Renewable Energy Laboratory Phase VI Research Articles

Prediction of the near wall flow behaviours of separation and transition for NREL phase VI wind turbine blade

ABSTRACT This study demonstrates the separation and transition flow behaviour around a 3-D wind turbine blade by analysing blending functions, intermittency, and transition onset Reynolds number in SST Gamma Theta model. The National Renewable Energy Laboratory Phase VI rotor was modelled for the analysis. The simulations were executed by using the ANSYS-CFX solver. Extensive comparisons were performed to demonstrate the accuracy of this numerical model for sectional pressure distribution, tangent force, normal force, thrust, and torque. Good agreement was observed in all wind speed cases. The discrepancies in pre-stall region were negligible, and those in the post-stall region were acceptable. Thus, the capability and accuracy of the SST Gamma Theta model in predicting separated and transitional flow effects were verified.

A Comparative Study of the Influences of Leading-Edge Suction and Blowing on the Aerodynamic Performance of a Horizontal-Axis Wind Turbine

The aerodynamic performance of a National Renewable Energy Laboratory Phase VI horizontal-axis wind turbine (HAWT) equipped with blades characterized by leading-edge suction and blowing was analyzed numerically in three dimensions using computational fluid dynamics. The effects of leading-edge suction and blowing flow control methods on the flow field and aerodynamic characteristics of the HAWT and its performance were compared. The results suggest that leading-edge suction is superior to leading-edge blowing in suppressing the flow separation on the suction side of the turbine blade and improving the turbine aerodynamic characteristics. However, compared with the baseline turbine, the bending moment acting on the rotating blade of the HAWT to which suction or blowing is applied increased. This increment was greater for turbines with leading-edge blowing flow control blades. The effects of different arrangements of suction slots on the performance of the HAWT were analyzed. The results showed that the net wind power coefficient of the HAWT after deducting the energy consumption of the suction control was 45% higher than that of conventional turbines, indicating great potential for practical applications.

Large-Eddy Simulation of a wind turbine using a Filtered Actuator Line Model

When dealing with multirotor devices such as quadcopters or wind farms, the cost of blade-resolved large-eddy simulation (LES) becomes prohibitive. Combining LES with a family of lower-fidelity models, called actuator line models (ALMs), has grown in popularity in the past decade. ALM replaces full blade resolution with an array of actuator points or lines parameterized by aerodynamic lift/drag polar plots along the blades. Body forces computed based on these actuator points are then projected onto the LES flow mesh, mimicking the effect of rotating blades on the flow. However, the optimal projection radius and the associated LES grid size is often too restrictive for multirotor simulations. Recently, a new tip-correction-based filtered ALM (F-ALM) was proposed by Martínez-Tossas and Meneveau (2019), which allows coarser-than-optimal grids by avoiding the associated overprediction of thrust. In this work, F-ALM is implemented into a high-order, in-house LES code to simulate National Renewable Energy Laboratory Phase VI wind turbine. It is then followed by a comparison between the baseline ALM and the newly implemented F-ALM in terms of instantaneous and time-averaged flow fields and blade loads, revealing the advantages of F-ALM in preventing the overprediction of power on coarse grids. This encourages accurate and affordable simulations of multirotor devices in the future.

Performance Enhancement Analysis of a Horizontal Axis Wind Turbine by Vortex Trapping Cavity

Abstract A wind turbine blade has the particularity of containing twisted and tapered thick airfoils. The challenge with this configuration is the highly separated flow in the region of high twist. This research presents a numerical investigation of the effectiveness of a vortex trapping cavity (VTC) on the aerodynamics of the National Renewable Energy Laboratory phase VI wind turbine. First, simulations are conducted on the S809 profile to study the fluid flow compared to the airfoil with the redesigned VTC. Second, the blade is simulated with and without VTC to assess its effect on the torque and the flow patterns. The results show that for high angles of incidence at Rec = 106, the lift coefficient increases by 10% and the wake region appears smaller for the case with VTC. For wind speeds larger than 10 m/s, the VTC improves the torque by 3.9%. This is due to the separation that takes place in the vicinity of the VTC and leads to trapping early separation eddies inside the cell. These eddies roll up forming a coherent laminar vortex structure, which in turn sheds periodically out of the cell. This phenomenon favorably reshapes excessive flow separation, reenergizes the boundary layer, and globally improves blade torque.

Aerodynamic measurements on wind turbines

This article reviews the aerodynamic measurement programs on wind turbines that have been performed in the last decades. It is largely based on results from four projects carried out under auspices of the International Energy Agency (IEA), which are denoted as IEA Tasks 14, 18, 20, and 29. The aim of these projects was to collect and analyze aerodynamic measurements on five field facilities (IEA Tasks 14 and 18), the National Renewable Energy Laboratory Phase VI turbine placed in the large NASA Ames wind tunnel (IEA Task 20), and the Mexico turbine placed in the Large Low Speed Facility of the German Dutch Wind tunnel DNW (IEA Task 29). Other experimental programs with an important aerodynamic content are touched upon as well. Research areas for which these measurements have led to important progress are identified. The progress is illustrated with analyses on these experiments. It is shown that detailed aerodynamic measurements are an absolute necessity for the validation and improvement of wind turbine design codes, where it is also concluded that the amount of measurement data which has been produced until now is still far too limited.This article is categorized under: Wind Power > Science and Materials

Effects of the Approximations Embodied in the Momentum Theory as Applied to the NREL PHASE VI Wind Turbine

This paper investigates the impact of the standard approximations embodied in the well-known Momentum Theory on its performance prediction capabilities. To this aim, the results of the momentum theory, which is still widely used in all Blade Element/Momentum codes for the analysis and/or design of wind turbines, are compared with those obtained with an actuator disk model based on Computational Fluid Dynamics techniques. In this method, the axisymmetric and steady Euler equations are solved with a classical finite volume approach, while the turbine effects are modelled through a set of axial and tangential body forces distributed over a disk shaped region representing the rotor swept surface. Since this method does not rely on the momentum theory simplifying assumptions, it can be suitably employed to verify the momentum theory validity. The analysis is carried out using the well documented experimental data of the National Renewable Energy Laboratory Phase VI wind turbine.

A new stall delay algorithm for predicting the aerodynamics loads on wind turbine blades for axial and yawed conditions

AbstractStall delay is a complicated phenomenon that has gained for many years the attention of industry and academics in the fields of helicopter and wind turbine aerodynamics. Since most of the potential flow theories still rely on the use of 2D aerofoil data for simulating loads on a rotating blade, less degree of accuracy is expected because of 3D rotational effects. In this work, a new model for correcting the 2D steady aerodynamic data for 3D effects is presented. The model can reduce the uncertainty in the blade design process and, subsequently, make wind turbines more cost‐effective. This model combines the stall delay model of Corrigan and Schillings, a modified version of an inviscid stall delay model, a new modification factor to account for the effect of the angle of attack changes and a new tip loss factor. Furthermore, the model applies the use of the separation factor of Du and Selig to evaluate the area on the rotor disc where stall delay is most prominent. The new stall delay model was embedded in a free‐wake vortex model to estimate the aerodynamic loads on the National Renewable Energy Laboratory Phase VI rotor blades consisting of the S809 aerofoil sections. The results in this study confirm the validity of the 3D corrections by the proposed new model under both axial and yawed flow conditions. Copyright © 2017 John Wiley & Sons, Ltd.

Aerodynamic noise prediction of a Horizontal Axis Wind Turbine using Improved Delayed Detached Eddy Simulation and acoustic analogy

Abstract This paper presents the results of the aerodynamic and aero-acoustic prediction of the flow field around the National Renewable Energy Laboratory Phase VI wind turbine. The Improved Delayed Detached Eddy Simulation turbulence model is applied to obtain the instantaneous turbulent flow field. The noise prediction is carried out using the Ffowcs Williams and Hawkings acoustic analogy. Simulations are performed for three different inflow conditions, U = 7, 10, 15 m/s. The capability of the Improved Delayed Detached Eddy Simulation turbulence model in massive separation is verified with available experimental data for pressure coefficient. The broadband noises of the turbulent boundary layers and the tonal noises due to the blade passing frequency are predicted via flow field noise simulation. The contribution of the thickness, loading and quadrupole noises are investigated, separately. The results indicated that there is a direct relation between the strength of the radiated noise and the wind speed. Furthermore, the effect of the receiver location on the Overall Sound Pressure Level is investigated.

Application and validation of incrementally complex models for wind turbine aerodynamics, isolated wind turbine in uniform inflow conditions

AbstractA comparison of several incrementally complex methods for predicting wind turbine performance, aeroelastic behavior, and wakes is provided. Depending on a wind farm's design, wake interference can cause large power losses and increased turbulence levels within the farm. The goal is to employ modeling methods to reach an improved understanding of wake effects and to use this information to better optimize the layout of new wind farms. A critical decision faced by modelers is the fidelity of the model that is selected to perform simulations. The choice of model fidelity can affect the accuracy, but will also greatly impact the computational time and resource requirements for simulations. To help address this critical question, three modeling methods of varying fidelity have been developed side by side and are compared in this article. The models from low to high complexity are as follows: a blade element‐based method with a free‐vortex wake, an actuator disc‐based method, and a full rotor‐based method. Fluid/structure interfaces are developed for the aerodynamic modeling approaches that allow modeling of discrete blades and are then coupled with a multibody structural dynamics solver in order to perform an aeroelastic analysis. Similar methods have individually been tested by researchers, but we suggest that by developing a suite of models, they can be cross‐compared to grasp the subtleties of each method. The modeling methods are applied to the National Renewable Energy Laboratory Phase VI rotor to predict the turbine aerodynamic and structural loads and then also the wind velocities in the wake. The full rotor method provides the most accurate predictions at the turbine and the use of adaptive mesh refinement to capture the wake to 20 radii downstream is proven particularly successful. Though the full rotor method is unmatched by the lower fidelity methods in stalled conditions and detailed prediction of the downstream wake, there are other less complex conditions where these methods perform as accurately as the full rotor method. Copyright © 2014 John Wiley & Sons, Ltd.

A solution‐based stall delay model for horizontal‐axis wind turbines

AbstractThis work proposes a new solution‐based stall delay model to predict rotational effects on horizontal‐axis wind turbines. In contrast to conventional stall delay models that correct sectional airfoil data prior to the solution to account for three‐dimensional and rotational effects, a novel approach is proposed that corrects sectional airfoil data during a blade element momentum solution algorithm by investigating solution‐dependent parameters such as the spanwise circulation distribution and the local flow velocity acting at a section of blade. An iterative process is employed that successively modifies sectional lift and drag data until the blade circulation distribution is converged. Results obtained with the solution‐based stall delay model show consistent good agreement with measured data along the National Renewable Energy Laboratory Phase VI and Model Experiments in Controlled Conditions rotor blades at low and high wind speeds. Copyright © 2014 John Wiley & Sons, Ltd.

An actuator line method with novel blade flow field coupling based on potential flow equivalence

AbstractA Reynolds‐averaged Navier–Stokes‐embedded actuator line model for wind and tidal turbine simulation has been implemented and validated using the National Renewable Energy Laboratory Phase VI wind tunnel experimental results. Actuator line models, first introduced by Sørensen and Shen, represent the blades virtually, enabling time‐resolved rotor simulations without requiring blade boundary layer discretization. This results in a lower computational cost than blade‐resolved simulations while preserving the predominant features of the rotor flow. The present method introduces a novel technique, based on potential flow equivalence, to determine the local flow velocity at the blade, and a method of projecting the resulting momentum sources to the flow field. These methods circumvent the requirement for smearing techniques used in other actuator line models. In addition, the model is adapted for use with an unstructured mesh, thereby enabling turbine components such as the tower and nacelle to be explicitly included in the domain. The model is validated through comparison of computed integrated loads and local force coefficients with the National Renewable Energy Laboratory Phase VI experimental results. Results for local force coefficients indicate general agreement with experiment, although discrepancies associated with three‐dimensional flow effects are observed at the tips. Copyright © 2014 John Wiley & Sons, Ltd.

Inverse design of horizontal axis wind turbine blades using a vortex line method

AbstractA new inverse design process for horizontal axis wind turbine blades is developed to account for three‐dimensional blade features such as non‐planar wing tip. The multidimensional Newton iteration method combined with a vortex line method is used to provide blade geometry parameters given desired aerodynamic behaviors such as lift coefficient and axial induction. The Jacobian matrix is visualized to show the effect of the change of the blade twist and chord on the change of the aerodynamic behaviors. The method is validated for a canonical straight blade with uniform lift coefficient and axial induction distributions. The results show an excellent agreement with those obtained by PROPID, which is a blade element momentum theory‐based inverse design code. The National Renewable Energy Laboratory Phase VI blade is used to validate the method for a straight blade with non‐uniform distributions of the lift coefficient and axial induction. The method is also applied successfully to a non‐straight blade design with a non‐planar wing tip. A noticeable change in the twist and chord for this non‐straight blade is seen compared with a straight blade. Finally, the inverse design code is used to make a large rotor blade, and the power output generated by this blade is computed. Copyright © 2014 John Wiley & Sons, Ltd.

NREL VI rotor blade: numerical investigation and winglet design and optimization using CFD

ABSTRACTThe main objectives of this study were to aerodynamically design and optimize a winglet for a wind turbine blade by using computational fluid dynamics (CFD) and to investigate its effect on the power production. For validation and as a baseline rotor, the National Renewable Energy Laboratory Phase VI wind turbine rotor blade is used. The Reynolds‐averaged Navier–Stokes equations are solved, and k–ε Launder–Sharma turbulence model was used. The numerical results have shown a considerable agreement with the experimental data. The genetic algorithm was used as the optimization technique with the help of artificial neural network to reduce the computational cost. In the winglet design, the variable parameters are the cant and twist angles of the winglet and the objective function the torque. Multipoint optimization is carried out for three different operating wind speeds, and a total of 24 CFD cases are run in the design. The final optimized winglet showed around 9% increase in the power production. Copyright © 2013 John Wiley & Sons, Ltd.

Numerical investigation of unsteady aerodynamics of a Horizontal‐axis wind turbine under yawed flow conditions

ABSTRACTIn the present study, unsteady flow features and the blade aerodynamic loading of the National Renewable Energy Laboratory phase VI wind turbine rotor, under yawed flow conditions, were numerically investigated by using a three‐dimensional incompressible flow solver based on unstructured overset meshes. The effect of turbulence, including laminar‐turbulent transition, was accounted for by using a correlation‐based transition turbulence model. The calculations were made for an upwind configuration at wind speeds of 7, 10 and 15 m/sec when the turbine rotor was at 30° and 60° yaw angles. The results were compared with measurements in terms of the blade surface pressure and the normal and tangential forces at selected blade radial locations. It was found that under the yawed flow conditions, the blade aerodynamic loading is significantly reduced. Also, because of the wind velocity component aligned tangent to the rotor disk plane, the periodic fluctuation of blade loading is obtained with lower magnitudes at the advancing blade side and higher magnitudes at the retreating side. This tendency is further magnified as the yaw angle becomes larger. At 7 m/sec wind speed, the sectional angle of attack is relatively small, and the flow remains mostly attached to the blade surface. At 10 m/sec wind speed, leading‐edge flow separation and strong radial flow are observed at the inboard portion of the retreating blade. As the wind speed is further increased, the flow separation and the radial flow become more pronounced. It was demonstrated that these highly unsteady three‐dimensional aerodynamic features are well‐captured by the present method. Copyright © 2012 John Wiley & Sons, Ltd.

Enhancement of wind turbine aerodynamic performance by a numerical optimization technique

Sectional aerodynamic design optimization was performed to enhance the aerodynamic performance of horizontal axis wind turbine rotor blades based on a computational fluid dynamics technique. The proposed sectional optimization framework consists of airfoil section contour modeling by the PARSEC shape function and a modified feasible direction search algorithm. To enhance the aerodynamic performance of wind turbine rotor blades, the objective of the design framework was set to maximize the lift-over-drag ratio for each design section. A two-dimensional Navier-Stokes flow solver coupled with a transition turbulence model was used to evaluate the aerodynamic performance during the iterative design optimization procedure. The sectional flow conditions were extracted from the flow of a three-dimensional rotor blade configuration. The design framework was applied to the National Renewable Energy Laboratory Phase VI rotor blade. The design optimization was conducted at nine spanwise sections of the rotor blade. To validate the present methodology, the aerodynamic performances of the original baseline rotor and the rotor after the design optimization were compared by using a three-dimensional Navier-Stokes flow solver. It was found that approximately 11% of torque enhancement was achieved after the aerodynamic shape design optimization.

Estimating the angle of attack from blade pressure measurements on the National Renewable Energy Laboratory phase VI rotor using a free wake vortex model: yawed conditions

AbstractWind turbine design codes for calculating blade loads are usually based on a blade element momentum (BEM) approach. Since wind turbine rotors often operate in off‐design conditions, such as yawed flow, several engineering methods have been developed to take into account such conditions. An essential feature of a BEM code is the coupling of local blade element loads with an external (induced) velocity field determined with momentum theory through the angle of attack. Local blade loads follow directly from blade pressure measurements as performed in the National Renewable Energy Laboratory (NREL) phase IV campaign, but corresponding angles of attack cannot (on principle) be measured. By developing a free wake vortex method using measured local blade loads, time‐dependent angle of attack and induced velocity distributions are reconstructed. In a previous paper, a method was described for deriving such distributions in conjunction with blade pressure measurements for the NREL phase VI wind turbine in axial (non‐yawed) conditions. In this paper, the same method is applied to investigate yawed conditions on the same turbine. The study considered different operating conditions in yaw in both attached and separated flows over the blades. The derived free wake geometry solutions are used to determine induced velocity distributions at the rotor blade. These are then used to determine the local (azimuth time dependent) angle of attack, as well as the corresponding lift and drag for each blade section. The derived results are helpful to develop better engineering models for wind turbine design codes. Copyright © 2008 John Wiley & Sons, Ltd.

Characterization of Three-Dimensional Effects for the Rotating and Parked NREL Phase VI Wind Turbine

This paper addresses three-dimensional effects which are pertinent to wind turbine aerodynamics. Two computational models were applied to the National Renewable Energy Laboratory Phase VI Rotor under rotating and parked conditions, a vortex line method using a prescribed wake, and a parallelized coupled Navier-Stokes/vortex-panel solver (PCS). The linking of the spanwise distribution of bound circulation between both models enabled the quantification of three-dimensional effects with PCS. For the rotating turbine under fully attached flow conditions, the effects of the vortex sheet dissipation and replacement by a rolled-up vortex on the computed radial force coefficients were investigated. A quantitative analysis of both radial pumping and Coriolis effect, known as the Himmelskamp effect, was performed for viscous as well as inviscid flow. For the parked turbine, both models were applied at various pitch angles corresponding to fully attached as well as stalled flow. For partially stalled flow, computed results revealed a vortical structure trailing from the blade’s upper surface close to the 40% radial station. This trailing vortex was documented as a highly unsteady flow structure in an earlier detached eddy simulation by another group, however, it was not directly observed experimentally but only inferred. Computed results show very good agreement with measured wind tunnel data for the PCS model. Finally, a new method for extracting three-dimensional airfoil data is proposed that is particularly well suited for highly stalled flow conditions.

Numerical Studies of the Effects of Active and Passive Circulation Enhancement Concepts on Wind Turbine Performance

The aerodynamic performance of a wind turbine rotor equipped with circulation enhancement technology (trailing-edge blowing or Gurney flaps) is investigated using a three-dimensional unsteady viscous flow analysis. The National Renewable Energy Laboratory Phase VI horizontal axis wind turbine is chosen as the baseline configuration. Experimental data for the baseline case is used to validate the flow solver, prior to its use in exploring these concepts. Calculations have been performed for axial and yawed flow at several wind conditions. Results presented include radial distribution of the normal and tangential forces, shaft torque, root flap moment, and surface pressure distributions at selected radial locations. At low wind speed (7m∕s) where the flow is fully attached, it is shown that a Coanda jet at the trailing edge of the rotor blade is effective at increasing circulation resulting in an increase of lift and the chordwise thrust force. This leads to an increased amount of net power generation compared to the baseline configuration for moderate blowing coefficients (Cμ⩽0.075). A passive Gurney flap was found to increase the bound circulation and produce increased power in a manner similar to Coanda jet. At high wind speed (15m∕s) where the flow is separated, both the Coanda jet and Gurney flap become ineffective. The effects of these two concepts on the root bending moments have also been studied.