Carlos Simao Ferreira

2D CFD Simulation of Dynamic Stall on a Vertical Axis Wind Turbine: Verification and Validation with PIV Measurements

After a decrease of interest in the 1990’s, the research on Vertical Axis Wind Turbines (VAWT) a has reappeared in the last years as a result of the its increasing application in the built environment, where VAWTs present several advantages over Horizontal Axis Wind Turbines (HAWT). The VAWT has an inherent unsteady aerodynamic behavior due to the variation of angle of attack with the angle of rotation µ, perceived velocity and consequentially Reynolds number. The phenomenon of dynamic stall is then an inherent efiect of the operation of a Vertical Axis Wind Turbine (VAWT) at low tip speed ratios (‚), having a signiflcant impact in both loads and power. The complexity of the unsteady aerodynamics of the VAWT makes it extremely attractive to be analyzed using Computational Fluid Dynamics (CFD) models, where an approximation of the continuity and momentum equations of the Navier-Stokes equations set is solved. The complexity of the problem and the need for new design approaches for VAWT for the built environment has driven the authors of this work to focus the research of CFD modeling of VAWT not in the perspective of creating one large academic model to test a particular situation, but to develop a work that would: † verify the sensitivity of the model to its grid reflnement (space and time), † evaluate the difierences between the difierent commonly used turbulence models (Laminar, Spalart i Allmaras and k i †), and † be evaluated using Particle Image Velocimetry (PIV) experimental data, thus determining the suitability of this data for model validation. The 2D model created represents the middle section of a single bladed VAWT with inflnite aspect ratio. The model simulates the experimental work b of ∞ow fleld measurement using Particle Image Velocimetry (PIV) by Sim~ao Ferreira et al 1 for a single bladed VAWT, for two difierent reference Reynolds numbers of Re = 52000 and Re = 70000 for three tip speed ratios: ‚ = 2;3;4. The results show the suitability of the PIV data for the validation of the model, the unsuitability of the application of a single turbulent model and the high sensitivity of the model to grid reflnement.

Validating BEM, Direct and Inverse Free Wake Models with the MEXICO Experiment

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.

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.

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) 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 ...

Airfoil optimization for stall regulated vertical axis wind turbines

Multi-megawatt Vertical Axis Wind Turbines (VAWTs) have inherent design and operational advantages that make them relevant for floating offshore applications. Many offshore VAWT concepts use stall for power regulation. Stall regulation imposes several constrains in the aerodynamic, structural and generator design. Due to the azimuthally varying and unsteady aerodynamics experienced by a VAWT, designing an airfoil for stall regulation is still a significant challenge. In this work, a family of airfoils for stall regulated VAWT is defined through numerical airfoil optimization, based on the original work of Simao Ferreira and Geurts [20]; the optimization is multi-objective, optimizing structural and aerodynamic performance. The control performance is not yet implemented in the function. The optimization is performed at Reynolds numbers representative of multi-megawatt VAWT. The performance of the VAWT, including operation in dynamic stall, is evaluated with an unsteady double-wake viscous-inviscid panel method and CFD simulations. The performance of the optimized airfoils is compared against a conventional airfoil, namely the NACA 0018 airfoil. The stall regulatory performance is assessed to provide insight for future optimizations. The aerodynamic performance is evaluated using three different numerical models: a single wake panel model coupled to airfoil polar data; a viscous-inviscid double wake panel model, capable of simulating dynamic stall; and an eulerian RANS CFD model. The airfoils are also design for a robust performance in the case of surface roughness. The preliminary results show that the design for surface roughness conflicts with the design for dynamic stall control. An extensive study of multi-objective optimisations with different weights of the different elements of aerodynamic performance is presented.

Creating a benchmark of vertical axis wind turbines in dynamic stall for validating numerical models

An experimental campaign using Particle Image Velocimetry (2C-PIV) technique has been conducted on a H-type Vertical Axis Wind Turbine (VAWT) to create a benchmark for validating and comparing numerical models. The turbine is operated at tip speed ratios (TSR) of 4.5 and 2, at an average chord-based Reynolds number of 1.6e and 0.8e. At both TSR, the velocity fields are presented in the mid (symmetry) plane of the blade for eight azimuthal positions. The velocity fields are directly derived from PIV, while the loads are obtained through an integral approach presented by Noca et al. The experimental data of the velocity fields around the airfoil and the loads on the blade are used for numerical validation. The aim of evaluating the two different TSR is identifying the effect of Dynamic Stall (DS), which is not present at the higher TSR, while dominant at the lower. The DS phenomenon is numerically very hard to model, so a solid benchmark for a VAWT in DS is of great interest. The aim of the paper is to present the experimental flow fields, and the validated loads on the blades for both TSR.

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.

Experimental investigation of the wind turbine blade root flow

Several methods from experimental to analytical are used to investigate the aerodynamics of a horizontal axis wind turbine. To understand 3D and rotational effects at the root region of a wind turbine blade, correct modeling of the flow field is essential. Aerodynamic models need to be validated by accurate experimental data. In this paper, the experimental results of the aerodynamic behavior of a model wind turbine blade, by focusing on the blade root flow, are presented. The measurements are performed on a 2 bladed rotor having 1 m radius by means of Stereo Particle Image Velocimetry in a wind tunnel. The spanwise velocity distribution on the suction side of the blade is determined in detail. It shows a complex flow pattern in the root region and positive spanwise flow component apparent at radial stations beyond r/R=0.4 at the leading edge (z/c=0.25).

Towards integral boundary layer modelling of vane-type vortex generators

This paper details an experimental investigation of submerged vane-type vortex generators (VGs) on a flat plate turbulent boundary layer inside a boundary layer wind tunnel. Spanwise planes at various positions downstream of the VG trailing edges are measured using stereoscopic particle image velocimetry (SPIV). The analysis focuses on the streamwise and spanwise development of the boundary layer, in light of the boundary layer integral parameters. The high spatial SPIV resolution enables an accurate resolution of the boundary layer with and without vortex generators. Through the study of the measured wake flow, insights towards an integral boundary layer modelling approach of vortex generators is gained through the verification of the vortex development properties as well as the net effect on the encompassing boundary layer.

Experimental and Numerical Quantification of Radial Flow in the Root Region of a HAWT

This paper explores the evolution of radial flow in a Horizontal Axis Wind Turbine (HAWT) blade root region. The radial flow is analyzed in the potential flow and viscous flow regions. An experiment carried out by means of stereo Particle Image Velocimetry to measure the velocity field produced by a HAWT blade. While the radial flow in the potential flow region was obtained from the measurements, the radial flow in the boundary layer was derived from CFD. By the direct observations obtained from the experiment, an insight is gained about the nature of the radial flow in the suction side of the blade as well as in the near wake. An outboard radial flow motion is observed in the root region. This tendency of the flow changes dramatically when it reaches the maximum chord position of the blade, where the radial flow moves inboard. The trace of the viscous region due to merging of the boundary layers and trailing vorticity are observed clearly in the radial velocity and vorticity distributions at 135o azimuth angle of the blade. In the viscous flow region the radial flow is more pronounced than in the potential flow region. The performed CFD simulations are able to predict the vortex formation in the maximum chord region and its interaction with the nacelle.