Abstract In the present study, a new hydrokinetic energy harvester concept based on the tandem oscillating hydrofoils configuration is numerically investigated. Unsteady turbulent two dimensional flow simulations were performed, by using the commercial finite volume computational fluid dynamics code FLUENT, to study the power extraction from tandem hydrofoils oscillating inside a convergent duct. A sinusoidal function was used to interpret the plunging/pitching motion of NACA 0015 hydrofoil. Computations were carried out for the different configurations of area ratios (AR) that is defined as the ratio of the upstream hydrofoil’s inlet cross section ( A 1 ) to the downstream hydrofoil’s inlet cross section ( A 2 ) ranging from 1.0 (without the convergent section) to AR = 2.6. The study revealed that the power extraction was enhanced by the increased incoming flow velocity enabled by the geometry of the convergent duct, which positively affected the total pressure as well as the hydrofoil-vortex interaction time and resulted the generation of larger vertical hydrodynamic force. Upon analysis of the obtained results, it is found that the energy extraction efficiency is improved, compared to that of the conventional tandem configuration where the hydrofoils move in an open flow space, and, as the area ratio (AR) increased, the amount of extracted power becomes higher in a proportion ranging from 4% to 80%. The concept was numerically tested for tandem and single configurations and for non-dimensional frequencies in the range of f ∗ = 0 . 04 - 0.20. One interesting finding is that, due to some positive effect of hydrofoil-vortex interaction, and at a specified area ratio and frequency, the downstream hydrofoil efficiency exceeds the efficiency of a single hydrofoil located at exactly same position as the downstream hydrofoil in the tandem configuration. The frequency and the area ratio are found to be critical factors that determine whether the downstream hydrofoil-vortex interaction is favorable or unfavorable. An optimal area ratio/frequency combination results in a positive hydrofoil-vortex interaction effect and allows the downstream hydrofoil to contribute positively to the total power extraction as a single hydrofoil. Based on the present results, and for an optimal efficient energy harvester design, a new correlation is suggested in order to properly control the area ratio in accordance with the variation of the oscillating frequency leading to more power production and high performance of the oscillating tandem hydrofoils. It is anticipated that this new innovative technique will be applied to improve the existing energy harvester devices.

中文翻译：

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会聚管道几何形状对串联振荡水翼系统能量提取性能的影响

摘要 在本研究中，对基于串联振荡水翼配置的新流体动能能量收集器概念进行了数值研究。通过使用商业有限体积计算流体动力学代码 FLUENT，进行了不稳定湍流二维流动模拟，以研究从收敛管道内振荡的串联水翼的功率提取。正弦函数用于解释 NACA 0015 水翼的俯冲/俯仰运动。对不同配置的面积比 (AR) 进行了计算，面积比 (AR) 定义为上游水翼入口横截面 (A 1 ) 与下游水翼入口横截面 ( A 2 ) 的比值，范围为 1.0（不含会聚截面） ) 到 AR = 2.6。研究表明，会聚管道的几何形状使进入流速增加，从而增强了功率提取，这对总压力以及水翼 - 涡流相互作用时间产生了积极影响，并导致产生更大的垂直流体动力。对所得结果进行分析后发现，与水翼在开放流动空间中移动的传统串联配置相比，能量提取效率有所提高，并且随着面积比 (AR) 的增加，提取的功率以 4% 到 80% 的比例变高。该概念已针对串联和单一配置以及 f ∗ = 0 范围内的无量纲频率进行了数值测试。04 - 0.20。一个有趣的发现是，由于水翼 - 涡流相互作用的一些积极影响，并且在指定的面积比和频率下，下游水翼效率超过与串联配置中与下游水翼位于完全相同位置的单个水翼的效率。发现频率和面积比是决定下游水翼-涡流相互作用是有利还是不利的关键因素。最佳面积比/频率组合会产生积极的水翼 - 涡流相互作用效应，并允许下游水翼作为单个水翼对总功率提取做出积极贡献。基于目前的结果，为了优化高效的能量收集器设计，提出了一种新的相关性，以便根据振荡频率的变化适当控制面积比，从而产生更多的功率和振荡串联水翼的高性能。预计这项新的创新技术将用于改进现有的能量收集器设备。