The turbulence boundary of a temporal jet
The turbulence boundary of a temporal jet
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DOI：
10.1017/jfm.2013.613
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发表时间：
2013-04
影响因子：
3.7
通讯作者：
M. van Reeuwijk;M. Holzner
中科院分区：
文献类型：
--
作者：
M. van Reeuwijk;M. Holzner
本文用统计方法研究了在拟能条件下，时间平面喷流的湍流边界结构。这些数据是通过直接数值模拟获得的，阈值跨度为24个数量级，范围从喷流外的基本上不旋转的流体到喷流核心内的完全湍流流体。基于拟能预算，我们使用两个独立的估计器来估计局部夹带速度${v}_{n}$。这些数据清楚地证明了存在包裹着湍流的粘性超层(VSL)。VSL是一种近一维的低表面曲率层。我们发现，它的面积和粘性输运速度都随卷吸速率的增加而变化，因此积分卷吸通量与阈值无关，尽管低雷诺数效应在所考虑的情况下起了作用。这种门槛独立性与积分夹带率的无粘性是一致的。建立了与实验数据吻合较好的VSL理论模型，并预测粘性输运和耗散对界面传播的贡献分别为$2{v}_(n})和$-{v}_(n})。我们还识别出连接VSL和TC的湍流核心区(TC)和缓冲区(BR)。BR在时间上生长，无粘性变位生产在这一地区很重要。BR与湍流-非湍流界面(TNTI)显示出许多相似之处，尽管TNTI似乎延伸到了TC。TC和VSL之间的平均距离，即BR厚度约为10Kolmogorov长度尺度或半个Taylor长度尺度，表明强烈湍流区和粘性主导区非常接近。
Abstract We examine the structure of the turbulence boundary of a temporal plane jet at $\mathit{Re}= 5000$ using statistics conditioned on the enstrophy. The data is obtained by direct numerical simulation and threshold values span 24 orders of magnitude, ranging from essentially irrotational fluid outside the jet to fully turbulent fluid in the jet core. We use two independent estimators for the local entrainment velocity ${v}_{n} $ based on the enstrophy budget. The data show clear evidence for the existence of a viscous superlayer (VSL) that envelopes the turbulence. The VSL is a nearly one-dimensional layer with low surface curvature. We find that both its area and viscous transport velocity adjust to the imposed rate of entrainment so that the integral entrainment flux is independent of threshold, although low-Reynolds-number effects play a role for the case under consideration. This threshold independence is consistent with the inviscid nature of the integral rate of entrainment. A theoretical model of the VSL is developed that is in reasonably good agreement with the data and predicts that the contribution of viscous transport and dissipation to interface propagation have magnitude $2{v}_{n} $ and $- {v}_{n} $ , respectively. We further identify a turbulent core region (TC) and a buffer region (BR) connecting the VSL and the TC. The BR grows in time and inviscid enstrophy production is important in this region. The BR shows many similarities with the turbulent–non-turbulent interface (TNTI), although the TNTI seems to extend into the TC. The average distance between the TC and the VSL, i.e. the BR thickness is about 10 Kolmogorov length scales or half a Taylor length scale, indicating that intense turbulent flow regions and viscosity-dominated regions are in close proximity.