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Technologies for Sustainable Built Environments Centre
Rosario Nobile | Dr Maria Vahdati | Dr Janet Barlow | Dr Anthony Mewburn-Crook
Click to edit Master title style
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: [email protected]
• 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.