Evaluation of different methods for determining the angle of attack on wind turbine blades with CFD results under axial inflow conditions

This work presents an investigation on different methods for the calculation of the angle of attack and the underlying induced velocity on wind turbine blades using data obtained from three-dimensional Computational Fluid Dynamics (CFD). Several methods are examined and their advantages, as well as shortcomings, are presented. The investigations are performed for two 10MW reference wind turbines under axial inflow conditions, namely the turbines designed in the EU AVATAR and INNWIND.EU projects. The results show that the evaluated methods are in good agreement with each other at the mid-span, though some deviations are observed at the root and tip regions of the blades. This indicates that CFD results can be used for the calibration of induction modeling for Blade Element Momentum (BEM) tools. Moreover, using any of the proposed methods, it is possible to obtain airfoil characteristics for lift and drag coefficients as a function of the angle of attack.Comment: This manuscript is Accepted at at Renewable Energy journal- online 13 March 2018 under the CC-BY-NC-ND 4.0 licens

Development of a model for computing tip loss corrections in wind turbines

A la fecha, la comprensión de la relación entre el comportamiento de la curva de potencia en una turbina eólica y su número de álabes parece ser insatisfactoria. Esto se debe en parte a que la teoría Blade Element Momentum (BEM), que es la herramienta más usada para modelar el comportamiento aerodinámico de las turbinas, asume un rotor con infinito número de álabes. Esta suposición simplifica el análisis en la medida en que el rotor, a diferencia de las turbinas eólicas reales, es descrito como un disco continuo que gira con una velocidad de rotación determinada. A su vez, hay diferentes fénomenos físicos que suceden en el álabe y que no pueden ser captados por esta teoría. Por ejemplo, la pérdida de potencia en la punta se explica en parte por la no-uniformidad de flujo causada por la presencia de un número finito de álabes en el rotor. Algunas correcciones realizadas para modelar la pérdida de potencia en las puntas tratan de incluir como parámetros importantes un número finito de álabes y la relación de velocidad en la punta. Los resultados de estas correcciones no son del todo satisfactorios y generan incertidumbres a la hora de calcular la distribución de cargas sobre el álabe. Es razonable suponer que también hay una relación entre la geometría del álabe y la pérdida de potencia en la punta del mismo; para incluir dicho efecto creemos que el párametro que es necesario considerar es la estructura de la cuerda del álabe en la punta. La primera corrección para incorporar un número finito de alabes fue presentada de forma analítica por Prandtl hacia la década de 1920. El mostró que la pérdida de potencia decae de manera exponencial en la cercanía de la punta del álabe. Esta corrección tiene en cuenta no sólo un número finito de álabes en el rotor sino también la posición radial en el alabe, y corrige la predicción de las fuerzas aerodinamicas calculadas con BEM. En 2005, Wen Z. Shen y sus colegas desarrollaron una corrección que mejora los resultados obtenidos por el modelo de Prandtl. Su propuesta, basada en el análisis empírico de los datos de un proyecto, consiste en agregar un factor de corrección al modelo desarrollado por Prandtl, de manera que aparece el efecto de la velocidad de rotación sobre las cargas en la punta. La función exponencial introducida por Shen (g) depende de la relacion de velocidad en la punta (TSR por su sigla en ingles)y fue hallada empíricamente (si g = 1 se obtiene el factor de corrección de Prandtl). A pesar de mejorar notablemente la precisión en el cálculo de las cargas, el modelo de Shen presenta algunas diferencias respecto a los casos de bajas TSR. Con el objetivo de mejorar los resultados obtenidos por Prandtl y Shen este trabajo presenta un nuevo factor de corrección que incluye parámetros que no han sido incluidos en los modelos existentes; por ejemplo, como ya se ha mencionado, la cuerda del álabe obteniendo así dos coefcientes empiricos y una nueva expresion para calcular las perdidas en la punta. Finalmente, vale la pena añadir que, más allá de estudiar y corregir los modelos de Prandtl y Shen, este trabajo pretende ser un aporte a la mejor comprensión de los fenómenos que ocurren en las puntas de los álabes eólicos

Development of a second order dynamic stall model

Dynamic stall phenomena bring risk for negative damping and instability in wind turbine blades. It is crucial to model these phenomena accurately to reduce inaccuracies in predicting design driving (fatigue) loads. Inaccuracies in current dynamic stall models may be due to the facts that they are not properly designed for high angles of attack, and that they do not 10 specifically describe vortex shedding behaviour. The Snel second order dynamic stall model attempts to explicitly model unsteady vortex shedding. This model could therefore be a valuable addition to DNV GL’s turbine design software Bladed. In this thesis the model has been validated with oscillating airfoil experiments and improvements have been proposed for reducing inaccuracies. The proposed changes led to an overall reduction in error between the model and experimental data. Furthermore the vibration frequency prediction improved significantly. The improved model has been implemented in Bladed and tested 15 against small scale turbine experiments at parked conditions. At high angles of attack the model looks promising for reducing mismatches between predicated and measured (fatigue) loading. Leading to possible lower safety factors for design and more cost efficient designs for future wind turbine

Wind Tunnel Test of Counter-Rotating Dual Rotor Wind Turbine With Double Rotational Armature Design

This study evaluates the performance of a counter-rotating dual rotor wind turbine (CR-DRWT) with 2 m2 rotor radius equipped with a double rotational armature in an open jet wind tunnel. With only one similar-sized design previously assessed in a wind tunnel, this study offers valuable validation material for the literature. Through wind tunnel testing, the CR-DRWT confirmed earlier findings in literature and achieved a 15% to 50% increase in power output and a 10% increase in efficiency (CP) compared to a single rotor configuration at higher wind speeds (> 7 m/s). Though these gains were not observed at lower wind speeds (4–7 m/s). The simplified mechanics of a double rotational armature show promise for SWTs, as financial viability depends on reducing LCOE through efficiency improvements that maximize energy capture. The design's maximum CP values were below those achieved in previous field tests at larger scale highlighting potential for improvement for smaller sized turbines. To further explore the aerodynamics of CR-DRWT's, computational fluid dynamics (CFD) simulations are recommended, as they could provide insights into optimizing flow dynamics around CR-DRWT's. Finally, the study emphasizes the need for precise pitch angle and rotational speed measurements to improve the value of future measurements

Investigation of the dynamic inflow effects due to collective and individual pitch steps on a wind tunnel setup

Dynamic inflow effects occur due to the rapid change of the rotor loading underconditions such as fast pitch steps. The paper presents a setup suitable for the investigation ofthose effects for non-axisymmetric rotor conditions, namely individual pitch steps. Furthermore, insights into the relevant phenomena are gathered. An individual pitch control capable model wind turbine is set up in a wind tunnel in order to conduct measurement under controllable conditions. During the execution of the collective and individual pitch steps, the loads and the operational parameters are recorded by the onboard sensors. Meanwhile, simulations engineering aeroelastic codes are run in order to evaluate their accuracy for predicting the relevant phenomena. Results show distinct behaviour of the rotor loads during an individual pitch step, which differs from the loads under collective steps. The free vortex wake simulations are able to predict the turbines’ response satisfactory while the blade element momentum tools show deviations from the measurements. The findings serve as a basis for discussion and future work.Dynamic inflow effects occur due to the rapid change of the rotor loading underconditions such as fast pitch steps. The paper presents a setup suitable for the investigation ofthose effects for non-axisymmetric rotor conditions, namely individual pitch steps. Furthermore, insights into the relevant phenomena are gathered. An individual pitch control capable model wind turbine is set up in a wind tunnel in order to conduct measurement under controllable conditions. During the execution of the collective and individual pitch steps, the loads and the operational parameters are recorded by the onboard sensors. Meanwhile, simulations engineering aeroelastic codes are run in order to evaluate their accuracy for predicting the relevant phenomena. Results show distinct behaviour of the rotor loads during an individual pitch step, which differs from the loads under collective steps. The free vortex wake simulations are able to predict the turbines’ response satisfactory while the blade element momentum tools show deviations from the measurements. The findings serve as a basis for discussion and future work