Showing papers on "Blade element theory published in 2013"

Modeling turbine wakes and power losses within a wind farm using LES: An application to the Horns Rev offshore wind farm

Abstract: A modeling framework is proposed and validated to simulate turbine wakes and associated power losses in wind farms. It combines the large-eddy simulation (LES) technique with blade element theory and a turbine-model-specific relationship between shaft torque and rotational speed. In the LES, the turbulent subgrid-scale stresses are parameterized with a tuning-free Lagrangian scale-dependent dynamic model. The turbine-induced forces and turbine-generated power are modeled using a recently developed actuator-disk model with rotation (ADM-R), which adopts blade element theory to calculate the lift and drag forces (that produce thrust, rotor shaft torque and power) based on the local simulated flow and the blade characteristics. In order to predict simultaneously the turbine angular velocity and the turbine-induced forces (and thus the power output), a new iterative dynamic procedure is developed to couple the ADM-R turbine model with a relationship between shaft torque and rotational speed. This relationship, which is unique for a given turbine model and independent of the inflow condition, is derived from simulations of a stand-alone wind turbine in conditions for which the thrust coefficient can be validated. Comparison with observed power data from the Horns Rev wind farm shows that better power predictions are obtained with the dynamic ADM-R than with the standard ADM, which assumes a uniform thrust distribution and ignores the torque effect on the turbine wakes and rotor power. The results are also compared with the power predictions obtained using two commercial wind-farm design tools (WindSim and WAsP). These models are found to underestimate the power output compared with the results from the proposed LES framework.

The Performance Test of Three Different Horizontal Axis Wind Turbine (HAWT) Blade Shapes Using Experimental and Numerical Methods

Abstract: Three different horizontal axis wind turbine (HAWT) blade geometries with the same diameter of 0.72 m using the same NACA4418 airfoil profile have been investigated both experimentally and numerically. The first is an optimum (OPT) blade shape, obtained using improved blade element momentum (BEM) theory. A detailed description of the blade geometry is also given. The second is an untapered and optimum twist (UOT) blade with the same twist distributions as the OPT blade. The third blade is untapered and untwisted (UUT). Wind tunnel experiments were used to measure the power coefficients of these blades, and the results indicate that both the OPT and UOT blades perform with the same maximum power coefficient, Cp = 0.428, but it is located at different tip speed ratio, λ = 4.92 for the OPT blade and λ = 4.32 for the UOT blade. The UUT blade has a maximum power coefficient of Cp = 0.210 at λ = 3.86. After the tests, numerical simulations were performed using a full three-dimensional computational fluid dynamics (CFD) method using the k-ω SST turbulence model. It has been found that CFD predictions reproduce the most accurate model power coefficients. The good agreement between the measured and computed power coefficients of the three models strongly

Optimization of power coefficient on a horizontal axis wind turbine using bem theory

Abstract: Aerodynamic optimization has widely become a issue of considerable interest to determine the geometry of an aerodynamic configuration amidst certain design constraints. Aerodynamic performance is calculated from a prescribed geometric shape, which is often performed in trial and error method. Numerous design methods are available for the aerodynamic design of the rotor. The goal in optimizing is to maximize the aerodynamic efficiency at a single design wind speed. However, single-design point methods do not automatically lead to the optimum design, since they consider only one point in the total operational range. Moreover they do not implicitly involve considerations on loads which require an experienced designer. The aerodynamic optimization of a Horizontal Axis Wind Turbine is a complex method characterized by numerous trade-off decisions aimed at finding the optimum overall performance. However researcher design the wind turbine is an enormous ways and more often decision-making is very difficult. Commercial turbines have been derived from both theoretical and empirical methods, but there is no clear evidence on which of these is optimal. Turbine blades are optimized with the aim to achieve maximum power coefficient for the given blade with solidity, ratio of coefficient of drag to lift, angle of attack and tip speed ratio. In this article, the blade element theory is used to find the optimum value analytically. The effect of power coefficient for different blade angle, tip speed ratio, ratio of coefficient of drag and coefficient of lift and blade solidity is presented and the optimized set value is obtained.

Impact vibration characteristics of a shrouded blade with asymmetric gaps under wake flow excitations

Abstract: The vibro-impact mechanism of a rotating shrouded blade with asymmetric gaps is investigated. The Frobenius method is employed to determine the dynamic frequencies and corresponding mode functions of the shrouded blade under the action of dynamic stiffness. The mode functions are used to truncate the governing partial differential equation of the shrouded blade to ordinary differential equations by using the Galerkin method. Taking into account the influence of the rotating speed, the contact stiffness of adjacent blades is assumed to be an equivalent contact spring stiffness which is determined by the bending dynamic stiffness of the blade. Then the average method is employed to obtain the dynamic responses of the primary, the sub-harmonic and the super-harmonic resonances of the shrouded blade with wake flow excitation, respectively. Finally, the dynamical responses of a shrouded blade with a set of typical material constants and geometrical parameters are given to illustrate the influence of the blade shroud gaps on the vibration amplitude.

Effect of Rotor Geometry and Blade Kinematics on Cycloidal Rotor Hover Performance

Abstract: This paper describes the systematic performance measurements conducted to understand the role of rotor geometry and blade pitching kinematics on the performance of a microscale cycloidal rotor. Key geometric parameters that were investigated include rotor radius, blade span, chord, and blade planform. Because of the flow curvature effects, the cycloidal-rotor performance was a strong function of the chord/radius ratio. The optimum chord/radius ratios were extremely high, around 0.5–0.8, depending on the blade pitching amplitude. Cycloidal rotors with shorter blade spans had higher power loading (thrust/power), especially at lower pitching amplitudes. Increasing the solidity of the rotor by increasing the blade chord, while keeping the number of blades constant, produced large improvements in power loading. Blade planform shape did not have a significant impact, even though trapezoidal blades with a moderate taper ratio were slightly better than rectangular blades. On the blade kinematics side, higher blad...

Accuracy of State-of-the-Art Actuator-Line Modeling for Wind Turbine Wakes

Abstract: The current actuator line method (ALM) within an OpenFOAM computational fluid dynamics (CFD) solver was used to perform simulations of the NREL Phase VI rotor under rotating and parked conditions, two fixed-wing designs both with an elliptic spanwise loading, and the NREL 5-MW turbine. The objective of this work is to assess and improve the accuracy of the state-of-the-art ALM in predicting rotor blade loads, particularly by focusing on the method used to project the actuator forces onto the flow field as body forces. Results obtained for sectional normal and tangential force coefficients were compared to available experimental data and to the in-house performance code XTurb-PSU. It was observed that the ALM results agree well with measured data and results obtained from XTurb-PSU except in the root and tip regions if a three-dimensional Gaussian of width, e, constant along the blade span is used to project the actuator force onto the flow field. A new method is proposed where the Gaussian width, e, varies along the blade span following an elliptic distribution. A general criterion is derived that applies to any planform shape. It is found that the new criterion for e leads to improved prediction of blade tip loads for a variety of blade planforms and rotor conditions considered.

Fluid structure interaction of a morphed wind turbine blade

Abstract: SUMMARY One serious challenge of energy systems design, wind turbines in particular, is the need to match the system operation to the variable load. This is so because system efficiency drops at off-design load. One strategy to address this challenge for wind turbine blades and obtain a more consistent efficiency over a wide load range, is varying the blade geometry. Predictable morphing of wind turbine blade in reaction to wind load conditions has been introduced recently. The concept, derived from fish locomotion, also has similarities to spoilers and ailerons, known to reduce flow separation and improve performance using passive changes in blade geometry. In this work, we employ a fully coupled technique on CFD and FEM models to introduce continuous morphing to desired and predetermined blade design geometry, the NACA 4412 profile, which is commonly used in wind turbine applications. Then, we assess the aerodynamic behavior of a morphing wind turbine airfoil using a two-dimensional computation. The work is focused on assessing aerodynamic forces based on trailing edge deflection, wind speed, and material elasticity, that is, Young's modulus. The computational results suggest that the morphing blade has superior part-load efficiency over the rigid NACA blade. Copyright © 2013 John Wiley & Sons, Ltd.

Dynamical characteristics of the tip vortex from a four-bladed rotor in hover

Abstract: Dynamical characteristics of tip vortices shed from a 1 m diameter, four-bladed rotor in hover are investigated using various aperiodicity correction tech- niques. Data are acquired by way of stereo-particle image velocimetry and comprises measurements up to 260 vor- tex age with 10 offsets. The nominal operating condition of the rotor corresponds to Rec = 248,000 and M = 0.23 at the blade tip. With the collective pitch set to 7.2 and a rotor solidity of 0.147, blade loading (CT/r )i s estimated from blade element momentum theory to be 0.042. The findings reveal a noticeable, anisotropic, ape- riodic vortex wandering pattern over all vortex ages mea- sured. These findings are in agreement with recent observations of a full-scale, four-bladed rotor in hover operating under realistic blade loading. The principal axis of wander is found to align itself perpendicular to the slipstream boundary. Likewise, tip vortices trailing from different blades show a wandering motion that is in phase instantaneously with respect to one another, in every direction and at every wake age in the measurement envelope.

Cavitation inception and simulation in blade element momentum theory for modelling tidal stream turbines

Abstract: Blade element momentum theory (BEMT) is an analytical modelling tool that describes the performance of turbines by cross-referencing one-dimensional momentum theory with blade element theory. Each blade is discretised along its length and the dynamic properties of torque and axial force are determined. A compatible cavitation detection model is introduced to indicate any cavitating blade elements. Cavitation occurrence is dependent on proximity to the free surface, the incident flow velocity and inflow angle and the blade cross-section aerofoil shape. The shock waves associated with cavitation can significantly damage the blade surface and reduce performance; therefore, this model is a useful addition to BEMT and can be used in turbine design to minimise cavitation occurrence. The results are validated using the cavitation experiment observations.

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

Abstract: In 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.

Effects of Biodynamic Feedthrough in Rotorcraft/Pilot Coupling: Collective Bounce Case

Abstract: This paper discusses the aeroelastic interaction between the helicopter and the pilot called collective bounce. The problem is mostly studied in the time domain, using the multibody system dynamics approach to model the dynamics of the vehicle and the aeroelasticity of the main rotor and a linear or quasilinear transfer function approach for the voluntary and involuntary dynamics of the pilot. Different models are considered for the aerodynamic forces acting on the rotor, ranging from blade-element/momentum theory to a boundary-element method used independently and in cosimulation with the multibody model. The problem is analyzed in hover and forward flight, highlighting modeling requirements and the sensitivity of the stability results to a variety of parameters of the problem.

Torsional Stiffness Effects on the Dynamic Stability of a Horizontal Axis Wind Turbine Blade

Abstract: Aeroelastic instability problems have become an increasingly important issue due to the increased use of larger horizontal axis wind turbines. To maintain these large structures in a stable manner, the blade design process should include studies on the dynamic stability of the wind turbine blade. Therefore, fluid-structure interaction analyses of the large-scaled wind turbine blade were performed with a focus on dynamic stability in this study. A finite element method based on the large deflection beam theory is used for structural analysis considering the geometric nonlinearities. For the stability analysis, a proposed aerodynamic approach based on Greenberg’s extension of Theodorsen’s strip theory and blade element momentum method were employed in conjunction with a structural model. The present methods proved to be valid for estimations of the aerodynamic responses and blade behavior compared with numerical results obtained in the previous studies. Additionally, torsional stiffness effects on the dynamic stability of the wind turbine blade were investigated. It is demonstrated that the damping is considerably influenced by variations of the torsional stiffness. Also, in normal operating conditions, the destabilizing phenomena were observed to occur with low torsional stiffness.

Unsteady Momentum Source Method for Efficient Simulation of Rotor Aerodynamics

Abstract: T HE flow field around a rotor is very complicated as we all understand. Prediction of aerodynamic performance of rotor including not only rotor loading but also rotor wake in computational fluid dynamics (CFD) is still an important issue and also very challenging. Various computational methods based on Euler or Navier–Stokes equations with or without wake modeling were developed to predict the flow field around rotor [1–4]. Unsteady rotor-airframe interaction problems requiring both moving and stationary domains was recently simulated by using various techniques such as overset or Chimera grid or sliding mesh scheme [5–7]. Computational burden becomes rather heavy for a full Navier– Stokes simulation of rotor-airframe interaction because rotating blades and stationary body have to be dealt with simultaneously. Some approaches to mitigate this computational burden were suggested that adopted an actuator disk concept for rotating blades. There are two types of actuator disk methods: a pressure-boundary approach and a momentum-source approach [8–13]. O’Brien and Smith [14] discussed rotor-fuselage interaction models of pressureboundary method and momentum-source method using various load distributions. Schweikhard [15] implemented the time-averaged momentum source method in an unstructured flow solver. These actuator-disk models treat the whole disk plane swept by rotors as pressure jump or momentum source plane so that they yield a timeaveraged representation of the flow. Considering that the flow is essentially unsteady, these actuator-disk models are not expected to simulate properly the unsteady-flow features of the rotor. Recently Boyd and Barnwell [16] developed an unsteady rotor/fuselage interactional model that loosely couples a Generalized Dynamic Wake Theory (GDWT) to thin layer Navier–Stokes code with an overset grid. The GDWT estimates the unsteady blade loading by adopting the inflow model of Peters and He [17]. Tadghighi and Anand [18] developed an unsteady rotor source model for the interactional rotor/fuselage aerodynamics. The blade was represented by an unsteadymomentum source distribution in the form of a lifting-line type representing the loading only along a radial line of the blade. Kim and Park [19] tried an unsteady momentum source method by using a Navier–Stokes solver. Themomentum sourcewas evaluated through the blade element theory with inflow model of Peters andHe [17]. Recently, apart from rotor aerodynamics, a simple momentum source method was used to simulate the flowfield around vortex-generator arrays [20,21]. The momentum source magnitude was determined simply by using only the lift force of the vortex generators with a model constant. According to O’Brien and Smith [14], the momentum source method is known to be more stable numerically than the pressure boundary method in the region where the wake is very close to the rotor disk. Moreover the momentum source method is known to represent the rotor aerodynamics better than the pressure boundary method. In the present study, we develop an unsteady momentum source method for unsteady Navier–Stokes solver without employing additionalmodels for induced velocity and tip loss, which is the very first attempt to the authors’ knowledge. The momentum sources are distributed along radial and chord-wise directions of a rotor. The method suggested is validated by several simulation results.

The Analysis of the Aerodynamic Character and Structural Response of Large-Scale Wind Turbine Blades

Abstract: A process of detailed CFD and structural numerical simulations of the 1.5 MW horizontal axis wind turbine (HAWT) blade is present. The main goal is to help advance the use of computer-aided simulation methods in the field of design and development of HAWT-blades. After an in-depth study of the aerodynamic configuration and materials of the blade, 3-D mapping software is utilized to reconstruct the high fidelity geometry, and then the geometry is imported into CFD and structure finite element analysis (FEA) software for completely simulation calculation. This research process shows that the CFD results compare well with the professional wind turbine design and certification software, GH-Bladed. Also, the modal analysis with finite element method (FEM) predicts well compared with experiment tests on a stationary blade. For extreme wind loads case that by considering a 50-year extreme gust simulated in CFD are unidirectional coupled to the FE-model, the results indicate that the maximum deflection of the blade tip is less than the distance between the blade tip (the point of maximum deflection) and the tower, the material of the blade provides enough resistance to the peak stresses the occur at the conjunction of shear webs and center spar cap. Buckling analysis is also included in the study.

Dynamic response of coupled shaft torsion and blade bending in rotor blade system

Abstract: This study develops a computational model for the dynamic characteristics of a rotor-blade system. The rotor-blade coupled model with pre-twisted blade attached to a rigid disk driven by a shaft is developed using the Lagrange equation in conjunction with the assumed mode method to discretize the blade deformation. The effects of axial shortening due to blade lagging deformation, centripetal force caused by the rotating blade, and gravity are included in the model. The coupled equation of motion is formulated based on the small deformation theory for the blade and shaft torsional deformation to obtain the dynamic characteristics of the system for various system parameters. Numerical simulations show that the pre-twist angle of the blade and the shaft torsional flexibility strongly influence the dynamic behavior of the rotor-blade system.

Twisted blade root

Abstract: The invention is related to a rotor blade for a wind turbine comprising a blade root, a transition piece and an aerodynamic part, wherein the blade root essentially is optimized for fixation of the blade to the hub and the aerodynamic part essentially is optimized to extract energy from the wind and wherein the transition part realizes a beneficial transition between the blade root and the aerodynamic part. According to the invention the rotor blade can perform better both aerodynamically and structurally compared to a classic design when the blade part located near the axis, approximately the part between 0%L and 50%L is provided with one or more of the following characteristics: more twist than usual, attached flow stimulating measures at the suction side, flow blocking measures at the pressure side, thicker profiles than usual, a triangular shape of the profile back and back twist.