Rotor power performance and flow physics in lateral sinusoidal gusts
Introduction
Wind is becoming a more and more popular energy source due to its biggest advantage of renewability and eco-friendliness. A vast amount of research on wind turbine including both horizontal axis wind turbine (HAWT) and vertical axis wind turbine (VAWT), is springing up these years, especially on some hot topics such as airfoil design and optimization [1], wind farm site selection [2], noise [3], atmospheric effects [4,5], etc. Among the various atmospheric conditions, gust, which is commonly generated by nearby buildings, trees and terrains, is one of the major factors influencing wind turbine greatly [6]. Previous studies have revealed that gust loads on wind turbine not only affect the power performance but also reduce the lifetime of fatigue [7]. However, it is still a big challenge to obtain a comprehensive and consistent conclusion of gust influence on wind turbines [8] and more extensive work is still needed for future wind energy exploitation.
A real three-dimensional (3-D) VAWT encounters three directions of gust in the nature, longitudinal, lateral and vertical [9], as shown in Fig. 1. Through literature survey it is found that nearly all the existing papers are related to horizontal and vertical gusts, except the one from our group [10] (Table 1). In addition, in regards to the methods modeling wind turbine rotation, there are either computational fluid dynamics (CFD) approach by solving the steady or unsteady Reynolds-Averaged Navier-Stokes (RANS) equations or analytical methods such as double multiple streamtube (DMS) [10] and blade element momentum (BEM) [11]. CFD numerical simulation is a superior choice especially for the needs of flowfield visualization and so far most of the relevant CFD researchers have adopted the sliding mesh technique in the commercial solver FLUENT as their meshing and calculation tools [[12], [13], [14], [15], [16]]. In the implementation of the sliding mesh technique, at least one static domain and one dynamic moving domain are needed to construct. In fact, the whole grid in the dynamic domain is rotating synchronously with the rotor rather than the real sense of grid movement relative to the rotor. Therefore, calculation errors or physical distortions can be easily yielded in practical applications. A good solution to this issue is the chimera mesh technique, which has an additional big advantage in mesh generations.
In our recent work [10], we adopted a synthetic method coupling the CFD TAU code and the double multiple streamtube (DMS) method to evaluate the influence of lateral wind gust on vertical axis wind turbine performance. However, the real rotor rotation motion could not directly be simulated using that method. Instead, the polar of the two-dimensional (2-D) airfoils is first obtained via CFD and then substituted into the DMS tool to output the power and torque coefficients of the 3-D turbines. The method is easy to implement with low computational cost, however, a biggest shortcoming comes up with its inability of knowing the real flowfield information. Thus, the method can be called as a partial simulation method. To overcome the abovementioned issue, this paper conducts a full simulation of a three-bladed VAWT rotation using the chimera mesh technique embedded in the DLR TAU code. Meanwhile, the influence of various sinusoidal gusts on the VAWT power performance and flowfield characteristics is predicted via the resolved gust approach (RGA) model [24] implemented also in TAU. The layout of the present work is as follows. First, a three-bladed NACA 0021 VAWT with real rotation in various sinusoidal gusts is simulated by using the chimera mesh technique and RGA model in TAU. Both the general aerodynamic model and the gust model are validated and mesh independence is examined in Section 3.1. Then, the power performance and the flow interaction between the gust and the rotor are analyzed in Section 3.2. Finally, the flowfield characteristics of the rotor under gusts are presented and the underlying physics are analyzed in Section 3.3.
Section snippets
Model geometry
The present work aims to analyze the unsteady performance of a three-bladed Darrieus NACA 0021 VAWT operating in lateral sinusoidal wind gusts. Previous studies have shown that a 2-D model is sufficient in revealing the factors influencing the performance and majority of flow physics that surround the VAWT [15,16,25,26], thus a 2-D CFD model is used to represent the VAWT in this work. The main geometrical feathers of the tested rotor are summarized in Table 2. The rotor azimuth θ is identified
Validation of CFD model
Sufficient temporal resolution is important in ensuring proper unsteady simulation of rotor flow. A number of studies have found that a time step size in the order of 0.1°ω−1 to 1°ω−1 is sufficient for VAWT simulations [12,15,18]. However, this order of time step size was found insufficient for the current method with the chimera mesh technique. In practice, we adopted time step sizes of 0.025°ω−1 for tip speed ratios λ = 1.44, 1.68, 2.04, 2.33 and 0.05°ω−1 for λ = 2.51, 2.64, 3.09, 3.3 to
Conclusions
In this study, numerical simulation was conducted to study the effects of sinusoidal gusts on a three-bladed VAWT power performance and flow characteristics. The chimera mesh technique was used to model the rotation of the grid around the rotor and the RGA model in the DLR TAU code for simulation of the gust traveling process across the whole domain. The results were compared with the experimental and numerical results from literature to show the validation of the general aerodynamic model. The
Acknowledgements
The author gratefully acknowledges the following institutions for support: the Alexander von Humboldt (AvH) Foundation (grant No. 1190117) for sponsoring the author nearly three years' research at the University of Stuttgart and the High Performance Computing Center Stuttgart (HLRS) for providing computational time in the CFD simulations.
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