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

Wind 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.peer-reviewe

A Possible Relation between Wind Conditions, Advanced Control and Early Gearbox Failures in Offshore Wind Turbines

AbstractDuring the past decades, great efforts have been undertaken to make wind power a competitive source for electrical energy. By the end of 2012, global installed wind capacity had risen to 264GW, almost a tenfold increase of the capacity in 2002. Nevertheless, the wind energy sector is still far too expensive to be profitable, especially the strong growing offshore branch. However, a significant part (about 25%) of the cost is related to operation and maintenance (O&M), in particular the failures of the main components (i.e. gearbox and drivetrain) resulting in long downtimes and hence high O&M costs. Various studies today discuss if condition monitoring systems, which allow the forecasting of failures at a very early stage, might be the cure to the problems related to the reliability of the gearbox. Rather than formulating yet another methodology to forecast upcoming failures, the aim of this paper is to identify the underlying cause of the reliability issues related to the gearbox

HAWT near-wake aerodynamics, part I : axial flow conditions

An improved physical understanding of the rotor aerodynamics of a horizontal axis wind turbine (HAWT) is required to reduce the uncertainties associated with today’s design codes. Wind tunnel experiments contribute to increased knowledge and enable valida- tion and construction of models. The present study focuses on the near-wake of a model HAWT in both axial and yawed flow conditions. At three downstream planes parallel to the rotor plane, single-sensor hot-film traverses are made. The phase-locked unsteady three- dimensional flow velocity vector is determined by a novel data reduction method. A series of two papers discusses the near-wake aerodynamics of a model HAWT. The main goals are to obtain a detailed understanding of the near-wake development and to arrive at a base for model construction and validation. The first paper presents the experimental setup, data reduction and the results for the baseline case (axial flow conditions). In the second paper, the results for the yawed flow cases are presented and the effect of yaw misalignment on the near-wake development is discussed. Copyrightpeer-reviewe

Effects of geometry and tip speed ratio on the HAWT blade's root flow

In this study, the effect of the parameters playing a role in the root flow behavior of HAWT are only partly understood. To better reveal the root flow properties, this study presents the progression of HAWT blade root flow at two different blade geometries and at two different tip speed ratios. The effects of the geometry and the tip speed ratio on the root flow behavior and on the evolution of the root flow features are investigated. This study aims to answer the following questions: (i) What are the effects of the blade geometry and tip speed ratio on the root flow behavior? (ii) How are the blade wake and the root vortex evolution affected by the change of these parameters? The analysis of the velocity fields shows that the radial flow behavior changes with different blade geometries but a remarkable difference in the radial flow behavior is not observed with the change of tip speed ratio. The formation of the wake is different at three test cases because of different loading that the blades are encountered. From the circulation distribution along the blades, while a strong root vortex can be observed in Blade 1, the bound vorticity along Blade 2 builds up gradually when moving outboard, and do not show a trace of a strong root vortex.peer-reviewe

Velocity measurements in the near wake of a horizontal axis wind turbine

Single film hot-film measurements in the near wake of a horizontal axis wind turbine are performed in the Delft University of Technology Open Jet Facility in order to capture the blade azimuth dependent velocity vectors. A novel approach to determine the average, blade azimuth dependent, velocity vector is proposed. It makes use of the asymmetric response of hot-films that have the film parallel to its probe. Contrary to the traditional approach in which the direction of velocity components cannot be determined and which poorly predicts relatively small velocity components, the proposed method yields velocity direction as part of the solution and consistently predicts relatively small velocity components. An uncertainty analysis is performed on the velocity magnitude and flow angle, quantifying the data quality. Furthermore, comparisons with previously published data on the same rotor, in the same wind tunnel are made. The comparisons increase confidence in the data and reveal the advantages of the proposed velocity determination method. Qualitatively, the velocity signals derived with the proposed approach are in agreement with expectations from a general vortex wake model, also the relatively small tangential and radial velocity components.peer-reviewe

An approach for the verification and validation of rotor aerodynamics codes based on free-wake vortex methods

This paper presents an approach to verify and validate a newly developed free-wake lifting line vortex code to model the wake generated by a wind turbine in axial flow conditions. Although the code is intended to simulate wind turbine wakes, it may be readily applied to propellers and helicopter rotors. The Delft University of Technology model wind turbine is used for this case study. Detailed hot-film inflow measurements in the near wake and smoke visualizations of tip vortex cores are used as a basis for validating the free-wake model. A parametric analysis was carried out to investigate how different levels of blade/wake descretizations and viscous modeling influence the accuracy of the inflow results. The tip vortex locations were compared with those predicted by the vortex model. In general very good agreement was obtained. It was found that the middle sections of the blades are rather insensitive to the choice of the viscous modeling parameters. However, high sensitivity to these parameters was observed at the blade tip and root regions.peer-reviewe

Generating nested quadrature rules with positive weights based on arbitrary sample sets

For the purpose of uncertainty propagation a new quadrature rule technique is proposed that has positive weights, has high degree, and is constructed using onl

Bayesian model calibration with interpolating polynomials based on adaptively weighted Leja nodes

An efficient algorithm is proposed for Bayesian model calibration, which is commonly used to estimate the model parameters of non-linear, computationally expens

Bayesian model calibration with interpolating polynomials based on adaptively weighted Leja nodes

An efficient algorithm is proposed for Bayesian model calibration, which is commonly used to estimate the model parameters of non-linear, computationally expensive models using measurement data. The approach is based on Bayesian statistics: using a prior distribution and a likelihood, the posterior distribution is obtained through application of Bayes' law. Our novel algorithm to accurately determine this posterior requires significantly fewer discrete model evaluations than traditional Monte Carlo methods. The key idea is to replace the expensive model by an interpolating surrogate model and to construct the interpolating nodal set maximizing the accuracy of the posterior. To determine such a nodal set an extension to weighted Leja nodes is introduced, based on a new weighting function. We prove that the convergence of the posterior has the same rate as the convergence of the model. If the convergence of the posterior is measured in the Kullback-Leibler divergence, the rate doubles. The algorithm and its theoretical properties are verified in three different test cases: analytical cases that confirm the correctness of the theoretical findings, Burgers' equation to show its applicability in implicit problems, and finally the calibration of the closure parameters of a turbulence model to show the effectiveness for computationally expensive problems