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... More advanced methods to analyse the local effects in detail can be found e.g. in [16]. ...

Design load simulations for wind turbines are traditionally based on the blade- element-momentum theory (BEM). The BEM approach is derived from a simplified representation of the rotor aerodynamics and several semi-empirical correction models. A more sophisticated approach to account for the complex flow phenomena on wind turbine rotors can be found in the lifting-line free vortex wake method. This approach is based on a more physics based representation, especially for global flow effects. This theory relies on empirical correction models only for the local flow effects, which are associated with the boundary layer of the rotor blades. In this paper the lifting-line free vortex wake method is compared to a state- of-the-art BEM formulation with regard to aerodynamic and aeroelastic load simulations of the 5MW UpWind reference wind turbine. Different aerodynamic load situations as well as standardised design load cases that are sensitive to the aeroelastic modelling are evaluated in detail. This benchmark makes use of the AeroModule developed by ECN, which has been coupled to the multibody simulation code SIMPACK.

The importance and the complexity of the phenomena related to the development of radial flows is demonstrated in the first part of this paper. In order to further study the radial flow effects and to extend the analysis to laminar and transitional flows, the authors used a CFD 3D model, validated in the wind tunnel owned by the University of Catania. In the second part of this paper, the authors describe the strategy which was used to post-process the simulation results. Furthermore, a comparison of the results was made. Several simulations were first carried out at various wind and rotational speeds. Angles of Attack and aerodynamic coefficients were evaluated on cylindrical surfaces at different radial stations using the ANSYS Fluent Solver and ANSYS Post. Local velocities and forces, related to the sectional airfoil, were obtained in each cylindrical surface along with pressure coefficient distributions. In this way, it was possible to demonstrate the close relationship between radial flows and the strong depressurization of the suction side of the blade. Moreover, the results proved that the increase of lift and drag coefficients is linked to rotational speed and Angle of Attack as well. The radial effects were found to be enforced by laminar and transitional flows related to low Reynolds numbers. This will affect both design and analysis of wind rotor performance, more so than that which was originally suggested by empirical stall delay models.

The radial flow along a rotating blade is a fluid dynamic behavior that specifically affects the flow field of HAWTs. The physical effects of such flow on the rotor performance are not yet fully understood due to the complexity of the phenomenon and its high dependence on three dimensionality and Reynolds numbers. In the first part of this paper the authors reviewed the State of the Ar tof physics and modeling of radial flows. Some researchers have proposed empirical models to take into account the centrifugal pumping inside 1D codes. It was found in general, that the radial flow acts on the blades, increasing the forces and delaying the stall. Compared to a simple 2D condition, the aerodynamic coefficients are hence increased. Obviously, this phenomenon is heavily dependent on rotational speed as the centrifugal force increases with the square of the angular velocity and only linearly with the radial distance. So, due to higher rotational speed, the aerodynamics of mini and micro rotors is mostly influenced by the radial flow rather than the large rotors. The combined effects of both transitional and radial flow were evaluated in the present work using an accurate CFD 3D model as there was no specific literature in this particular field. This model, developed by the authors, was based on a RANS, four equations, transition turbulence model and it was calibrated and validated on a suitably designed micro rotor. The rotor was tested in the subsonic wind tunnel owned by the University of Catania. A review of the modeling and validation strategy is presented in the first part of this paper while the extrapolated data and the post-processing is presented in the second part, thus finding results of significant interest.

The simulation of wind turbine aerodynamics can be improved with more physically realistic models than the ones currently in use in engineering practice. In this report the mathematical and numerical aspects and the practical use of a new wind turbine aerodynamics simulation module is described. The new simulation code is based on non-linear lifting line vortex wake theory which is a more accurate model for rotor wake physics; an important aspect in wind turbine load simulations. The new model is verified for some test cases with analytical solutions. Wake dynamics are shown to behave as anticipated. The new simulation module, designated AWSM, is expected to substantially reduce the number of uncertainties that accompany currently used blade-element-momentum methods. To assert the quality of AWSM results a comparison with experimental data is recommended. Numerical tests should be performed to investigate the influence of AWSM simulation parameters on the computed results. Further functionality extensions and algorithm optimizations are suggested.

AeroDyn is a set of routines used in conjunction with an aeroelastic simulation code to predict the aerodynamics of horizontal axis wind turbines. These subroutines provide several different models whose theoretical bases are described in this manual. AeroDyn contains two models for calculating the effect of wind turbine wakes: the blade element momentum theory and the generalized dynamic-wake theory. Blade element momentum theory is the classical standard used by many wind turbine designers and generalized dynamic wake theory is a more recent model useful for modeling skewed and unsteady wake dynamics. When using the blade element momentum theory, various corrections are available for the user, such as incorporating the aerodynamic effects of tip losses, hub losses, and skewed wakes. With the generalized dynamic wake, all of these effects are automatically included. Both of these methods are used to calculate the axial induced velocities from the wake in the rotor plane. The user also has the option of calculating the rotational induced velocity. In addition, AeroDyn contains an important model for dynamic stall based on the semi-empirical Beddoes-Leishman model. This model is particularly important for yawed wind turbines. Another aerodynamic model in AeroDyn is a tower shadow model based on potential flow around a cylinder and an expanding wake. Finally, AeroDyn has the ability to read several different formats of wind input, including single-point hub-height wind files or multiple-point turbulent winds.

The effect of wind turbine blade tip geometry is numerically analysed using Computational Fluid Dynamics (CFD). Three different rotating blade tips are compared for attached flow conditions and the flow physics around the geometries are analysed. To this end, the pressure coefficient (Cp) is defined based on the stagnation pressure rather than on the inflow dynamic pressure. The tip geometry locally modifies the angles of attack (AOA) and the inflow dynamic pressure at each of the studied sections. However not all 3D effects could be reduced to a change of these two variables. An increase in loadings (particularly the normal force) towards the tip seem to be associated to a spanwise flow component present for the swept-back analysed tip. Integrated loads are ranked to asses wind turbine tip overall performance. It results from the comparison that a better tip shape that produced better torque to thrust ratios in both forces and moments is a geometry that has the end tip at the pitch axis. The work here presented shows that CFD may prove to be useful to complement 2D based methods on the design of new wind turbine blade tips.

This document describes the enhancement of the aeroelastic stability analysis with the program system ARLIS by applying aerodynamic results obtained from 3D CFD computations. As a main goal a coupling between the CFD-solver and ARLIS by exchanging aerodynamic loads and deformations is envisioned.

The flow field past the rotating blade of a horizontal axis wind turbine has been modeled with a full 3–D steady–RANS approach. Flow computations have been performed using the commercial finite–volume solver Fluent. A number of blade sections from the 3–D rotating geometry were chosen and the corresponding 2–D flow computations have been carried out for comparison, for different angles of attack and in stalled conditions. In order to investigate the effects of rotation a postprocessing tool has been developed, allowing the evaluation of the terms in the boundary layer equations. Examples of the output are proposed for the analyzed flow situations. The main features of the boundary layer flow are described, for both the rotating blade and the corresponding 2–D profiles. Computed pressure distributions and aerodynamic coefficients evidence less lift losses after separation in the 3–D rotating case, mostly for the inward sections of the blade and the highest angles of attack, in agreement with the literature.

A method for calculating the output power from large horizontal-axis wind turbines is presented. Modifications to the airfoil characteristics and the momentum portion of classical blade element-momentum theory are given that improve correlation with measured data. Improvement is particularly evident at low tip-speed ratios where aerodynamic stall can occur as the blade experiences high angles of attack. Output power calculated using the modified theory is compared with measured data for several large wind turbines. These wind turbines range in size from the DOE/NASA 100 kW Mod-0 (38 m rotor diameter) to the 2000 kW Mod-1 (61 m rotor diameter). The calculated results are in good agreement with measured data from these machines.

Summary
This paper presents an approach to compute fluid-structure interactions on wind turbines. It is a contribution to the development of future design tools and aims to improve the quality of numerical simulations of the fluid-structure interaction process, leading to a better understanding of the underlying physics. The presented approach is widely discussed in literature and is referred to as tight or strong coupling.
Strong coupling means an exchange of fluid loads and structural deformations at each time step. Since the analysis methods and codes for both domains have independently reached a high level of sophistication, this approach is effectuated in a fully modular manner and data is exchanged between separate codes. The underlying coupling schemes are classified by the character of time integration on fluid and structure side, respectively. Several combinations are possible, but this paper focuses on a first order implicit-explicit scheme.
So far the strong coupling focuses on rotor only computations. The respective models on both fluid and structure side are presented and discussed. The contribution presents coupled fluid-structure computations at the rotor of a 2.75 MW wind turbine. The results are compared to and validated against state of the art simulation tools.

For wind turbines the eects of rotation lead to larger aerodynamic power and thrust compared to predictions based on 2D aerodynamic coecients, which has been the subject of investigation for decades. Most models that account for the eects of rotation are in terms of increased lift or delay of stall to larger angles of attack. A model is pre- sented on basis of a description of the separated flow at the trailing-edge. It includes the eect of the local speed ratio and it also gives a correction for the drag. This approach led to a so-called 'cen- trifugal pumping' correction model for the normal force coecient together with a delay of separation

This paper summarizes the developments of transition prescription and transition prediction techniques which were implemented into the DLR Reynolds-averaged Navier-Stokes (RANS) solver FLOWer in the framework of the DLR projects MEGAFLOW and MEGAFLOW II and the German research project MEGAFLOW. The very basic transition handling functionalities which FLOWer provided before the projects started were generalized in order to prescribe arbitrary transition lines on very complex aircraft geometries with different components, such as wings, fuselages or nacelles. A number of transition prediction methods were incorporated into the code and an infrastructure was built up in order to handle the underlying transition prediction strategy which results in an iteration process within the solution process of the RANS equations. Finally, physical models for the modeling of transitional flow were implemented and tested.

Design and analysis methods for wind turbines are presently based on relatively simple models of rotor blade aerodynamics, such as 2-D blade element/momentum theory (BEMT). Field investigations over the past few years have shown discrepancies between predicted and measured performance, owing to the effect of rotation on the wind turbine blade boundary layer distribution. The present paper is aimed at describing a fundamental phenomenon: the effect of rotation on the blade boundary layer of a wind turbine. In this paper, 3-D incompressible steady momentum integral boundary layer equations are employed to study this complex problem. By solving the 3-D integral boundary layer equations with the assumed velocity profiles and a closure model (including both laminar and turbulent boundary layer models), the effects of rotation on blade boundary layers are investigated. Several key parameters, such as separation position and momentum thickness, are calculated and compared for the rotation and non-rotation cases. It is concluded that the stall is postponed due to rotation and the separation point is delayed as a result of increasing rotation speed or decreasing blade spanwise position. Possible modifications that should be considered to the existing 2-D BEMT method are suggested.

Wind turbine generator systems -Part 1: Design requirements

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Extraction of lift, drag and angle of attack from computed 3-D viscous flow around a rotating blade

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