An insight into the capability of the Actuator Line Method to resolve tip vortices
Abstract. The Actuator Line Method (ALM) is being increasingly preferred to the ubiquitous Blade Element Momentum (BEM) approach in several applications related to wind turbine simulation, thanks to the higher level of fidelity required by the design and analysis of modern machines. This approach, however, still falls behind other medium-fidelity methodologies such as the Lifting Line Theory (LLT) when it comes to resolving the vortex-like structures shed at the blade tip (i.e., tip vortices) and their effect on the load profile along the blade. Despite the numerical strategies proposed so far to overcome this limitation, the reason for such behaviour is still unclear. Moving from this background, in this study the ALM’s capability to simulate tip effects is investigated. To this end, the ALM tool developed by the authors in the ANSYS® FLUENT® environment (v. 20.2) was employed for the simulation of a NACA0018 finite wing, for different pitch angles. Three different test cases were considered: high-fidelity blade-resolved CFD simulations, to be used as a benchmark, standard ALM, and ALM with the spanwise force distribution coming from blade-resolved data (frozen ALM). The last option was included to isolate the effect of force projection, using three different smearing functions. For the post-processing of the results, two different techniques were applied: the LineAverage sampling of the local angle of attack along the blade and state-of-the-art Vortex Identification Methods (VIM) to outline the blade vortex system. The analysis showed that the ALM can account for tip effects without the need of additional corrections, provided that the correct angle of attack sampling and force projection strategies are adopted.
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