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Fast and effective numerical models describing the effect of platform motion on the performance of floating offshore wind turbines (FOWTs) are fundamental to assess energy harvesting potential in large offshore wind farms. The purpose of this paper is to implement a CFD-based Computationally-Efficient approach based on an actuator line model (ALM) for FOWTs aerodynamics. Such a tool aims at complementing reasonable accuracy and affordable computational effort while being able to investigate the effects of the platform motions on the wake evolution. The actuator line model for FOWTs is developed by implementing a dedicated C++ library in the OpenFOAM toolbox. In addition, a tip treatment is applied to involve the tip effects induced by the pressure equalization from the suction and pressure sides. Results show that employing ALM decreases computational cost and preprocessing time for producing appropriate computational grids, as just about 400k and 600k grids are necessary for solving two representative test cases of fixed-bottom turbines (NREL Phase VI and NREL 5-MW) with reasonable accuracy. The inclusion of platform motion is then introduced, and the results showed that ALM is capable of capturing vortices trajectory, potential blade-vortex interactions, and vortex pairing and vortex ring state phenomenon in FOWTs. [ABSTRACT FROM AUTHOR]

In a large wind farm, the wakes of upstream and downstream wind turbines can interfere with each other, affecting the overall power output of the wind farm. To further improve the numerical accuracy of the turbine wake dynamics under atmosphere turbulence, this work proposes some improvements to the actuator line-large-eddy simulation (AL-LES) method. Based on the dynamic k-equation large-eddy simulation (LES), this method uses a precursor method to generate atmospheric inflow turbulence, models the tower and nacelle wakes, and improves the body force projection method based on an anisotropic Gaussian distribution function. For these three improvements, three wind tunnel experiments are used to validate the numerical accuracy of this method. The results show that the numerical results calculated in the far-wake region can reflect the characteristics of typical onshore and offshore wind conditions compared with the experimental results. After modeling the tower and nacelle wakes, the wake velocity distribution is consistent with the experimental result. The radial migration velocity of the tip vortex calculated by the improved blade body force distribution model is 0.32 m/s, which is about 6% different from the experimental value and improves the prediction accuracy of the tip vortex radial movement. The method proposed in this paper is very helpful for wind turbine wake dynamic analysis and wind farm power prediction. [ABSTRACT FROM AUTHOR]

The wake characteristics of a variable-speed pitch-regulated floating offshore wind turbine (FOWT) under surge motions are studied using a Computational Fluid Dynamics (CFD) method which combines the Actuator Curve Embedding (ACE) method and collective blade pitch control strategy. Verifications and validations show the method is accurate with affordable computational cost. Comparative analysis of a FOWT under specified surge motion at rated wind speed indicates the controller significantly reduces the mean wake deficits, which highlights the necessity of considering controller for FOWT(s) wake analysis and floating offshore wind farm layout optimizations. The mean wake deficits and wake widths are observed to reduce with increasing surge amplitudes for a controlled FOWT. At rated wind speed, the helical tip vortices of a controlled FOWT under surge motions are observed transitioning into well-organized vortex ring structures and the vortex ring distances are found almost independent on surge amplitudes. The total wake vortex transition distance and vortex ring distance are observed increasing linearly with surge periods. These findings contribute to a better understanding of FOWT wake dynamics in real operating conditions. [ABSTRACT FROM AUTHOR]

Wind turbine farms suffer from wake losses, where the downstream turbines generate less power, and/or the leading turbines are throttled to reduce the downstream power losses. In this paper, we focus on possible external modifications that can enhance the wind turbines' performance when they are operating in a farm environment. In particular, this study is interested in enhancing the performance of the downstream turbines in wind farms. The idea is to move each turbine's wake down and away from subsequent turbines. This goal is achieved by using stationary external airfoils that are placed in proximity to the rotating blades. A number of different designs are tested and the design concepts are tested using Reynolds‐Averaged Navier‐Stokes simulations of an aligned array of 2 wind turbines. The turbines are modeled as actuator disks with axial induction and are placed in a velocity field that is modeled as a turbulent atmospheric boundary layer. It is found that fixed external airfoils can enable partial or full power recovery at turbine separations of as small as 3 rotor diameters downstream. We will also demonstrate that some devices can also improve the performance of the upstream turbine. The physical reasons for these power recovery phenomena are discussed. [ABSTRACT FROM AUTHOR]

We apply a modified proper orthogonal decomposition (POD) to large eddy simulation data of a wind turbine wake in a turbulent atmospheric boundary layer. The turbine is modeled as an actuator disk. Our analysis mainly focuses on the pragmatic identification of spatial modes, which yields a low order description of the wake flow. This reduction to a few degrees of freedom is a crucial first step for the development of simplified dynamic wake models based on modal decompositions. It is shown that only a few modes are necessary to capture the basic dynamical aspects of quantities that are relevant to a turbine in the wake flow. Furthermore, we show that the importance of the individual modes depends on the relevant quantity chosen. Therefore, the optimal choice of modes for a possible model could in principle depend on the application of interest. We additionally present a possible interpretation of the extracted modes by relating them to the specific properties of the wake. For example, the first mode is related to the horizontal large-scale movement. [ABSTRACT FROM AUTHOR]

An actuator line method combining the actuator line technique, in which the body forces representing the blades of the wind turbine with a three-dimensional Navier-Stokes solver, is proposed to get flow fields about wind turbines. It avoids numerous meshes and huge computational for the simulation of blade boundary layers, and in replacement, more meshes and computational resource can be used to simulate the wind turbine wake flow filed, so the actuator line method is very suitable for the research of the wind turbine wake. Computational results for NH1500 blade are compared among the actuator line method with the Gauss distribution, BEM theory, CFD and wind tunnel experiment in the load distribution on the blade and wind turbine power coefficient, validating that the actuator line method is effective and efficient for wind turbine aerodynamic calculation. For completeness, the wind turbine wake flow filed calculated by the actuator line method is analyzed. [ABSTRACT FROM AUTHOR]

On a wind farm, the wake has an important impact on the performance of the wind turbines. For example, the wake of an upstream wind turbine affects the blade load and output power of the downstream wind turbine. In this paper, a modified actuator line model with blade tips, root loss, and an airfoil three-dimensional delayed stall was revised. This full-scale modified actuator line model with blades, nacelles, and towers, was combined with a Large Eddy Simulation, and then applied and validated based on an analysis of wind turbine wakes in wind farms. The modified actuator line model was verified using an experimental wind turbine. Subsequently, numerical simulations were conducted on two NREL 5 MW wind turbines with different staggered spacing to study the effect of the staggered spacing on the characteristics of wind turbines. The results show that the output power of the upstream turbine stabilized at 5.9 MW, and the output power of the downstream turbine increased. When the staggered spacing is R and 1.5R, both the power and thrust of the downstream turbine are severely reduced. However, the length of the peaks was significantly longer, which resulted in a long-term unstable power output. As the staggered spacing increased, the velocity in the central near wake of the downstream turbine also increased, and the recovery speed at the threshold of the wake slowed down. The modified actuator line model described herein can be used for the numerical simulation of wakes in wind farms. [ABSTRACT FROM AUTHOR]

Abstract: Experimental results on the wake properties of a non-rotating simplified wind turbine model, based on the actuator disc concept, and a rotating model, a three-blade wind turbine, are presented. Tests were performed in two different test sections, one providing a nominally decaying isotropic turbulent inflow (turbulence intensity of 4% at rotor disc location) and one providing a neutral atmospheric boundary layer above a moderately rough terrain at a geometric scale of 1:300 (determined from the combination of several indicators), with 13% of turbulence intensity at hub height. The objective is to determine the limits of the simplified wind turbine model to reproduce a realistic wind turbine wake. Pressure and high-order velocity statistics are therefore compared in the wake of both rotor discs for two different inflow conditions in order to quantify the influence of the ambient turbulence. It has been shown that wakes of rotating model and porous disc developing in the modeled atmospheric boundary layer are indistinguishable after 3 rotor diameters downstream of the rotor discs, whereas few discrepancies are still visible at the same distance with the decaying isotropic turbulent inflow. [Copyright &y& Elsevier]

Abstract: Experimental results on the unsteady behaviour of the wake of a modelled wind turbine in an atmospheric boundary layer (ABL) wind tunnel are presented. Tests were performed by modelling in the wind tunnel an ABL above a neutral moderately rough terrain at a geometric scale of 1:400, and a wind turbine with the help of the actuator disk concept. In order to characterise the meandering properties of its wake, velocity spectra and space–time correlations were measured through hot wire anemometry, both in the wake of the modelled wind turbine and in the wake of a solid disk. Comparing these two configurations allowed the examination of the differences between the random motion of the whole wind turbine wake characterising the meandering in the first case, and the periodic oscillations of the well-known vortex shedding, which appears behind a bluff-body, in the second case. Finally, the same experiments were performed in homogeneous and isotropic turbulent flows to exhibit the role of the large atmospheric turbulent scales in the meandering process. [Copyright &y& Elsevier]