Darrieus vertical-axis turbines are known for their complex aerodynamics connected to the continuous change in the angle of attack experienced by the blades, which often exceeds the static stall limit. Low fidelity tools such as the Blade Element Momentum Theory have been shown lately not to provide sufficient levels of accuracy, while the medium-fidelity Actuator Line Method (ALM) has been increasingly applied to Darrieus rotors. In this method, the blade-flow interaction is modeled as an equivalent momentum loss calculated introducing equivalent aerodynamic forces into the computed Computational Fluid Dynamics (CFD) domain. This strongly reduces the computational cost in comparison to blade-resolved CFD, allowing ALM to be used in three-dimensional problems, e.g., multiple rotors, floating offshore, etc. While several corrections and guidelines have been recently proposed to tailor ALM to Darrieus turbines, issues are still open on how to improve accuracy.

The present study aims at assessing to what extent the three main factors of the ALM theory, namely the quality of input polar, the dynamic stall modeling, and the force insertion in the domain, influence the overall accuracy of the method. In particular, this unprecedented understanding is enabled by the novel use of a “frozen ALM”, i.e., an ALM method fed by the aerodynamic forces calculated by blade-resolved CFD, which allowed to separate the contributions coming from airfoil performance analysis and force projection in the domain. Based on the results, three main important conclusions are drafted out: i) for high and medium tip-speed ratios, provided that the aerodynamic forces are correct, the ALM method is able to generate extremely accurate solutions of the flow field, almost equivalent to blade-resolved CFD; ii) the relevance of the kernel’s shape and smearing function is largely overestimated and current knowledge is adequate for the model to be set; iii) a better dynamic stall model is indeed the real key factor that could lead to an improvement of the ALM accuracy.

Darrieus 垂直轴涡轮机以其复杂的空气动力学特性而闻名，这些复杂的空气动力学特性与叶片所经历的攻角的连续变化有关，这通常超过静态失速极限。近来，诸如叶片元素动量理论之类的低保真工具已被证明无法提供足够的精度，而中等保真执行器线法 (ALM) 已越来越多地应用于 Darrieus 转子。在该方法中，叶片-流动相互作用被建模为等效动量损失计算，将等效气动力引入计算的计算流体动力学 (CFD) 域。与叶片解析 CFD 相比，这大大降低了计算成本，允许 ALM 用于三维问题，例如多转子、海上漂浮等。

本研究旨在评估 ALM 理论的三个主要因素，即输入极性的质量、动态失速建模和域中的力插入，在多大程度上影响该方法的整体精度。特别是，这种前所未有的理解是通过“冻结 ALM”的新颖使用实现的，即由叶片分辨 CFD 计算的空气动力提供的 ALM 方法，它允许分离来自翼型性能分析和力投射的贡献在域中。基于这些结果，得出了三个主要的重要结论：i）对于中高叶速比，只要气动力正确，ALM 方法能够生成极其精确的流场解，几乎相当于叶片分辨 CFD；ii) 内核形状和拖尾函数的相关性在很大程度上被高估了，并且当前的知识对于要设置的模型来说是足够的；iii) 更好的动态失速模型确实是可以提高 ALM 精度的真正关键因素。