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Lifting line vortex models have been widely used to predict flow fields around wind turbine rotors. Such models are known to be deficient in modelling flow fields close to the blades due to the assumption that blade vorticity is concentrated on a line and consequently the influences of blade geometry are not well captured. The present study thoroughly assessed the errors arising from this approximation by prescribing the bound circulation as a boundary condition on the flow using a lifting line free-wake vortex approach. The bound circulation prescribed to free-wake vortex model was calculated from two independent sources using (1) experimental results from SPIV and (2) data generated from a 3D panel free-wake vortex approach, where the blade geometry is fully modelled. The axial and tangential flow fields around the blades from the lifting line vortex model were then compared with those directly produced by SPIV and the 3D panel model. The comparison was carried out for different radial locations across the blade span. The study revealed the cumulative probability error distributions in lifting line model estimations for the local aerofoil flow field under both 3D rotating and 2D non-rotating conditions. It was found that the errors in a 3D rotating environment are considerably larger than those for a wing of infinite span in 2D flow. Finally, a method based on the Cassini ovals theory is presented for defining regions around rotating blades for which the lifting line model is unreliable for estimating the flow fields., Wind Energy

Copyright 2016 AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing., Lifting line vortex models have been widely used to predict flow fields around wind turbine rotors. Such models are known to be deficient in modelling flow fields close to the blades due to the assumption that blade vorticity is concentrated on a line and consequently the influences of blade geometry are not well captured. The present study thoroughly assessed the errors arising from this approximation by prescribing the bound circulation as a boundary condition on the flow using a lifting line free-wake vortex approach. The bound circulation prescribed to free-wake vortex model was calculated from two independent sources using (1) experimental results from SPIV and (2) data generated from a 3D panel free-wake vortex approach, where the blade geometry is fully modelled. The axial and tangential flow fields around the blades from the lifting line vortex model were then compared with those directly produced by SPIV and the 3D panel model. The comparison was carried out for different radial locations across the blade span. The study revealed the cumulative probability error distributions in lifting line model estimations for the local aerofoil flow field under both 3D rotating and 2D non-rotating conditions. It was found that the errors in a 3D rotating environment are considerably larger than those for a wing of infinite span in 2D flow. Finally, a method based on the Cassini ovals theory is presented for defining regions around rotating blades for which the lifting line model is unreliable for estimating the flow fields., Peer Reviewed, Postprint (published version)

The tip vortex of a wind turbine rotor blade originates as a result of a complex distribution of vorticity along the blade tip thickness. While the tip vortex evolution was extensively studied previously in other work, the mechanism of the initiation of the tip vorticity in a 3D rotating environment is still somewhat obscured due to lack of detailed experimental evidence. This paper therefore aims at providing an understanding of how tip vorticity is formed at the wind turbine blade tip and what happens just behind the tip trailing edge. Stereo Particle Image Velocimetry (SPIV) is used to measure the flow field at the tip of a 2m diameter, two- bladed rotor at the TU Delft Open Jet Facility (OJF). The rotor has a rectangular blade tip. Spanwise measurements were performed for both axial and yawed flow conditions with a very small azimuthal increment. A 3D, unsteady, potential flow panel method is also used for the purpose of better understanding the tip bound vorticity. A validation study is carried out with positive results. This paper is focused on axial flow results. A complex distribution of vorticity is found along the blade tip thickness. Just after release, the tip vortex becomes almost immediately round and well defined. Observations from the MEXICO rotor are confirmed again by a slight inboard convection of the tip vortex. This is explained by means of the effect of chordwise vorticity at the tip from the numerical solutions. The results presented in this work suggest that a more physical interpretation of the tip loss effect is required. Currently, inclusion of tip effects are based primarily on either wake induced effects or on an empirical 3D correction for airfoil data. This research should stimulate a more rigorous approach, where the effects of the blade tip chordwise vorticity are implemented in tip correction models., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

The scope of this work was to investigate radial flow component for a Horizontal Axis Wind Turbine in axial flow conditions and to assess its impact on the turbine operation. This was done by means of Particle Image Velocimetry and numerical simulation with a 3D unsteady potential-flow panel model. A direct comparison between the numerical and experimental radial velocity results show differences in the tip and root regions. These differences have important implications on the wake development just at the moment of release of the tip vortex. Moreover, the impact of the radial velocities on the blade loading has been studied using the numerical results. The contribution of the radial velocity to the normal load on the blade is only slightly appreciable in the tip and root regions of the blade. However, as the numerical model does not account for viscous effects, further analysis of impact on boundary layer development is necessary., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

The main motivation behind this work is to create a purely analytical engineering model for wind turbine wake upward deflection due to shear flow, by developing a closed form solution of the velocity field due to an oblique vortex ring. The effectiveness of the model is evaluated by comparing the results with those of a free-wake model. The solution of the velocity field due to an oblique vortex ring is obtained by using the result of an upright ring along with an equivalent point method. The wake model is derived using oblique ring elements with a number of suitable assumptions. Results of wake vertical deflection are compared with a free-wake solution. A linear trend between wake deflection and shear flow exponent is found with both models. The oblique ring model shows some discrepancies from the free-wake result in terms of the dependence of the deflection on the reference tip speed ratio. The oblique ring model needs further refinements and validation with experimental work and is only currently suited for the determination of general wake kinematics. It however provides immediate results for a given input and can be useful in generating databases with wake geometry information., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

The primary objective of the MEXICO (Model Experiments in Controlled Conditions) project was to generate experimental data for validation of models for wind turbines. Kulite©pressure sensors were used for pressure measurements while Particle Image Velocimetry was used with the aim of tracking the tip vortex trajectory. The pressure measurements were carried out for both axial and yawed flow conditions with yaw angles of 15º; 30º and 45º. For the Particle Image Velocimetry measurements data was gathered for axial flow and for the ±30º yaw cases at a single tip speed ratio. In this work, an inverse free wake lifting line model, a direct free wake model and a BEM model are validated with the MEXICO data. Particular emphasis is placed on the study of yawed flow conditions. The inverse free-wake model makes use of the experimental loads as input in order to find the distribution of inductions and angle of attack. The predictive capability of BEM may therefore be assessed based on this. Validation of the inverse free-wake model was performed by investigating the stagnation pressureprediction as well as the vortex trajectory prediction. This was done by means of the PIV data gathered from the MEXICO experiment. This PIV data was also used for validation purposes of the direct free-wake model. The differences in the angle of attack distributions in yawed flow with these models was studied in order to assess the difference in results between the use of 2D and 3D airfoil data., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

Wind turbine structures and components suffer excessive loads and premature failures when key aerodynamic phenomena are not well characterized, fail to be understood, or are inaccurately predicted. Turbine blade rotational augmentation remains incompletely characterized and understood, thus limiting robust prediction for design. Pertinent rotational augmentation research including experimental, theoretical, and computational work has been pursued for some time, but large scale wind tunnel testing is a relatively recent development for investigating wind turbine blade aerodynamics. Because of their large scale and complementary nature, the MEXICO and UAE Phase VI wind tunnel experiments offer unprecedented synergies to better characterize and understand rotational augmentation of blade aerodynamics. Cn means, Cn standard deviations, two-dimensional cp distributions, and three-dimensional planform surface pressure topologies from these two experiments were analyzed and compared. Rotating blade data were evaluated against analogous stationary blade data. Rotational augmentation effects were found to be pervasive and were present over the blade radius and throughout blade operating envelopes at all radial locations investigated. Rotational effects manifested themselves in both mean and time varying statistics, in both two-dimensional sectional data as well as three-dimensional planform data. Comparative analyses of MEXICO and UAE data validated and generalized current knowledge regarding rotationally augmented blade flow fields. In addition to confirming prior research, results also provided new insights not attainable by considering either data set in isolation of the other., Aerodynamics & Wind Energy, Aerospace Engineering

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 validation and construction of models. The present study focuses on the near-wake of a model HA WT in both axial and yawed flow conditions. A t 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. Copyright © 2008 John Wiley & Sons, Ltd.

Aerospace Engineering

Aerospace Engineering

The mining licence situation in Turkey is reviewed including the total number of exploration and operation licences for the period Dec 2008 - Dec 2015 as well as the new exploration licences issued per year by direct application and by auction for the period 2002 - 2016. Strategies employed by exploration companies to add mineral resource projects to their portfolios - greenfield or brownfield exploration and acquisition of existing project - are described. At the present time, owing to an amendment to Turkish mining law in 2015, ground can be accessed solely by auction or by licence transfer between parties. Data required (e.g. licence documents and publicly available geological information) by an exploration company to allow a property to be evaluated as part of a potential licence transfer are discussed., The mining licence situation in Turkey is reviewed including the total number of exploration and operation licences for the period Dec 2008 - Dec 2015 as well as the new exploration licences issued per year by direct application and by auction for the period 2002 - 2016. Strategies employed by exploration companies to add mineral resource projects to their portfolios - greenfield or brownfield exploration and acquisition of existing project - are described. At the present time, owing to an amendment to Turkish mining law in 2015, ground can be accessed solely by auction or by licence transfer between parties. Data required (e.g. licence documents and publicly available geological information) by an exploration company to allow a property to be evaluated as part of a potential licence transfer are discussed.