Phase VI Blade Research Articles

On the influence of twist and taper of HAWT blades on the rotational augmentation phenomenon: the NREL Phase VI – Phase II comparison

The physics of 3D flows on rotating blades is currently one of the most important research fields related to wind turbines. Although many authors have studied the phenomenon thoroughly and they have proposed meaningful physical explanation of the mechanism which triggers the onset of the rotational augmentation, a universal correlation between rotor geometry, operating conditions and centrifugal pumping effects has not been derived yet. For instance, all the proposed corrections for rotational stall delay within 1D codes have demonstrated fairly good accuracy but in limited operating range or for specific airfoils or rotor geometries. In the present work the authors applied a consolidated methodology, based on the use of accurate CFD 3D models and of an inverse BEM code, to thoroughly analyze the differences in terms of rotational augmentation effects between the two widely known NREL Phase VI and Phase II HAWTs. This choice was made precisely since both the rotors used the S809 airfoil for the active part of the blade, had equal radial dimensions and the operating conditions were approximately the same. Thus, the substantial difference between the rotors was the fact that the Phase VI blade was twisted and tapered while the Phase II one had constant chord and pitch along the radial direction. In this way, the possible influence due to the twist and taper of the blade on the physics of the centrifugal pumping could be highlighted more easily. The CFD models were developed in Ansys Fluent and validated against experimental measurements available in the literature. The inverse BEM code, already implemented in a previous work, allowed the authors to extrapolate and compare sectional data obtained through the CFD simulations. The post-processing of the results demonstrated the strong influence of the twist and taper of the blade on the dynamics of the rotational augmentation. These results lead the way for a better understanding of the relation between rotor geometry and centrifugal pumping physics.

Fluid–Structure Interaction Analysis of a Wind Turbine Blade with Passive Control by Bend–Twist Coupling

The idea of improving the energy output for small wind turbines without compromising the remaining aspects of the technology, such as costs and structural integrity, is very appealing in the context of the growing concern for global warming and the goal of providing electricity to remote and isolated regions. This work aims to contribute to the development of distributed wind generation by exploring the effects of bend–twist coupling on the performance of a wind turbine with a focus on a small rotor based on the NREL Phase VI blade geometry. After defining a structure in composite materials exhibiting the coupling behavior along with a reference counterpart, a comparative numerical analysis is performed using a Fluid–Structure Interaction (FSI) analysis. The main numerical framework is based on commercial software and consists of a finite-volume solver for fluid physics, a finite-element solver for solid physics, and a coupling interface for the interaction problem. The results, complemented with the predictions from a one-way analysis based on the blade-element momentum theory are used to define the increments in rotor torque. The analysis of the annual energy yield shows a 3% increase due to the bend–twist coupling used as a passive pitch mechanism, considering a Rayleigh distribution with an 11 m/s average wind speed. Simultaneously, the coupling causes increments of 0.2% and 0.3% for the blade root flapwise moment and the rotor thrust force, respectively, when considering parked conditions and a simplified extreme wind model.

Performance enhancement of small-scale wind turbine featuring morphing blades

The demand for renewable energy is driven by the depletion and adverse environmental impacts of fossil fuels. There is a growing global consensus for research and development of renewable energy, including wind. In the current study, National Renewable Energy Laboratory (NREL) Phase VI wind turbine blade is integrated with morphing trailing-edge, installed on the aft-30% blade chord, across outboard 75% blade span. The morphing trailing-edge generates unique topology for each wind speed such that the glide ratio is maximized along the blade span. Three-dimensional transient computational fluid dynamics (CFD) analyses are conducted over low to medium wind speeds to investigate the blade aerodynamics. The analyses exhibit significant increments in the low-speed shaft torque and power of the morphed blades compared to the baseline. The integration of morphing trailing-edge high-lift flow control mechanism on the NREL Phase VI blade enhanced energy harvesting and reduced the wind turbine cut-in wind speed. Comparative investigations are also conducted to assess the improvements in thrust, bending moment, and aerodynamic load distribution, as well as alterations in the pressure, flow field, turbulence, surface flow, and wake. The aeroacoustics directivity of the wind turbines exhibits marginal far-field noise increment in case of morphing trailing-edge integrated blades.

Airfoil Shape Optimization of a Horizontal Axis Wind Turbine Blade using a Discrete Adjoint Solver

In this study, airfoil shape optimization of a wind turbine blade is performed using the ANSYS Fluent Adjoint Solver. The aim of this optimization process is to increase the wind turbine output power, and the objective function is to maximize the airfoil lift to drag ratio (Cl/CD ). This study is applied to the NREL phase VI wind turbine, therefore, the S809 airfoil is used as a reference profile. First, for the validation of the applied numerical model, steady-state simulations are carried out for the S809 airfoil at various angles of attack. Then, the optimization is performed with the airfoil set at a fixed angle of attack, , considering three Reynolds numbers, Re =3 105,4.8 105 and 106. Next, computations are performed for the fluid flow around the optimized airfoils at angles of attack AOA= 6.1° ranging from 0° to 20°. The results show that (i) the lift to drag ratios of the optimized airfoils are significantly improved compared to the baseline S809 airfoil, (ii) this improvement is sensitive to the Reynolds number, and (iii) the Cl/CD ratios are also improved for another angle of attack values. Thereafter, the optimized airfoils are used for the design of the NREL Phase VI blade and the aerodynamic performances of this new wind turbine are assessed using the open-source code QBlade. These latter results indicate that when the blades are designed with the optimized airfoils, the wind turbine aerodynamic performances increase significantly. Indeed, at a wind speed of 10 m/s, the power output of the wind turbine is improved by about 38% compared to that of the original turbine.

Rotational effects on the blade flow of a horizontal axis wind turbine under axial and yawed inflow conditions

Unsteady aerodynamic loads and fluctuating output power of wind turbines remain challenging to predict accurately due to the incomplete understanding of blade flow mechanisms. Rotational effects can significantly impact the wind turbine aerodynamic performance. Therefore, this paper presents a careful investigation into rotational flow characteristics of the NREL Phase VI blade under axial and yawed inflow conditions based on the DDES method. Rotational effects change the flow regime from the massive 2D trailing-edge separation into the moderate 3D leading-edge separation on the NREL Phase VI blade. Flow separation is found sufficient but not necessary for the radial flow. Under the axial inflow, rotational effects can effectively suppress the flow separation and increase the nonlinear aerodynamic loads. Under the yawed inflow, rotational effects also effectively suppress the unsteady separated flow and accelerate the flow reattachment, particularly on the downwind side. Sectional lift coefficient is therefore greatly increased at the high angles of attack due to the pronounced rotational effects. Increasing the yaw angle and wind speed may also strengthen the rotational effects and widen the difference between the 2D airfoil flow and 3D sectional flow. These findings imply that flow regime and aerodynamic hysteresis on the inboard blade can be substantially changed in comparison with the 2D airfoil flow due to strong rotational effects. This study might deepen the understanding of rotational effects on the near-wall flow of wind turbine blades and also improve the aerodynamic modelling.

Numerical investigation into rotational augmentation with passive vortex generators on the NREL Phase VI blade

Passive vortex generators (VGs) have been widely used on wind turbines to improve the aerodynamic power. Nevertheless, most studies of VGs are conducted on flat plates and airfoils rather than rotating blades. The effect of VGs on rotational blade flow therefore remains unclear. This paper presents a comparative study between the NREL S809 airfoil flow and NREL Phase VI blade flow, to highlight rotational augmentation with VGs. There are 36 pairs of rectangular VGs installed at about 20% c between 25% and 50% spans of the blade. Fully-resolved RANS simulations are used to identify the airfoil and blade flow characteristics with VGs. Although VGs greatly increase the maximum lift coefficient of airfoil by almost 60%, they cause unexpected decreases in aerodynamic forces on the inboard blade. This is mainly because VGs can reduce the radial flow and thus undermine rotational augmentation. Furthermore, increasing VG size amplifies the VG effect on radial flow and enlarges the separation bubble at 30% span from 43% c to 57% c in height. Positioning VGs in a skewed layout, however, effectively diminishes this negative VG effect and reduces the separation bubble height to 28% c. These findings indicate a huge difference between the airfoil flow and blade flow with VGs due to rotational effects. This work might deepen the understanding of rotational augmentation with VGs on a rotating blade.

Power generation enhancement in a horizontal axis wind turbine blade using split blades

The positive effects of a split on the aerodynamic performance of fixed wings are well known. However, for horizontal axis wind turbines (HAWTs), changes in the local angle of attack (AoA) and relative velocities along the blade create complex three-dimensional flow over blades, making the analysis of the split location very challenging. In this paper, the effects of the radial position of a split are investigated using CFD simulations of five different split versions of the NREL Phase VI blade and the original blade. The CFD models are solved as transient simulations at different tip speed ratios (TSRs). Results demonstrate that the power generated by all of the split blades at any TSR<3.5 is higher than the No-split blade. Also, when the split is located at an outer radius, the split blade generates more power. Besides, it was observed that for this stall-regulated wind turbine, at high winds and especially at a stall condition, a blade in which the split extends from the blade root to the blade tip generates considerably higher power than the No-split blade. Finally, to visualize the physical phenomenon behind each case, flow patterns and streamlines of different cases are compared and discussed.

Effects of blunt trailing-edge optimization on aerodynamic characteristics of NREL phase VI wind turbine blade under rime ice conditions

To reduce the adverse effects of the ice on aerodynamic characteristics, a new NREL Phase VI wind turbine blade which is suitable to rime ice environments is developed through the blunt trailing-edge optimization. The parametric control equations of blunt trailing-edge airfoil are established by adopting the airfoil profile integration theory and B-spline curve, and the curve fitting of the airfoil’s rime ice from LEWICE software is carried out using the linear interpolation algorithm with equidistant and equiangular step lengths. The S809 airfoil under rime ice conditions is optimized to maximize the lift coefficient by the particle swarm optimization (PSO) coupled with GAMBIT and FLUENT, and a NREL Phase VI blade is formed with the optimized airfoil S809-BT (with BT the blunt trailing-edge). The blade’s rime ice is obtained through using the polynomial fitting to deal with projection point coordinates of airfoils’ ice shapes in lagging and flapping surfaces, and the pressure coefficient, flow characteristics, torque and output power of icy sharp and blunt trailing-edge blades are investigated. The results indicate that in rime ice conditions, compared with those of sharp trailing-edge blade, the pressure difference and vortex size of blunt trailing-edge blade become larger, and the torque and output power increase by 4.36 %, 1.55 % and 2.88 % at v= 7 m/s, 15 m/s and 20 m/s, respectively. The research provides significant guidance for improving the aerodynamic performance of wind turbine blade considering the icing effects.

Aerodynamic performance prediction of NREL phase VI blade adopting biplane airfoil

Abstract The NREL Phase VI wind turbine is taken as an example to study the changes in aerodynamics performance and 3D flow structures due to the adoption of the biplane airfoil in the inboard region of the blade. The NREL Phase VI blade is taken as the baseline blade, and the biplane blades are designed by replacing the original inboard region with biplane airfoil. The flows around the blades are simulated by solving the Reynolds averaged Navier–Stokes equations with the Spalart–Allmaras model. Results show that evident performance improvement of biplane blades compared with baseline blade. The rotor flap moment to shaft torque ratios of the biplane blade drop, which proves its applicability in large blades. Detailed analysis of sectional loads along the blades further reveals that the increment in shaft torque of the biplane blades mainly comes from the inboard region, and the gap and stagger of the biplane airfoil have pronounced effects on the performance and flow structures of biplane blades. The biplane blades show a good capacity in suppressing flow separation and radial flow. In addition, the transition section from the biplane airfoil to monoplane airfoil must be carefully designed using 3D simulation approaches to reduce its negative effect.

Aerodynamic Sensitivity Analysis for a Wind Turbine Airfoil in an Air-Particle Two-Phase Flow

In this paper, the Navier-Stokes equations coupled with a Lagrangian discrete phase model are described to simulate the air-particle flows over the S809 airfoil of the Phase VI blade, the NH6MW25 airfoil of a 6 MW wind turbine blade and the NACA0012 airfoil. The simulation results demonstrate that, in an attached flow, the slight performance degradation is caused by the boundary layer momentum loss. After flow separation, the performance degradation becomes significant and is dominated by a more extensive separation due to particles, since the aerodynamic coefficient increments and the moving distance of separation point present similar variation trends with increasing angle of attack. Unlike the NACA0012 airfoil, a most particle-sensitive angle of attack is found in the light stall region for a wind turbine airfoil, at which the lift decrement and the drag increment reach their peak values. For the S809 airfoil, the most sensitive angle of attack is about 3° higher than that for the maximum lift-to-drag ratio. Hence, the aerodynamic performance of a wind turbine is very susceptible to particles. Based on the most sensitive angles of attack, the more sensitive scope of angles of attack of a blade airfoil and the more sensitive range of rotor tip speed ratios are predicted sequentially. The present study clarifies the principles for the performance degradation of a wind turbine airfoil due to particles and the conclusions are useful for the wind turbine design reducing the particle influences.

Effects of leading edge slat on flow separation and aerodynamic performance of wind turbine

The flow separation could affect the aerodynamic performance of the wind turbine blades and induce to occur the stall of the wind turbine blades at high angle of attack. The leading edge slat could effectively control and delay the flow separation. In this paper, the effects of the leading edge slat on the aerodynamic performance of the S809 airfoil and the Phase VI blade were investigated and the effects of the geometric parameters were considered. The aerodynamic performance of the S809 airfoil and the Phase VI blade was obtained using the RANS approach, and involved the standard transition model SST, k-omega turbulence model. It was shown that the leading edge slat had a great influence on the aerodynamic performance of the S809 airfoil and the Phase VI blade. For Case-1, when the angle of attack was 16.22°, the point of flow separation was shifted from x/c = 0.47 to 0.67, the lift coefficient increased by 52.99%. The torques of the total (included the wind turbine blade and leading edge slat) were increased except the torque of Blade-C1at 10 m/s.

Aeroelastic analysis of membrane blade via panel-BEM coupling

This paper aims at introducing a methodology for aeroelastic analysis of membrane blades using panel-BEM coupling. The proposed methodology is used for evaluating the performance of a membrane blade with the baseline platform similar to the NASA-Ames Phase VI blade. The performance of the membrane blade is compared with the rigid baseline blade. The studied membrane blade has certain aerodynamic advantages over its rigid counterpart. For the studied blade and at higher wind speeds, the axial induction factor for membrane blade is nearer to the Betz optimum value of 1/3 and as a result the membrane blade is more efficient in producing power. The performance of the membrane blade is further examined via high fidelity FSI analysis. The same trend as in panel-BEM coupling is also observed in the high fidelity model, namely for higher wind speeds the studied membrane blades is more efficient than its rigid counterpart.

Numerical simulation of rime ice on NREL Phase VI blade

Wind turbine installation in cold regions is prone to icing events. In this study, rime ice on the rotating NREL Phase VI blade is simulated. First, an icing model is developed and validated to gain a better understanding of blade icing. For the fixed airfoil, the root-mean-square error of the collection efficiency is 8.6%. The ice shape area error is 1.5%. For the rotating blade, the ice shape area error is 2.4%. The validation results indicate that the accuracy of the icing model is satisfactory. Second, rime ice on the NREL Phase VI blade is examined. The results suggest that ice mainly accretes on the blade leading edge. The ice thickness gradually increases along the radial direction. The maximum ice thickness can reach 7.7 cm. Finally, ice mass and power degeneration distributions are investigated. The results indicate that ice mass increases along the radial direction. The increase in the ice mass is up to 105.3%. The power mainly degenerates at the outer blade. The decrease in the power is up to 13.3%. With respect to anti/de-icing, increased attention should be focused on the outer blade.

Calculation of Collection Efficiency on NREL Phase VI Blade

Icing on wind turbines is a major problem in cold regions. To study blade icing, water droplet collection efficiency is calculated on the National Renewable Energy Laboratory (NREL) phase VI blade. First, water droplet conservation equations are embedded into ANSYS Fluent, and the results calculated by the Eulerian method are validated. For the two-dimensional (2D) airfoil, the peak collection efficiency error is 3.7%; for the three-dimensional (3D) blade, the peak collection efficiency error is 2.8%. Second, collection efficiency on the NREL phase VI blade is investigated. The results indicate that water droplets mainly impact on the blade leading edge, and the collection efficiency increases along the radial direction. Finally, the 3D rotating effect on collection efficiency is studied. The results demonstrate that, at a wind speed of 7 m/s, the 3D rotating effect has almost no influence on collection efficiency; however, the effect must be considered in water droplet collection at a wind speed of 10 m/s.

Preliminary Analysis of a Concept Wind Turbine Blade with Piecewise Constant Chord and Constant Twist Angle Using BEM Method

Preliminary analysis of a concept wind turbine blade is performed using Blade Element Momentum method. The concept wind turbine blade consists of sections with constant chord and twist angle. The spanwise length of each section and the twist angle in each section is obtained numerically with using an optimization algorithm together with Blade Element Momentum theory. Using the optimization algorithm several different concept wind turbine blades are analysed using the very well-known National Renewable Energy Laboratory’s Unsteady Aerodynamics Experiment Phase VI wind turbine blade geometry. The aerodynamic performances of these concept wind turbine blades are compared with the results of the original UAE Phase VI blade. The concept design offers the wind turbine blades to be manufactured easily using cheaper manufacturing techniques with the highest quality and also with the possibility of being designed as modular while having almost the same aerodynamic efficiency of a tapered and twisted wind turbine blade.

Trailed vorticity modeling for aeroelastic wind turbine simulations in standstill

Abstract. Current fast aeroelastic wind turbine codes suitable for certification lack an induction model for standstill conditions. A trailed vorticity model previously used as an addition to a blade element momentum theory based aerodynamic model in normal operation has been extended to allow computing the induced velocities in standstill. The model is validated against analytical results for an elliptical wing in constant inflow and against standstill measurements from the NREL/NASA Phase VI unsteady experiment. The extended model obtains good results in the case of the elliptical wing but underpredicts the steady loading for the Phase VI blade in attached flow. The prediction of the dynamic force coefficient loops from the Phase VI experiment is improved by the trailed vorticity modeling in both attached flow and stall in most cases. The exception is the tangential force coefficient in stall, where the codes and measurements deviate and no clear improvement is visible. This article also contains aeroelastic simulations of the DTU 10 MW reference turbine in standstill at turbulent inflow with a fixed and idling rotor. The influence of the trailed vorticity modeling on the extreme flapwise blade root bending moment is found to be small.

Effect on Torque and Thrust of the Pointed Tip Shape of a Wind Turbine Blade

This paper presents the effect of the tip shape of a wind turbine blade on aerodynamic forces, including the effects of separation, transition and stall. A National Renewable Energy Laboratory (NREL) Phase-VI wind turbine blade was used, in which the shape of the tip was modified to a pointed tip. Computational fluid dynamics (CFD) simulations were employed for the analysis and the results were compared with the original NREL blade CFD and experimental data using ANSYS CFX (Ansys Inc., Delaware, PA, USA). To predict the separation and separation-induced transition on both near wall and far away, the shear-stress-transport (SST) Gamma-Theta turbulent model was used. The stall onset of a 20° angle of attack and its effects were also analyzed and presented. The value of torque with the pointed tip blade was found to be 3%–8% higher than the original NREL blade showing the benefit of the pointed tip. Normal force coefficient is lower at the tip for the pointed tip blade, which results in lower deformation of the blade. It was found that the pointed-tip blade is more efficient in terms of generating torque than the original NREL Phase-VI blade in the dynamic stall region of 10–15 m/s wind speeds.

Trailing-edge flow control for wind turbine performance and load control

This paper reports an investigation into the performance of trailing-edge flow control devices on horizontal axis wind turbines by solving the three dimensional Reynolds averaged Navier-Stokes equations in the rotational framework. The validation case selected for this work is the NREL Phase VI blade with wind tunnel experimental data. The trailing-edge flow control devices studied include microtabs and microjets installed near the trailing-edge of the rotating blade. The divergent trailing-edge is also included in the study as a passive flow control device due to its practical interest. These trailing-edge devices are implemented on the fixed-pitch NREL Phase VI blade, using the original performance and flow characteristics as a benchmark. Both 2D and 3D simulations are carried out in order to investigate the suitability of the 2D blade sectional design analysis and control for the actual 3D rotating blades.

Trailed vorticity modeling for aeroelastic wind turbine simulations in stand still

Current fast aeroelastic wind turbine codes suitable for certification lack an induction model for standstill conditions. A trailed vorticity model previously used as addition to a blade element momentum theory based aerodynamic model in normal operation has been extended to allow computing the induced velocities in standstill. The model is validated against analytical results for an elliptical wing in constant inflow and against stand still measurements from the NREL/NASA Phase VI unsteady experiment. The extended model obtains good results in case of the elliptical wing, but underpredicts the steady loading for the Phase VI blade in attached flow. The prediction of the dynamic force coefficient loops from the Phase VI experiment is improved by the trailed vorticity modeling in both attached flow and stall in most cases. The exception is the tangential force coefficient in stall, where the codes and measurements deviate and no clear improvement is visible.

Fluid-Structure interaction analysis and performance evaluation of a membrane blade

Examining the potential of a membrane blade concept is the goal of the current work. In the sailwing concept the surface of the wing, or the blade in this case, is made from pre-tensioned membranes which meet at the pre-tensioned edge cable at the trailing edge. Because of the dependency between membrane deformation and applied aerodynamic load, two-way coupled fluid-structure interaction analysis is necessary for evaluation of the aerodynamic performance of such a configuration. The in-house finite element based structural solver, CARAT++, is coupled with OpenFOAM in order to tackle the multi-physics problem. The main aerodynamic characteristics of the membrane blade including lift coefficient, drag coefficient and lift to drag ratio are compared with its rigid counterpart. A single non-rotating NREL phase VI blade is studied here as a first step towards analyzing the concept for the rotating case. Compared with the rigid blade, the membrane blade has a higher slope of the lift curve. For higher angles of attack, lift and drag coefficients as well as the lift to drag ratio is higher for the membrane blade. A single non-rotating blade is studied here as a first step towards analyzing the concept for the rotating case.