Tidal turbines represent a new frontier for extracting energy from a huge potential of renewable source such as the tides. Despite the technology being mature enough, new solutions aimed at improving the machines efficiency with reduced environmental impact and installation costs are currently under investigation. A novel tidal turbine, characterized by being self-balanced and not requiring structures and foundations, was recently proposed. The machine geometry presents an open-center, which affects the flow field and improves the machine performance in terms of extracted power and power coefficient. At the best of authors’ knowledge, despite being open-center turbines produced and installed, an evaluation of the fluid-dynamics effects of the open-center on machine performance is still missing. The main objective of the present work is to fill this gap and to contribute to have a deeper insight in the flow-field in the wake region and to investigate its effects on machine performance. The analysis is carried out by means of a CFD tool, for three different machine geometries: the open-center machine, which is modeled in single-rotor and double-rotors configurations, and the traditional wind-like machine. A comparison with literature experimental data is also included for validation purpose. For each analyzed case, the flow analysis is carried out for the optimal tip-speed ratio. A free stream velocity of 3 m/s is selected for simulations and a scaled model of the turbine having the diameter of about 1m is considered. The results show that the open-center configurations allow the achievement of higher performance in terms of power coefficient and extracted power, in comparison to the traditional wind-like turbines. In particular, for the new double-rotor prototype, the estimated Cp value is about 0.43, with improvements of about 15% with respect to traditional configurations. Moreover, the corresponding increase in the extracted power, which for 1m diameter prototype increases from 5.2 kW to 6 kW, is achieved with a lighter machine. The weight/power ratio, in fact, reduces from about 2 kg/kW for the standard machine configuration to 1.6 kg/kW for the new double-rotor prototype. The improvements in machine performance are related to the fluid dynamics characteristics of the central opening. This, in fact, causes a suction effect, which contributes to reduce the energy losses due to radial and tangential kinetic energy displaced out from the rotor disk that for a standard machine amount to about 36% of the inlet kinetic energy. Only part of these energy losses is recovered as mechanical power to the turbine, while the remaining part increases the flow kinetic energy lost in the open-center region. The amount of energy recovered as mechanical power at the turbine increases considerably in the double-rotor configuration, where, besides the suction effect of the open center, the external counter-rotating rotor gives the major contribution to the recovery of the kinetic energy lost in tangential and radial components of the inner rotor, which is then reduced down to 12.5%.

潮汐涡轮机代表了从潮汐等巨大可再生资源中提取能量的新领域。尽管技术已经足够成熟，但目前正在研究旨在提高机器效率，减少对环境的影响和降低安装成本的新解决方案。最近提出了一种新颖的潮汐涡轮机，其特征在于自平衡并且不需要结构和基础。机器的几何形状呈现出开放中心，这会影响流场并在提取的功率和功率系数方面提高机器的性能。据作者所知，尽管已生产并安装了开心式涡轮机，但仍缺少对开心式流体对机器性能的流体动力学影响的评估。当前工作的主要目的是填补这一空白，并有助于对尾流区域的流场有更深入的了解，并研究其对机器性能的影响。分析是通过CFD工具针对三种不同的机器几何形状进行的：以单转子和双转子配置为模型的开放式中心机器，以及传统的类似风的机器。还包括与文献实验数据的比较，以进行验证。对于每种分析的情况，都针对最佳的叶尖速比进行流量分析。选择3 m / s的自由流速度进行仿真，并选择直径约为的涡轮机比例模型。分析是通过CFD工具针对三种不同的机器几何形状进行的：以单转子和双转子配置为模型的开放式中心机器，以及传统的类似风的机器。还包括与文献实验数据的比较，以进行验证。对于每种分析的情况，均会针对最佳的叶尖速比进行流量分析。选择3 m / s的自由流速度进行仿真，并选择直径约为的涡轮机比例模型。分析是通过CFD工具针对三种不同的机器几何形状进行的：以单转子和双转子配置为模型的开放式中心机器，以及传统的类似风的机器。还包括与文献实验数据的比较，以进行验证。对于每种分析的情况，都针对最佳的叶尖速比进行流量分析。选择3 m / s的自由流速度进行仿真，并选择直径约为的涡轮机比例模型。进行流量分析以获得最佳叶尖速比。选择3 m / s的自由流速度进行仿真，并选择直径约为的涡轮机比例模型。进行流量分析以获得最佳叶尖速比。选择3 m / s的自由流速度进行仿真，并选择直径约为的涡轮机比例模型。考虑1m 。结果表明，与传统的风力涡轮机相比，开放式中心配置在功率系数和提取功率方面实现了更高的性能。特别地，对于新的双转子原型，估计的Cp值约为0.43，与传统配置相比，提高了约15％。而且，提取功率相应增加，持续时间为1m原型机的直径从5.2 kW增加到6 kW，这是通过打火机实现的。实际上，重量/功率比从标准机器配置的约2 kg / kW降低到了新的双转子原型的1.6 kg / kW。机器性能的提高与中央开口的流体动力学特性有关。实际上，这会引起吸力效应，这有助于减少由于从转子盘移出的径向和切向动能而导致的能量损失，对于标准机器，径向和切向动能相当于入口动能的36％。这些能量损失中只有一部分作为对涡轮机的机械能被回收，而其余部分则增加了在开中心区域损失的流动动能。