A critical analysis on low-order simulation models for Darrieus VAWTs: How much do they pertain to the real flow?

To improve the efficiency of Darrieus wind turbines, whichstill lacks from that of horizontal-axis rotors, ComputationalFluid Dynamics (CFD) techniques are now extensively applied,since they only provide a detailed and comprehensive flowrepresentation. Their computational cost makes them, however,still prohibitive for routine application in the industrial context,which still makes large use of low-order simulation models likethe Blade Element Momentum (BEM) theory. These modelshave been shown to provide relatively accurate estimations ofthe overall turbine performance; conversely, the description ofthe flow field suffers from the strong approximations introducedin the modelling of the flow physics.In the present study, the effectiveness of the simplified BEMapproach was critically benchmarked against a comprehensivedescription of the flow field past the rotating blades comingfrom the combination of a two-dimensional unsteady CFDmodel and experimental wind tunnel tests; for both data sets,the overall performance and the wake characteristics on themid plane of a small-scale H-shaped Darrieus turbine wereavailable. Upon examination of the flow field, the validity ofthe ubiquitous use of induction factors is discussed, togetherwith the resulting velocity profiles upstream and downstreamthe rotor. Particular attention is paid on the actual flowconditions (i.e. incidence angle and relative speed) experiencedby the airfoils in motion at different azimuthal angles, forwhich a new procedure for the post-processing of CFD data ishere proposed. Based on this model, the actual lift and dragcoefficients produced by the airfoils in motion are analyzed anddiscussed, with particular focus on dynamic stall. The analysishighlights the main critical issues and flaws of the low-orderBEM approach, but also sheds new light on the physicalreasons why the overall performance prediction of thesemodels is often acceptable for a first-design analysis.