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

Evaluation of the lifting line vortex model approximation for estimating the local blade flow fields in horizontal-axis wind turbines

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) experi- mental 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 estima- tions 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 consider- ably 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-reviewe

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

Body and wake influences on the 3D flow close to a HAWT rotor

This work focuses on the hypothesis that flow three-dimensionality in the rotor proximity is strongly affected by blade blockage and bound circulation. The extent of flow three-dimensionality in the blade proximity needs to be assessed particularly in sensitive regions such as the root and tip of the blade. Moreover the induction needs to be decomposed into two components; that resulting due to the blade presence and that due to the wake. In this regard, the estimation of bound vorticity is essential such that its effects and those due to trailing vorticity can be understood. To investigate the hypothesis, an experimental and a numerical approach is used. A two bladed, 2m diameter rotor was tested in the Open Jet Facility at TU Delft measuring 6m×6.5m×13.5m using Stereo Particle Image Velocimetry (SPIV). The flow field around the tested HAWT blade was successfully measured and the velocity components around the entire blade were studied. The bound vorticity was measured using the experimental data and compared well with the numerical measurements. The radial flow, responsible for 3D effects was found to have high values not at the 100% span position but rather at around 90% span. The flow components were also decomposed using the potential flow model. Whilst the wake causes an increase in the flow three-dimensionality, the blade effect was found to be indeed the opposite and hence to decrease the inductions (and hence flow three-dimensionality). While momentum approaches utilize correction functions to account for the finite nature of the HAWT blade, we provide new understanding of the three-dimensional flow field which may prove useful for future work in refining momentum formulations.peer-reviewe

The influence of a cubic building on a roof mounted wind turbine

The performance of a wind turbine located above a cubic building is not well understood. This issue is of fundamental importance for the design of small scale wind turbines. One variable which is of particular importance in this respect is the turbine height above roof level. In this work, the power performance of a small wind turbine is assessed as a function of the height above the roof of a generic cubic building. A 3D Computational Fluid Dynamics model of a 10m x 10m x 10m building is used with the turbine modelled as an actuator disc. Results have shown an improvement in the average power coefficient even in cases where the rotor is partially located within the roof separation zone. This goes against current notions of small wind turbine power production. This study can be of particular importance to guide the turbine installation height on building roof tops.peer-reviewe

Assessment of the lifting line approximation for wind turbine blade modelling

Lifting line vortex approaches have been widely used to predict rotor flow fields. Nonetheless, there could be some deficiencies in the flow field close to the blade due to the assumption that blade vorticity is concentrated on a line. The present study thoroughly assessed the errors arising from this approximation by prescribing the bound circulation as a boundary condition on the flow, using an inverse lifting-line free- wake vortex approach. The prescribed bound circulation was calculated front two independent sources using (1) experimental results from PIV and (2) data generated from a 3D panel free- wake vortex approach, where the blade geometry is fully modelled. The flow field around the blades from the inverse lifting line vortex model was then compared with those directly produced by SPIV and the 3D panel model.peer-reviewe

Combined Structural Optimization and Aeroelastic Analysis of a Vertical Axis Wind Turbine

Floating offshore wind energy poses challenges on the turbine design. A possible solution is vertical axis wind turbines, which are possibly easier to scale-up and require less components (lower maintenance) and a smaller floating structure than horizontal axis wind turbines. This paper presents a structural optimization and aeroelastic analysis of an optimized Troposkein vertical axis wind turbine to minimize the relation between the rotor mass and the swept area. The aeroelastic behavior of the different designs has been analyzed using a modified version of the HAWC2 code with the Actuator Cylinder model to compute the aerodynamics of the vertical axis wind turbine. The combined shape and topology optimization of a vertical axis wind turbine show a minimum mass to area ratio of 1.82 kg/m2 for blades with varying blade sections from a NACA 0040 at the attachment points to a NACA 0015 in the equatorial region. During an aeroelastic analysis of the wind turbine a maximum flapwise deflection of 0.45 m and a maximum edgewise deflection of 0.47 m were found. While the turbine is aeroelastically stable, an oscillation as a result of resonance reduces the fatigue life

Effect of Oxygen-Reducing Atmospheres on the Safety of Packaged Shelled Brazil Nuts during Storage

This work reports the application of oxygen-(O2-) reducing atmosphere methods on stored shelled Brazil nut (Bertholletia excelsa H.B.K.) packs aiming to evaluate the degree of aflatoxin degradation, nuts lipid oxidative stability, fungi control, and hygienic conditions improvement. The methods applied were (a) ozone: O3, (b) carbon dioxide: CO2, and (c) O2 absorber pads with and without vacuum. From all modified atmospheres evaluated, the best performance was obtained with O3, either with or without vacuum. It was the only nut treatment that was able to degrade aflatoxins. None of the spiked (AFLs: 15 μg·kg−1) nut samples O3- treated had aflatoxins detected up to the LC-MS/MS method LOQ (0.36 μg·kg−1 for total AFLs), thus producing safer nuts. Also it kept the fatty acid oxidation indicator—malondialdehyde stable and improved the sensory attributes for consumer acceptance. In addition, the destruction of fungi and yeast was observed since the O3 application (from 1.8 × 104 cfu/g to NG = no growth). All other treatments stabilized and/or inhibited microorganisms' growth only. By adding CO2 gas also played an important role in the nut quality. Regarding cost, gaseous O3 showed to be of low cost for application in the nut packs

Wake skew angle variation with rotor thrust for wind turbines in yaw based on the MEXICO experiment

The primary objective of the MEXICO (Model Experiments in Controlled Conditions) project was to generate experimental data from which the uncertainties of the computational tools employed to predict wind turbine performance and loads. Pressure sensors were used for pressure measurements while PIV was used with the major aim of tracking the tip vortex trajectory. The aerodynamic forces on the blades were derived found from the pressure measurements and were used in an inverse free wake lifting line model to compute the positions of the tip vortices. From these the wake skew angle was derived. A relationship between the skew angle and the thrust coefficient was thus drawn.peer-reviewe