The present study uses results of eddy-resolving numerical simulations to investigate the open channel flow over large clusters of freshwater mussels (Unio elongatulus) partially buried in a rough, gravel bed. The density of the mussels forming the array varies from 26 to 500 mussels/m². The flow structure is analyzed at large distances from the leading edge of the mussel bed, where the flow can be considered fully developed. The effects of changing the mussel bed density, the filtering discharge, the burial level and the roughness of the bed surface in which mussels are burrowed, are investigated in terms of flow field, turbulent structures, drag forces, and bed shear stresses. It is found that strong interactions occur between energetic eddies generated by the larger gravels on the exposed bed surface and by the mussel shells. Simulations results show that for a burial depth close to 50% and a ratio between the average gravel size and the mussel protruding height of 0.13, the shell induced turbulence becomes dominant for mussel bed densities around 50 mussels/m². The influence of the bed roughness becomes less relevant with increasing mussel density, as the generation of energetic eddies is mostly controlled by mussel-to-mussel interactions. For fixed bed roughness, burial level and filtering velocity, the mean streamwise drag force and the associated drag coefficient for the exposed part of each mussel decrease with increasing mussel density, even if strong variations are observed for individual mussels. For constant mussel bed density and burial level, the mean streamwise drag force and the mean drag coefficient decrease slightly with increasing bed roughness. Increasing the burial level decreases not only the drag forces but also the drag coefficients because of the more streamlined shape of the top of the mussels. Strong active filtering acts toward decreasing the mean streamwise force and the mean drag coefficient. The spanwise drag forces contribute significantly to the total drag force, especially for high mussel bed densities. Based on smooth bed calculations, bed-averaged shear stresses are reduced in highly dense clusters.

中文翻译：

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完全发育的明渠流过部分埋在砾石床上的淡水贻贝群


本研究利用涡旋解析数值模拟的结果来研究部分埋在粗糙的砾石床中的大群淡水贻贝（ Unio elongatulus ）上的明渠流动。形成阵列的贻贝密度在26至500个贻贝/m ²之间变化。在距离贻贝床前缘很远的地方分析流动结构，在该处流动可以被认为是充分发展的。从流场、湍流结构、阻力和床剪应力方面研究了改变贻贝床密度、过滤流量、埋藏水平和贻贝挖洞的床表面粗糙度的影响。研究发现，裸露河床表面较大的砾石和贻贝壳产生的高能涡流之间会发生强烈的相互作用。模拟结果表明，当埋深接近50%且平均砾石尺寸与贻贝突出高度之比为0.13时，贝壳引起的湍流在贻贝床密度约为50只/m ²时占主导地位。河床粗糙度的影响随着贻贝密度的增加而变得不太相关，因为高能涡流的产生主要是由贻贝与贻贝之间的相互作用控制的。对于固定床粗糙度、埋藏水平和过滤速度，每个贻贝暴露部分的平均流向阻力和相关阻力系数随着贻贝密度的增加而减小，即使观察到单个贻贝的强烈变化。对于恒定的贻贝床密度和埋藏水平，平均流向阻力和平均阻力系数随着床粗糙度的增加而略有下降。 增加埋藏高度不仅会降低阻力，还会降低阻力系数，因为贻贝顶部的形状更加流线型。强主动过滤的作用是降低平均流向力和平均阻力系数。展向阻力对总阻力有显着影响，特别是对于高密度贝类床。根据光滑床层计算，高密度簇中的床层平均剪应力会降低。