Turbulence comes in bursts in stably stratified flows.
Turbulence comes in bursts in stably stratified flows.
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湍流在稳定的分层流中突然出现。
DOI：
10.1103/physreve.89.043002
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
A. Pouquet
中科院分区：
文献类型：
--
作者：
C. Rorai;P. Mininni;A. Pouquet
简单的层流和复杂的湍流之间有明显的区别；然而，在某些情况下，对于夜间的行星边界层，稳定和有序的流动可以形成强烈的和零星的湍流活动爆发，并随着时间的推移慢慢消失。这一现象没有得到很好的理解和建模，但它是我们理解天气和气候动力学的核心。我们给出了由20483个点组成的网格上分层湍流的直接数值模拟的数据，这些数据显示了对于更稳定的流动来说，可测量的更强事件的结果有些自相矛盾，不仅在湍流中常见的温度和垂直速度导数上，而且在场本身的振幅上，与均匀各向同性湍流的情况相反。流动可视化表明，极值发生在开尔文-赫尔莫茨，颠覆了在场变量中发展的事件和锋面。这些结果被我们提出的一个简单模型的分析所证实。该模型只考虑了垂直速度和温度的波动及其垂直导数。这表明，在稳定分层湍流中，当预期流动较稳定时，可能会出现较强的猝发。爆发是由存储在波中的能量的快速非线性放大产生的，并与垂直速度和温度(或密度)波动之间的能量交换有关，这些波动对应于众所周知的分层湍流饱和制度。
There is a clear distinction between simple laminar and complex turbulent fluids; however, in some cases, as for the nocturnal planetary boundary layer, a stable and well-ordered flow can develop intense and sporadic bursts of turbulent activity that disappear slowly in time. This phenomenon is ill understood and poorly modeled and yet it is central to our understanding of weather and climate dynamics. We present here data from direct numerical simulations of stratified turbulence on grids of 20483 points that display the somewhat paradoxical result of measurably stronger events for more stable flows, not only in the temperature and vertical velocity derivatives as commonplace in turbulence, but also in the amplitude of the fields themselves, contrary to what happens for homogenous isotropic turbulent flows. A flow visualization suggests that the extreme values take place in Kelvin-Helmoltz overturning events and fronts that develop in the field variables. These results are confirmed by the analysis of a simple model that we present. The model takes into consideration only the vertical velocity and temperature fluctuations and their vertical derivatives. It indicates that in stably stratified turbulence, the stronger bursts can occur when the flow is expected to be more stable. The bursts are generated by a rapid nonlinear amplification of energy stored in waves and are associated with energetic interchanges between vertical velocity and temperature (or density) fluctuations in a range of parameters corresponding to the well-known saturation regime of stratified turbulence.