Abstract
In this study, large eddy simulation (LES) is combined with a turbine model to investigate the wake behind a vertical-axis wind turbine (VAWT) in a three-dimensional turbulent flow. Two methods are used to model the subgrid-scale (SGS) stresses: (a) the Smagorinsky model; and (b) the modulated gradient model. To parameterize the effects of the VAWT on the flow, two VAWT models are developed: (a) the actuator swept-surface model (ASSM), in which the time-averaged turbine-induced forces are distributed on a surface swept by the turbine blades, i.e., the actuator swept surface; and (b) the actuator line model (ALM), in which the instantaneous blade forces are only spatially distributed on lines representing the blades, i.e., the actuator lines. This is the first time that LES has been applied and validated for the simulation of VAWT wakes by using either the ASSM or the ALM techniques. In both models, blade-element theory is used to calculate the lift and drag forces on the blades. The results are compared with flow measurements in the wake of a model straight-bladed VAWT, carried out in the Institute de Méchanique et Statistique de la Turbulence (IMST) water channel. Different combinations of SGS models with VAWT models are studied, and a fairly good overall agreement between simulation results and measurement data is observed. In general, the ALM is found to better capture the unsteady-periodic nature of the wake and shows a better agreement with the experimental data compared with the ASSM. The modulated gradient model is also found to be a more reliable SGS stress modeling technique, compared with the Smagorinsky model, and it yields reasonable predictions of the mean flow and turbulence characteristics of a VAWT wake using its theoretically-determined model coefficient.
Highlights
Revived attention towards vertical axis wind turbines (VAWTs) has been observed in recent years.vertical-axis wind turbine (VAWT) are claimed to have several advantages over the conventional horizontal axis wind turbines (HAWTs), such as: insensitivity to the wind direction and, the absence of any yaw equipment; lower noise, due to the lower tip-speed of the blades; and placement of the drive train system on the ground [1]
Dabiri [2] has claimed that the power density of wind farms consisting of counter-rotating VAWTs can be potentially greater than HAWT wind farms by an order of magnitude
We have chosen the experimental study by Brochier et al [18] to compare our simulation results with, because it contains data on mean velocity and turbulence intensity at several downstream positions in the wake of a model VAWT, even though it was conducted under uniform flow conditions
Summary
Revived attention towards vertical axis wind turbines (VAWTs) has been observed in recent years. Battisti et al [20] carried out wind tunnel measurements on the wake of a VAWT and provided data on time-averaged and phase-resolved velocity and turbulence intensity in a vertical plane located 1.5 rotor diameters downstream of the turbine axis. They showed that large-scale aerodynamic unsteadiness is greater in the tip region than in the central region. Simulation of VAWTs by solving the Navier–Stokes equations with numerical techniques was pioneered by Rajagopalan and Fanucci [26], who introduced the actuator swept-surface method in a two-dimensional space as a cylindrical porous shell on which the time-averaged blade forces continuously act on the fluid.
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