The actuator line technique was introduced as a numerical tool to be employed in combination with large eddy simulations to enable the study of wakes and wake interaction in wind farms. The technique is today largely used for studying basic features of wakes as well as for making performance predictions of wind farms. In this paper, we give a short introduction to the wake problem and the actuator line methodology and present a study in which the technique is employed to determine the near-wake properties of wind turbines. The presented results include a comparison of experimental results of the wake characteristics of the flow around a three-bladed model wind turbine, the development of a simple analytical formula for determining the near-wake length behind a wind turbine and a detailed investigation of wake structures based on proper orthogonal decomposition analysis of numerically generated snapshots of the wake.

The power production of the Lillgrund wind farm is determined numerically using large-eddy simulations and compared with measurements. In order to simulate realistic atmospheric conditions, pre-generated turbulence and wind shear are imposed in the computational domain. The atmospheric conditions are determined from data extracted from a met mast, which was erected prior to the establishment of the farm. In order to allocate most of the computational power to the simulations of the wake flow, the turbines are modeled using an actuator disc method where the discs are imposed in the computational domain as body forces which for every time step are calculated from tabulated airfoil data. A study of the influence of imposed upstream ambient turbulence is performed and shows that higher levels of turbulence results in slightly increased total power production and that it is of great importance to include ambient turbulence in the simulations. By introducing ambient atmospheric turbulence, the simulations compare very well with measurements at the studied inflow angles. A final study aiming at increasing the farm production by curtailing the power output of the front row turbines and thus letting more kinetic energy pass downstream is performed. The results, however, show that manipulating only the front row turbines has no positive effect on the farm production, and therefore, more complex curtailment strategies are needed to be tested.

The flow upstream a wind turbine is studied in order to investigate blockage effects. We use rotating wind turbine models in a wind tunnel, where velocity measurements have been made both with hot‐wire anemometry up to approximately 4.5 diameters (D) upstream the turbine, as well as laser particle image velocimetry measurements close to the turbine rotor. Also, numerical simulations have been carried out by means of a finite volume code. The measurements show, among other things, that the flow is affected more than 3 D upstream the rotor plane. Copyright © 2011 John Wiley & Sons, Ltd.

Two modal decomposition techniques are employed to analyse the stability of wind turbine wakes. A numerical study on a single wind turbine wake is carried out focusing on the instability onset of the trailing tip vortices shed from the turbine blades. The numerical model is based on large-eddy simulations (LES) of the Navier-Stokes equations using the actuator line (ACL) method to simulate the wake behind the Tjaereborg wind turbine. The wake is perturbed by low-amplitude excitation sources located in the neighbourhood of the tip spirals. The amplification of the waves travelling along the spiral triggers instabilities, leading to breakdown of the wake. Based on the grid configurations and the type of excitations, two basic flow cases, symmetric and asymmetric, are identified. In the symmetric setup, we impose a 120• symmetry condition in the dynamics of the flow and in the asymmetric setup we calculate the full 360• wake. Different cases are subsequently analysed using dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD). The results reveal that the main instability mechanism is dispersive and that the modal growth in the symmetric setup arises only for some specific frequencies and spatial structures, e.g. two dominant groups of modes with positive growth (spatial structures) are identified, while breaking the symmetry reveals that almost all the modes have positive growth rate. In both setups, the most unstable modes have a non-dimensional spatial growth rate close to π/2 and they are characterized by an out-of-phase displacement of successive helix turns leading to local vortex pairing. The present results indicate that the asymmetric case is crucial to study, as the stability characteristics of the flow change significantly compared to the symmetric configurations. Based on the constant non-dimensional growth rate of disturbances, we derive a new analytical relationship between the length of the wake up to the turbulent breakdown and the operating conditions of a wind turbine.

The aim of the present paper is to obtain a better understanding of the stability properties of wakes generated by wind turbine rotors. To accomplish this, a numerical study on the stability of the tip vortices of the Tjaereborg wind turbine has been carried out. The numerical model is based on large eddy simulations of the Navier–Stokes equations using the actuator line method to generate the wake and the tip vortices. To determine critical frequencies, the flow is disturbed by inserting harmonic perturbations, giving rise to spatially developing instabilities. The results show that the instability is dispersive and that growth arises only for some specific frequencies and type of modes, in agreement with previous instability studies. The result indicates two types of modes; one where oscillations of neighboring vortex spirals are out of phase and one where oscillations in every vortex spiral in phase. The mode with spirals out of phase results in the largest growth with the main extension of the disturbance waves in radial and downstream directions. The out‐of‐phase disturbance leads to vortex pairing once the development leaves the linear stage. The study also provides evidence of a relationship between the turbulence intensity and the length of the near wake. The relationship, however, needs to be calibrated against measurements. Copyright © 2010 John Wiley & Sons, Ltd.

The New European Wind Atlas project will create a freely accessible wind atlas covering Europe and Turkey, develop the model chain to create the atlas and perform a series of experiments on flow in many different kinds of complex terrain to validate the models. This paper describes the experiments of which some are nearly completed while others are in the planning stage. All experiments focus on the flow properties that are relevant for wind turbines, so the main focus is the mean flow and the turbulence at heights between 40 and 300 m. Also extreme winds, wind shear and veer, and diurnal and seasonal variations of the wind are of interest. Common to all the experiments is the use of Doppler lidar systems to supplement and in some cases replace completely meteorological towers. Many of the lidars will be equipped with scan heads that will allow for arbitrary scan patterns by several synchronized systems. Two pilot experiments, one in Portugal and one in Germany, show the value of using multiple synchronized, scanning lidar, both in terms of the accuracy of the measurements and the atmospheric physical processes that can be studied. The experimental data will be used for validation of atmospheric flow models and will by the end of the project be freely available.This article is part of the themed issue ‘Wind energy in complex terrains’.

Numerical simulations using CFD are performed for wind turbine applications. The aim of the project is to get a better understanding of wake behaviour that cannot be obtained by standard industrial design codes for wind power applications. Such codes are based on the Blade Element Momentum (BEM) technique, extended with a number of empirical corrections that are not entirely based on physical flow features. The importance of accurate design models also increases as the turbines become larger. Therefore, the research is today undergoing a shift toward more fundamental approaches, aiming at understanding basic aerodynamic mechanisms.Theoretically, the bound circulation on the blades is equal to the circulation behind the rotor, i.e. in the wake. For inviscid flows, the sum of the circulation of the tip and the root vortex should ideally be zero. However, this is not entirely correct for viscous flow. The tip and root vortex do, however, both for inviscid and viscous flows, have different sense of rotation, i.e. different signs of the circulation. A steep decline of circulation toward the tip will lead to a rapid concentration of the vortex at the tip (occurring a few chord lengths behind the tip). The sign of the circulation gradient along the blade will also determine the sense of rotation of the vortex behind the blade.The simulations are performed using the CFD program "EllipSys3D" developed at DTU (The Technical University of Denmark) and Risø. The socalled Actuator Line Method is used, where the blade is represented by a line instead of a large number of panels. The forces on that line are introduced by using tabulated aerodynamic coefficients. In this way, computer resources are used more efficiently since the number of node points locally around the blade is decreased, and they are instead concentrated in the wake behind the blades. The actuator line method was introduced by Sørensen and Shen [1] and later implemented into the EllipSys3D code by Mikkelsen [2]. EllipSys3D is a general purpose 3D solver developed by Sørensen [3] and Michelsen [4,5].

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