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首页> 外文期刊>Journal of Fluid Mechanics >On the structure of shear stress and turbulent kinetic energy flux across the roughness layer of a gravel-bed channel flow
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On the structure of shear stress and turbulent kinetic energy flux across the roughness layer of a gravel-bed channel flow

机译：横穿砾石床流粗糙面的剪切应力和湍动能通量的结构

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摘要

This study examines the structure of shear stress and turbulent kinetic energy(TKE) flux across the roughness layer of a uniform, fully rough gravel-bed channelflow (k,+> 100, 61k= 20) using high-resolution acoustic Doppler velocity profilermeasurements. The studied gravel-bed roughness layer exhibits a complex randommulti-scale roughness structure in strong contrast with conceptualized k- or d-typeroughness in standard rough-wall flows. Within the roughness layer, strong spatialvariability of all time-averaged flow quantities are observed affecting up to 40 % of theboundary layer height. This variability is attributed to the presence of bed zones withemanating bed protuberances (or gravel clusters) acting as local flow obstacles and bedzones of more homogenous roughness of densely packed gravel elements. Consideringthe strong spatial mean flow variability across the roughness layer, a spatio-temporalaveraging procedure, called double averaging (DA), has been applied to the analysedflow quantities. Three aspects have been addressed: (a) the DA shear stress andDA TKE flux in specific bed zones associated with three classes of velocity profilesas previously proposed in Mignot, Barthelemy & Hurther (J. Fluid Mech., vol. 618,2009, p. 279), (b) the global and per class DA conditional statistics of shear stressand associated TKE flux and (c) the contribution of large-scale coherent shear stressstructures (LC3S) to the TKE flux across the roughness layer. The mean Reynoldsand dispersive shear structure show good agreement between the protuberance bedzones associated with the S-shape/accelerated classes and recent results obtained instandard k -type rough-wall flows (Djenidi et al., Exp. Fluids, vol. 44, 2008, p. 37;Pokrajac, McEwan & Nikora, Exp. Fluids, vol. 45, 2008, p. 73). These gravel-bedprotuberances act as local flow obstacles inducing a strong turbulent activity in theirwake regions. The conditional statistics show that the Reynolds stress contributionis fairly well distributed between sweep and ejection events, with threshold valuesranging from H = 0 to H = 8. However, the TKE flux across the roughness layerprimarily results from the residual shear stress between ejection and sweep of veryhigh magnitude (H = 10-20) and of small turbulent scale. Although LC3S are seento penetrated the interfacial roughness layer, their TKE flux contribution is found tobe negligible compared to the very energetic small-scale sweep events. These sweepsare dominantly produced in the bed zones of local gravel protuberances where thevelocity profiles are inflexional of S-shape type and the mean flow properties are ofmixing-layer flow type as previously shown in Mignot et al. (2009).
机译：这项研究使用高分辨率声多普勒速度剖面仪测量了均匀，完全粗糙的砾石层通道流（k，+> 100，61k = 20）的粗糙层上的剪切应力和湍动能（TKE）通量的结构。所研究的砾石层粗糙度层表现出复杂的随机多尺度粗糙度结构，与标准粗糙壁流中概念化的k型或d型粗糙度形成强烈对比。在粗糙层内，观察到所有时间平均流量的强烈空间变化，影响了边界层高度的40％。这种可变性归因于存在带局部隆起作用的河床隆起（或砾石团簇）的河床区域和密集堆积的砾石元素具有更均匀粗糙度的河床区域。考虑到整个粗糙层的空间平均流量变化很大，已将时空平均程序（称为双重平均（DA））应用于分析流量。已经解决了三个方面：（a）特定床层区域中的DA剪切应力和DA TKE通量，与先前在Mignot，Barthelemy＆Hurther（J. Fluid Mech。，vol.618,2009，p。 279），（b）剪切应力和相关TKE通量的全局和每类DA条件统计数据，以及（c）整个相干剪切应力结构（LC3S）对整个粗糙度层上TKE通量的贡献。平均雷诺分散弥散剪切结构显示与S形/加速类相关的隆起床层与标准k型粗糙壁流中获得的最新结果（Djenidi等，Exp.Fluids，vol.44，2008，第37页; Pokrajac，McEwan和Nikora，流体研究中心，第45卷，2008年，第73页）。这些砾石床隆起物是局部流动障碍，在其苏醒区引起强烈的湍流活动。条件统计数据表明，雷诺应力贡献在扫掠和喷射事件之间分布得相当好，阈值范围从H = 0到H =8。但是，横穿粗糙层的TKE通量主要是由弹道和扫掠之间的残余剪切应力产生的。震级极高（H = 10-20），湍流尺度小。尽管发现LC3S穿透了界面粗糙度层，但与非常活跃的小规模扫掠事件相比，发现它们的TKE通量贡献可忽略不计。这些吹扫作用主要发生在局部砾石隆起的河床区域，那里的速度剖面呈S形弯曲，而平均流动特性则属于混合层流动类型，如Mignot等人先前所述。 （2009）。

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