Three-Dimensional Simulation of a Complete Vertical Axis Wind Turbine Using Overlapping Grids

Three-dimensional simulations of the aerodynamic field around a three-blade straight-axis Vertical Axis Wind Turbine (VAWT) are presented for two values of the Tip Speed Ratio λ (TSR), namely λ=1.52 and λ=2.5. Numerical simulations were carried out using the over-set grid solver ROSITA (ROtorcraft Software ITAly). The Reynolds-Averaged Navier-Stokes equations are completed by the Spalart-Allmaras turbulence model. A strong interaction between the blade and the blade wakes is evidenced. Dynamic stall is observed in the case λ=2.5. The computed flow-field presents diverse three-dimensional effects, including the interaction between the blades and the tip vortices and the aerodynamic disturbances from the turbine shaft and the support arms. Three-dimensional effects are more relevant for λ=2.5. The comparison to experimental data confirms the general features of the flow

A Numerical Model of an Axial Wind Turbine

Since the beginning of the industrial revolution, there has never been more of a constant in the world than that of the demand for energy. For years a stable source has been fossil fuels, but with the growing impacts of global warming, it is important to look for renewable sources. Wind energy’s use has become more and more prevalent throughout the world. This preliminary work runs through the creation of a three-dimensional numerical model of a conventional wind turbine that was created using ANSYS-Fluent© commercial software. This work provides the base wind turbine and a blade element and momentum theory (BEMT) based MATLAB© code that can create the blade coordinates if the wind turbine design is modified. The numerical simulations created in this work are compared with the results of the bare wind turbine simulations and the experimental power generation data obtained from (Ohya et al., 2008). The geometry consists of a large one-third cylindrical enclosure with an interior rotating mesh around the turbine blade with periodic boundary conditions on either side. The torque of the blade can be calculated and compared to the original experimental power production values. Three comparison speeds are chosen; four, six, and eight meters per second based on physical data available. In experiments done on a bare wind turbine, the power values compare very well to the expected results from the research group from Japan. For 4, 6, and 8 m/s wind speeds, the percent differences in power production between the actual results and numerical results were 0.81%, 3.39%, and 5.23% different respectively. The values produced numerically were all higher than the experimental data, which should be the case because general wind turbine losses that are briefly introduced in Section 1.3 are not considered in this study. It is the hope that this work will be continued further by having another researcher to work on a shrouded numerical model by benchmarking it according to the experimental flanged diffuser results from a research team in Japan that performed many experiments regarding shrouding on wind turbines. It is the hope that shrouding is added because adding a shroud to a normal wind turbine can almost quintuple power production compared standard wind turbines (Ohya et al., 2008)

Physical and numerical modeling of cross-flow turbines

Cross-flow (often vertical-axis) turbines (CFTs), despite being thoroughly investigated and subsequently abandoned for large scale wind energy, are seeing renewed interest for smaller scale wind turbine arrays, offshore wind, and marine hydrokinetic (MHK) energy applications. Though they are similar to the large scale Darrieus wind turbines, today\u27s CFT rotors are often designed with higher solidity, or blade chord-to-radius ratios, which makes their behavior more difficult to predict with numerical models. Furthermore, most experimental datasets used for numerical model validation were acquired with low solidity rotors. An experimental campaign was undertaken to produce high quality open datasets for the performance and near-wake flow dynamics of CFTs. An automated experimental setup was developed using the University of New Hampshire\u27s towing tank. The tank\u27s linear motion, control, and data acquisition systems were redesigned and rebuilt to facilitate automated cross-flow turbine testing at large laboratory (on the order of 1 meter) scale. Two turbines were designed and built---one high solidity (dubbed the UNH Reference Vertical-Axis Turbine or UNH-RVAT) and one medium-to-low solidity, which was a scaled model of the US Department of Energy and Sandia National Labs\u27 Reference Model 2 (RM2) cross-flow MHK turbine. A baseline performance and near-wake dataset was acquired for the UNH-RVAT, which revealed that the relatively fast wake recovery observed in vertical-axis wind turbine arrays could be attributed to the mean vertical advection of momentum and energy, caused by the unique interaction of vorticity shed from the blade tips. The Reynolds number dependence of the UNH-RVAT was investigated by varying turbine tow speeds, indicating that the baseline data had essentially achieved a Reynolds number independent state at a turbine diameter Reynolds number $Re_D \sim 10^6$ or chord based Reynolds number $Re_c \sim 10^5$. A similar study was undertaken for the RM2, with similar results. An additional dataset was acquired for the RM2 to investigate the effects of blade support strut drag on overall performance, which showed that these effects can be quite significant---on the order of percentage points of the power coefficient---especially for lower solidity rotors, which operate at higher tip speed ratio. The wake of the RM2 also showed the significance of mean vertical advection on wake recovery, though the lower solidity made these effects weaker than for the UNH-RVAT. Blade-resolved Reynolds-averaged Navier--Stokes (RANS) computational fluid dynamics (CFD) simulations were performed to assess their ability to model performance and near-wake of the UNH-RVAT baseline case at optimal tip speed ratio. In agreement with previous studies, the 2-D simulations were a poor predictor of both the performance and near-wake. 3-D simulations faired much better, but the choice of an appropriate turbulence model remains uncertain. Furthermore, 3-D blade-resolved RANS modeling is computationally expensive, requiring high performance computing (HPC), which may preclude its use for array analysis. Finally, an actuator line model (ALM) was developed to attempt to drive down the cost of 3-D CFD simulations of cross-flow turbines, since previously, the ALM had only been investigated for a very low Reynolds number 2-D CFT. Despite retaining some of the disadvantages of the lower fidelity blade element momentum and vortex methods, the ALM, when coupled with dynamic stall, flow curvature, added mass, and end effects models, was able to predict the performance of cross-flow turbines reasonably well. Near-wake predictions were able to match some of the important qualitative flow features, which warrants further validation farther downstream and with multiple turbines. Ultimately, the ALM provides an attractive alternative to blade-resolved CFD, with computational savings of two to four orders of magnitude for large eddy simulation and RANS, respectively