Numerical and experimental methods to investigate the behaviour of vertical-axis wind turbines with stators M. Burlando n , A. Ricci, A. Freda, M.P. Repetto University of Genoa, Department of Civil, Chemical and Environmental Engineering (DICCA), Via Montallegro 1,16145 Genoa, Italy article info Keywords: Vertical axis wind turbine Stator vanes Wind tunnel End plate Numerical wind tunnel Computational fluid dynamics abstract The practice of reproducing wind tunnel tests by means of CFD numerical simulations, which is known as numerical wind tunnel (NWT), is becoming quite common in many research fields of wind engineering. Wind tunnel tests can provide the indispensable validation data needed for CFD numerical simulations; at the same time, CFD can be considered a complementary support for wind tunnel tests in order to obtain a more comprehensive description of the flow field. In the present paper, NWT technique is applied to study the flow around and inside a multi-stage vertical-axis wind turbine (VAWT) surrounded by stator vanes. At first, the flow field has been studied in the wind tunnel by means of experimental tests. Then the experimental results have been used to validate a CFD model. The numerical model has finally been used to study and describe how the results obtained by means of the physical model can be extended to more general conditions. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction In the last decades, because of lagging in the sources of con- ventional fuels, a great effort has been made to improve the effi- ciency of renewable energy sources. Among them, both horizontal axis wind turbines (HAWT) and vertical axis wind turbines (VAWT) experienced a very fast development. In particular, small- size wind turbines seem very promising for decentralised power generation (Conejo et al., 2011). This is due to the fact that they have a low environmental impact, they do not cause instabilities in the power network distribution, and they do not need large power storage capabilities (e.g. Beller, 2009; Mertens, 2006). Besides, in very complex contexts, VAWT have some advantages with respect to HAWT, like the independency on the direction of the incoming flow (Balduzzi et al., 2012) and their simpler and cheaper con- struction and maintenance. VAWTs appear to be particularly promising for conditions corresponding to low wind speed and urban areas, even if they suffer from a poorer efficiency with respect to HAWTs. However, as stated by Akwa et al. (2012), rotor performance is affected by operational conditions, geometric and air flow parameters, and all of these variables can be changed to improve the performance of VAWTs. For instance, Fujisawa (1996) studied the effects of dif- ferent overlap ratios on the flow fields in and around a Savonius rotor through particle imaging velocimetry; Saha et al. (2008) conducted wind tunnel tests to evaluate the aerodynamics per- formance of different rotors in order to optimise the number of stages, and the number and the geometry of the blades; Howell et al. (2010) tested the effect of different surface roughness of the rotor blades on the turbine's performance. Apart from the rotor design optimisation, Akwa et al. (2012) report that performance gains of up to 50% can be reached with the use of stators. At the authors' knowledge, one of the first studies focusing on this subject dates back to 1978, when Alex- ander and Holownia (1978) published an experimental research on a Savonius rotor, testing in wind tunnel the effects of geometry (blade aspect ratio, blade overlap and gap) as well as the effects of different stators (end plates and shielding obstacles) on the air flow. More recently, Mohamed et al. (2011) have shown numeri- cally that it is possible to obtain performance gains of around 30% using an obstacle plate ahead of a Savonius rotor. They also pro- vide a brief review on the use of stators around this kind of rotors, reporting for example the work done by Irabu and Roy (2007) using a guide-box tunnel, and by Altan et al. (2008) using a frontal nozzle. Note, however, that the adoption of stators to increase the performance of wind turbines can be considered a strategy that applies independently on the rotor type, as shown for example by Pope et al. (2010) and Chong et al. (2013), who report the role of stator vanes in increasing turbine performance for VAWTs with non-Savonius rotor types. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jweia Journal of Wind Engineering and Industrial Aerodynamics http://dx.doi.org/10.1016/j.jweia.2015.04.006 0167-6105/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ39 010 353 2509. E-mail address: massimiliano.burlando@unige.it (M. Burlando). J. Wind Eng. Ind. Aerodyn. 144 (2015) 125–133