Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session L12: Wind Turbines: Wind Farms II |
Hide Abstracts |
Chair: Claire VerHulst, United States Military Academy Room: 200 |
Monday, November 23, 2015 4:05PM - 4:18PM |
L12.00001: Enhancing kinetic energy entrainment in LES of large wind farms by unconventional forcing at the turbine rotors Claire VerHulst, Charles Meneveau Vertical entrainment of mean kinetic energy is believed to be a limiting factor for power generation in very large wind farms, which operate in the turbulent atmospheric boundary layer and experience detrimental wake effects. A new approach, meant to increase vertical entrainment and aid wake recovery, is proposed and evaluated with a preliminary ``proof of concept'' test using Large Eddy Simulation (LES) with periodic boundary conditions to obtain realistic fully developed flow. In addition to the traditional actuator thrust force, a synthetic vertical force is applied at the turbine rotors to force high-speed flow downward and low-speed flow upward. The ratio of the vertical force and the thrust force, held constant within each case, ranges from 0 to 1 across six cases and is applied independently at each turbine. The proposed approach is found to increase the power extraction and mean kinetic energy entrainment significantly, by up to 95{\%} when the vertical force is similar in magnitude to the thrust force. The effect of the forcing scheme on the mean velocity field is considered in detail. In addition, a quadrant analysis is performed to determine how the synthetic forcing changes the statistical characteristics of the mean kinetic energy entrainment within the wind farm. [Preview Abstract] |
Monday, November 23, 2015 4:18PM - 4:31PM |
L12.00002: Effect of topography on wind turbine power and load fluctuations Christian Santoni, Umberto Ciri, Stefano Leonardi Onshore wind turbines produce more than $17GW$ in the US, which constitutes $4.4\%$ of all the energy produced. Sites selection is mostly determined by the atmospheric conditions and the topographical characteristics of the region. While the effect of the atmospheric boundary layer had been widely studied, less attention has been given to the effect of the topography on the wind turbine aerodynamics. To address how the topography affects the flow, Large Eddy Simulations of the flow over a wind turbine placed over wavy wall are performed. The wavelength of the wavy terrain, $\lambda$, is $1.7D$ where $D$ is the turbine rotor diameter. Two different values of the height of the wavy wall, $a/D=0.05$ and $a/D=0.10$ have been considered. In addition, two positions of the turbine with respect to the wavy wall had been studied, on the crest and trough of the wavy wall and compared with a wind turbine over a flat wall. For the turbine located at the crest, the pressure gradient due to the wavy wall caused a recirculation behind the wind tower $2.5D$ larger than that of the smooth wall. When placed at the trough of the wavy terrain, the favorable pressure gradient increases the wake velocity near the wall and promotes entrainment into the turbine wake. [Preview Abstract] |
Monday, November 23, 2015 4:31PM - 4:44PM |
L12.00003: Coupling the Weather Research and Forecasting (WRF) model and Large Eddy Simulations with Actuator Disk Model: predictions of wind farm power production Edgardo Javier Garcia Cartagena, Christian Santoni, Umberto Ciri, Giacomo Valerio Iungo, Stefano Leonardi A large-scale wind farm operating under realistic atmospheric conditions is studied by coupling a meso-scale and micro-scale models. For this purpose, the Weather Research and Forecasting model (WRF) is coupled with an in-house LES solver for wind farms. The code is based on a finite difference scheme, with a Runge-Kutta, fractional step and the Actuator Disk Model. The WRF model has been configured using seven one-way nested domains where the child domain has a mesh size one third of its parent domain. A horizontal resolution of 70 m is used in the innermost domain. A section from the smallest and finest nested domain, 7.5 diameters upwind of the wind farm is used as inlet boundary condition for the LES code. The wind farm consists in six-turbines aligned with the mean wind direction and streamwise spacing of 10 rotor diameters, (D), and 2.75D in the spanwise direction. Three simulations were performed by varying the velocity fluctuations at the inlet: random perturbations, precursor simulation, and recycling perturbation method. Results are compared with a simulation on the same wind farm with an ideal uniform wind speed to assess the importance of the time varying incoming wind velocity. [Preview Abstract] |
Monday, November 23, 2015 4:44PM - 4:57PM |
L12.00004: Optimal coordinated control of energy extraction in LES of wind farms: effect of turbine arrangement patterns Johan Meyers, Wim Munters, Jay Goit We investigate optimal control of wind-farm boundary layers, considering the individual wind turbines as flow actuators. By controlling the thrust coefficients of the turbines as function of time, the energy extraction can be dynamically regulated with the aim to optimally influence the flow field and the vertical energy transport. To this end, we use Large-Eddy Simulations (LES) of wind-farm boundary layers in a receding-horizon optimal control framework. Recently, the approach was applied to fully developed wind-farm boundary layers in a 7D by 6D aligned wind-turbine arrangement [1]. For this case, energy extraction increased up to 16\%, related to improved wake mixing by slightly anti-correlating the turbine thrust coefficient with the local wind speed at the turbine level. Here we discuss optimal control results for finite wind farms that are characterized by entrance effects and a developing internal boundary layer above the wind farm. Both aligned and staggered arrangement patterns are considered, and a range of different constraints on the controls is included.\\[4pt] [1] Goit JP, Meyers J. 2015 ``Optimal control of energy extraction in wind-farm boundary layers,'' Journal of Fluid Mechanics 768, 5-50. [Preview Abstract] |
Monday, November 23, 2015 4:57PM - 5:10PM |
L12.00005: Low-order representations of a wind turbine array boundary layer via double POD Nicholas Hamilton, Murat Tutkun, Ra\'{u}l Bayo\'{a}n Cal Experimental data from stereo particle image velocimetry enables access to the full Reynolds stress tensor in planes parallel to the scale-model turbine rotor. Proper orthogonal decomposition (POD) is applied to isolate structures in the wake. Modes resulting from the decomposition indicate that structures evolve along the streamwise coordinate. Secondary application of the POD, double proper orthogonal decomposition (DPOD), is applied to modes of common rank yielding a refined set of projections. The DPOD describes sub-modal organization in terms of projections of POD modes common to the span of the wake, followed by a series of spatially explicit corrections. Sub-modal structures that persist through the wake combine linearly with amplitudes and account for the evolution of the POD modes. Eigenvalues from the DPOD indicate that the wind turbine wake can be described with a very small subset of the original mode basis. The truncated basis of sub-modes represents a total reduction to 0.015\% of the original degrees of freedom in the wake. Low-order description of the stress tensor is corrected to account for energy excluded from the truncated basis. Root-mean-square error associated with low-order statistics is less than 15\% for normal stresses and 3\% for shear stresses. [Preview Abstract] |
Monday, November 23, 2015 5:10PM - 5:23PM |
L12.00006: A simplified model for average kinetic energy flux within large wind turbine arrays Corey Markfort, Wei Zhang, Fernando Porte-Agel We investigate the kinetic energy distribution within an array of wind turbines using a 1-D model for the interactions between large-scale wind farms and the atmospheric boundary layer (ABL). Obstructed shear flow scaling is used to predict the development length of the wind farm flow as well as vertical momentum flux. Within the region of flow development, momentum and energy is advected into the wind farm and wake turbulence draws excess momentum in from between turbines. This is characterized by large dispersive fluxes. Once the flow within the farm is developed, the area - averaged velocity profile exhibits an inflection point, characteristic of obstructed shear flows. The inflected velocity profile is responsible for a characteristic turbulence eddy scale, which may be responsible for a significant amount of the vertical momentum and energy flux. Prediction of this scale is useful for determining the amount of available power for harvesting. The model result for kinetic energy flux is compared to wind tunnel measurements. The model is useful for optimizing wind turbine spacing and layout, and for assessing the impacts of wind farms on nearby wind resources and the environment. [Preview Abstract] |
Monday, November 23, 2015 5:23PM - 5:36PM |
L12.00007: A simple and complete two-interface model for spatially developing flow in rigid and flexible canopies Samaneh Sadri, Paolo Luzzatto-Fegiz At the front of a canopy, flow deceleration is associated with strong vertical fluxes of mass and momentum. Accurately describing this region is important in many applications, including terrestrial and aquatic vegetation, as well as large wind farms. Simple models can provide a framework to analyze these flows, thereby guiding and complementing more refined and computationally intensive tools. Jerram \textit{et al.} (2003) introduced a linearised model that describes the flow field through sparse canopies, albeit at the cost of solving a PDE. A simpler approach involves vertically integrating the governing equations across the canopy, yielding scalings that relate key variables (e.g. Chen {\&} Nepf 2013), which in turn can be used to construct empirical fits. We build a simple and complete model, by separating the flow in three horizontal layers. These comprise the canopy, the overlying boundary layer, and the outer flow, such that exchanges of mass and momentum occur at two interfaces. We parameterize turbulent exchanges by means of the entrainment hypothesis; this is a closure that has been used extensively in other problems in geophysical fluid dynamics. We neglect pressure gradients inside the canopy, but account for upstream pressure variations and retain nonlinear terms. Our two-interface model quantitatively describes the flow velocities and boundary layer heights in developing canopy flows, and successfully accounts for the effect of ambient stratification. Finally, we discuss developments accounting for the effects of flexibility in vegetation canopies. [Preview Abstract] |
Monday, November 23, 2015 5:36PM - 5:49PM |
L12.00008: Physical Model Study of the Fully Developed Wind Turbine Array Boundary Layer in the UNH Flow Physics Facility John Turner, Martin Wosnik Results from an experimental study of an array of up to 100 model wind turbines with 0.25 m diameter are reported. The study was conducted in the UNH Flow Physics Facility (FPF), which has test section dimensions of 6.0 m wide, 2.7 m high and 72.0 m long. For a given configuration (spacing, initial conditions, etc.), the model wind farm reaches a “fully developed” condition, in which turbulence statistics remain the same from one row to the next within and above the wind turbine array. Of interest is the transport of kinetic energy within the wind turbine array boundary layer (WTABL). Model wind farms of up to 20 rows are possible in the FPF at the wind turbine scale used. The present studies in the FPF are able to achieve the fully developed WTABL condition, which can provide valuable insight to the optimization of wind farm energy production. The FPF can achieve a boundary layer height on the order of 1 m at the beginning of the wind turbine array. The wind turbine array was constructed of porous disks, which where drag (thrust) matched to wind turbines at typical operating conditions and therefore act as momentum sinks similar to wind turbines. The flow in the WTABL was measured with constant temperature anemometry using an X-wire. [Preview Abstract] |
Monday, November 23, 2015 5:49PM - 6:02PM |
L12.00009: Assessing the Impacts of Low Level Jets over Wind Turbines Walter Gutierrez Rodriguez, Guillermo Araya, Arquimedes Ruiz-Columbie, Murat Tutkun, Luciano Castillo Low Level Jets (LLJs) are defined as regions of relatively strong winds in the lower part of the atmosphere. They are a common feature over the Great Plains in the United States. This paper is focused on the determination of the static/dynamic impacts that real LLJs in West Texas have over wind turbines and wind farms. High-frequency (50Hz) observational data from the 200-m meteorological tower (Reese, Texas) have been input as inflow conditions into the NREL FAST code in order to evaluate the LLJ's structural impacts on a typical wind turbine. Then, the effect of the LLJ on the wind turbine's wake is considered to evaluate the overall impact on the wind farm. It has been observed that during a LLJ event the levels of turbulence intensity and turbulence kinetic energy are significantly much lower than those during unstable conditions. Also, low-frequency oscillations prevail during stable conditions when LLJs are present, as opposed to high-frequency oscillations which are more prevalent during unstable conditions. Additionally, in LLJs the energy concentrates in particular frequencies that stress the turbine whereas turbine signals show frequencies that are also present in the incoming wind. [Preview Abstract] |
Monday, November 23, 2015 6:02PM - 6:15PM |
L12.00010: Properties of wind turbine wakes under various atmospheric stability conditions Shengbai Xie, Cristina Archer Large-eddy simulations (LES) are performed to study the properties of wind turbine wakes under various atmospheric stability conditions. The Wind Turbine and Turbulence Simulator (WiTTS), a 4$^{\mathrm{th}}$-order finite-difference LES code is used for stable, neutral, and unstable conditions. The Coriolis forcing is also considered. Three cases are studied: isolated turbine, finite-size turbine array, and infinite wind farm. The results show strong correlations with stability. For the stable condition, the power extraction by an isolated turbine is highest, but the wake is also longest, thus the relative performance inside the array is lowest. In contrast, although the single-turbine power extraction is low for the unstable condition, the performance of downstream turbines is improved due to faster wake recovery. The wake shape is distorted by the stability-related wind veering. Therefore, the self-similar Gaussian wake deficit is not accurate. Here, a new wake model is proposed for correction. The infinite wind-farm case shows that the temperature near the ground is warmed by about 1 K for the stable condition, but the influence is almost negligible for the unstable and neutral conditions. For all conditions, the near-ground shear stress is reduced. [Preview Abstract] |
Monday, November 23, 2015 6:15PM - 6:28PM |
L12.00011: Outer layer effects in wind-farm boundary layers: Coriolis forces and boundary layer height Dries Allaerts, Johan Meyers In LES studies of wind-farm boundary layers, scale separation between the inner and outer region of the atmospheric boundary layer (ABL) is frequently assumed, i.e., wind turbines are presumed to fall within the inner layer and are not affected by outer layer effects. However, modern wind turbine and wind farm design tends towards larger rotor diameters and farm sizes, which means that outer layer effects will become more important. In a prior study, it was already shown for fully-developed wind farms that the ABL height influences the power performance [1]. In this study, we use the in-house LES code SP-Wind to investigate the importance of outer layer effects on wind-farm boundary layers. In a suite of LES cases, the ABL height is varied by imposing a capping inversion with varying inversion strengths. Results indicate the growth of an internal boundary layer (IBL), which is limited in cases with low inversion layers. We further find that flow deceleration combined with Coriolis effects causes a change in wind direction throughout the farm. This effect increases with decreasing boundary layer height, and can result in considerable turbine wake deflection near the end of the farm. \\[4pt] [1] Allaerts D. and Meyers J., Phys. Fluids, 27, 065108 (2015) [Preview Abstract] |
Monday, November 23, 2015 6:28PM - 6:41PM |
L12.00012: Influence of Coriolis forces on the structure and evolution of wind-turbine wakes Mahdi Abkar, Fernando Porté-Agel In this study, large-eddy simulation (LES) is combined with a turbine model to investigate the effect of Coriolis forces on the structure and evolution of wind-turbine wakes. In order to isolate the Coriolis effect on the turbulent wake flow, two set of simulations are performed. In the first set of simulations, atmospheric boundary layer (ABL) flow is driven by the geostrophic forces including the effect of Earth's rotation, while in the second case, the ABL flow is driven by a unidirectional pressure gradient forcing. Both cases have the same mean horizontal velocity and turbulence intensity at the hub height. The simulation results show that the Coriolis forces significantly affect the spatial distribution of the mean velocity deficit and turbulence statistics in the wake region. In particular, it is found that the Coriolis effect, responsible for vertical wind veer, has important lateral wake stretching effects, which in turn significantly impacts the wake recovery and wake meandering characteristics downwind of the turbines. We also apply the proper orthogonal decomposition (POD) to LES data of the wake. The results indicate a very high correlation between the most energetic modes and both maximum velocity deficit and wake meandering characteristics. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2019 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
1 Research Road, Ridge, NY 11961-2701
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700