Wind Turbine Aerodynamics Research Articles

Prediction of Roughness Effects on Wind Turbine Aerodynamics

Wind turbines may suffer power loss after a long operation period due to degradation of the surface structure around the leading edge, by accumulation of contaminants and/or by erosion. For understanding the underlying physics, and validating the available prediction methods, the aerodynamics of smooth and artificially roughened airfoil profiles are investigated by different modelling procedures. A spectrum of turbulence models are applied encompassing RANS, URANS and DES. Two approaches are used to model the roughness. In one approach, the roughness is not geometrically resolved but its effect is modelled via the wall-functions of the turbulence models. In the alternative approach, the roughness structures are geometrically resolved, which necessitates a three-dimensional formulation, whereas the wallfunctions based approach may also be applied in two-dimensions. Computational results are compared with the experimental results of other authors, and the predictive capability of different modelling procedures are assessed.

Improved linearised models of wind turbine aerodynamics and control system dynamics using harmonic linearisation

Where non-linearities are not too strong, linearised frequency-domain approaches offer fast calculations, which can be valuable for preliminary design of wind turbine blades, foundations and floating platforms. But the aerodynamic and control system behaviour of a wind turbine is noticeably non-linear. Here we show for the first time that the technique of harmonic linearisation can reduce error in the approximation of aerodynamic and control system non-linearities, compared to the more common tangent linearisation. After deriving the linearised models, comparing linearised results to non-linear simulations for the NREL 5 MW turbine shows that: (1) harmonic linearisation captures aero-elastic effects and non-linearity in aerodynamic forces, giving a 2–4x reduction in error compared to the tangent linearisation; (2) harmonic linearisation can capture non-linear wake dynamics; and (3) the torque and pitch controller behaviour can be approximated with good results away from the rated wind speed but with some challenges when the two controllers interact. Further improvements in the linearised model of the control system have been identified. By improving the accuracy of linearised models, harmonic linearisation is a promising means to extend the applicability of frequency-domain approaches for initial design and optimisation of wind turbines.

Iterative Frequency-Domain Response of Floating Offshore Wind Turbines with Parametric Drag

Methods for coupled aero-hydro-servo-elastic time-domain simulations of Floating Offshore Wind Turbines (FOWTs) have been successfully developed. One of the present challenges is a realistic approximation of the viscous drag of the wetted members of the floating platform. This paper presents a method for an iterative response calculation with a reduced-order frequency-domain model. It has heave plate drag coefficients, which are parameterized functions of literature data. The reduced-order model does not represent more than the most relevant effects on the FOWT system dynamics. It includes first-order and second-order wave forces, coupled with the wind turbine structural dynamics, aerodynamics and control system dynamics. So far, the viscous drag coefficients are usually defined as constants, independent of the load cases. With the computationally efficient frequency-domain model, it is possible to iterate the drag, such that it fits to the obtained amplitudes of oscillation of the different members. The results show that the drag coefficients vary significantly across operational load conditions. The viscous drag coefficients converge quickly and the method is applicable for concept-level design studies of FOWTs with load case-dependent drag.

Experimental Investigation of Wind Turbine Emulator for PMSM-Based Energy Conversion Systems

This paper presents a hardware-in-the-loop emulator of wind turbine generator. In order to test the developed control of energy conversion system, emulators are widely used to prototype wind turbine. Indeed, the proposed HIL emulator is based on DC motor controlled by DC-DC converter and taking into account the aerodynamics of wind turbine in different operation modes. Experimental and simulation results show the performance of the proposed hardware-in-the-loop emulator which provide an effective platform to prove the wind turbine control.

Computer Modeling of Wind Turbines: 1. ALE-VMS and ST-VMS Aerodynamic and FSI Analysis

This is the first part of a two-part article on computer modeling of wind turbines. We describe the recent advances made by our teams in ALE-VMS and ST-VMS computational aerodynamic and fluid–structure interaction (FSI) analysis of wind turbines. The ALE-VMS method is the variational multiscale version of the Arbitrary Lagrangian–Eulerian method. The VMS components are from the residual-based VMS method. The ST-VMS method is the VMS version of the Deforming-Spatial-Domain/Stabilized Space–Time method. The ALE-VMS and ST-VMS serve as the core methods in the computations. They are complemented by special methods that include the ALE-VMS versions for stratified flows, sliding interfaces and weak enforcement of Dirichlet boundary conditions, ST Slip Interface (ST-SI) method, NURBS-based isogeometric analysis, ST/NURBS Mesh Update Method (STNMUM), Kirchhoff–Love shell modeling of wind-turbine structures, and full FSI coupling. The VMS feature of the ALE-VMS and ST-VMS addresses the computational challenges associated with the multiscale nature of the unsteady flow, and the moving-mesh feature of the ALE and ST frameworks enables high-resolution computation near the rotor surface. The ST framework, in a general context, provides higher-order accuracy. The ALE-VMS version for sliding interfaces and the ST-SI enable moving-mesh computation of the spinning rotor. The mesh covering the rotor spins with it, and the sliding interface or the SI between the spinning mesh and the rest of the mesh accurately connects the two sides of the solution. The ST-SI also enables prescribing the fluid velocity at the turbine rotor surface as weakly-enforced Dirichlet boundary condition. The STNMUM enables exact representation of the mesh rotation. The analysis cases reported include both the horizontal-axis and vertical-axis wind turbines, stratified and unstratified flows, standalone wind turbines, wind turbines with tower or support columns, aerodynamic interaction between two wind turbines, and the FSI between the aerodynamics and structural dynamics of wind turbines. Comparisons with experimental data are also included where applicable. The reported cases demonstrate the effectiveness of the ALE-VMS and ST-VMS computational analysis in wind-turbine aerodynamics and FSI.

Computational and experimental investigation of the aerodynamics and aeroacoustics of a small wind turbine with quasi-3D optimization

Aerodynamics and aeroacoustics of a small horizontal axis wind turbine are investigated experimentally and computationally. The purpose has been to develop a procedure to predict and optimize the aerodynamic and aeroacoustic performance of such turbines. Aeroacoustic measurements are performed by an acoustic camera, while monitoring the plant’s power output. In the computational analysis, an unsteady, 3D analysis is applied, modelling the turbulence by the Improved Delayed Detached Eddy Simulation (IDDES), and the aeroacoustics by the Ffowcs Williams and Hawkings (FW-H) approach. In the first part of the study, the original turbine blade is analyzed. A satisfactory agreement between the predictions and measurements is observed for the power conversion efficiency and the emitted sound. In the second part, the aerodynamics of the blade is optimized computationally, in the sense of a Quasi-3D approach, by a previously developed automated procedure. In this part, where a large number of 2D blade profiles are analyzed and optimized, a Reynolds Averaged Numerical Simulation approach is adopted for turbulence. In the third part, it is computationally verified (IDDES, FW-H) that the 3D blade, re-constructed based on the Quasi-3D optimization, exhibits a higher efficiency and lower sound emission. It is shown that the latter holds not only for the overall sound pressure, but also for the criteria pertaining to human perception of sound. Application of high-resolution methods with verification by on-site measurements, demonstration of the proposed Quasi-3D optimization procedure to lead to improved aerodynamic and aeroacoustic performance are innovative aspects of the present investigation. The presented, validated procedure can be applied to predict and improve the aerodynamic and aeroacoustic performance of small horizontal axis wind turbines.

Aeroelastic analysis of a floating offshore wind turbine in platform‐induced surge motion using a fully coupled CFD‐MBD method

AbstractModern offshore wind turbines are susceptible to blade deformation because of their increased size and the recent trend of installing these turbines on floating platforms in deep sea. In this paper, an aeroelastic analysis tool for floating offshore wind turbines is presented by coupling a high‐fidelity computational fluid dynamics (CFD) solver with a general purpose multibody dynamics code, which is capable of modelling flexible bodies based on the nonlinear beam theory. With the tool developed, we demonstrated its applications to the NREL 5 MW offshore wind turbine with aeroelastic blades. The impacts of blade flexibility and platform‐induced surge motion on wind turbine aerodynamics and structural responses are studied and illustrated by the CFD results of the flow field, force, and wake structure. Results are compared with data obtained from the engineering tool FAST v8.

Numerical investigation into the blade and wake aerodynamics of an H-rotor vertical axis wind turbine

Blade aerodynamics is of critical importance in the design of an isolated vertical axis wind turbine (VAWT). In contrast, wake aerodynamics plays a crucial role in the optimum placement of multiple VAWTs. In this study, both the blade and the wake aerodynamics of a straight-bladed VAWT are investigated using a three-dimensional computational fluid dynamics (CFD) model. The algebraic wall-modeled large eddy simulation (LES) was used for turbulence modeling. The LES predictions were compared with and had good agreement with the published experimental data. A semi-empirical method was proposed to estimate the convergence time of CFD calculations. Further, guidelines for the LES modeling of VAWTs were provided. Relatively thick airfoils are recommended for VAWTs to reduce fatigue loading on blades. Torque analysis suggested that winds in built environments would benefit the self-starting performance. Comparisons between transient and time-averaged velocity fields indicated a statistically stable wake. Moreover, the blade speed ratio effect on the development of wake velocities was assessed. The underlying causes of the significant wake asymmetry were addressed. The periodical vortex-ring structures, counter-rotating vortical motions, and wake asymmetry were analyzed and concluded as the signature features of H-rotor VAWTs' wake.

Influence of wake asymmetry on wind turbine blade aerodynamic and aeroelastic performance in shear/yawed wind

Wind turbines operate in atmospheric shear layer and often in yawed flow condition, which, among others, produce cyclic fluctuating loads on the blades. The unsteady aerodynamics of wind turbine in both shear and yawed wind are studied in the present work. The rotor aeroelastic behaviors considering blade flexibility are also discussed. An advanced aeroelastic model based on free wake lifting surface model and geometrically exact beam theory is established to conduct the study. The aerodynamic simulations show that the wake is asymmetric in both shear and yawed conditions. Comparison is made between the free wake lifting surface model and the skewed wake correction model used in BEM theory for the induced velocity on blade at different azimuth positions. The correction model for yawed flow in BEM theory seems to overpredict the induced velocity variations. The skewed wake induction in yawed condition causes a phase shift of angle of attack variation as a function of azimuth angle, particularly on the outboard of the blade. In addition, the blade aeroelastic deformations are found to further change the phase of circumferential distribution of aerodynamic loads. Rotor moments are influenced by the phase shifts of aerodynamic loads.

Numerical investigation of the aerodynamics and wake structures of horizontal axis wind turbines by using nonlinear vortex lattice method

Abstract Wind turbines are emerging as one of the most promising and cost-effective renewable energy sources, due to their economical merits and technical maturity. It is important to accurately predict the aerodynamic performance of rotor blades for efficient design of wind turbine. Among the various numerical approaches, the vortex lattice method (VLM) is one of the most suitable models for wind turbine aerodynamics because the wind turbine mostly operates in the subsonic flow. However, it inherently cannot predict the nonlinear aerodynamic characteristics at a high angle of attack. In the current paper, a nonlinear vortex lattice method (NVLM) has been suggested to extend the existing VLM for handling the nonlinear stall and post-stall behaviors. This can be possible by finding a control point in the airfoil where the effective angle of attack is applied. This paper mainly discusses the development and validation of the NVLM for predicting the aerodynamic performances and the wake geometry against the measurements on the MEXICO rotor model. The comparison results show that the aerodynamic loads and tip vortex trajectories computed by NVLM are in significantly good agreement with the measured data. In addition, the complicated and unsteady wake structures are also analyzed using vortex particle method.

Effect of different atmospheric boundary layers on the wake characteristics of NREL phase VI wind turbine

Abstract In this study, the interaction of horizontal axis wind turbine (HAWT) with neutrally stratified atmospheric boundary layer (ABL) and its wake characteristics are investigated. Important wake characteristics of wind turbine such as velocity deficit and turbulence level are analyzed. For this purpose, Unsteady Reynolds-Averaged Navier-Stokes (URANS) using k-e turbulence closure models are performed using commercial Computational Fluid Dynamics (CFD) software called ANSYS FLUENT. Full rotor CFD simulations of the NREL Phase VI wind turbine by virtually placing on a flat surface with different aerodynamic roughness lengths are performed. Discussions on effective modelling of horizontal homogeneity for the undisturbed ABL is included. The influence of inflow ABL's turbulence level in the wake velocity recovery and the ground effect on the wake turbulence intensity (TI) is analyzed. In addition, comparison of rotor aerodynamics of wind turbine in different terrains is performed using pressure coefficient distributions. Finally, the necessity of inclusion of TI recovery in addition to velocity recovery in the wake for the wind farm alignment is discussed.

Improving VAWT performance through parametric studies of rotor design configurations using computational fluid dynamics

Vertical axis wind turbines present several advantages over the horizontal axis machines that make them suitable to a variety of wind conditions. However, due to the complexity of vertical axis wind turbine (VAWT) aerodynamics, available literature on VAWT performance in steady and turbulent wind conditions is limited. This paper aims to numerically predict the performance of a 5 kW VAWT under steady wind conditions through computational fluid dynamics modeling by varying turbine configuration parameters. Two-dimensional VAWT models using a cambered blade (1.5%) were created with open field boundary extents. Turbine configuration parameters studied include blade mounting position, blade fixing angle, and rotor solidity. Baseline case with peak Cp of 0.31 at tip-speed ratio of 4 has the following parameters: mounting position at 0.5c, zero fixing angle, and three blades (solidity = 0.3). Independent parametric studies were carried out and results show that a blade mounting position of 0.7c from the leading edge produces the best performance with maximum Cp = 0.315 while the worst case is a mounting position of 0.15c with peak Cp = 0.273. Fixing angle study reveals a toe-out setting of −1° producing the best performance with peak Cp of 0.315 and the worst setting at toe-in of 1.5° with peak Cp of 0.287. The solidity study resulted in the best case of four blades (solidity = 0.4) with peak Cp = 0.316 and the worst case of two blades (solidity = 0.2) with peak Cp = 0.283.

Comparison of Experimental and Numerically Predicted Three-Dimensional Wake Behavior of Vertical Axis Wind Turbines

The evolution of the wake of a wind turbine contributes significantly to its operation and performance, as well as to those of machines installed in the vicinity. The inherent unsteady and three-dimensional (3D) aerodynamics of vertical axis wind turbines (VAWT) have hitherto limited the research on wake evolution. In this paper, the wakes of both a troposkien and a H-type VAWT rotor are investigated by comparing experiments and calculations. Experiments were carried out in the large-scale wind tunnel of the Politecnico di Milano, where unsteady velocity measurements in the wake were performed by means of hot wire anemometry. The geometry of the rotors was reconstructed in the open-source wind-turbine software QBlade, developed at the TU Berlin. The aerodynamic model makes use of a lifting line free-vortex wake (LLFVW) formulation, including an adapted Beddoes-Leishman unsteady aerodynamic model; airfoil polars are introduced to assign sectional lift and drag coefficients. A wake sensitivity analysis was carried out to maximize the reliability of wake predictions. The calculations are shown to reproduce several wake features observed in the experiments, including blade-tip vortex, dominant and minor vortical structures, and periodic unsteadiness caused by sectional dynamic stall. The experimental assessment of the simulations illustrates that the LLFVW model is capable of predicting the unsteady wake development with very limited computational cost, thus making the model ideal for the design and optimization of VAWTs.

A novel hybrid free-wake model for wind turbine performance and wake evolution

Abstract A new free-wake analysis for wind turbine aerodynamics is developed to accurately predict turbine performance and downstream wake evolution. A key feature is the Constant Circulation Contour Method (CCCM) which is a novel free-wake model for wind turbine wakes. This method characterizes a turbine wake by a number of resultant vortex filaments, avoiding the numeric artifact of a vortex lattice comprised of trailing and shedding vortices. The natural capture of wake roll-up and distortion using CCCM is illustrated and its computational complexity is demonstrated to be lower than Vortex Lattice Method (VLM). For accurate blade downwash calculation, a hybrid free-wake model is developed by combining CCCM with a VLM wake immediately behind turbine blades, which transitions to CCCM further downstream. Important properties of this hybrid wake are discussed and optimized to enhance model accuracy and efficiency. Blade static stall and unsteady effects are included. The hybrid model is validated through comparison to the wind tunnel experiments UAE Phase VI and MEXICO. Simulation examples are presented showing the utilization of this free-wake analysis to investigate wake steering, demonstrating the potential application of this method to wind farm scale wake simulations.

Aerodynamic effects of compressibility for wind turbines at high tip speeds

In the present work two dimensional airfoil computations are used to investigate the effects of compressibility in the tip region of large scale wind turbines of 20 MW+ size. In the past application of incompressible CFD solvers have been wide spread for wind turbine aerodynamics, due to their efficiency and robustness at the near incompressible conditions experienced near the rotor center. With the increasing size of modern wind turbines and the desire to approach high tip speeds, the incompressible assumption might be violated in the tip region of the turbine.To investigate the effects of compressibility and the possibility of correcting incompressible flow solutions using explicit compressibility corrections, a CFD study of 2D airfoil aerodynamics at conditions of a large scale wind turbine is performed. The present study show that classical compressibility corrections can be successfully applied as a post-processing step to incompressible solutions, reducing the error in the predicted lift and drag to within a few percent for attached flow conditions where viscous effects are limited at Mach numbers upto 0.3.

Generating atmospheric turbulence using passive grids in an expansion test section of a wind tunnel

Generating atmospheric turbulence in wind tunnels is an important issue in the study of wind turbine aerodynamics. A turbulent inlet is usually generated using passive grids. However, to obtain an atmospheric-like flow field relatively large length scales (L∼30 cm) and high turbulence intensities (I∼15%) need to be reproduced. In this work, the passive grid technique has been used in combination with a downstream expansion test section in order to investigate the generation of atmospheric like turbulence, with the possibility of varying both the turbulence intensity and the integral length scale of the flow field independently. Four passive grids with different mesh and bar sizes were used with four wind velocities and five downstream measurement positions. It was found that the flow field is isotropic and homogeneous for distances less than what is recommended in literature (x/M∼5). The effect of the expansion on the turbulence characteristics is also investigated in detail for the first time. The study confirms that by adding an expansion test section it is possible to increase both turbulence intensity and integral length scale downstream from the grid with limited impact on the overall flow quality in terms of anisotropy and energy spectra.

Numerical studies on two turbulence models and a laminar model for aerodynamics of a vertical-axis wind turbine

The purpose of this study is the analysis of flow around a one-bladed Darrieus-type wind turbine using computational fluid dynamics (CFD). The rotor geometry consists of a NACA 0015 airfoil with chord length of 0.15 m. Numerical simulations are performed using ANSYS Fluent, employing laminar model and two turbulence models: SST k-ω and RNG k-e. The obtained numerical results of unsteady aerodynamic blade loads are compared with available experimental results from literature. Computed aerodynamic characteristics of normal and tangential forces comply with the experiment results. The RNG k-e turbulence model has a good accuracy in determining aerodynamic blade loads for the upwind and downwind parts of the rotor. The laminar model and the SST k-ω turbulence model slightly overestimate the tangential aerodynamic blade loads at the downwind part of the rotor. An averaged wind turbine velocity profile computed at one rotor radius downstream of the rotor has a Gaussian shape. The steady-state airfoil characteristics are computed for the Reynolds number comparable to the Reynolds number of a moving blade employing the SST k-ω and RNG k-e turbulence models and using the same computational grid as for unsteady simulations of the rotor.

Harmonic balance Navier‐Stokes aerodynamic analysis of horizontal axis wind turbines in yawed wind

AbstractMultimegawatt horizontal axis wind turbines often operate in yawed wind transients, in which the resulting periodic loads acting on blades, drive‐train, tower, and foundation adversely impact on fatigue life. Accurately predicting yawed wind turbine aerodynamics and resulting structural loads can be challenging and would require the use of computationally expensive high‐fidelity unsteady Navier‐Stokes computational fluid dynamics. The high computational cost of this approach can be significantly reduced by using a frequency‐domain framework. The paper summarizes the main features of the COSA harmonic balance Navier‐Stokes solver for the analysis of open rotor periodic flows, presents initial validation results on the basis of the analysis of the NREL Phase VI experiment, and it also provides a sample application to the analysis of a multimegawatt turbine in yawed wind. The reported analyses indicate that the harmonic balance solver determines the considered periodic flows from 30 to 50 times faster than the conventional time‐domain approach with negligible accuracy penalty to the latter.

The unsteady aerodynamics of floating wind turbine under platform pitch motion

The aerodynamic performance of floating platform wind turbines is much more complex than fixed-base wind turbines because of the flexibility of the floating platform. Due to the extra six degrees-of-freedom of the floating platform, the inflow of the wind turbine rotors is highly influenced by the motions of the floating platform. It is therefore of interest to study the unsteady aerodynamics of the wind turbine rotors involved with the interaction of the floating platform induced motions. In the present work, a lifting surface method with a free wake model is developed for analysis of the unsteady aerodynamics of wind turbines. The aerodynamic performance of the NREL 5 MW floating wind turbine under the prescribed floating platform pitch motion is studied. The unsteady aerodynamic loads, the transient of wind turbine states, and the instability of the wind turbine wakes are discussed in detail.

2D Numerical Simulation and Sensitive Analysis of H-Darrieus Wind Turbine

Recently, a lot of attention has been devoted to the use of Darrieus wind turbines in urban areas. The aerodynamics of a Darrieus turbine are very complex due to dynamic stall and changing forces on the turbine triggered by changing horizontal angles. In this study, the aerodynamics of H-rotor vertical axis wind turbine (VAWT) has been studied using computational fluid dynamics via two different turbulence models. Shear stress transport (SST) k-ω turbulence model was used to simulate a 2D unsteady model of the H-Darrieus turbine. In order to complete this simulation, sensitivity analysis of the effective turbine parameters such as solidity factor, airfoil shape, wind velocity and shaft diameter were done. To simulate the flow through the turbine, a 2D simplified computational domain has been generated. Then fine mesh for each case consisting of different turbulence models and dimensions has been generated. Each mesh in this simulation dependent on effective parameters consisted of domain size, mesh quality, time step and total revolution. The sliding mesh method was applied to evaluate the unsteady interaction between the stationary and rotating components. Previous works just simulated turbine, while in our study sensitivity analysis of effective parameters was done. The simulation results closely match the experimental data, providing an efficient and reliable foundation to study wind turbine aerodynamics. This also demonstrates computing the best value of the effective parameter. The sensitivity analysis revealed best value of the effective parameter that could be used in the process of designing turbine. This work provides the first step in developing an accurate 3D aerodynamic modeling of Darrieus wind turbines.Article History: Received :August 19th 2017; Received: December 15th 2017; Accepted: Januari 14th 2018; Available onlineHow to Cite This Article: Saryazdi, S. M. E. and Boroushaki, M. (2018) 2D Numerical Simulation and Sensitive Analysis of H-Darrieus Wind Turbine. Int. Journal of Renewable Energy Development,7(1),23-34https://doi.org/10.14710/ijred.7.1.23-24