Spatially evolving cascades in temporal planar jets
Spatially evolving cascades in temporal planar jets
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DOI：
10.1017/jfm.2020.1002
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发表时间：
2021-01
影响因子：
3.7
通讯作者：
A. Cimarelli;J. Mollicone;M. van Reeuwijk;E. De Angelis
中科院分区：
文献类型：
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作者：
A. Cimarelli;J. Mollicone;M. van Reeuwijk;E. De Angelis
本文从湍流场的另一种分解出发，提出了一种描述和理解自由剪切流中湍流的多维统计形式，并应用于平面时间射流的对称性。理论框架是基于两点速度增量的二阶矩的精确方程，并允许我们跟踪，第一次，空间演变的叶栅过程的湍流混合和夹带的基础上。在界面区域，长而宽的结构的产生是由快速反向能量级联机制引起的。类似于二维湍流，由这些空间上升的反向级联提供的能量被发现最终消散的粘度在大尺度上通过摩擦剪切过程，涉及这些大尺度结构的薄横流层。最后，外部的非湍流区域的射流也被发现是活跃的从一个充满活力的角度来看。据发现，压力介导的非本地现象的位移几乎静止的流体引起的非湍流的波动，在时间上，通过过渡机制，将有助于增长的湍流射流。总的来说，在组合的物理/尺度空间，这是一个相当大的新奇，相对于已知的描述湍流混合和夹带的尺度能量通量所采取的意想不到的路径，可能会对我们的理论理解和建模产生重大影响，在这里预期的简化方程能够给出一个简单的尺度相关的描述丰富的动态的流动。
Abstract Starting from an alternative decomposition of the turbulent field, a multi-dimensional statistical formalism for the description and understanding of turbulence in free-shear flows is proposed and applied to the symmetries of planar temporal jets. The theoretical framework is based on the exact equation for the second-order moment of the two-point velocity increment and allows us to trace, for the first time, the spatially evolving cascade processes at the basis of turbulence mixing and entrainment. Fascinating reverse energy cascade mechanisms are found to be responsible for the generation of long and wide structures in the interface region. Analogously to two-dimensional turbulence, the energy provided by these spatially ascending reverse cascades is found to be eventually dissipated by viscosity at large scales through friction shearing processes involving a thin cross-flow layer of these large-scale structures. Finally, the external non-turbulent region of the jet is also found to be active from an energetic point of view. It is found that pressure-mediated non-local phenomena of displacement of almost quiescent fluid give rise to non-turbulent fluctuations that in time, through transitional mechanisms, would contribute to the growth of the turbulent jet. Overall, the unexpected paths taken by the scale-energy flux in the combined physical/scale space, which are a substantial novelty with respect to known descriptions of turbulent mixing and entrainment, may have major repercussions on our theoretical understanding and modelling, as anticipated here by reduced equations capable of giving a simple scale-dependent description of the rich dynamics of the flow.