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Coupled Fluid-Structure Simulations of a Wind Turbine Rotor
Abstract
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.
... An improved approach for load simulations able to solve the complex flow phenomena described above is provided by 3-dimensional unsteady RANS (URANS) computations [6]. This method can also consider aeroelastic effects by including the fluid-structure interaction as demonstrated by Streiner [7]. Since such simulations consume a great deal of computational time it is, at least for the present, not yet realistic to simulate all the load cases as defined in the guidelines with such an approach. ...
... Different turbulence models are available in FLOWer. However, due to good experiences in former studies [7] the k-ω SST turbulence model is the sole model used for the present study. In previous projects (e.g. ...
... Grid setup showing blade (green), hub (red) and background grid (blue)[7] ...
... This motion has been approximated by two appended sine funtions, as presented in Figure 8.1. Following the work of Streiner [108], a sufficient timestep size corresponding to an azimuth movement of ∆Ψ = 5° per timestep was chosen. Before starting the CFD calculation of the prescribed floating motion, 4 rotor rotations have been pre-calculated to ensure, that the wake behind the rotor is fully developed in the background grid when the motion starts. ...
... Before starting the CFD calculation of the prescribed floating motion, 4 rotor rotations have been pre-calculated to ensure, that the wake behind the rotor is fully developed in the background grid when the motion starts. Due to good experiences in former studies [108], the k-ω SST turbulence model is used. ...
The objectives of Task 4.3 within UpWind Work Package 4 are to enhance the currently available design tools and methods for the efficient design of large numbers of structures at deep-water site, and to actively support the development of dedicated international standards which specify best practice for the design of offshore wind farms (e.g. site-specific design, aerodynamic and hydrodynamic impact, low-risk structures, floating concepts).
Therefore, the report focuses on the development of design requirements and standards.
... A better but more complicated method to determine the loads is the coupling between aerodynamics and structure. Such a method has been developed in cooperation at the IAG and SWE [4]. Therefore the actual CFD simulation code is coupeled to a multi body simulation code, so that data between both codes is exchanged at every time step. ...
... Mesh and solution of a wind turbine simulation; without tower[4] ...
This paper describes some methods being used at the Uni- versity of Stuttgart to do research on wind turbine aero- dynamics and aeroacoustics. The first part is about wind turbine aerodynamics in general by using a process chain based on an RANS solver. The second part deals with aero- dynamics noise reduction concepts to design low noise air- foils for wind turbine blades. The third part of this paper gives a short description of a method used for airfoil de- sign and an airfoil optimization process. The last part give s a short overview of the experimental facilities used at the IAG.
... IMEX schemes are widely used in multiscale problems including atmospheric [31,32], ocean [33], sea-ice [34], shallow-water [29], and wind turbine models [35] and in plasma simulations [36]. By treating the fastest waves implicitly, IMEX methods overcome the stringent time step size of explicit methods and simplify the fully implicit system solves by using an explicit integrator for the nonstiff components. ...
We propose entropy-preserving and entropy-stable partitioned Runge–Kutta (RK) methods. In particular, we extend the explicit relaxation Runge–Kutta methods to IMEX–RK methods and a class of explicit second-order multirate methods for stiff problems arising from scale-separable or grid-induced stiffness in a system. The proposed approaches not only mitigate system stiffness but also fully support entropy-preserving and entropy-stability properties at a discrete level. The key idea of the relaxation approach is to adjust the step completion with a relaxation parameter so that the time-adjusted solution satisfies the entropy condition at a discrete level. The relaxation parameter is computed by solving a scalar nonlinear equation at each timestep in general; however, as for a quadratic entropy function, we theoretically derive the explicit form of the relaxation parameter and numerically confirm that the relaxation parameter works the Burgers equation. Several numerical results for ordinary differential equations and the Burgers equation are presented to demonstrate the entropy-conserving/stable behavior of these methods. We also compare the relaxation approach and the incremental direction technique for the Burgers equation with and without a limiter in the presence of shocks.
... In Santo et al. (2020b), gusts were introduced showing that in the case considered, flow separation was occurring working as a passive load control. Streiner et al. (2008), Meister (2015) and Klein et al. (2018) worked sequentially on an FSI coupling between the CFD code FLOWer and the multibody simulation (MBS) commercial solver SIMPACK. In Klein et al. (2018), the NREL 5 MW wind turbine was simulated, including the drivetrain torsion, the foundation flexibility and the controller for variation in revolutions per minute (rpm) and pitch angle, examining thereby the origin of low-frequency noise sources and seismic excitation. ...
This paper shows high-fidelity fluid–structure interaction (FSI) studies applied to the research wind turbine of the WINSENT (Wind Science and Engineering in Complex Terrain) project. In this project, two research wind turbines are going to be erected in the south of Germany in the WindForS complex-terrain test field. The FSI is obtained by coupling the CFD URANS–DES code FLOWer and the multiphysics FEM solver Kratos Multiphysics, in which both beam and shell structural elements can be chosen to model the turbine. The two codes are coupled in both an explicit and an implicit way. The different modeling approaches strongly differ with respect to computational resources, and therefore the advantages of their higher accuracy must be correlated with the respective additional computational costs. The presented FSI coupling method has been applied firstly to a single-blade model of the turbine under standard uniform inflow conditions. It could be concluded that for such a small turbine, in uniform conditions a beam model is sufficient to correctly build the blade deformations. Afterwards, the aerodynamic complexity has been increased considering the full turbine with turbulent inflow conditions generated from real field data, in both flat and complex terrains. It is shown that in these cases a higher structural fidelity is necessary. The effects of aeroelasticity are then shown on the phase-averaged blade loads, showing that using the same inflow turbulence, a flat terrain is mostly influenced by the shear, while the complex terrain is mostly affected by low-velocity structures generated by the forest. Finally, the impact of aeroelasticity and turbulence on the damage equivalent loading (DEL) is discussed, showing that flexibility reduces the DEL in the case of turbulent inflow, acting as a damper that breaks larger cycles into smaller ones.
... In Santo et al. (2020b), gusts were introduced showing that in the considered case, flow separation was occurring working as a passive load control. Streiner et al. (2008), Meister (2015) and Klein et al. (2018) worked sequentially on a FSI coupling between the CFD code FLOWer and the Multi-Body Simulation (MBS) commercial solver SIMPACK. In Klein et al. (2018), the NREL 5 MW wind 60 turbine has been simulated, including the drive train torsion, the foundation flexibility and the controller for variation of RPM and pitch angle, examining thereby the origin of low frequency noise sources and seismic excitation. ...
This paper shows high-fidelity Fluid Structure Interaction (FSI) studies applied on the research wind turbine of the WINSENT project. In this project, two research wind turbines are going to be erected in the South of Germany in the WindForS complex terrain test field. The FSI is obtained by coupling the CFD URANS/DES code FLOWer and the multiphysics FEM solver Kratos, in which both beam and shell structural elements can be chosen to model the turbine. The two codes are coupled in both an explicit and an implicit way. The different modelling approaches strongly differ with respect to computational resources and therefore the advantages of their higher accuracy must be correlated with the respective additional computational costs. The presented FSI coupling method has been applied firstly to a single blade model of the turbine under standard uniform inflow conditions. It could be concluded that for such a small turbine, in uniform conditions a beam model is sufficient to correctly build the blade deformations. Afterwards, the aerodynamic complexity has been increased considering the full turbine with turbulent inflow conditions generated from real field data, in both a flat and complex terrains. It is shown that in these cases a higher structural fidelity is necessary. The effects of aeroelasticity are then shown on the phase-averaged blade loads, showing that using the same inflow turbulence, a flat terrain is mostly influenced by the shear, while the complex terrain is mostly affected by low velocity structures generated by the forest. Finally, the impact of aeroelasticity and turbulence on the Damage Equivalent Loading (DEL) is discussed, showing that flexibility is reducing the DEL in case of turbulent inflow, acting as a damper breaking larger cycles into smaller ones.
... IMEX schemes are widely used in multiscale problems including atmospheric [3,4], ocean [5], sea-ice [6], shallow-water [7], and wind turbine models [8], and plasma simulations [9]. By treating the fastest waves implicitly, IMEX methods overcome the stringent time step size of explicit methods and simplify the fully implicit system solves by using an explicit integrator for the nonstiff components. ...
... IMEX schemes are widely used in multiscale problems including atmospheric [3,4], ocean [5], sea-ice [6], shallow-water [7], and wind turbine models [8], and plasma simulations [9]. By treating the fastest waves implicitly, IMEX methods overcome the stringent time step size of explicit methods and simplify the fully implicit system solves by using an explicit integrator for the nonstiff components. ...
We propose entropy-preserving and entropy-stable partitioned Runge-Kutta(RK) methods. In particular we develop entropy conditions for implicit-explicit methods and a class of second-order multirate methods. We extend relaxation ideas for explicit methods to partitioned RK methods. We show that the proposed methods support fully entropy-preserving and entropy-stability properties at a discrete level. Numerical results for ordinary differential equations and the Burgers equation are presented to demonstrate the behavior of these methods.
... Following the work of Streiner (Streiner, Hauptmann, Kühn and Krämer, 2008), a sufficient timestep size corresponding to an azimuth movement of ΔΨ = 5° per timestep was chosen. Before starting the CFD calculation of the prescribed floating motion, four rotor rotations were pre-calculated to ensure that transients had decayed and the wake behind the rotor was developed in the background grid when the motion starts. ...
This paper presents the current major modeling challenges for floating offshore wind turbine design tools. It also describes aerodynamic and hydrodynamic effects due to rotor and platform motions and usage of non-slender support structures. The applicability of advanced potential flow and computational fluid dynamics-based aerodynamic and hydrodynamic simulation methods to represent these effects-exceeding state-of-the-art design tool capabilities-is analyzed and the results are presented. Different techniques for the representation of mooring-line dynamics, including quasi-static, finite element, and multibody methods, and their impact on global system loads are investigated. Conclusions are drawn about the importance of the relevant effects, strengths and weaknesses of the different methods are discussed, and development needs of future tools are described. Copyright © 2011 by the International Society of Offshore and Polar Engineers (ISOPE).
The low-frequency emissions from a generic 5MW wind turbine are investigated numerically. In order to regard airborne noise and structure-borne noise simultaneously, a process chain is developed. It considers fluid–structure coupling (FSC) of a computational fluid dynamics (CFD) solver and a multi-body simulations (MBSs) solver as well as a Ffowcs-Williams–Hawkings (FW-H) acoustic solver. The approach is applied to a generic 5MW turbine to get more insight into the sources and mechanisms of low-frequency emissions from wind turbines. For this purpose simulations with increasing complexity in terms of considered components in the CFD model, degrees of freedom in the structural model and inflow in the CFD model are conducted. Consistent with the literature, it is found that aeroacoustic low-frequency emission is dominated by the blade-passing frequency harmonics. In the spectra of the tower base loads, which excite seismic emission, the structural eigenfrequencies become more prominent with increasing complexity of the model. The main source of low-frequency aeroacoustic emissions is the blade–tower interaction, and the contribution of the tower as an acoustic emitter is stronger than the contribution of the rotor. Aerodynamic tower loads also significantly contribute to the external excitation acting on the structure of the wind turbine.
The low-frequency emissions from a generic 5MW turbine are investigated numerically. In order to regard airborne noise and structure-borne noise simultaneously a process chain was developed. It considers fluid-structure coupling (FSC) of a computational fluid dynamics (CFD) solver and multibody simulations (MBS) solver as well as a Ffowcs Williams-Hawkings (FW-H) acoustic solver. The approach was applied to a generic 5MW turbine to get more insight into the sources and mechanisms of low-frequency emissions from wind turbines. For this purpose simulations with increasing complexity in terms of considered components in the CFD model, degrees of freedom in the structural model and inflow in the CFD model were conducted. Consistent with literature, it has been found that aeroacoustic low-frequency emission is dominated by the blade-passing frequency harmonics. The tower base loads, which excite seismic emission, tend to be dominated by structural eigenfrequencies with increasing complexity of the model. The main source of aeroacoustic emissions is the blade-tower interaction and the contribution of the tower as an acoustic emitter is stronger than the contribution of the rotor. Aerodynamic tower loads also significantly contribute to the external excitation acting on the structure of the wind turbine.
In the design, certification, and optimization of offshore wind turbines, extensive loads simulation is inevitable to develop reliable and cost‐effective turbines. Description of the marine environment is based on a variety of techniques taking the stochastic nature of both, the wind and the water waves into account. The wind turbine is a highly dynamic system including effects of heavy rotating machinery and other significant nonlinearities leading to static, cyclic, transient, and stochastic loads. Due to the nature of the external loading, the system properties and the turbine design lifetime, offshore wind turbines are prone to fatigue‐driven failure. For loads assessment, aero‐hydro‐servo‐elastic tools are used including the coupled effects of the environment and the turbine to simulate the overall lifetime of the turbine in the harsh marine environment. These tools are constantly further developed and adapted to the needs of a fast‐growing industry and newly arising turbine concepts. Engineering approaches to allow for certification and reliable design are summarized in extensive guidelines and standards supporting engineers in daily design work.
This article is categorized under: Wind Power > Science and Materials
Wind Power > Systems and Infrastructure
There is an increasing interest to refine the numerical simulation of the unsteady aerodynamics, the wake development and its interaction with atmospheric boundary layer and gust to finally predict the transient loads and extreme loads acting on the blades of turbines in the first row and the ones behind. For this purpose a CFD based process chain is being developed and applied at the institute of aerodynamics and gasdynamics (IAG), University of Stuttgart. The process chain consists of a grid generator with scripting capability for automated meshing, the block structured flow solver FLOWER, which is provided by the German Aerospace Center (DLR) and optional of a multibody simulation code with a grid morpher for aeroelastic simulations. The aeroelastic coupling scheme is described in [1] along with application examples. A library of scripts has been developed that enables a fast generation of high-quality structured RANS meshes by means of the commercial mesh generator Gridgen. The scripts include a parametric representation of the blade geometry and the possibility to control the discretisation and Reynolds-dependent mesh refinement. With this approach it is possible to create case optimized blade grids on a desktop computer within a few minutes. The meshing of the blades bases on airfoil sections of the blade geometry. Therefore the number of cuts and their location on the geometry are defined manually. The airfoil sections are then directly extracted from an IGES-file which contains the blade geometry and imported to the meshing process. The blade boundary layer is fully resolved by about 30 cells in wall normal direction with an anticipated nominated wall distance of the first cell of about y + = 1 even in span width. To enable better quality the mesh is twisted to fit to the local incidence of each airfoil section. Moreover the mesh in the near wake of the blades is refined to reduce the numerical dissipation and to enable a detailed simulation of the wake development. An example of a meshed blade and its IGES geometry is shown in Fig. 1.
The phenomenon of wake interaction between two wind turbines was analysed using the actuator line technique and full unsteady Navier–Stokes computations. Results are presented for varying mutual distances between the two turbines and both full wake and half wake situations were considered. Furthermore, simulations were carried out at different degrees of ambient turbulence intensity representing laminar, offshore and onshore conditions. From the simulations, the main characteristics of the interacting wakes were extracted including the averaged velocity and turbulence fields as well as the development of wake generated vortex structures. Moreover, the influence of the wake of the upstream turbine on the external aerodynamic loading on the blades of the downstream turbine was studied. Copyright © 2010 John Wiley & Sons, Ltd.
The Blade Element Momentum theory is based on the work of Glauert who formulated the equations to derive thrust, torque and power as a function of the tip speed ratio λ and axial induction. The moment of momentum applied to the wake, is taken into account in his theory with one major simplification. The radial pressure distribution required to maintain the swirl in the wake is ignored, and not included in the momentum balance and the final results. For high λ the swirl is negligible so this simplification has no effect, but for low λ this is questionable. After Glauert, many authors have published solutions for the low λ regime of discs with constant circulation, all with different assumptions on the radial distribution of flow quantities, the momentum balance or the impact of the infinite pressure in case of a discrete vortex at the disc axis. A common property of the results is that the power coefficient Cp increases above the Lanchester-Betz-Joukowsky limit. As for most other authors, the present analysis proceeds from actuator discs with constant circulation, yielding a wake flow characterized by a singular vortex at the downstream disc axis. Without any further assumption, the impact of the radial pressure distribution on the momentum balance and the Bernoulli equation is taken into account, as is the effect of the infinite pressure at the disc axis due to the discrete vortex. The results are presented in analytical expressions for the axial and azimuthal velocities through the disc and in the far wake, and similarly for the thrust, power and torque. The inclusion of the pressure-due-to- swirl gives Vaxial-disc > 0.5(Vwake+Vundisturbed) and the maximum power coefficient indeed tends to infinity for λ→0. Furthermore, the maximum Cp-value is always greater than the Lanchester-Betz-Joukowsky limit at 16/27. The explanation for this is that the axial velocity through the disc becomes larger than the undisturbed velocity: air at a radial distance that is larger than the disc radius is sucked through the disc due to the under-pressure induced by the swirl. This behavior has been the subject of many considerations and explanations. Glauert (1935) stated that the condition of constant circulation cannot be fully realized in practice since it implies that near the roots of the blades the angular velocity imparted to the air is greater than the angular velocity of the propeller itself. Other investigators, such as de Vries (1979) and Wilson and Lissaman (1978), dealt the view point of Glauert that the solution is unphysical as it results in infinite values of power and circulation when the tip speed ratio tends to zero. In a recent work, Sharpe (2004) argues that the theory in principle establishes that there is no loss of efficiency associated with the rotating wake and that it is possible, at least in theory, to exceed the Lanchester-Betz-Joukowsky limit. Thus, still today, there seems not be fully agreement in the validity of the model.
Actuator disc calculations can be divided in two categories: force models where, for a prescribed force field, the flow is calculated using a CFD method, and kinematic models, where the wake is calculated based on wake boundary conditions and the force field is known when the velocities are known. In both categories, but specifically for the kinematical models, results are reported that differ some 10% from momentum models. Furthermore, most calculations which give details about the flow through the disc do not satisfy the condition derived by Xyros & Xyros (2007) that the axial velocity through the disc is uniform for discs with a uniform surface load. Apart from this, the inconsistency in the momentum models discussed by van Kuik (2003) is still unresolved. These observations raise the questions: what is the relation between force- and flow field, what are the requirements for a steady axisymmetric force field to generate vorticity in an Euler flow?
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.
Two new versions of the kappa-omega two-equation turbulence model will be presented. The new Baseline (BSL) model is designed to give results similar to those of the original kappa-omega model of Wilcox, but without its strong dependency on arbitrary freestream values. The BSL model is identical to the Wilcox model in the inner 50% of the boundary-layer but changes gradually to the standard kappa-epsilon model (in a kappa- omega formulation) towards the boundary-layer edge. The free shear layers. The second version of the model is called Shear-Stress Transport (SST) model. It is a variation of the BSL model with the additional ability to account for the transport of the principal turbulent shear stress in adverse pressure gradient boundary-layers. The model is based on Bradshaw's assumption that the principal shear-stress is proportional to the turbulent kinetic energy, which is introduced into the definition of the eddy-viscosity. Both models are tested for a large number of different flowfields. The results of the BSL model are similar to those of the original kappa-omega model, but without the undesirable freestream dependency. The predictions of the SST model are also independent of the freestream values but show better agreement with experimental data for adverse pressure gradient boundary-layer flows.
On the Timewise Accuracy of Staggered Aeroelastic Simulations of Rotary Wings
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