Turbulence in transient channel flow
Turbulence in transient channel flow
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DOI：
10.1017/jfm.2012.498
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发表时间：
2013-01-25
影响因子：
3.7
通讯作者：
Seddighi, M.
中科院分区：
文献类型：
--
作者：
He, S.;Seddighi, M.
直接数值模拟（DNS）的瞬态通道流的流量从最初的湍流快速增加。结果表明，低雷诺数的湍流可以经历一个过渡过程，类似于层流湍流过渡。随着流量的快速增加，流动并不是从初始湍流结构逐渐演化为新的湍流结构，而是经历了一个包含三个不同阶段（转捩前、转捩和完全湍流）的过程，相当于边界层旁路转捩的三个区域，即冲击层流区、间歇流区和完全湍流区。这种瞬时通道流代表了自由流湍流（FST）诱导转捩的替代旁路转捩场景，其中用作扰动的初始流是具有预先存在的条纹结构的低雷诺数湍流壁面剪切流。然而，流动经历了一个“感受性”的过程，在此过程中，初始结构被一个随时间发展的边界层所调制，形成明显特定的有利间隔（约为新边界层厚度的两倍）的条纹，这些条纹在预过渡期间被拉长。结构是稳定的，最初的流动是层流状的;但后来在过渡阶段，产生局部湍流斑点，这些斑点在空间上生长，彼此合并，最终占据整个壁面，当流动变得完全湍流时。看来，初始湍流结构的存在并不促进早期过渡相比，边界层过渡类似的FST强度。新的湍流结构首先出现在高波数处，随后随着湍流斑的生长并结合在一起而延伸到较低波数谱中。根据瞬态能量增长理论，在预过渡阶段中的最大湍流动能线性增长，但仅以u'表示，而v'和w'基本保持不变。在这个阶段，尽管u '的水平很高，但能量产生和耗散率都很低。压力-应变项在那时保持不变，但在随后的过渡期间沿着湍流斑的产生而迅速增加，因此为过渡的开始提供了明确的测量。
Direct numerical simulations (DNS) are performed of a transient channel flow following a rapid increase of flow rate from an initially turbulent flow. It is shown that a low-Reynolds-number turbulent flow can undergo a process of transition that resembles the laminar-turbulent transition. In response to the rapid increase of flow rate, the flow does not progressively evolve from the initial turbulent structure to a new one, but undergoes a process involving three distinct phases (pre-transition, transition and fully turbulent) that are equivalent to the three regions of the boundary layer bypass transition, namely, the buffeted laminar flow, the intermittent flow and the fully turbulent flow regions. This transient channel flow represents an alternative bypass transition scenario to the free-stream-turbulence (FST) induced transition, whereby the initial flow serving as the disturbance is a low-Reynolds-number turbulent wall shear flow with pre-existing streaky structures. The flow nevertheless undergoes a 'receptivity' process during which the initial structures are modulated by a time-developing boundary layer, forming streaks of apparently specific favourable spacing (of about double the new boundary layer thickness) which are elongated streamwise during the pre-transitional period. The structures are stable and the flow is laminar-like initially; but later in the transitional phase, localized turbulent spots are generated which grow spatially, merge with each other and eventually occupy the entire wall surfaces when the flow becomes fully turbulent. It appears that the presence of the initial turbulent structures does not promote early transition when compared with boundary layer transition of similar FST intensity. New turbulent structures first appear at high wavenumbers extending into a lower-wavenumber spectrum later as turbulent spots grow and join together. In line with the transient energy growth theory, the maximum turbulent kinetic energy in the pre-transitional phase grows linearly but only in terms of u' whilst v' and w' remain essentially unchanged. The energy production and dissipation rates are very low at this stage despite the high level of u'. The pressure-strain term remains unchanged at that time, but increases rapidly later during transition along with the generation of turbulent spots, hence providing an unambiguous measure for the onset of transition.