The Eighth Asia-Pacific Conference on Wind Engineering,
December 10–14, 2013, Chennai, India
Influence of Boundary Layer control on wind turbine
blade aerodynamic characteristics – Part I –
Computational Study
T. A. Sundaravadivel1, Dr. S. Nadaraja Pillai2, Dr. C. Senthil Kumar3
1Doctoral Student, Department of Aeronautical Engineering, Madras Institute of Technology, Chennai, TN,
India, aerovelu@yahoo.com
2
Department of Aeronautical Engineering, J. J. College of Engineering & Technology, Tiruchirapalli, TN,
India, aeropillai@gmail.com
3
Department of Aerospace Engineering, Madras Institute of Technology, Anna University, chennai, TN, India,
cskumar@mitindia.edu
ABSTRACT
Wind turbine blades are shaped to generate the maximum power from the wind at the minimum cost.
The shape of rotor blade plays an important role in determining the overall aerodynamic performance of a
horizontal axis wind turbine. The focus is on research regarding a kind of Passive flow control (PFC) as it is
applied to alter wind turbine performance and loads. Boundary Layer Fence (vortex generator) , a well known
passive flow control device, which is previously used for aircraft application for stopping the 3Dimensional
nature of the flow, is employed for wind turbine application in this work. The passive flow control is installed in
different radial locations viz., r/R= 0.3, 0.4, 0.5, 0.6, 0.7and0.8 and the variation of Coefficient of Lift (Cl) and
Coefficient of Drag (Cd) for various cross sections were reported. In all these locations, aerodynamic noise is
also computed in the downstream and it is also discussed.
Results show that installing a vortex generator increases the generated torque and power up to a
maximum of 7.4% and 11.77% respectively the measured wind turbine noise reduced up to 12.97%.
Keywords: Airfoil, Chord, Boundary Layer Fence, Aerodynamic Noise.
Nomenclature
R = rotor radius
r = spanwise distance
TSR = tip speed ratio
ȍ = rotational speed
W = wind speed
CP = mechanical power coefficient
CT = thrust coefficient
A = rotor area
SPL=Sound Pressure Level
1. Introduction
Wind turbines interact with the wind, capturing a part of its kinetic energy and convert it in
to usable energy. This energy conversion is because of several phenomena. The efficiency of
wind turbine blade is an important one. From the theory of Incompressible flow over finite
aircraft wings, when the incoming wind is flown over the Blade, there is a quantifiable
amount of span wise flow, called as third dimensional flow which is other than axial and
lateral component. Due to this span wise flow, vortices are created near the wing tip. The
strength of the vortices contributes to the total drag of the blade by the name induced drag. In
Proc. of the 8th Asia-Pacific Conference on Wind Engineering – Nagesh R. Iyer, Prem Krishna, S. Selvi Rajan and P. Harikrishna (eds)
c 2013 APCWE-VIII. All rights reserved. Published by Research Publishing, Singapore. ISBN: 978-981-07-8011-1
Copyright doi:10.3850/978-981-07-8012-8 211
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wind turbines, the resulting pressure difference causes inward span wise flow on the suction
side and outward span wise flow on the pressure side near the tip. At the trailing edge,
vortices are generated, which is the origin of induced drag. The purpose of adding a wing
fence to the rotor blade design is to decrease the induced drag from the blade by changing the
downwash distribution. Reduction of total drag is obtained if the additional drag from the
vortex generator (a fence, normal to the blade) is less than the reduction of the induced drag
on the remaining blade. The art is then to design a vortex generator, which reduce the induced
drag and increase the power production.
Wind turbines generate sound via various routes, both mechanical and aerodynamic. As the
technology has advanced, wind turbines have gotten much quieter, but sound from wind
turbines is still an important siting criterion. Sound emissions from wind turbine have been
one of the more studied environmental impact areas in wind energy engineering. In general,
the effects of noise on people can be classified into three general categories: 1. Subjective
effects including annoyance, nuisance, dissatisfaction, 2. Interference with activities such as
speech, sleep, and learning and 3. Physiological effects such as anxiety, tinnitus, or hearing
loss.
In almost all cases, the sound levels associated with wind turbines large & small produce
effects only in the first two categories, with modern turbines typically producing only the first.
The third category includes such situations as work inside industrial plants and around
aircraft. Whether a sound is objectionable will depend on the type of sound (tonal, broadband,
low frequency, or impulsive) and the circumstances and sensitivity of the person (or receptor)
who hears it. Because of the wide variation in the levels of individual tolerance for noise,
there is no completely satisfactory way to measure the subjective effects of noise or of the
corresponding reactions of annoyance and dissatisfaction. Operating sound produced from
wind turbines is considerably different in level and nature than most large scale power plants,
which can be classified as industrial sources. Wind turbines are often sited in rural or remote
areas that have a corresponding ambient sound character. Furthermore, while noise may be a
concern to the public living near wind turbines, much of the sound emitted from the turbines
is masked by ambient or the background sounds of the wind itself. The sound produced by
wind turbines has diminished as the technology has improved. As blade airfoils have become
more efficient, more of the wind energy is converted into rotational energy, and less into
acoustic energy. Vibration damping and improved mechanical design have also significantly
reduced noise from mechanical sources. Due to this research on wind turbine noise plays a
significant role.
The objective of the present work is to install a vortex generator in the various span wise
locations (r/R = 0.3,0.4, 0.5, 0.6, 0.7, 0.8 and 0.9 ) on a rotating wind turbine blade and to
quantify the amount of variation happen in the flow components as well as the aerodynamic
noise downstream of the turbine computationally.
2. Methodology
2.1 Problem Definition
In the Present work, an attempt is made to study the effectiveness of installing a vortex
generator in the various span wise locations of an NREL Phase VI wind turbine blade. The
reason for selecting the NREL Phase VI blade for the study is many researchers have did
experiments with this blade and many used this blade as a bench mark for their studies. The
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NREL blades have a linear taper and a non linear twist with S809 aerofoil sections. The length
of the blade is 5.029 m. The simulation is conducted for 7, 10, 15 and 25 m/s wind speeds.
The angular velocity of 72 RPM is given for the rotor for all the simulations. The numerical
simulations were carried out for a conventional NREL blade initially, after getting satisfactory
results, vortex generators are installed
in different span wise locations r/R =
0.3,0.4,0.5,0.6,0.7,0.8 and 0.9. The model of NREL blade with vortex generator installed is
shown in Figure 1.
In addition to the above, an attempt is made to calculate the aerodynamic noise
generated due to the rotation of the wind turbine blades with and without installing vortex
generators at various span wise locations say r/R = 0.3,0.4,0.5,0.6,0.7,0.8 and 0.9. In fluent, an
inbuilt specialized code, the FFWH code is available for noise calculation and that is
employed for the calculation purpose. For activating the code, receivers should be set at
different required locations and the intensity of the noise is computed on that particular
locations. As per the international standard (ref.5), the aerodynamic noise downstream wind
turbine is also observed at a length of 17m . The setup is shown in Figure 2.
The height of the vortex generator is calculated by considering the displacement
thickness over the surface of airfoil. Based on the boundary layer thickness, the vortex
generator height is determined. The boundary layer thickness is calculated at the location
where the vortex generator is installed , for an optimum velocity of 15m/s by taking sea level
density and viscosity into account and the vortex generator is designed in such a way that the
height of it is set above than the calculated boundary layer thickness at that location.
2.2 CFD Solver
The geometric model of the NREL rotor blade was constructed according to the
available data (ref.2) using Catia. A Well known CFD solver, ANSYS –Fluent is used for the
present study. It is an unstructured, multipurpose, finite volume based, Navier strokes CFD
solver. It offers a wide range of physical models, starting with the basic two equation models
k-epsilon, k-Omega up to the latest SST k-Omega (4 equations), LES and DES. Several
numerical solvers are available, pressure/density based, steady/transient state,
Implicit/Explicit etc.
The Ffowcs Williams and Hawkings (FW-H) equation is used to compute the
aerodynamic noise. It is essentially an inhomogeneous wave equation that can be derived by
manipulating the continuity equation and the Navier-Stokes equations. The FW-H formulation
in fluent can handle rotating surfaces as well as stationary surfaces. The procedure for
computing sound using the FW-H acoustics model in fluent consists largely of two steps. In
the first step, a time-accurate flow solution is generated, from which time histories of the
relevant variables (e.g., pressure, velocity, and density) on the selected source surfaces are
obtained. In the second step, sound pressure signals at the user-specified receiver locations are
computed using the source data collected during the first step. In computing sound pressure
using the FW-H integral solution, fluent uses a so-called forward-time projection" to account
for the time delay between the emission time (the time at which the sound is emitted from the
source) and the reception time (the time at which the sound arrives at the receiver location).
The forward-time projection approach enables to compute sound at the same time as the
transient flow solution progresses, without having to save the source data (Ref. 9).
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Figure 1 :- Model of NREL Blade used for the study with vortex generator installed.
Figure2:- Details of the receivers for noise measurement
The domain consists of 4.7 million cells. Boundary layer mesh is formed near the
blade for capturing the flow properties. Grid independence study was carried out and finally a
grid independent solution is arrived at 10 layers adjacent to the blade with a Y+ value of 1
and the first cell length is 0.034 mm. Unstructured grid is formed in the domain other than the
above said layers. The mesh is shown in Figure 3.The blade is kept stationary and is given the
“WALL” boundary condition. The blades are arranged inside a small cylindrical layer which
is made to rotate with 72 RPM for the study. “Symmetry” conditions are given in the far field
domain. It is shown in Figure. 4. All the simulations are conducted with pressure based steady
state solver. SST k-Omega model is used for the study.
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Figure 3 :- Meshed model used for the present study
Figure 4:- Boundary conditions used for the present study
3. Results and Discussion:
The Wind speed Vs computed shaft torque is shown in Figure 5.The simulated torque
values are found good agreement with the experimental one, after refining the grid to an
optimized cell size of 4.7 million. For the blade with vortex generators, the results show that for
the wind velocities of 7 and 10 m/s the torque does not have a typical change. When the inlet
velocity is increased to 15m/s and 25m/s a good increase in the torque values are seen. A
maximum of 7.4% increase in the torque values are found. A plot of Power Vs Tip Speed Ratio
(Ratio of Blade tip speed to inlet velocity) is shown in Figure 6. Power Coefficient and Pressure
coefficient are computed using the formula, CPc =(Tן)/(0.5ȡʌR2V3) and Cp = (P-P’)
/0.5ȡ{(rŸ)2+V2}, Both agree well with the experimental results. As reported in the literatures,
maximum power coefficient is obtained for tip speed ratio between 6 to 7 for all the cases. A
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maximum of 11.77% increase in power coefficient is found in blade with vortex generator
installed at r/R=40% for a tip speed ratio of 6.545.
From the Figure 7 it is clear that there is a considerable amount of decrease in sound
pressure level due to the addition of a vortex generator at the various span wise stations. In all the
cases (r/R =0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9) the sound pressure level is found to be reduced
when compared with the conventional blade results. A maximum of 12.97% reduction in the
sound pressure level is found in the blade with vortex generator installed at r/R = 0.9. As stated
in ref. 5, it is inferred that the sound pressure level at the root is lower than that of the tip.
Figure 5:- Wind Speed Vs Torque
Figure 6:- Tip Speed Ratio Vs Power Coefficient
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Figure 7: Variation of SPL for different V.G. locations
4. References
1. Bertagnolio.F , N. Sorensen, J. Johansen, P. Fuglsang, Wind Turbine Airfoil Catalogue, Riso National
Laboratory, Roskilde, Denmark, August 2001.
2. Design and Experimental Results for the S809 Airfoil NREL/SR-440-6918
3. Johansen, J.; Sorensen, N.; Aerodynamic Investigation of Winglets on Wind turbine blades using CFD.
Technical ReportRISO-R-1543, Denmark,EN,2006.
4. Ramsay R.R , J. M. JaniszewskaG.M., Gregorek, The Ohio State University, Columbus, Ohio Wind Tunnel
Testing of Three S809 Aileron Configurations for use on Horizontal Axis Wind Turbines
5. Tadamasa et.al. (2011), “Numerical Prediction of wind turbine noise” J.Renewable Energy,Vol. 36, pp.1902
to1912.
6. Yen-Pin Chen et.al.,(2009), “ A Study of the Aerodynamic Behavior of A NREL Phase VI Wind Turbine Using
the CFD Methodology”Master’s Thesis, Wright State University.
7. Walter P. Wolfe Stuart S. Ochs, AIAA 97- 0973 CFD Calculations of S809Aerodynamic Characteristics
8. Williams, D.; Reeder, F.; Maple, C., Solfelt, A. Modelling, Simulation, and Flight Tests for a T-38 Talon with
Wing Fences. Journal of Aircraft, 2010, vol. 47. 9. Fluent user manual, fluent inc 2006. 1217