Y. Bazilevs Department of Structural Engineering, University of California–San Diego, La Jolla, CA 92093 e-mail: yuri@ucsd.edu A. Korobenko Department of Structural Engineering, University of California–San Diego, La Jolla, CA 92093 X. Deng Department of Structural Engineering, University of California–San Diego, La Jolla, CA 92093 J. Yan Department of Structural Engineering, University of California–San Diego, La Jolla, CA 92093 M. Kinzel Department of Aerospace Engineering, Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125 J. O. Dabiri Department of Aerospace Engineering, Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125 Fluid–Structure Interaction Modeling of Vertical-Axis Wind Turbines Full-scale, 3D, time-dependent aerodynamics and fluid–structure interaction (FSI) simu- lations of a Darrieus-type vertical-axis wind turbine (VAWT) are presented. A structural model of the Windspire VAWT (Windspire energy, http://www.windspireenergy.com/) is developed, which makes use of the recently proposed rotation-free Kirchhoff–Love shell and beam/cable formulations. A moving-domain finite-element-based ALE-VMS (arbi- trary Lagrangian–Eulerian-variational-multiscale) formulation is employed for the aero- dynamics in combination with the sliding-interface formulation to handle the VAWT mechanical components in relative motion. The sliding-interface formulation is aug- mented to handle nonstationary cylindrical sliding interfaces, which are needed for the FSI modeling of VAWTs. The computational results presented show good agreement with the field-test data. Additionally, several scenarios are considered to investigate the tran- sient VAWT response and the issues related to self-starting. [DOI: 10.1115/1.4027466] 1 Introduction In recent years, the wind-energy industry has been moving in two main directions: off shore, where energy can be harvested from stronger and more sustained winds, and urban areas, which are closer to the direct consumer. In the offshore environments, large-size horizontal-axis wind turbines (HAWTs) are at the lead- ing edge. They are equipped with complicated pitch and yaw con- trol mechanisms to keep the turbine in operation for wind velocities of variable magnitude and direction, such as wind gusts. The existing HAWT designs are currently more efficient for large-scale power production compared with the VAWT designs. However, smaller-size VAWTs are more suitable for urban envi- ronments and are currently employed for small-scale wind-energy generation. Nevertheless, wind-energy technologies are maturing, and several studies were recently initiated that involve placing VAWTs off shore [2,3]. There are two main configurations of VAWTs, employing the Savonius or Darrieus rotor types [4]. The Darrieus configuration is a lift-driven turbine. It is more efficient than the Savonius con- figuration, which is a drag-type design. Recently, VAWTs resur- faced as a good source of small-scale electric power for urban areas. The main reason for this is their compact design. The gener- ator and drive train components are located close to the ground, which allows for easier installation, maintenance, and repair. Another advantage of VAWTs is that they are omidirectional (i.e., they do not have to be oriented into the main wind direction), which obviates the need to include expensive yaw control mechanisms in their design. However, this brings up issues related to self-starting. The ability of VAWTs to self-start depends on the wind conditions as well as on airfoil designs employed [5]. Stud- ies in Refs. [6,7] reported that a three-bladed H-type Darrieus rotor using a symmetric airfoil is able to self-start. In Ref. [8], the author showed that significant atmospheric wind transients are required to complete the self-starting process for a fixed-blade Darieus turbine when it is initially positioned in a dead-band region defined as the region with the tip-speed-ratio values that result in negative net energy produced per cycle. Self-starting remains an open issue for VAWTs, and an additional starting sys- tem is often required for successful operation. Due to increased recent emphasis on renewable energy, and, in particular, wind energy, aerodynamics modeling and simulation of HAWTs in 3D have become a popular research activity [9–17]. FSI modeling of HAWTs is less developed, although, recently, several studies were reported showing validation at full-scale against field-test data for medium-size turbines [18], and demon- strating feasibility for application to larger-size offshore wind- turbine designs [10,19]. However, 3D aerodynamics modeling of VAWTs is lagging behind. The majority of the computations for VAWTs are reported in 2D [20–22], while a recent 3D simulation in Ref. [23] employed a quasi-static representation of the air flow instead of solving the time-dependent problem. A detailed 3D aerodynamics analysis of a VAWT used for laboratory testing was recently performed by some of the authors of the present paper in Ref. [24]. The studies included full 3D aerodynamic simulations, validated using experimental data, and a simulation of two side- by-site counter-rotating turbines. The aerodynamics and FSI computational challenges in VAWTs are different than in HAWTs due to the differences in their aerodynamic and structural design. Because the rotation axis Manuscript received March 25, 2014; final manuscript received April 14, 2014; accepted manuscript posted April 22, 2014; published online May 7, 2014. Assoc. Editor: Kenji Takizawa. Journal of Applied Mechanics AUGUST 2014, Vol. 81 / 081006-1 Copyright V C 2014 by ASME Downloaded From: http://appliedmechanics.asmedigitalcollection.asme.org/ on 06/19/2014 Terms of Use: http://asme.org/terms