Technologies for Sustainable Built Environments Centre

Rosario Nobile | Dr Maria Vahdati | Dr Janet Barlow | Dr Anthony Mewburn-Crook

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Mesh

The mesh, as shown in Figure 2, is mainly

composed of three sub-domains: one fixed sub-

domain outside the rotor, one dynamic sub-

domain around the blades of the rotor and one

fixed sub-domain for the remaining part of the

rotor.

In Figure 2, the local mesh around the blades of

the rotor is refined for accurate and efficient

resolution of the boundary layer and wakes.

Boundary conditions and turbulence

method

Symmetrical boundaries were used for the top

and bottom parts of the 2-D model with no-slip

boundary conditions at the two sides. An opening

boundary was chosen for the output and a

constant wind was defined for the inlet.The three

Reynolds-Averaged Navier-Stokes (RANS)

turbulence methods are: the standard k-ω model,

the standard k-ε model and the SST model 4.

Dynamic stall

As shown in Figure 3, dynamic stall is mainly

characterised by flow separations at the suction

side of the airfoil 5. This can be summarised in

four crucial stages:

• Leading edge separation starts,

• Vortex build-up at the leading edge,

• Detachment of the vortex from leading edge and build-up of trailing edge vortex,

• Detachment of trailing edge vortex and breakdown of leading edge vortex

Overview

Dynamic stall is an intrinsic phenomenon of

Vertical Axis Wind Turbines (VAWTs) at low

tip speed ratios (TSRs) and its nature can

affect fatigue life and energy output of a

wind turbine. A two-dimensional Vertical

Axis Wind Turbine (VAWT) is explored. The

analysis is conducted by using

Computational Fluid Dynamics (CFD) tools.

The numerical results are compared with

experimental data.

Introduction

The last few years have proved that Vertical Axis

Wind Turbines (VAWTs) are more suitable for urban

areas than Horizontal Axis Wind Turbines (HAWTs) 1, 2, 3. However, the aerodynamic analysis of a

VAWT is very complicated than conventional wind

turbines and at low tip speed ratios (TSRs<5),

VAWTs are subjected to a phenomenon called

dynamic stall. This can really affect the fatigue

life of a VAWT if it is not well understood.

Recently, to study a full scale wind turbine in the

wind tunnel is an infeasible task due to size

limitations and costs involved. Therefore , a

Computational Fluid Dynamics (CFD) Software,

ANSYS 12.0, is selected for this study and in

order to reduce time and memory costs only a 2-D

case is explored.

MethodGeometry

As shown in Figure 1, the 3-D solid model of the

rotor was generated with ProEngineer 4.0. And the

2-D model of the VAWT was generated from the

middle plane and imported into ANSYS CFX 12.0.

References1. S. Mertens, Wind energy in the built environment: concentrator effects

of buildings. TU Delft, 2006, pp. 3-14.

2. S. Stankovic, N. Campbell, and A. Harries, Urban Wind Energy.

Earthscan, 2009

3. C. J. Ferreira, G. van Bussel, and G. van Kuik, 2D CFD simulation of

dynamic stall on a Vertical Axis Wind Turbine: verification and validation

with PIV measurements, presented at the 45th AIAA Aerospace Sciences

Meeting and Exhibit, 2007, pp. 1-11.

4. D. C. Wilcox, Turbulence Modeling for CFD. DCW industries La Canada,

2006.

5. J. Larsen, S. Nielsen, and S. Krenk, Dynamic stall model for wind turbine

airfoils, Journal of Fluids and Structures, vol. 23, no. 7, 2007, pp. 959-

982.

6. S. Wang, D. B. Ingham, L. Ma, M. Pourkashanian, and Z. Tao, Numerical

investigations on dynamic stall of low Reynolds number flow around

oscillating airfoils, Computers & Fluids, vol. 39, no. 9, 2010, pp. 1529-

1541.

Acknowledgements• The author would like to thank my academic supervisors Dr M. Vahdati and Dr

J. Barlow and my industrial supervisor Dr A. Mewburn-Crook for their supports

for this work.

• The author is also grateful to the EPRSC and MatildasPlanet for funding this

project.

Contact information• Department of Technologies for Sustainable Built Environments, University of

Reading, Whiteknights, RG6 6AF

• Email: r.nobile@reading.ac.uk

• www.reading.ac.uk/tsbe

Dynamic Stall in Vertical Axis Wind Turbines

Figure 1. Three dimensional rotor of a straight-bladed Darrieus wind turbine obtained with ProEngineer 4.0 and two dimensional rotor extrapolated from middle plane.

3-D 2-D

Fixed Sub-domain

Fixed Sub-domain

Dynamic Sub-domain

Figure 2. Mesh and sub-domains for the two-dimensional .VAWT

Results The numerical simulations obtained during the

present study are mainly compared with an

experimental study carried out in 2010 6. The SST

method shows a good agreement with the

experimental data than the k- and k-ε methods.

Figure 4 shows how the lift and the drag

coefficients, Cl and Cd, are affected by different

angles of attack and TSRs. The curve shapes are in

good agreement with the experimental data, which

is the red line on the right side.

Figure 4. Lift and Drag coefficient (Cl and CD ) from numerical studies and experimental data.

Figure 3. An example of dynamic stall for the 2-D simulation at low TSR and different positions of the blades.

ConclusionsThe key conclusion of this numerical study is that a

CFD tool will allow the visualisation of the flow

aerodynamics involved during the operation of a

VAWT that is not possible with ordinary wind

tunnel tests. In general the CFD code adopted is

able to show dynamic stall that is typical found in

VAWTs at low TSRs. Also, from this numerical

analysis appears that in order to achieve a good

agreement between numerical and experimental

data , the right selection of the turbulent method

is fundamental. However, it is strongly suggested

to develop a more sophisticated 3-D model that is

more realistic than 2-D.

In Figure 4, strong instability is observed for large

angle of attacks and low TSRs due to deep

dynamic stall. In addition, the development of

several peaks, especially for negative angle of

attacks and low TSRs can be associated with the

development of upstream wakes that interact with

the downstream blades.