University of California Santa Barbara, Science Daily, 4 October 2018
Depending on how wind turbines are situated relative to each other and to the prevailing wind, those not directly in the path of the wind could be left to extract energy from significantly depleted airflow. However, according to researchers, there are ways to get around this issue of diminishing wind returns.
Artem Korobenko, Yuri Bazilevs, Kenji Takizawa, Tayfun Tezduyar, Archives of Computational Methods in Engineering, September 2018
This is the first part of a two-part article on computer modeling of wind turbines. We describe the recent advances made by our teams in ALE-VMS and ST-VMS computational aerodynamic and fluid–structure interaction (FSI) analysis of wind turbines. The ALE-VMS method is the variational multiscale version of the Arbitrary Lagrangian–Eulerian method. The VMS components are from the residual-based VMS method. The ST-VMS method is the VMS version of the Deforming-Spatial-Domain/Stabilized Space–Time method. The ALE-VMS and ST-VMS serve as the core methods in the computations. They are complemented by special methods that include the ALE-VMS versions for stratified flows, sliding interfaces and weak enforcement of Dirichlet boundary conditions, ST Slip Interface (ST-SI) method, NURBS-based isogeometric analysis, ST/NURBS Mesh Update Method (STNMUM), Kirchhoff–Love shell modeling of wind-turbine structures, and full FSI coupling. The VMS feature of the ALE-VMS and ST-VMS addresses the computational challenges associated with the multiscale nature of the unsteady flow, and the moving-mesh feature of the ALE and ST frameworks enables high-resolution computation near the rotor surface. The ST framework, in a general context, provides higher-order accuracy. The ALE-VMS version for sliding interfaces and the ST-SI enable moving-mesh computation of the spinning rotor. The mesh covering the rotor spins with it, and the sliding interface or the SI between the spinning mesh and the rest of the mesh accurately connects the two sides of the solution.
Sayed Hossein Hezaveh, Elie Bou-Zeid, Physical Review Fluids, September 2018
Vertical-axis wind turbines (VAWTs) are the subject of renewed interest due to the potential for higher power generation per unit land used, as well as their lower center of mass (the generator is at the bottom of tower), which renders them favorable for offshore deployment. However, VAWT farms have hardly been studied. In this paper, using a previously tested actuator line model in a large eddy simulation code, we investigate the transport of the mean kinetic energy (MKE) that replenishes the power in the farm. The primary sources of MKE are (1) the initial advective streamwise influx through the frontal area and (2) the vertical planform influx through the top and bottom interfaces of the farm. The results show that, for realistic finite-size farms, the planform MKE transport is a loss term over the first six rows: in this initial zone the mean flow adjusts by slowing down, and an upward mean advection develops that results in an efflux loss of MKE from the farm volume. The power extracted from farms is thus mainly from the frontal advection over the first few rows. When the initial streamwise advective flux is exhausted, the planform regeneration of MKE from above the wind farm becomes the dominant source; it is primarily affected by turbulent-mean interaction. This regeneration continues to adjust until rows 8 to 10 in our setups, beyond which a fully developed flow (similar to an infinite wind farm) can be observed. In the fully developed region, actual mechanical power generation by the turbines is about one third of replenishment. A primary conclusion is that more irregular farms designs should be studied, while the current literature continues to focus on the very classic layouts.
Sayed Hossein Hezaveh, Elie Bou-Zeid, John Dabiri, Matthias Kinzel, Gerard Cordina, Luigi Martinelli, 6 July 2018
Vertical-axis wind turbines (VAWTs) are being reconsidered as a complementary technology to the more widely used horizontal-axis wind turbines (HAWTs) due to their unique suitability for offshore deployments. In addition, field experiments have confirmed that vertical-axis wind turbines can interact synergistically to enhance the total power production when placed in close proximity. Here, we use an actuator line model in a large-eddy simulation to test novel VAWT farm configurations that exploit these synergistic interactions. We first design clusters with three turbines each that preserve the omni-directionality of vertical-axis wind turbines, and optimize the distance between the clustered turbines. We then configure farms based on clusters, rather than individual turbines. The simulations confirm that vertical-axis wind turbines have a positive influence on each other when packed in well-designed clusters: such configurations increase the power generation of a single turbine by about 10 percent. In addition, the cluster designs allow for closer turbine spacing resulting in about three times the number of turbines for a given land area compared to conventional configurations. Therefore, both the turbine and wind-farm efficiencies are improved, leading to a significant increase in the density of power production per unit land area.