RESEARCH ARTICLE Model improvements for evaluating the effect of tower tilting on the aerodynamics of a vertical axis wind turbine K. Wang 1,2 , M. O. L. Hansen 2,3 and T. Moan 2 1 NOWITECH, Norwegian University of Science and Technology, 7491 Trondheim, Norway 2 Centre for Ships and Ocean Structures, Norwegian University of Science and Technology, 7491 Trondheim, Norway 3 DTU Wind Energy, Department of Mechanical Engineering, Technical University of Denmark, Lyngby, Denmark ABSTRACT If a vertical axis wind turbine is mounted offshore on a semi-submersible, the pitch motion of the platform will dominate the static pitch and dynamic motion of the platform and wind turbine such that the effect of tower tilting on the aerodynam- ics of the vertical axis wind turbine should be investigated to more accurately predict the aerodynamic loads. This paper proposes certain modifications to the double multiple-streamtube (DMS) model to include the component of wind speed parallel to the rotating shaft. The model is validated against experimental data collected on an H-Darrieus wind turbine in skewed flow conditions. Three different dynamic stall models are also integrated into the DMS model: Gormont’s model with the adaptation of Strickland, Gormont’s model with the modification of Berg and the Beddoes–Leishman dynamic stall model. Both the small Sandia 17 m wind turbine and the large DeepWind 5 MW are modelled. According to the experimental data, the DMS model with the inclusion of the dynamic stall model is also well validated. On the basis of the assumption that the velocity component parallel to the rotor shaft is small in the downstream part of the rotor, the effect of tower tilting is quantified with respect to power, rotor torque, thrust force and the normal force and tangential force coefficients on the blades. Addition- ally, applications of Glauert momentum theory and pure axial momentum theory are compared to evaluate the effect of the velocity component parallel to the rotor shaft on the accuracy of the model. Copyright © 2013 John Wiley & Sons, Ltd. KEYWORDS vertical axis wind turbine; platform pitch motion; aerodynamics; dynamic stall; tower tilting Correspondence K. Wang, Centre for Ships and Ocean Structures, Norwegian University of Science and Technology, 7491 Trondheim, Norway. E-mail: kai.wang@ntnu.no Received 19 December 2012; Revised 23 August 2013; Accepted 3 October 2013 NOMENCLATURE U ∞ Wind velocity at the equator level U Local upwind wind velocity Z EQ Elevation of the equatorial level Z Elevation of the streamtube with respect to the ground γ Atmospheric wind shear exponent ρ Air density a u Induction factor at the upwind rotor a d Induction factor at the downwind rotor ϕ Tilt angle θ Azimuthal angle δ Angle between the blade normal and the equatorial plane ΔI u , ΔI d Rate of change of momentum for upwind and downwind streamtubes U u Upwind horizontal velocity at the rotor V u Upwind vertical velocity at the rotor U e Equilibrium velocity in the middle plane between the upstream and downstream zone U d Downwind induced velocity at the rotor WIND ENERGY Wind Energ. 2015; 18:91–110 Published online 18 November 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/we.1685 Copyright © 2013 John Wiley & Sons, Ltd. 91