Turbulent flux events in a nearly neutral atmospheric boundary layer
Turbulent flux events in a nearly neutral atmospheric boundary layer
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DOI：
10.1098/rsta.2006.1949
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发表时间：
2007-03
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通讯作者：
R. Narasimha;S. R. Kumar;A. Prabhu;S. Kailas
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作者：
R. Narasimha;S. R. Kumar;A. Prabhu;S. Kailas
本文提出了一种新的分析湍流动量通量信号的方法。用于分析的数据来自接近中性的大气边界层，并在地面以上4 m的高度（对应于1.1×105个壁单元）采集，在平均速度的对数层内。分析方法涉及检查超过给定阈值的瞬时通量分布，对于该阈值，发现最佳值为1 s. d。的通量信号。它被发现是可行的，以确定归一化通量变化签名分别为积极和消极的“通量事件”的符号被确定的通量本身。使用这些签名，通量信号被转换为事件的发生时间，持续时间和强度的特征之一。它还发现，连续事件之间的平均持续时间和平均时间间隔的顺序为1秒，约四个数量级高于一个壁单位的时间。在时域中的湍流通量的这种情景描述使我们能够分别识别生产，反生产和空闲期（分别占时间的36%，15%和49%），作为标准的动量通量的产生。发现数据的“突发性”指数为0.72。与实验室数据的比较表明，较高（/较低）的喷射（/扫描）象限占用，但较低（/较高）的贡献通量从喷射（/扫描）象限在高雷诺数的大气边界层。本文简要地讨论了与湍流边界层中主动和被动运动概念的可能联系。
We propose here a novel method of analysing turbulent momentum flux signals. The data for the analysis come from a nearly neutral atmospheric boundary layer and are taken at a height of 4 m above ground corresponding to 1.1×105 wall units, within the log layer for the mean velocity. The method of analysis involves examining the instantaneous flux profiles that exceed a given threshold, for which an optimum value is found to be 1 s.d. of the flux signal. It is found feasible to identify normalized flux variation signatures separately for positive and negative ‘flux events’—the sign being determined by that of the flux itself. Using these signatures, the flux signal is transformed to one of events characterized by the time of occurrence, duration and intensity. It is also found that both the average duration and the average time-interval between successive events are of order 1 s, about four orders of magnitude higher than a wall unit in time. This episodic description of the turbulence flux in the time domain enables us to identify separately productive, counter-productive and idle periods (accounting, respectively, for 36, 15 and 49% of the time), taking as criterion the generation of the momentum flux. A ‘burstiness’ index of 0.72 is found for the data. Comparison with laboratory data indicates higher (/lower) ejection (/sweep) quadrant occupancy but lower (/higher) contributions to flux from the ejection (/sweep) quadrant at the high Reynolds numbers of the atmospheric boundary layer. Possible connections with the concept of active and passive motion in a turbulent boundary layer are briefly discussed.