Predictions of unsteady HAWT aerodynamics in yawing and pitching using the free vortex method

To predict the unsteady aerodynamic loads of horizontal-axis wind turbines (HAWTs) during operations under yawing and pitching conditions, an unsteady numerical simulation method is proposed. This method includes a nonlinear lifting line method to compute the aerodynamic loads on the blades and a time-accurate free-vortex method to simulate the wake. To improve the convergence property in the nonlinear lifting line method, an iterative algorithm based on the Newton–Raphson method is developed. To increase the computational efficiency and the accuracy of the calculation, a new wake vortex model consisting of the vortex core model, the vortex sheet model and the tip vortex model is used. Wind turbines with different diameters, such as NREL Phase VI, the TU Delft model turbine and the Tjæreborg wind turbine, are used to validate the method for rotors operating at given yaw and/or pitch angles and during yawing and/or pitching processes at different wind speeds. The results, including the blade loads, the rotor torque and the locations of the tip vortex cores in the wake, agree well with the measured data and the computed data. It is shown that the proposed method can be used for predictions of unsteady aerodynamic loads and rotor wakes in the operational processes of blade pitching and/or rotor yawing.     

实度对低风速风电机组风轮气动性能影响研究

为提高低风速地区的风能利用率,研究风轮实度对低风速风电机组气动性能的影响。考虑影响风轮实度因素（叶片数量、弦长及安装角）,设计2组不同弦长叶片与可调安装角轮毂。安装角改变时不仅会引起实度变化,还会使叶尖速比发生改变。通过车载试验验证安装角不同时对风轮气动性能的影响主要与叶尖速比相关。根据不同风轮表面压力分布数值模拟结果得出：相同风速下,弦长由叶根到叶尖逐渐增大的叶片更易启动。相同条件下,试验机组输出功率与数值模拟机组输出功率最大相差5.37%,说明数值模拟结果可信。随着风轮实度的增加,风速5 m/s时,其风能利用系数呈增大趋势,风速8 m/s时,其风能利用系数呈减小趋势,两趋势相交时实度为25.38%,得出该实度下风轮气动性能较优,即可得到适合低风速地区的风轮实度。     

Experimental measurements of the hydrodynamic performance and structural loading of the Transverse Horizontal Axis Water Turbine: Part 3

This paper is the third of three, which describe the procedures and results for a set of experiments on various configurations of the Transverse Horizontal Axis Water Turbine (THAWT), which is a horizontally orientated variant of the Darrieus cross-flow turbine. Tests were conducted in the combined wind, wave and current tank at Newcastle University on a 0.5 m diameter rotor, while the flow depth and velocity were varied over a range of realistic Froude numbers for tidal streams. Various configurations of the device were tested to assess the merits of varied blade pitch, rotor solidity, blockage ratio and truss oriented blades. Experiments were carried out using a speed-controlled motor/generator, allowing quasi-steady results to be taken over a range of tip speed ratios. Measurements of power, thrust, blade loading and free surface deformation provide extensive data for future validation of numerical codes and demonstrate the ability of the device to exceed the Lanchester–Betz limit for kinetic efficiency by using high blockage. This paper covers the instrumentation, hydrodynamic performance and loading of the truss bladed variant of the THAWT device. The first paper covers the experimental setup and hydrodynamic performance of the parallel bladed rotor and the second paper covers the instrumentation and hydrodynamic loading of the parallel bladed rotor.     

Wind turbine aerodynamics and loads control in wind shear flow

Wind turbine is subjected to some asymmetrical effects like wind shear, which will lead to unsteady blade airloads and performance. Fatigue loads can lead to damage of turbine components and eventually to failures. It is evident that the variation of the velocity over the rotor disc has an influence on the blade and introduces both flap-wise and edge-wise fatigue damage on the blade as a result of moment fluctuations in the two directions. The flap-wise moments on the blade are the origin of the rotor yaw and tilt moments which transmit to the turbine structure through the drive train to the yaw system and the tower. A lifting surface method with time marching free wake model is used to investigate the periodic unsteady nature in the wind shear. Individual pitch control (IPC) that is applied nowadays is the most advanced active control to reduce the fatigue. The blade airloads and performance of the turbine are also predicted under IPC control. It is found that IPC of the fluctuating blade root flap-wise moment can reduce the flap-wise fatigue damage remarkably while the blade root edge-wise moments are less sensitive to the varying blade pitch than the blade root flap-wise moments.     

Experimental measurements of the hydrodynamic performance and structural loading of the transverse horizontal axis water turbine: Part 2

This paper is the second of three, which outline the procedures and results for a set of experiments on various configurations of the Transverse Horizontal Axis Water Turbine (THAWT), which is a horizontally orientated variant of the Darrieus cross-flow turbine. Tests were conducted in the combined wind, wave and current tank at Newcastle University on a 0.5 m diameter rotor, while the flow depth and velocity were varied over a range of realistic Froude numbers for tidal streams. Various configurations of the device were tested to assess the merits of varied blade pitch, rotor solidity, blockage ratio and truss oriented blades. Experiments were carried out using a speed controlled motor/generator, allowing quasi-steady results to be taken over a range of tip speed ratios. Measurements of power, thrust, blade loading and free surface deformation provide extensive data for future validation of numerical codes and demonstrate the ability of the device to exceed the Lanchester–Betz limit for kinetic efficiency, by exploiting high blockage. This second paper covers the instrumentation and analysis for the structural loading for the parallel bladed variant of the THAWT device. The first paper covers the experimental setup and hydrodynamic performance of the parallel bladed rotor, and the third paper covers both performance and loading of the truss configured THAWT device.     

Experimental measurements of the hydrodynamic performance and structural loading of the Transverse Horizontal Axis Water Turbine: Part 1

This paper is the first of three, which outline the procedures and results for a set of experiments carried out on various configurations of the Transverse Horizontal Axis Water Turbine (THAWT), which is a horizontally orientated variant of the Darrieus cross-flow turbine. Tests were conducted in the combined wind, wave and current tank at Newcastle University on a 0.5 m diameter rotor, while the flow depth and velocity were varied over a range of realistic Froude numbers for tidal streams. Various configurations of the device were tested to assess the merits of varied blade pitch, rotor solidity, blockage ratio and truss oriented blades. Experiments were carried out using a speed controlled motor/generator, allowing quasi-steady results to be taken over a range of tip speed ratios. Measurements of power, thrust, blade loading and free surface deformation provide extensive data for future validation of numerical codes and demonstrate the ability of the device to exceed the Lanchester-Betz limit for kinetic efficiency by using high blockage. This paper covers the experimental procedures and results for the hydrodynamic performance for the parallel bladed variant of the THAWT device. The second paper covers the hydrodynamic loading of the parallel bladed rotor and the third covers both hydrodynamic performance and loading of the truss configured THAWT device.     

A hybrid actuator disc – Full rotor CFD methodology for modelling the effects of wind turbine wake interactions on performance

The performance of individual wind turbines is crucial for maximum energy yield, however, their performance is often reduced when turbines are placed together in an array. The wake produced by the rotors interacts with downstream turbines, resulting in a reduction in power output. In this paper, we demonstrate a new and faster modelling technique which combines actuator disc theory, modelled using wind tunnel validated Computational Fluid Dynamics (CFD), and integrated into full rotor CFD simulations. This novel hybrid of techniques results in the ability to analyse performance when simulating various array layouts more rapidly and accurately than using either method on its own.It is shown that there is a significant power reduction from a downstream turbine that is subjected to the wake of an upstream turbine, and that this is due to both a reduction in power in the wind and also due to changes in the aerodynamics. Analysis of static pressure along the blade showed that as a result of wake interactions, a large reduction in the suction peak along the leading edge reduced the lift generated by the rotor and so reduced the torque production and the ability for the blade to extract energy from the wind.     

Modelling the aerodynamics of vertical‐axis wind turbines in unsteady wind conditions

Most numerical and experimental studies of the performance of vertical‐axis wind turbines have been conducted with the rotors in steady, and thus somewhat artificial, wind conditions—with the result that turbine aerodynamics, under varying wind conditions, are still poorly understood. The vorticity transport model has been used to investigate the aerodynamic performance and wake dynamics, both in steady and unsteady wind conditions, of three different vertical‐axis wind turbines: one with a straight‐bladed configuration, another with a curved‐bladed configuration and another with a helically twisted configuration. The turbines with non‐twisted blades are shown to be somewhat less efficient than the turbine with helically twisted blades when the rotors are operated at constant rotational speed in unsteady wind conditions. In steady wind conditions, the power coefficients that are produced by both the straight‐bladed and curved‐bladed turbines vary considerably within one rotor revolution because of the continuously varying angle of attack on the blades and, thus, the inherent unsteadiness in the blade aerodynamic loading. These variations are much larger, and thus far more significant, than those that are induced by the unsteadiness in the wind conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

Application of the actuator surface concept to wind turbine rotor aerodynamics

The structure of blade tip vortices is recognized as a key issue in wind turbine aerodynamic modelling by many researchers in the field. In the search for an intermediate model between full Navier–Stokes and blade‐element momentum simulations, this article presents a method using rotating actuator surfaces to model wind turbine aerodynamics. An actuator surface is a simple planar surface, porous to the flow, which is characterized by velocity and pressure discontinuities, whose action on the flow is achieved through an attached system of forces. These discontinuities and forces are determined from blade‐element analysis and the Kutta–Joukowski relation. After implementing this concept in a three‐dimensional CFD (Computational Fluid Dynamics) method, results are produced for the experimental rotors of NREL and TUDelft. The method is validated against both experimental measurements and the predictions of three other numerical models for wind turbine aerodynamic analysis. Qualitative and quantitative comparisons show that the actuator surface concept agrees well with the other numerical models. In addition to rotor aerodynamic analysis, the actuator surface concept can be used in the study of wake aerodynamics, or as the Eulerian flow solver in hybrid methods. Copyright © 2009 John Wiley & Sons, Ltd.     

Investigation of potential extreme load reduction for a two‐bladed upwind turbine with partial pitch

This paper presents a wind turbine concept with an innovative design combining partial pitch with a two‐bladed (PP‐2B) turbine configuration. Special emphasis is on extreme load reduction during storm situations at standstill, but operational loads are also investigated. In order to compare the loads and dynamics of the PP‐2B turbine, a partial pitch three‐bladed (PP‐3B) turbine and a normal pitch regulated three‐bladed (3B) turbine are introduced on the basis of solidity similarity scaling. From the dynamic comparisons between two‐ and three‐bladed turbines, it has been observed that the blade vibrations are transferred differently from the rotor to the tower. For a three‐bladed turbine, blade vibrations seen in a fixed frame of reference are split with ±1P only. A two‐bladed turbine has a similar split of ±1P but also includes contributions on higher harmonics (±2P, ±3P, … etc.). Further on, frequency split is also seen for the tower vibrations, where an additional ±2P contribution has been observed for the two‐bladed turbine. Regarding load comparisons, the PP‐2B turbine produces larger tower load variations because of 2P excitation during the operational cases. However, extreme loads are reduced by approximately 20% for the PP‐2B and 18% for the PP‐3B compared with the 3B turbine for the parked condition in a storm situation. Moreover, a huge potential of 60% is observed for the reduction of the extreme tower bottom bending moment for the PP‐2B turbine, when the wind direction is from ±90° to the turbine, but this also requires that the turbine is parked in a T‐configuration. © 2014 The Authors. Wind Energy published by John Wiley & Sons, Ltd.     

On the aerodynamics of variable-geometry oval-trajectory Darrieus wind turbines

A new computational model for the aerodynamics of vertical-axis wind turbines is introduced. It is based on the double-multiple streamtube concept and it incorporates the capacity of dealing with rotors whose blades follow oval-trajectories at variable setting-angles. We applied this model to the study of the aerodynamics of an innovative concept in extra-large wind-power plants: the VGOT (variable-geometry oval-trajectory) Darrieus wind turbine. Due to the especial geometric characteristics of the VGOT Darrieus, it was necessary to propose three new non-dimensional parameters to quantify its performance under different wind-conditions: the equivalent power coefficient, the equivalent solidity coefficient and the trajectory efficiency. We show some numerical results testing several rotor configurations working under different wind scenarios.     

Helical vortex theory and blade element analysis of multi-bladed windmills

Multi-bladed windmills usually pump water for agriculture and domestic consumption, often in remote locations. Although they have been around for over 150 years, their aerodynamic performance is still poorly understood. This paper describes the use of helical vortex theory (HVT) and blade element momentum (BEM) analysis to predict windmill thrust, torque, and extracted power. We emphasize the unusual features of windmills: low Reynolds numbers and tip speed ratios and high solidity, all related to the generation of high torque at low wind speeds. Wind tunnel tests on a model rotor with 3, 6, 12, and 24 circular-arc, constant-chord blades determined the thrust, torque, and extracted power over a range of tip speed ratio that extended to runaway. For comparison, BEM was implemented with a correction for finite blade number derived from HVT, as well as the classical Prandtl tip loss factor. The HVT correction predicted the rotor power coefficient to within 3% of the test data on the average. At low tip speed ratios and smaller blade numbers, HVT was consistently more accurate than the Prandtl factor. At all blade numbers, the measured rotor torque exceeded the BEM predictions at the lowest tip speed ratios indicating stall delay which became more important (and more beneficial for windmill performance) as the blade number increased. The Prandtl formulation predicted the thrust to within a mean accuracy of 13% and was more accurate than the HVT method.     

Performance enhancement and load reduction of a 5 MW wind turbine blade

A wind turbine rotor blade, based on the U.S. National Renewable Energy Laboratory (NREL) 5 MW reference turbine, is optimized for minimum cost of energy through simultaneous consideration of aerodynamics and bend-twist coupling. Eighty-three total design variables are considered, encompassing airfoil shapes, chord and twist distributions, and the degree of bend-twist coupling in the blade. A recently developed method requiring significantly less computation than finite element analysis is used for planning and predicting the bend-twist coupling behavior of the rotor. Airfoil performance is computed using XFOIL, while the wind turbine loads and performance are computed using the NREL FAST code. The objective function is annual cost of energy (COE), where reductions in flapwise bending loads and blade surface area are assumed to decrease rotor cost through reduced material requirements. The developed optimization process projects decreased blade loads while maintaining wind turbine performance.     

Kinetic energy extraction of a tidal stream turbine and its sensitivity to structural stiffness attenuation

The hydrodynamic forces imparted on a tidal turbine rotor, whilst causing it to rotate and hence generate power, will also cause the blades to deform. This deformation will affect the turbine's performance if not included in the early design phase and could lead to a decrease in power output and a reduction in operational life. Conversely, designing blades to allow them to deform slightly may reduce localised stress and therefore prolong the life of the blades and allow the blades to deform in to their optimum operational state. The aim of this paper is to better understand the kinetic energy extraction by varying the material modulus of a turbine blade. Shaft torque/power, blade tip displacement, and axial thrust results are presented for 2, 3 and 4 bladed rotor configurations at peak power extraction. For the rotor design studied the FSI model data show that there is a low sensitivity to blade deformation for the 2, 3 and 4 bladed rotors. However, the results reveal that the 3 bladed rotor displayed maximum hydrodynamic performance as a rigid structure which then decreased as the blade deformed. The 2 and 4 bladed rotor configurations elucidated a slight increase in hydrodynamic performance with deflection.     

A full-scale prediction method for wind turbine rotor noise by using wind tunnel test data

The development of a low-noise wind turbine rotor and propeller is often cost-effective and is in fact a race against time to those who wish to build and test a small-scale rotor instead of an expensive full-scale rotor. The issue of this approach has to do with the interpretation of wind tunnel model test data in terms of both the frequency band and sound pressure level information for the noise scaling effect.This paper discusses a prediction method for the estimation of the noise generated from a full-scale wind turbine rotor using wind tunnel test data measured with both a small-scale rotor and a 2D section of the blade. The 2D airfoil self-noise and the scaled rotor noise were investigated with a series of wind tunnel experiments. Wind tunnel data post-processing considered four aspects: removal of the test condition effect, scaling to full scale, consideration of the wind turbine rotor operating conditions, and the most important terms of full-scale rotor noise as adjustments to address the differences between the wind tunnel test conditions and the full-scale operating conditions.A full-scale rotor noise prediction results comparison was performed by initially dividing the test conditions into the condition of a 2D section noise test and the condition of a small-scale rotor noise test. Based on an airfoil section, the rotor was selected from a blade section at r/R = 0.75. The small-scale rotor was scaled down by a factor of 5.71 for the wind tunnel test.Finally, the full-scale rotor noise data was compared with the wind tunnel test data using a scaling estimation method.     

Analysis of aerodynamic load on straight-bladed vertical axis wind turbine

This paper presents a wind tunnel experiment for the evaluation of energy performance and aerodynamic forces acting on a small straight-bladed vertical axis wind turbine (VAWT) depending on several values of tip speed ratio. In the present study, the wind turbine is a four-bladed VAWT. The test airfoil of blade is symmetry airfoil (NACA0021) with 32 pressure ports used for the pressure measurements on blade surface. Based on the pressure distributions which are acted on the surface of rotor blade measured during rotation by multiport pressure-scanner mounted on a hub, the power, tangential force, lift and drag coefficients which are obtained by pressure distribution are discussed as a function of azimuthally position. And then, the loads which are applied to the entire wind turbine are compared with the experiment data of pressure distribution. As a result, it is clarified that aerodynamic forces take maximum value when the blade is moving to upstream side, and become small and smooth at downstream side. The power and torque coefficients which are based on the pressure distribution are larger than that by torque meter.     

Dynamic characteristics and enhanced performance of a novel wind turbine rotor analyzed via experiments and numerical simulation

The enormous demand for large wind turbine rotors has led to a need to develop high‐performance and reliable wind turbine rotors. The flexibility of the huge blade was a challenge in creating a balanced design with regard to dynamic behavior, mass, and power output. To enhance the wind turbine rotor, a newly designed wind turbine system with a supporting rod and damper was proposed and investigated. A scaled blade was experimentally tested, with the results indicating an increase in both frequency and damping of the system. Through the use of a self‐coded numerical model, the correlations between the design constraints and the dynamic behavior, tip displacement, and additional mass of the rotor were demonstrated. This showed that the novel rotor has some preferable characteristics in both static and dynamic aspects. In particular, this blade is stiffer and has a smaller tip displacement compared with a traditional cantilevered blade. These characteristics enabled the effective application of the novel rotor to a 5‐MW wind turbine to achieve a 15.16% power output increase based on the blade element momentum theory with Prandtl correction, as well as 5.1% mass savings. Copyright © 2016 John Wiley & Sons, Ltd.     

Experimental investigation of the influence of solidity on the performance and flow field aerodynamics of vertical axis wind turbines at low Reynolds numbers

This paper shows the results of an experimental investigation into the effect of changes in solidity on the performance of a Vertical Axis Wind Turbine. Two VAWT configurations are used, one of solidity σ = 0.26 (chord C = 0.03 m) and the other with σ = 0.34 (C = 0.04 m). The turbine performance coefficient (C_p) was measured over a range of tip speed ratios and Particle Image Velocimetry (PIV) was used to determine the flow field around both turbine configurations.Performance (C_p–λ) curves for the two VAWTs are compared at the same Reynolds numbers to investigate the effects of solidity alone on the performance and aerodynamics of each configuration. The higher solidity (σ = 0.34) VAWT attained a similar maximum C_p but with a narrower C_p–λ curve than the lower solidity VAWT. The performance differences between the two VAWT configurations at two tip speed ratios are explained in detail using PIV around both VAWT rotor blades. This allows the linking of detailed aerodynamics to the performance and it was shown that the generation and shedding of stall vortices started earlier on the lower solidity VAWT than the higher solidity VAWT, thus limiting the rotor efficiency.     

A study of the performance benefits of closely-spaced lateral wind farm configurations

Scaled wind turbine experiments were conducted in order to evaluate the beneficial effect of closely-spaced lateral wind turbine configurations on the performance of a wind farm. Two outer wind turbines were spaced apart with a particular gap distance and the longitudinal setback of a central rotor was varied at each gap width. The turbine placement resulted in tip-to-tip separation distances that ranged from 1 diameter (D) to 0.25D. Additionally, the performance of a wind farm layout in rough and smooth boundary layers, designed to mimic onshore and offshore conditions, respectively, was evaluated. It was observed that a narrow gap between several laterally-aligned rotors creates an in-field blockage effect that results in beneficial flow acceleration through the gap. This increase in speed increases the power output of the central turbine when its longitudinal setback is between 0D and 2.5D. A cumulative increase in power output of 17% was observed when 3 rotors were aligned in a lateral plane with a blade tip separation of 0.5D or 0.25D, compared to the same number of rotors in isolation. While the benefits of closely-spaced wind turbines were observed in both of the tested boundary layers, the performance benefits with a smooth boundary layer were smaller than with a rough boundary layer. These results may lead to new wind farm design methodologies for certain topology- and wind distribution-specific sites and suggest that wind turbines can be closely-spaced in the lateral direction in order to obtain substantial increases in power.     

Investigating the influence of dimensional scaling on aerodynamic characteristics of wind turbine using CFD simulation

Wind turbines are used in a variety of applications with different performance requirements. Investigating the influence of scaling on wind turbine characteristics can pave the way to utilize the experience gained from a smaller turbine for a larger one. In this paper, the effects of wind turbine size on aerodynamic characteristics of a rotor blade are examined using CFD simulation. NREL phase VI wind turbine rotor was simulated in order to validate the results and ensure the accuracy of the CFD model. A 2 MW wind turbine was then chosen as a large turbine and a scaled down model of its rotor was simulated numerically. The results of the simulation were introduced to Similarity Theory relations in order to predict the aerodynamic characteristics of the 2 MW wind turbine. The 2 MW turbine was also simulated and the results of the simulation were compared to predictions of Similarity Theory. It was observed that the results of the simulation completely follow the values predicted by Similarity Theory. Both Similarity Theory predictions and simulation results demonstrated that the torque increases with the cube of change in rotor diameter whereas the thrust value and aerodynamic forces grow with the square of change in diameter.