C.J. Simao Ferreira

Comparison of aerodynamic models for Vertical Axis Wind Turbines

Multi-megawatt Vertical Axis Wind Turbines (VAWTs) are experiencing an increased interest for floating offshore applications. However, VAWT development is hindered by the lack of fast, accurate and validated simulation models. This work compares six different numerical models for VAWTS: a multiple streamtube model, a double-multiple streamtube model, the actuator cylinder model, a 2D potential flow panel model, a 3D unsteady lifting line model, and a 2D conformal mapping unsteady vortex model. The comparison covers rotor configurations with two NACA0015 blades, for several tip speed ratios, rotor solidity and fixed pitch angle, included heavily loaded rotors, in inviscid flow. The results show that the streamtube models are inaccurate, and that correct predictions of rotor power and rotor thrust are an effect of error cancellation which only occurs at specific configurations. The other four models, which explicitly model the wake as a system of vorticity, show mostly differences due to the instantaneous or time averaged formulation of the loading and flow, for which further research is needed.

Kinetic energy entrainment in wind turbine and actuator disc wakes: an experimental analysis

The present experimental study focuses on the comparison between the wake of a two-bladed wind turbine and the one of an actuator disk. The flow field at the middle plane of the wake is measured with a stereoscopic particle image velocimetry setup, in the low-speed Open Jet Facility wind tunnel of the Delft University of Technology. The wind turbine wake is characterized by the complex dynamics of the tip vortex development and breakdown. Analysis of the flow statistics show anisotropic turbulent fluctuations in the turbine wake, with stronger components in the radial direction. The wake of the actuator disc is instead characterized by isotropic random fluctuations. The mixing process in the shear layer is further analysed in terms of flux of mean flow kinetic energy, to show the main differences between the kinetic energy entrainment in the actuator and the turbine wake. This project is intended to provide the basis for understanding the origin of the limitations of the current wake models based on the actuator disc assumption.

The origins of a wind turbine tip vortex

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.

PIV visualization of dynamic stall VAWT and blade load determination

The increasing awareness of the need for environmentally sustainable housing and cities has driven the promotion of wind energy conversion systems for the built environment. One of the results of the development of solutions for the built environment is the reappearance of Vertical Axis Wind Turbines (VAWTs). In the built environment, the VAWT presents several advantages over the more common Horizontal Axis Wind Turbines (HAWTs), namely: its low sound emission (consequence of its operation at lower tip speed ratios), better esthetics due to its three-dimensionality, its insensitivity to yaw wind direction and its increased power output in skewed ∞ow (see Mertens et al 1 and Sim~ao Ferreira et al 2 ). The phenomenon of dynamic stall is an inherent efiect of the operation of a VAWT at low tip speed ratios (‚). The presence of dynamic stall has signiflcant impact on both load and power. The paper focuses on evaluating the feasibility of estimating loads on Vertical Axis Wind Turbine (VAWT) blades in dynamic stall by velocity data acquired with Particle Image Velocimetry (PIV). The work uses both numerical and experimental data. Simulated velocity data from a Detached Eddy Simulation (DES) at space and time reflnement equivalent to that obtained with PIV is used to estimate the error associated with the method. The method is then applied to experimental data to verify the in∞uence of the complexity of the ∞ow and determination of space and time derivatives. The acquired data over the entire rotation is used to calculate the blade forces from the velocity data and its derivatives (solving the momentum equation), following the methodology presented by Noca et al 3 and Scarano et al. 4 The integration of the forces from the velocity fleld should overcome the di‐culties and limitations presented by pressure sensors for local section loads, but involves the referred di‐culties in determining the correct time-derivatives.

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.

Experimental comparison of a wind-turbine and of an actuator-disc near wake

The actuator disc (AD) model is commonly used to simplify the simulation of horizontal-axis wind-turbine aerodynamics. The limitations of this approach in reproducing the wake losses in wind farm simulations have been proven by a previous research. The present study is aimed at providing an experimental analysis of the near-wake turbulent flow of a wind turbine (WT) and a porous disc, emulating the actuator disc numerical model. The general purpose is to highlight the similarities and to quantify the differences of the two models in the near-wake region, characterised by the largest discrepancies. The velocity fields in the wake of a wind turbine model and a porous disc (emulation of the actuator disc numerical model) have been measured in a wind tunnel using stereo particle image velocimetry. The study has been conducted at low turbulence intensity in order to separate the problems of the flow mixing caused by the external turbulence and the one caused by the turbulence induced directly by the AD or the WT presence. The analysis, as such, showed the intrinsic differences and similarities between the flows in the two wakes, solely due to the wake-induced flow, with no influence of external flow fluctuations. The data analysis provided the time-average three-component velocity and turbulence intensity fields, pressure fields, rotor and disc loading, vorticity fields, stagnation enthalpy distribution, and mean-flow kinetic-energy fluxes in the shear layer at the border of the wake. The properties have been compared in the wakes of the two models. Even in the absence of turbulence, the results show a good match in the thrust and energy coefficient, velocity, pressure, and enthalpy fields between wind turbine and actuator disc. However, the results show a different turbulence intensity and turbulent mixing. The results suggest the possibility to extend the use of the actuator disc model in numerical simulation until the very near wake, provided that the turbulent mixing is correctly represented.

Comparison between PIV measurements and computations of the near-wake of an actuator disc

Experimental stereoscopic PIV measurements in the wake of a two-bladed rotor and a porous actuator disc are compared to numerical simulation of an actuator disc. Compared to previous literature, the focus of the present analysis is on the near wake, where the actuator discs fail to represent the complex flow structures correctly, which affects the downstream representation of the full wake behind a real rotor. The near wake region is characterised by the instability and breakdown of the tip-vortex helical system, which constitutes the onset of a stronger mixing process. The comparison focuses on the turbulent structures in the shear layer at the borders of the wake through the analysis of the Reynolds stresses and by employing POD on two separate regions. The analysis shows that the actuator discs fail to capture the details of the complex flow behind a rotor, but that the experimental and numerical actuator discs are generally comparable at a certain distance behind the actuator disc. This project is intended to provide the basis for understanding the origin of the limitations of the current wake models based on the actuator disc assumption.

Evaluation of different methods of determining the angle of attack on wind turbine blades under yawed inflow conditions

As part of the AVATAR and Mexnext projects, this study compares several methods used to derive lifting line variables from CFD simulations of the MEXICO rotor in yawed inflow. The results from six partners within the AVATAR/Mexnext consortium using five different methods of extraction were compared. Overall comparison of the induced velocities at the mid and tip parts of blade shows fairly good agreement between the tested methods, where the derived angle of attack differs within 1°, within the linear range this accounts to ˂ 10% uncertainty on the aerodynamic forces. The presented comparison shows inadequate agreement between the methods for application towards the root.