ORIGINAL ARTICLE Performance of SB-VAWT using a modified double multiple streamtube model E. Saber a , R. Afify a, * , H. Elgamal b a Mechanical Engineering Department, Faculty of Engineering, Arab Academy for Science, Technology and Maritime Transport (AASTMT), Abu kir, Alexandria, P.O. Box 1029-Miami, Egypt b Mechanical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt Received 2 March 2018; revised 23 June 2018; accepted 2 July 2018 Available online 15 November 2018 KEYWORDS Vertical axis wind turbine; Darrieus rotor; DMST; Aerodynamic performance; Solidity Abstract The present study introduces a new relation to evaluate the equilibrium induced air velocity in the mid plane between the upstream and downstream rotor zones to give a good improvement in prediction of the maximum power coefficient. The relation is derived as a function of both interference factor in upwind and downwind rotor zones determined using classical DMST model. The root-finding algorithm is used to obtain the interference factor both in upwind and downwind zones of the turbine at each streamtube. The comparison between the numerical results and the experimental data of 5-m, 17-m, and 34-m Darrieus VAWTs shows that the use of the intro- duced relation gives a sensible improvement in the prediction of maximum value for turbine power coefficient. A theoretical study is presented based on the use of well documented symmetrical NACA0012 turbine blade profile. The effect of wind speed and rotor solidity on maximum power coefficient and corresponding optimal speed ratio on the turbine rotor is evaluated in order to deter- mine an improved aerodynamic configuration for a given application. Ó 2018 Faculty of Engineering, Alexandria University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction For the design and analysis of VAWTs, the use of Blade Element Momentum (BEM) models has aroused a large credit both in research and academic communities as well as in industry. The BEM theory was first introduced by Glauert [1] for predicting the performance of airplane pro- pellers. Templain [2] adjusted the momentum based model for VAWT aerodynamics, obtaining the Single-Streamtube (SST) model. Strikland [3] extended the SST model to the Multiple-Streamtube (MST) model within dependent parallel streamtubes. Several authors [4–8] improved the MST model by placing two actuator disks behind each other to represent the turbine as a set of two tandem actuator disks, modeling the upwind and downwind passes of blades, the model named the Double Multiple Streamtube (DMST). Castelli [9] studied the non-stationary aerodynamic behavior during the rotation of the rotor. This phenomenon has a significant effect on dynamic loads acting on the rotor and on gener- ated power. Wind tunnel experiments were carried out [10–13] to study the turbine performance, especially for high * Corresponding author. E-mail address: rola@aast.edu (R. Afify). Peer review under responsibility of Faculty of Engineering, Alexandria University. Alexandria Engineering Journal (2018) 57, 3099–3110 HOSTED BY Alexandria University Alexandria Engineering Journal www.elsevier.com/locate/aej www.sciencedirect.com https://doi.org/10.1016/j.aej.2018.07.009 1110-0168 Ó 2018 Faculty of Engineering, Alexandria University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).