A Critical Analysis on Low-Order Simulation Models for Darrieus Vawts: How Much Do They Pertain to the Real Flow?

Abstract

To improve the efficiency of Darrieus wind turbines, which still lacks from that of horizontal-axis rotors, computational fluid dynamics (CFD) techniques are now extensively applied, since they only provide a detailed and comprehensive flow representation. 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 like the blade element momentum (BEM) theory. These models have been shown to provide relatively accurate estimations of the overall turbine performance; conversely, the description of the flow field suffers from the strong approximations introduced in the modeling of the flow physics. In this study, the effectiveness of the simplified BEM approach was critically benchmarked against a comprehensive description of the flow field past the rotating blades coming from the combination of a two-dimensional (2D) unsteady CFD model and experimental wind tunnel tests; for both data sets, the overall performance and the wake characteristics on the midplane of a small-scale H-shaped Darrieus turbine were available. Upon examination of the flow field, the validity of the ubiquitous use of induction factors is discussed, together with the resulting velocity profiles upstream and downstream the rotor. Particular attention is paid on the actual flow conditions (i.e., incidence angle and relative speed) experienced by the airfoils in motion at different azimuthal angles, for which a new procedure for the postprocessing of CFD data is here proposed. Based on this model, the actual lift and drag coefficients produced by the airfoils in motion are analyzed and discussed, with particular focus on dynamic stall. The analysis highlights the main critical issues and flaws of the low-order BEM approach, but also sheds new light on the physical reasons why the overall performance prediction of these models is often acceptable for a first-design analysis.

Three-Dimensional CFD Simulation and Experimental Assessment of the Performance of a H-Shape Vertical-Axis Wind Turbine at Design and Off-Design Conditions

“…The detailed analysis of the airfoil aerodynamics reveals that the low power extraction is caused by the severe drop of torque caused by the onset of super-stall at the end of the upwind phase. In fact, such effect is not consistent with the estimates of aerodynamic forces coming from the application of engineering methods (double multiple streamtube model) to this machine which, instead, led to an excellent agreement with the experimental performance [21].…”

Darrieus vertical-axis water turbines: deformation and force measurements on bioinspired highly flexible blade profiles

“…This model reduction allows for a clear observation of the flow characteristics at the blade level. However, it neglects some effects applying to the real turbine, such as a reduced angle of attack in the downstream region of the rotor reported by Delafin (2014) or Bianchini et al (2018). During the experimental investigations, w is set constant, since the flow speed v ch in a water channel cannot be varied in the required timescales.…”

Current status and grand challenges for small wind turbine technology

“…Based on recent analyses, this aerodynamic solution seems to provide unprecedented specific power values for small rotors (Bianchini et al, 2015a). On the other hand, these models showed the significant shortcomings of existing simulation models (Bianchini et al, 2019), which were resolved largely by the new understanding of the role of flow curvature effects (Bianchini et al, 2015b(Bianchini et al, , 2016. Renewed research efforts are being undertaken to determine whether VAWTs can fit the scope of distributed energy production in complex installation areas, as testified to by the recent EU project (Aeolus4Future, n.d.).…”