Recent direct numerical simulations (DNS) and experiments in turbulent channel flow have found intermittent low- and high-drag events in Newtonian fluid flows, at Reτ=uτh/ν between 70 and 100, where uτ, h and ν 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 Reτ between 70 and 250. For numerical simulations, DNS of a channel flow is performed in an extended domain at Reτ = 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 Reτ = 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.

中文翻译：

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湍流通道流动中的低阻力和高阻力间歇

最近的直接数值模拟 (DNS) 和湍流通道流实验发现牛顿流体流动中存在间歇性低阻力和高阻力事件，Reτ=uτh/ν 介于 70 和 100 之间，其中 uτ、h 和 ν 是摩擦速度，分别为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“休眠”和“过度活跃”，在本文中，使用实验和数值技术对这些间歇性事件进行了进一步调查。对于实验，分别使用热膜风速仪 (HFA) 和激光多普勒测速仪 (LDV) 在通道流动设施中同时测量壁面剪切应力和速度，Reτ 介于 70 和 250 之间。对于数值模拟，通道流的 DNS 在 Reτ = 70 和 85 的扩展域中执行。这些间歇性事件是通过基于组合的阈值幅度和持续时间标准对壁剪应力数据进行条件采样来选择的。探索了使用三种不同的标度（所谓的外部、内部和混合）作为条件事件的持续时间标准。发现如果时间-持续时间标准在内部单位中保持恒定，则这些条件事件的发生频率对雷诺数保持不敏感。条件事件的发生频率与其持续时间呈指数分布，这意味着潜在的无记忆过程。对在低阻力（或高阻力）事件之前和之后的整体平均壁面剪应力数据中存在尖峰（或倾角）的解释进行了研究。在低阻力事件期间，对于所有研究的雷诺数，条件平均的流向速度更接近壁附近的 Virk 最大阻力减少 (MDR) 渐近线。对于 Reτ = 70 和 85，研究了这些条件事件期间的雷诺剪切应力 (RSS) 特性。除了非常靠近壁外，条件平均 RSS 高于低阻力事件期间的时间平均值。