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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Aerodynamics and rotor design' taking place on Wednesday, 12 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Hervé Le Sourne ICAM, France
Co-authors:
jean-francois largeau (1) F P abdelhaq abdelqari (1) herve le sourne (1)
(1) ICAM, carquefou, France

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Abstract

Using experimental and CFD models for selecting blades profile of a small vertical axis wind turbine

Introduction

The recent years have seen more focus on the development of small and medium sized wind turbines aimed to urban or remote areas. For this reason, the vertical axis wind turbines (VAWTs) present an attractive alternative to the traditional horizontal axis wind turbines (HAWTs) in the environment of fluctuating wind.


Approach

In France more particularly, the intention is to develop small generators that can be used as power supply for remote residences, ships, or any residence in urban areas in order to reduce the consumption from the general power grid. This served the basis for the development of a series of VAWTs in the framework of the Aerojoules Project financed by Nantes region (fig1 first VAWT prototype ).

This work is based on the study and development of a small VAWT, with a specific focus on its aerodynamic optimization, in order to maximize the power extracted from the wind. A design loop is presented, detailing the procedure for a decoupled fluid-structure analysis. Two main pillars of the design process are, therefore, the aerodynamic and the structural analysis.

The first step was to develop a correct 2D modeling able to define quickly the best profiles to design a VAWT. These results are used to define structural load. The second step was to modeling correctly the flow over a complete 3D blade. Different configurations were tested, extruded, twisted or helicoidal ()with or without movement. The extruded case has permitted to compare the results with previous studies in order to obtain a reference case. At the end the complete Vawt was modeled with a sliding mesh and unsteady flow. These results were compared with experimental work and Qblade Software. The aim is to propose a methodology to design a VAWT with an overview, a good feeling, of the robustness of the method used (2D,3D, turbulence models …) and his impact on the prediction quality (Power coefficient, torque…).


Main body of abstract

This work has started with a less complex two dimensions (2D) study of NACA profiles with CFD for different wind velocities and different inclination angles. The sensitivity of the solution to the grid density, to the time step and to the adopted turbulence model is studied. By comparing 2D numerical results with experimental ones, the best CFD solution parameters (cell size, time step, etc.) are determined and re-used for developing a 2D CFD model of a section perpendicular to the rotation axis of the vertical axis wind turbine ( example of calculation ).
A pre-selection of design alternatives for the turbine is then carried out and a series of simplified 2D CFD simulations are performed in order to define candidate solutions for detailed three-dimensional CFD models. Those simulations are performed using the sliding mesh functionality of STAR CCM+ software.
Because 2D simulation could not well predict the topology of flow over the profile (separation point, recirculation zone, helicoidal or twisted shapes of VAWT blades), full three dimensions (3D) unsteady simulations are needed. In order to obtain a robust CFD approach, this 3D study starts with extruded NACA profiles (no rotation). These shapes are well known and the results were compared with previous works. The impact of several turbulence models are studied (k-epsilon, k-omega, Spalart-Amalart) for RANS method VS LES (Smagorinsky, Wale subgrid method). Of course, sensitivity of the mesh and particularly close to the blade wall are studied (ie first cell height in term of Y+ and wall function impact). In addition, small NACA profiles are created using a 3D printer and tested in a wind tunnel. Pressure fields, velocity fields (hot-wire anemometry) and aerodynamic forces (drag and lift) are measured for different wind velocities and different inclination angles. Some visualization technics were used to highlight the topology of the flow (separation point, recirculation zone). Different roughnesses were tested to show the impact on flow topology, and aerodynamic force. Endeed, the parameter is often forgotten but the cad used in CFD software is perfect, without roughness witch could modify the separation point and also the complete topology of flow aver the blade.
Based on this work, twisted and helicoidal blades are modeled in CFD and studied in a wind tunnel. At least CDF simulation of the complete VAWT were done to obtained results in accordance with a complete vertical axis wind turbine tested in a wind-tunnel. These results are compared with QBlade predictions. This software, based on Boundary Element Method (BEM), has recently proposed an update for VAWT prediction with a Double Multiple Streamtube (DMS) algorithm method.

With respect to the structural analysis, a simplified method to estimate, by using 2D models, the pressure loads on the blades for preliminary verifications is presented and its results are compared to those of available 3D CFD simulations. A methodology to modeling a complete 3D VAWT is proposed to obtain results in accordance with experimental work. A general discussion on the structural design of the blades is introduced, setting the basis for further work in this aspect.
Special thanks to the region Pays de Loire for his support.


Conclusion

A procedure for selecting Blades profile of a vertical axis wind turbine is presented. Wind tunnel test are performed on small NACA profiles and experimental results are used as reference to validate 2D numerical models. Two dimensions CFD simulations of a section of a VAWT are then performed to choose the best design alternative, which is then re-used for a detailed 3D CFD simulation. A simplified method is proposed to calculate, using a 2D model, the pressure loads on the blade of the turbine.
Starting from the 2D results, a 3D study was done in order to obtain more precise physical values. 3D extruded profiles were studied in comparison with previous works and experimental approach (one blade realized by 3D printer) in order to build the base of our CFD methodology. The difficulties were increased by using twisted and helicoidal profiles both in CFD and in wind tunnel. At the end, a complete VAWT were simulated with sliding mesh method and compare with experimental data.
Finally, 2D(RANS) and 3D simulations (RANS (steady/unsteady)/LES) are compared with Qblade Software with Double Multiple Streamtube in order to define what is the best tool in accordance with the results and the precision needed to design a vertical axis wind turbine. This work will be complete by the development of new methodology using neuronal network and genetic code. The new approach is study with Dublin City University. The goal is to reduce the computational time required by CFD code (2D or 3D) because of unsteady calculations.




Learning objectives
This work aims to demonstrate that small tests and CFD simulations may be useful to design a vertical axis wind turbine and to optimize the profile of the rotor blades. A procedure to assess rapidly the pressure fields on blades will also be presented.


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