Abstract Large-Eddy Simulation on a grid composed of about two billion points is utilized to characterize the turbulent wake of an axial-flow hydrokinetic turbine. The dominant role of the tip vortices is revealed, both in the near wake, where they are very coherent, and downstream, after development of instability. As long as the tip vortices are stable, sharp peaks of Reynolds stresses populate the outer boundary of the wake, where turbulence is strongly anisotropic and dominated by the fluctuations of the radial velocity component. Such anisotropy is also confirmed by the shear stresses, with the most significant one tied to the fluctuations of the radial and streamwise velocities, originating from the interaction between tip vortices. When the system of tip vortices develops instability, the behavior of turbulence is substantially modified. The outer maxima become more diffused and move gradually towards the wake core. In contrast with the near wake, the fluctuations of the radial velocity become lower than those of the azimuthal and streamwise velocities. Instead, the shear stress associated to the radial and streamwise velocities keeps the most significant one, contributing to momentum recovery via turbulent transport. The present results demonstrate a strong anisotropy of turbulence within the wake, which should be taken properly into account when lower-fidelity methodologies, relying on turbulence modeling, are utilized to simulate this class of flows.

中文翻译：

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通过大涡模拟表征轴流式流体动力涡轮机的湍流尾流

摘要 利用由大约 20 亿个点组成的网格上的大涡模拟来表征轴流式流体动力涡轮机的湍流尾流。尖端涡流的主导作用被揭示，无论是在近尾流中，它们非常连贯，在不稳定发展后的下游。只要尖端涡是稳定的，雷诺应力的尖峰就会出现在尾流的外边界，在那里湍流具有很强的各向异性，并由径向速度分量的波动主导。剪切应力也证实了这种各向异性，其中最重要的一个与径向和流向速度的波动有关，源于尖端涡流之间的相互作用。当尖端涡流系统变得不稳定时，湍流的行为会得到显着的改变。外部最大值变得更加分散并逐渐向尾流核心移动。与近尾流相反，径向速度的波动变得低于方位角和流向速度的波动。相反，与径向和流向速度相关的剪应力保持最重要的剪应力，通过湍流传输促进动量恢复。目前的结果表明尾流内湍流具有很强的各向异性，当使用依赖湍流建模的低保真方法来模拟此类流动时，应适当考虑这一点。与径向和流向速度相关的剪应力保持最重要的剪应力，通过湍流传输促进动量恢复。目前的结果表明尾流内的湍流具有很强的各向异性，当使用依赖湍流建模的低保真方法来模拟此类流动时，应适当考虑这一点。与径向和流向速度相关的剪应力保持最重要的剪应力，通过湍流传输促进动量恢复。目前的结果表明尾流内的湍流具有很强的各向异性，当使用依赖湍流建模的低保真方法来模拟此类流动时，应适当考虑这一点。