DTU 10MW Reference Wind Turbine Research Articles

Experimental and CFD analysis of a floating offshore wind turbine under imposed motions

The rotor of a floating offshore wind turbine experiences intricate aerodynamics due to significant motion in the floating foundation, necessitating a holistic understanding through a synergistic blend of experimental and numerical methodologies. This study investigates rotor loads and the emergence of unsteady phenomena for a floating offshore wind turbine under motion. The approach compares a wind tunnel experimental campaign on a moving scale model with large-eddy simulations. Importantly, both experimental and numerical setups were co-designed simultaneously to match conditions and allow a fair comparison. The experimental setup features a 1:148 scale model of the DTU 10MW reference wind turbine on a six degrees of freedom robotic platform, tested in a wind tunnel. Numerically, the LES code YALES2, employing an actuator line approach undergoing imposed motions, is used. Harmonic motions on one degree of freedom in surge and pitch directions are explored at various frequencies. Thrust force variation aligns with quasi-steady theory for both numerical and experimental results at low frequencies. However, higher frequencies reveal the rise of unsteady phenomena in experiments. Large-eddy simulations, coupled with an actuator line approach, provide additional insights into the near- and mid-wake response to imposed motions. This co-design approach between numerical and experimental tests enhances the comprehension of aerodynamic behaviour in floating offshore wind turbines, offering valuable insights for future designs.

Experimental study of floating wind turbine control on a TetraSub floater with tower velocity feedback gain

We present an experimental study of floating wind turbine control with inclusion of tower motion feedback. The study is conducted for a 1:60 scaled-model of the DTU 10MW reference wind turbine on a design variant of the TetraSub floater. A review of control methods for floating wind turbines is provided, covering both de-tune and tower motion feedback methods. We next detail the implementation in the experimental setup that includes real-time integration and low-pass filtering of the measured tower-top acceleration. We demonstrate how the tower feedback loop is able to stabilize an otherwise unstable land-based controller. The results further show that the stability is maintained for increasing values of the feedback gain until a certain limit. For the chosen control parameters and the lab-generated wind field, which had limited low-frequency energy, we observe that the variations in rotor speed, blade pitch and platform motion are generally larger for the tower loop controller. This finding is not generalizable due to the special inflow conditions and because controller optimization is not performed. For the tower loop controller, a substantial response at the floater roll frequency is identified. This is caused by the influence of blade pitch on the aerodynamic torque in combination with the constant generator torque. Mitigation through generator torque control is proposed.

Fatigue study of an inverse-designed low induction rotor using open-source tools

The current trend in wind turbine design leans toward increasing rotor diameter for increased energy capture. This presents a notable fatigue concern as blade loads increase with blade length. One solution to potentially alleviate the additional blade loading and improve the fatigue behavior of large-diameter wind turbines, while also reducing overall cost of energy, is the low-induction rotor. The purpose of this work is to perform a holistic analysis to assess the attainable benefits of a notional low-induction rotor variant of the DTU 10MW Reference Wind Turbine using multi-disciplinary open-source design and analysis tools. The notional low-induction rotor blade is designed using an inverse-design routine that allows for the direct specification of desired blade properties. To assess viability of the new rotor design, annual energy production and levelized cost of energy are considered along with fatigue at the blade root and the associated drivetrain and tower response to changes in blade loading. It is estimated that the proposed early-concept low-induction rotor design improves annual energy production by 6.35% (or 3 GW-h), resulting in a 4% (0.25 ¢/kW-h) reduction in levelized cost of energy. The low-thrust nature of the low-induction rotor design reduces root flap bending moment fatigue by up to 21.5%, and it is proposed that changes in blade root edgewise fatigue for the longer, heavier blades of the low-induction rotor can be alleviated with mass redistribution.

Dynamic Investigations between Two- and Three-bladed Wind Turbines with Various Yaw, Tilt and Cone Angles

Tilt and cone are included in wind turbine design to increase tower clearance. Yaw misalignments take place often during turbine operation. The combined effects of these parameters modify the relative wind speed experienced by wind turbine blades, thereby altering the turbine’s dynamics. Consequently, power and loads, key design considerations, are affected. In the absence of exhaustive research available on this topic, this paper provides a quantitative analysis on the impact of tilt, cone and yaw on power and loads. The DTU 10MW Reference Wind Turbine and its two-bladed equivalent are evaluated. The results are obtained using the aeroelastic code HAWC2. It is demonstrated that the ultimate and fatigue loads are greater in the two-bladed than in the three-bladed model during operations. Tilt, cone and ±10º yaw reduce the annual energy production by up to 1%. These three parameters generally increase ultimate and fatigue loads. However, the relationship between these parameters and their impact on the bending moments of both wind turbines is complex, and the results are configuration dependent. It is concluded that the combination of these three parameters must be included as part of the design load examination of any target wind turbine.

Potential of Mini Gurney Flaps as a Retrofit to Mitigate the Performance Degradation of Wind Turbine Blades Induced by Erosion

Leading edge erosion of wind turbine blades is still an important challenge for wind energy professionals, both at research and industrial level. While the efficiency and durability of materials and coatings are improving rapidly, it is important to explore innovative solutions at the aerodynamic design level to mitigate the adverse effects of surface erosion. For that, a preliminary analysis on the use of Mini Gurney Flaps (MGFs) is presented in the study. High-fidelity Computational Fluid Dynamic (CFD) simulations are used to evaluate the impact of severe leading edge (LE) erosion on the performance of the FFAW3-241 airfoil on the rotor blade of the DTU 10-MW Reference Wind Turbine, which is used as test case. CFD lift and drag polars of the eroded airfoil show that MGFs are able to partially recover aerodynamic efficiency caused by erosion; this suggested evaluating their use as a retrofit solution for blades that already experienced leading edge erosion damage. When tested on the DTU 10MW RWT blade, results show how, if sized correctly, MGFs perform as predicted: the lift curve is shifted back to its design value and performance is improved with respect to the eroded blade. Moreover, as one would expect, higher than optimal MGFs resulted in excessive lift increases and thus decreased performance, even in the case of LE-erosion. Although these devices behave as intended however, based on the results of this paper, performance decreases are noted at high tip-speed-ratios (TSR), due to the blade operating in off-design conditions.

Efficient multibody modeling of offshore wind turbines with flexible substructures

Offshore wind turbines, especially floating wind turbines, are often simulated assuming rigid substructures to obtain computationally efficient simulation models for preliminary parameter variation studies. This causes large errors in the determination of coupled natural frequencies and internal loads, particularly with increasing turbine sizes. Finite Element models for flexible substructures were developed by several researchers, often resulting in a high simulation effort. In this paper, a modally reduced Finite Element model, precomputed by the SubDyn module of OpenFAST, is directly included in the generalized Equation of Motion of the Simplified Low Order Wind turbine model SLOW. The approach was tested with the DTU10MW reference wind turbine mounted on a flexible monopile. It shows a high agreement with the former beam-based Multibody System in the calculated coupled natural frequencies and steady state results both for the linear and nonlinear model. A basis has been established to integrate flexible bodies of any shape even into computational efficient Multibody Systems of reduced order, such as SLOW, without coupling of two modules as in OpenFAST. This might improve numerical stability due to unified equations of motion.

Multidisciplinary aeroelastic optimization of a 10MW-scale wind turbine rotor targeting to reduced LCoE

In the present paper, multidisciplinary optimization (MDAO) is applied with the aim to reduce the levelized cost of energy (LCoE) of the DTU-10MW Reference Wind Turbine (RWT) rotor. As application paradigm, the widely applied in the literature passive load control method of Bend Twist Coupling (BTC) is considered. The integrated optimization framework combines in a common loop, rotor aerodynamic and full wind turbine structural elasto-dynamic analyses, aiming at determining the optimum rotor diameter, the planform of the blade in terms of twist and chord distributions, the offset ply angle for BTC and the inner structure of the blade with cost function directly the LCoE. It is based on an in-house servo-aero-elastic analysis tool for determining the ultimate loads along the span of the blades and the power yield, whereas a cross-sectional analysis tool is employed for acquiring structural properties of the modified blade and stresses distributions over the blade sections. A cost model of the overall wind turbine is implemented by combining existing in the literature models and open data. The new rotor design is found to have a reduced LCoE by 0.71% and to produce 2.4% higher energy annually due to its increased by 3.7% diameter.

Numerical modal analysis of DTU 10-MW reference wind turbine’s blade using modified Myklestad´s method

Since the rotor blade system of an horizontal axis wind turbine is responsible for converting wind energy into mechanical, which turns into electrical and predicting its dynamic behavior is of vital importance. In that sense, this paper deals with performing a modal analysis of a blade belonging to the DTU 10-MW reference wind turbine by using a modified Myklestad’s method. The blade model was built on two different keystones, as follows: first, considering uncoupled bending in out-of-plane (flapwise) and in-plane (edgewise) directions and considering a coupled bending-torsion motion also in both directions. In order to accomplish these objectives, a Python code was implemented. The computed eigenfrequencies were compared to the results obtained for the blade by using the finite element method. Besides, the mode shapes were plotted and the centrifugal stiffening was also taken into account. Results suggest the feasibility of the modified Myklestad’s method for modal analysis purposes, since good agreement with reference data was achieved and considering that the Myklestad’s method has a considerably less complex implementation than the finite element method.

Simulations of 10MW wind turbine under seismic loadings

In this study, a shell-based finite element model based on Technical University of Denmark 10MW reference wind turbine (DTU 10 MW RWT) is used to study its dynamic responses under seismic loads. The natural vibration frequencies of blades and wind turbines from this study and the results from the literature Bak et al. (2013) are compared. The difference in natural vibration frequencies of the blades arises from simple assigned orientation in composites blades in Bak et al. (2013). The differences in natural vibration frequencies of the wind turbines are suggested to be due to shell elements used in this study whereas beam elements are used in Bak et al. (2013). The built shell-based finite element model of DTU 10MW reference Wind Turbine is then subject to artificially generated earthquake converted from codes of European union earthquake response spectrum. The simulation results show that the foundation has a significant impact on the responses of DTU 10 MW RWT. That is, the DTU 10 MW RWT suffers less stress in the flexible foundation than in the more rigid foundation . The results also show that neglecting the self-weight effect could significantly underestimate the dynamic responses of wind turbines under seismic loadings. This study suggests that shell-element based model should be used in company in the process for certification.

A simplified model for transition prediction applicable to wind-turbine rotors

Abstract. This work aims to develop a simple framework for transition prediction over wind-turbine blades, including effects of the blade rotation and spanwise velocity without requiring fully three-dimensional simulations. The framework is based on a set of boundary-layer equations (BLEs) and parabolized stability equations (PSEs), including rotation effects. An important element of the developed BL method is the modeling of the spanwise velocity at the boundary-layer edge. The two analyzed wind-turbine geometries correspond to a constant airfoil and the DTU 10-MW Reference Wind Turbine blades. The BL model allows an accurate prediction of the chordwise velocity profiles. Further, for regions not too close to the stagnation point and root of the blade, profiles of the spanwise velocity agree with those from Reynolds-averaged Navier–Stokes (RANS) simulations. The model also allows predicting inflectional velocity profiles for lower radial positions, which may allow crossflow transition. Transition prediction is performed at several radial positions through an “envelope-of-envelopes” methodology. The results are compared with the eN method of Drela and Giles, implemented in the EllipSys3D RANS code. The RANS transition locations closely agree with those from the PSE analysis of a 2D mean flow without rotation. These results also agree with those from the developed model for cases with low 3D and rotation effects, such as at higher radial positions and geometries with strong adverse pressure gradients where 2D Tollmien–Schlichting (TS) waves are dominant. However, the RANS and PSE 2D models predict a later transition in the regions where 3D and rotation effects are non-negligible. The developed method, which accounts for these effects, predicted earlier transition onsets in this region (e.g., 19 % earlier than RANS at 26 % of the radius for the constant-airfoil geometry) and shows that transition may occur via highly oblique modes. These modes differ from 2D TS waves and appear in locations with inflectional spanwise velocity. However, except close to the root of the blade, crossflow transition is unlikely since the crossflow velocity is too low. At higher radial positions, where 3D and rotation effects are weaker and the adverse pressure gradient is more significant, modes with small wave angles (close to 2D) are found to be dominant. Finally, it is observed that an increase in the rotation speed modifies the spanwise velocity and increases the Coriolis and centrifugal forces, shifting the transition location closer to the leading edge. This work highlights the importance of considering the blade rotation and the three-dimensional flow generated by that in transition prediction, especially in the inner part of the blade.

Operational and extreme responses of a new concept of 10MW semi-submersible wind turbine in intermediate water depth: An experimental study

A floating wind turbine concept, named as SPIC, is composed of the DTU 10MW reference wind turbine and a newly designed semi-submersible platform featured with three partially inclined side columns. The dynamic responses of the SPIC concept considering an intermediate water depth of 60 m are investigated through the basin model test approach. After calibrating the wind and wave conditions and identifying the model wind turbine system, the operational and short-term extreme responses of the floating wind turbine concept were studied. This innovative concept meets reasonable natural period requirements and achieves great stability under design environmental conditions. For operational responses, low-frequency turbulent wind increases the surge resonance response but provides aerodynamic damping to reduce pitch resonance and tower vibration. The wind also excites the responses at 1P, 2P, and 3P frequencies, but the current slightly suppress these responses. The extreme surge, mooring line tension and nacelle acceleration occur under extreme environments, while the extreme pitch, tower-top shear force and tower-base bending moment should be examined under operational conditions with rated wind as well. These extreme responses are proved to be smaller than the limited values recommended by standards, demonstrating the reasonability of the SPIC concept in intermediate water depth.

Practical Considerations on Wind Turbine Blade Leading Edge Erosion Modelling and its Impact on Performance and Loads

Wind turbines operate in all sorts of weather conditions around the globe, exploiting installation sites with high power production potential. These machines operate within the atmospheric boundary layer and are therefore subject to impacts with rain, hail dust particles and insects, often leading to rapid blade deterioration and performance drops. This wok aims to explore different ways of modelling wind turbine blade leading edge erosion with a medium-fidelity approach and to show how different models impact performance and loads. This provides a quick way of evaluating the effects of blade deterioration. A 2D airfoil erosion model is developed and lift and drag coefficients are calculated with Computational Fluid Dynamics (CFD). An aero-servo-elastic model of the DTU 10MW Reference Wind Turbine (RWT) is then used to evaluate the impacts of the models on performance. Differences in airfoil performance are discussed as well as differences in the turbine’s power coefficient and fatigue loading levels.

Calibration of a super-Gaussian wake model with a focus on near-wake characteristics

Offshore wind turbine near wakes can extend downstream up to 5D due to low atmospheric turbulence intensities. They are characterised by strong velocity deficits, a transitioning Gaussian shape, and strong added turbulence intensities. Classical analytical wake models are still used due to their low computational costs, but they mainly focus on far-wake characteristics. A super-Gaussian wake model valid in near-and far-wake regions has recently been developed at IFP Energies nouvelles. This wake model requires calibration and validation. To this end, large-eddy simulations of the large DTU-10MW reference wind turbine under different neutrally stratified atmospheric flows are carried out with the LES Meso-NH model. A database is generated based on these results and used to calibrate and validate the super-Gaussian model.

Active tip deflection control for wind turbines

This paper studies the use of blade tip sensors for load reductions and blade-tower clearance control. Typically, modern blade tip sensors measure flapwise tip deflection distances at a high sampling rate, and such measurements can be utilised as feedback signals for control operations. Thus, this paper proposes a novel blade pitch control design based on the tip deflection measurements and individual pitch control (IPC). Firstly, an IPC system design is presented, using the tip deflection measurements to alleviate turbine fatigue loads caused by differential loads such as wind shear, yaw misalignment and turbulence. Secondly, a novel implementation of IPC with tip trajectory tracking feature is proposed where the blade tips are guided along a fixed trajectory to maximise blade-tower clearance. The motivation of this implementation is to reduce the chance of blade-tower interactions for large and flexible rotors. The presented controller is implemented in HAWC2, and high fidelity load measurements are produced using the DTU10MW reference wind turbine. The simulation results showed that the fatigue damage reduction on key turbine components and the improved blade-tower clearance can be achieved simultaneously. Lifetime equivalent load reductions were seen in both rotating and fixed frame components under the normal operating conditions.

Variable-speed Variable-pitch control for a wind turbine scale model

The present work deals with the issues faced by authors in the implementation of a variable-speed variable-pitch (VS-VP) controller on a wind turbine scale model for wind tunnel tests. The PoliMi 1/75 model of the DTU 10MW reference wind turbine (RWT) is presented, focusing on the mechatronic system used to control the machine. The main differences between the PoliMi wind turbine model (WTM) and the DTU 10MW RWT are highlighted out pointing out their influence on the behavior of the controlled machine. Then, the control logic adopted for the PoliMi WTM is introduced. Finally, results from wind tunnel tests on the controlled model are shown and the performance of the closed-loop system is discussed.

State-of-the-art model for the LIFES50+ OO-Star Wind Floater Semi 10MW floating wind turbine

This paper describes a state-of-the-art model of the DTU 10MW Reference Wind Turbine mounted on the LIFES50+ OO-Star Wind Floater Semi 10MW floating substructure, implemented in FAST v8.16. The purpose of this implementation is to serve as a reference for different activities carried out within the LIFES50+ project. Attention is given to the changes necessary to adapt the numerical model of the onshore DTU 10MW Reference Wind Turbine to a floating foundation. These changes entail controller, tower structural properties, floating substructure hydrodynamics and mooring system. The basic DTU Wind Energy controller was tuned in order to avoid the “negative damping” problem. The flexible tower was extended down to the still water level to capture some of the floater flexibility. The mooring lines were implemented in MoorDyn, which includes dynamic effects and allows the user to define multi-segmented mooring lines. Hydrodynamics were precomputed in the radiation-diffraction solver WAMIT, while viscous drag effects are captured by the Morison drag term. The floating substructure was defined in HydroDyn to approximate the main drag loads on the structure, keeping in mind that only circular members can be modelled. A first set of simulations for system identification purposes was carried out to assess system properties such as natural frequencies and response to regular waves. The controller was tested in a simulation with uniform wind ranging from cut-in to cut-out wind speed. A set of simulations in stochastic wind and waves was carried out to characterize the global response of the floating wind turbine. The results are presented and the main physical phenomena are discussed. The model will form the basis for further studies in the LIFES50+ project and is available for free use.

NAUTILUS-DTU10 MW Floating Offshore Wind Turbine at Gulf of Maine: Public numerical models of an actively ballasted semisubmersible

This study presents two numerical multiphysics models of the NAUTILUS-10 floating support structure mounting the DTU10 MW Reference Wind Turbine at Gulf of Maine site, and analyses its dynamics. With the site conditions and the FAST model of the onshore turbine as the starting point, the floating support structure: tower, floating substructure with its corresponding active ballast system and station keeping system, was designed by NAUTILUS. The numerical models were developed and the onshore DTU wind energy controller was tuned to avoid the resonance of the operating FOWT by TECNALIA, in the framework of H2020 LIFES50+ project. This concept and its subsystems are fully characterised throughout this paper and implemented in opensource code, FAST v8.16. Here, the mooring dynamics are solved using MoorDyn, and the hydrodynamic properties are computed using HydroDyn. Viscous effects, not captured by radiation-diffraction theory, are modelled using two different approaches: (1) through linear and quadratic additional hydrodynamic damping matrices and (2) by means of Morison elements. A set of simulations (such as, decay, wind only and broadband irregular waves tests) were carried out with system identification purposes and to analyse the differences between the two models presented. Then, a set of simulations in stochastic wind and waves were carried out to characterise the global response of the FOWT.

Comparison of unsteady aerodynamics on wind turbine blades using methods of ranging fidelity

In this paper the unsteady aerodynamic loads for aeroelastic bend-twist coupling predicted by methods of various fidelity are compared. These are: a fast and simplistic strip theory model based on the analytical theories by Greenberg and Loewy, an Euler and a RANS unsteady CFD simulation. The test object is the DTU 10MW reference wind turbine. The comparison uses transfer functions with bending deflections and blade torsion as an input, aerodynamic thrust and pitching moment as an output. A structural feedback is not considered. The RANS simulation has the highest fidelity and is taken as a baseline for all other methods. The temporal discretization and simulation time of the CFD have to be chosen carefully as they influence the transfer behavior. As long as the flow separation is small, the Euler simulation is very accurate. The frequency response by the strip theory has a good agreement over a broad range of flow conditions in attached flow when a distributed torsion or a flap-wise deflection is applied. Severe deviations are found for an edge-wise deformation and in-plane forces.

Airfoil Blender for Blade Optimizations

An airfoil database was developed in the present work. The database includes a multivariate, unstructured grid, Radial Basis Function (RBF), optimization-compatible interpolator for airfoil aerodynamic coefficients. The database was designed to aid complex blade design optimizations carried out in a novel optimization framework, HAWTOpt2, being currently developed at DTU Wind Energy. In the database, the lift, drag and moment coefficients are stored as functions of airfoil family, thickness, Reynolds number and angle of attack. Aerodynamic add-ons and airfoil modifications are included in the database as separate airfoil families. Additionally, the database includes two different 3D correction methods and two different 360 degree extrapolation methods. Further, the database stores airfoil coordinates as functions of airfoil family and thickness. Those coordinates may also be interpolated using the RBF method. Present work included a demonstration of the coefficient interpolator in an optimization aimed at indicating the optimal layout of Vortex Generators on eroded blades of the DTU 10MW Reference Wind Turbine in order to maximize the Annual Energy Production.

Rotor‐tower interactions of DTU 10MW reference wind turbine with a non‐linear harmonic method

AbstractIn this paper, a computational study of the DTU 10MW reference wind turbine unsteady aerodynamics is presented. The whole wind turbine assembly was considered, including the complete rotor and the tower. The FINE/Turbo flow solver developed by NUMECA International was employed for the simulations. In particular, the Non‐Linear Harmonic (NLH) method was applied in order to accurately model flow unsteadiness at reduced computational cost. Important vortex shedding structures were identified at low blade span range and all along the tower height. A strong interaction between rotor and tower flows was also observed. Lastly, the performance of the NLH approach was compared against a standard Unsteady Reynolds‐Averaged Navier Stokes simulation. The same complex unsteady flow phenomena were captured by both technologies. Nevertheless, the NLH approach was found to be 10 times faster than the Unsteady Reynolds‐Averaged Navier Stokes method for this particular application. Copyright © 2016 John Wiley & Sons, Ltd.