2.5D large eddy simulation of vertical axis wind turbine in consideration of high angle of attack flow

Abstract

A renewed interest in vertical axis wind turbines (VAWTs) has been seen recently. Computational fluid dynamics (CFD) is regarded as a promising technique for aerodynamic studies of VAWTs. In particular, 2D unsteady Reynolds-averaged Navier-Stokes (URANS) is commonly adopted, although past studies on VAWTs revealed the limited accuracy of 2D URANS. This paper investigated the feasibility and accuracy of three different CFD approaches, namely 2D URANS, 2.5D URANS and 2.5D Large Eddy Simulations (LES), in the aerodynamic characterization of straight-bladed VAWT (SBVAWT), with a focus on the capability of the 2.5D LES approach in CFD simulation of high angle of attack (AOA) flow. The 2.5D model differs from a full 3D model in that only a certain length of blades is modeled with periodic boundaries at the two extremities of the domain. The applications of these three approaches were systematically examined in the aerodynamic simulations of a single static airfoil and a 3-blade SBVAWT at different rotating speeds. Their capabilities to predict the aerodynamic forces were evaluated through a comparison with the wind tunnel results obtained by other researchers, with particular attention to high AOA flow beyond stall. Among the three methods, 2.5D LES yielded the best agreement with the experimental results in both cases. Detailed examinations of simulated flow field revealed that 2.5D LES produces more realistic 3D vortex diffusion after flow separation, resulting in more accurate predictions of aerodynamic coefficients in static or dynamic stall situations. It is noteworthy that 2.5D LES cannot capture the effect of tip vortex and vertical flow divergence in VAWTs, which used to be regarded by some researchers as the major cause of overprediction of VAWT power in 2D URANS. In this study, the considerably improved results achieved by 2.5D LES imply that the poor accuracy of URANS method is mainly due to its inherent limitation in vortex modeling. In general, 2.5D LES showed good agreement with experimental results at a relatively low tip speed ratio (TSR), but only fair agreement at a high TSR. Compared with the other two approaches, 2.5D LES is regarded as a more promising and effective CFD tool for investigating the aerodynamic characteristics of VAWTs, particularly their self-starting features corresponding to very low rotation speeds.

Three-dimensional numerical simulations of straight-bladed vertical axis tidal turbines investigating power output, torque ripple and mounting forces

“…Numerous authors [11,13,18] have suggested that 3D CFD models are not practical for turbine performance investigations due to their excessive simulation time and computational requirements.…”

“…It has previously been suggested that URANS methods are unable to accurately predict vertical axis turbine blade vortex shedding and flow diffusion, requiring higher order CFD methods such as Large Eddy Simulation (LES) [13]. However the authors believe that the accuracy of 3D CFD simulations when compared to EFD results, as demonstrated in Figure 10, suggests that reasonable estimates of performance coefficients such as Cp can be obtained by URANS methods and that resolution of small- …”

“…Large Eddy Scale (LES) simulations which treat the blades as possessing infinite length [13], and three-dimensional (3D) CFD models [14,15], with increasing levels of simulation complexity and computational resource requirements. Commercial CFD software such as ANSYS Fluent and CFX are commonly used to simulate turbine power output and hydrodynamics [3,8,[12][13][14][15][16][17][18], with most CFD simulations performed in 2D as 3D models require lengthy simulation times [10,13,17].…”

3D CFD Analysis of a Vertical Axis Wind Turbine

“…The TSR, which is also referred to as λ in the manuscript, is defined as U/V ∞ , where U is the tip peripheral velocity of the rotor, namely ωR, ω is the angular velocity of the blade and R the rotor radius, while V ∞ is the free stream velocity. A 2.5D model [16] is also presented in the literature, which differs from a 3D simulation, because only a segment of the airfoil blades is modeled with periodic boundaries at the extremities of the domain. The limitation of this model is that it can only be used with Darrieus-type straight-bladed VAWT.…”

Noise Emission of a 200 kW Vertical Axis Wind Turbine

“…Simulations of VAWT noise emissions from first principles would require an accurate determination of the flow close to the blade, which is a computationally expensive task. However, recent progress with large eddy simulations [10][11][12][13][14] might render this approach feasible further on.…”