Low- and High-Drag Intermittencies in Turbulent Channel Flows.
Low- and High-Drag Intermittencies in Turbulent Channel Flows.
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DOI：
10.3390/e22101126
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发表时间：
2020-10-04
期刊：
影响因子：
--
通讯作者：
Poole RJ
中科院分区：
文献类型：
--
作者：
Agrawal R;Ng HC;Davis EA;Park JS;Graham MD;Dennis DJC;Poole RJ
最近的直接数值模拟（DNS）和湍流通道流动的实验发现，在牛顿流体流动中，在70到100之间存在间歇性的低阻力和高阻力事件，其中h和分别是摩擦速度，通道半高和运动粘度。这些低阻力和高阻力的间隔分别被称为“冬眠”和“过度活跃”，在本文中，使用实验和数值技术对这些间歇事件进行了进一步的研究。在实验中，分别使用热膜测速仪（HFA）和激光多普勒测速仪（LDV）在通道流设备中同时测量了70至250之间的壁面剪切应力和速度。对于数值模拟，在= 70和85的扩展域内对通道流进行DNS。这些间歇事件是通过对基于阈值大小和时间持续时间标准的墙体剪应力数据进行有条件采样来选择的。对于条件事件的时间-持续时间标准，探索了三种不同的缩放（所谓的外部，内部和混合）的使用。研究发现，当时间-持续准则在内部单位保持不变时，这些条件事件的发生频率对雷诺数不敏感。条件事件的发生频率相对于它们的持续时间呈指数分布，这意味着一个潜在的无记忆过程。研究了在低阻力（或高阻力）事件前后的整体平均壁面剪切应力数据中出现尖峰（或陡降）的原因。在低阻力事件中，对于所有研究的雷诺数，条件平均流向速度更接近靠近壁面的Virk最大阻力减小（MDR）渐近线。在这些条件事件中，研究了= 70和85时的雷诺数剪切应力（RSS）特性。除非常靠近井壁外，在低阻力情况下，条件平均RSS高于时间平均RSS。
Recent direct numerical simulations (DNS) and experiments in turbulent channel flow have found intermittent low- and high-drag events in Newtonian fluid flows, at between 70 and 100, where , h and are the friction velocity, channel half-height and kinematic viscosity, respectively. These intervals of low-drag and high-drag have been termed “hibernating” and “hyperactive”, respectively, and in this paper, a further investigation of these intermittent events is conducted using experimental and numerical techniques. For experiments, simultaneous measurements of wall shear stress and velocity are carried out in a channel flow facility using hot-film anemometry (HFA) and laser Doppler velocimetry (LDV), respectively, for between 70 and 250. For numerical simulations, DNS of a channel flow is performed in an extended domain at = 70 and 85. These intermittent events are selected by carrying out conditional sampling of the wall shear stress data based on a combined threshold magnitude and time-duration criteria. The use of three different scalings (so-called outer, inner and mixed) for the time-duration criterion for the conditional events is explored. It is found that if the time-duration criterion is kept constant in inner units, the frequency of occurrence of these conditional events remain insensitive to Reynolds number. There exists an exponential distribution of frequency of occurrence of the conditional events with respect to their duration, implying a potentially memoryless process. An explanation for the presence of a spike (or dip) in the ensemble-averaged wall shear stress data before and after the low-drag (or high-drag) events is investigated. During the low-drag events, the conditionally-averaged streamwise velocities get closer to Virk’s maximum drag reduction (MDR) asymptote, near the wall, for all Reynolds numbers studied. Reynolds shear stress (RSS) characteristics during these conditional events are investigated for = 70 and 85. Except very close to the wall, the conditionally-averaged RSS is higher than the time-averaged value during the low-drag events.
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