL case, and in pipe flows with no-slip and slip boundary conditions under circumferential mode inlet disturbance, respectively. The peak probability of such events is located in the late stage of transition between the streamwise stations where the minimum and maximum mean skin friction are attained. The magnitude of the peak probability is substantially larger than the corresponding probability in the downstream fully turbulent region: in the case of no-slip pipe flow, the difference is two orders of magnitude and is primarily due to an enhanced number density of the backflow events coupled with a notable increase in the wall footprint size of such events. For both the boundary layer and pipe flows considered here, and in both the transitional and fully turbulent regions, the incipient negative skin friction is found, through instantaneous flow visualization and conditional averaging, to be induced by the head element of a reverse hairpin vortex.