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

Abstract

The paper presents the results of a computational study on the aerodynamics and the performance of a small-scale Vertical-Axis Wind Turbine (VAWT) for distributed micro-generation. The complexity of VAWT aerodynamics, which are inherently unsteady and three-dimensional, makes high-fidelity flow models extremely demanding in terms of computational cost, limiting the analysis to mainly 2D or 2.5D Computational Fluid-Dynamics (CFD) approaches. This paper discusses how a proper setting of the computational model opens the way for carrying out fully 3D unsteady CFD simulations of a VAWT. Key aspects of the flow model and of the numerical solution are discussed, in view of limiting the computational cost while maintaining the reliability of the predictions. A set of operating conditions is considered, in terms of tip-speed-ratio (TSR), covering both peak efficiency condition as well as off-design operation. The fidelity of the numerical predictions is assessed via a systematic comparison with the experimental benchmark data available for this turbine, consisting of both performance and wake measurements carried out in the large-scale wind tunnel of the Politecnico di Milano. The analysis of the flow field on the equatorial plane allows highlighting its time-dependent evolution, with the aim of identifying both the periodic flow structures and the onset of dynamic stall. The full three-dimensional character of the computations allows investigating the aerodynamics of the struts and the evolution of the trailing vorticity at the tip of the blades, eventually resulting in periodic large-scale vortices. technology dominates the market today but, in the aforementioned non-conventional environments, the Vertical-Axis Wind Turbine (VAWT) can be an interesting alternative, for several reasons: it is inherently 'aerodynamically' robust to yawed and skewed flows; it can provide a proper structural coupling with floating foundations (as demonstrated in the frame of the DeepWind EU program, [1]); it is characterized by lower costs of installation and maintenance as the gearbox and the generator can be placed on the ground; it produces lower acoustic pollution due to the lower optimal tip speed ratio with respect to HAWT. However, VAWT rotors are characterized by very complex aerodynamics, inherently unsteady, and fully three-dimensional, resulting in periodic fluctuating forces which ultimately complicate the structural design and affect the turbine performance.Several engineering methods were specifically developed for VAWTs, such as the multiple stream-tube and the double multiple stream-tube, that are analyzed in detail in [2]. In recent years, higher fidelity methods based on Computational Fluid Dynamics (CFD) have been applied to VAWTs, obtaining physically-sound representations of the flow field around the rotor [3-7], as well as quantitatively reliable performance estimates [8]. However, such simulations cause very high computational cost that has historically led the researchers to employ simplified actuator-line [9] or 2D...

Numerical investigation of h-Darrieus wind turbine aerodynamics at different tip speed ratios

“…Li et al (2013) achieved one of the first incompressible large eddy simulations (LES) on a limited span (what they called 2.5D) of a 3-NACA0015 blade H-Darrieus VAWT including the mast, at one single operating condition close to maximum efficiency. When compared to uRANS it improved performance predictions confirming results in Franchina et al (2019), and showed more turbulent structures shed by the blades at an operating condition close to maximum efficiency. However, the short span simulation may have triggered too coherent turbulent structures and both grid and numerical schemes too dissipative to yield any significant interaction between the blades and the mast.…”

Multi-objective Optimization and Experimental Testing of a Laminated Vertical-Axis Wind Turbine Blade

“…Seeing that the details of numerical set-up and numerous obtained results can be found in [27], only a short summary is given here. Other extremely useful examples of flow simulations around VAWTs can be found in [28][29][30][31][32][33][34].…”

“…the relative extents of the rotational and stationary parts of computational domain, mesh features and its level of medium fineness, material properties, zone and boundary conditions, adopted turbulence model, numerical schemes, etc. ), the complete modeling, meshing and computing processes were repeated on a VAWT of comparable dimensions, rotor solidity and employed airfoil (of slightly greater relative thickness) whose description and experimentally obtained data in a wind tunnel are available in [34,35]. The only difference is in the smaller turbulence intensity of 1% assumed along the inlet boundary (corresponding to the flow conditions in the wind tunnel).…”

Design of Rotor Blades for Vertical Axis Wind Turbine with Wind Flow Modifier for Low Wind Profile Areas

“…However, this article proposes an involute-type rotor blade in the VAWT model, because it supports the lift force as well as lift supporting drag force through an available conical-shaped space. Furthermore, the proposed involute-type blades with a wind accelerator, termed the wind flow modifier (WFM), are also adapted for simulation using 3D CFD ANSYS Fluent software [25][26][27] for the effective and reliable outcomes. The core objectives of this article are as follows:…”