16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012 - 1 - Exploring the Flow around a Savonius Wind Turbine Ivan Dobrev 1,* , Fawaz Massouh 1 1: Fluid Mechanics Laboratory, Arts et Métiers ParisTech, 151 bd de l’Hopital, 75013 Paris, France * correspondent author: [email protected]Abstract This paper presents the study of flow through a vertical axis wind turbine of Savonius type. For this kind of wind turbines the axis of rotation is perpendicular to wind and the flow is always unsteady. Due to operation as a drag type wind turbine and the continuous variation of flow angle with respect to blades, strong unsteady effects including separation and vortex shedding are observed. In these conditions, it is supposed that CFD analysis with turbulence modeling by means of detached eddy simulation can give good results. In order to confirm this supposition, experimental investigation of flow around a Savonius wind turbine is carried out in the wind tunnel of Arts et Metiers ParisTech. The studied rotor has two blades and a height which is approximately equal to the rotor diameter. Initially, the aerodynamic performance of the rotor is measured. Then, for optimal point of operation, the flow field through the rotor is investigated using particle image velocimetry. By means of conditional sampling technique, the velocity fields around the blades and in the wake are obtained for several angular positions of the rotor. The investigation permits to determine the structure of the real flow and to create a database which is helpful for CFD validation. The numerical simulation is carried out by means of three-dimensional unsteady solver which gives comparable results with experiments. These results confirm the capability of detached eddy simulations modelling to represent well the turbulent detached flow. Thus this kind of turbulence modelling can be applied to analyse and to optimize the Savonius wind turbine as well as another drag type wind turbine. 1. Introduction Wind turbines are generally classified as two families: horizontal axis and vertical axis machines. This classification refers to the position of rotor axis relatively to wind. The Savonius wind turbine is thus classified as vertical axis wind turbine like the Darrieus, Gyromill or H-rotor etc. The basic version of this rotor has S-shaped cross-section formed by two semi-circular blades with a small overlap between them. The rotor has good starting characteristics and operates at relatively low wind speeds. It does not need an orientation device and can work for all wind directions. This wind turbine operates at low tip speed ratio and presents real interest for small or miniaturized machines. Akwa et al (2012) publish recently an interesting review which presents the Savonius wind turbine as and the influence of different parameters on its aerodynamic performance. There are several experimental studies concerning a flow structure around Savonius rotor. One of the first attempts to study the torque mechanism and the flow field is carried out by Fujisawa (1992) and Fujisawa & Gotoh (1992). The authors study the influence of the blade overlapping on aerodynamic performance of two-blade rotor. The flows visualizations allow to authors make conclusion that a Coanda like flow on a convex side of advancing blade permits to improve the torque. Lately Fujisawa (1996) performs the similar study carried out by means of particle image velocimetry (PIV). The author compares the phase-averaged velocity distributions around Savonius rotors for different overlap ratio with numerical calculations by a discrete vortex method. The author found a significant difference between the velocity distribution on a rotating rotor and that
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16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 1 -
Exploring the Flow around a Savonius Wind Turbine
Ivan Dobrev1,*
, Fawaz Massouh1
1: Fluid Mechanics Laboratory, Arts et Métiers ParisTech, 151 bd de l’Hopital, 75013 Paris, France
Abstract This paper presents the study of flow through a vertical axis wind turbine of Savonius type. For this kind of wind turbines the axis of rotation is perpendicular to wind and the flow is always unsteady. Due to operation as a drag type wind turbine and the continuous variation of flow angle with respect to blades, strong unsteady effects including separation and vortex shedding are observed. In these conditions, it is supposed that CFD analysis with turbulence modeling by means of detached eddy simulation can give good results. In order to confirm this supposition, experimental investigation of flow around a Savonius wind turbine is carried out in the wind tunnel of Arts et Metiers ParisTech.
The studied rotor has two blades and a height which is approximately equal to the rotor diameter. Initially, the aerodynamic performance of the rotor is measured. Then, for optimal point of operation, the flow field through the rotor is investigated using particle image velocimetry. By means of conditional sampling technique, the velocity fields around the blades and in the wake are obtained for several angular positions of the rotor. The investigation permits to determine the structure of the real flow and to create a database which is helpful for CFD validation. The numerical simulation is carried out by means of three-dimensional unsteady solver which gives comparable results with experiments. These results confirm the capability of detached eddy simulations modelling to represent well the turbulent detached flow. Thus this kind of turbulence modelling can be applied to analyse and to optimize the Savonius wind turbine as well as another drag type wind turbine.
1. Introduction
Wind turbines are generally classified as two families: horizontal axis and vertical axis machines.
This classification refers to the position of rotor axis relatively to wind. The Savonius wind turbine
is thus classified as vertical axis wind turbine like the Darrieus, Gyromill or H-rotor etc. The basic
version of this rotor has S-shaped cross-section formed by two semi-circular blades with a small
overlap between them. The rotor has good starting characteristics and operates at relatively low
wind speeds. It does not need an orientation device and can work for all wind directions. This wind
turbine operates at low tip speed ratio and presents real interest for small or miniaturized machines.
Akwa et al (2012) publish recently an interesting review which presents the Savonius wind turbine
as and the influence of different parameters on its aerodynamic performance.
There are several experimental studies concerning a flow structure around Savonius rotor. One of
the first attempts to study the torque mechanism and the flow field is carried out by Fujisawa (1992)
and Fujisawa & Gotoh (1992). The authors study the influence of the blade overlapping on
aerodynamic performance of two-blade rotor. The flows visualizations allow to authors make
conclusion that a Coanda like flow on a convex side of advancing blade permits to improve the
torque. Lately Fujisawa (1996) performs the similar study carried out by means of particle image
velocimetry (PIV). The author compares the phase-averaged velocity distributions around Savonius
rotors for different overlap ratio with numerical calculations by a discrete vortex method. The
author found a significant difference between the velocity distribution on a rotating rotor and that
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 2 -
for a stationary rotor. However, the results of flow simulation are found to be quite satisfactory in
the case of rotation.
Interesting study is presented by Murai et al (2007). Here the authors study velocity field obtained
by means of particle tracking velocimetry (PTV) in order to calculate the pressure field around the
turbine and on the blade, and also to evaluate the rotor torque. The authors confirm a supplementary
effect of lift force on torque and the importance of the overlap effect of the blades to increase
torque. Nakajima et al (2008) investigate the aerodynamic performance of different rotor types
using a pigment streak-line method. The flow visualization and the torque measurement permit to
obtain information about single and two-stage Savonius rotor.
McWilliams & Johnson (2008) present a study of flow around different model of Savonius wind
turbine by means of PIV. The flow fields were obtained at six constant phase angles of the rotor.
Examination of the flow shows for all rotor positions vortices that shed from blade surfaces. The
authors show a considerable interaction between flow over the forward blade and the wake of the
trailing blade and note the importance of lift forces on the torque.
Another PIV exploration in the case of one or two Savonius rotors is presented by Shigetomi et al
(2011). Here the authors present the study of flow field around two Savonius turbines in close
configurations, which permits to understand quantitatively the interaction mechanism and to
revealed two types of power-improvement. The presented visualization and measurement of flow
through the Savonius rotor permit to understand the torque mechanism and the influence of gap
between blades. However, the obtained flow field results did not permit comparison between
experiments and simulation, which is needed to choose an appropriate turbulence model.
The research presented in this paper aims to create database for flow around a Savonius wind
turbine. This database contains the flow fields for different phase angles, which are needed for the
comparison with flow simulation. Initially, the PIV exploration is carried out, and then numerical
simulations are performed for the experimentally studied cases. Finally, the analysis and
comparison of experimental and numerical results permit to validate the applied numerical
modeling.
2. Test bench
The aim of this experimental study is to obtain the aerodynamic performance of studied wind
turbine and also the detailed information about velocity field around the rotor. The experimental
aerodynamic performance permits to qualify the validity of numerical tests. The PIV measuring is
needed to obtain detailed data concerning the mechanism of torque creation and the vortex structure
downstream of the rotor.
The experiments are conducted in the wind tunnel of Arts et Métiers - ParisTech, fig.1. The wind
tunnel is a closed circuit type and has a three-blade axial fan with a rotor diameter of 3m. The fan is
driven by frequency controlled asynchronous motor with a power of 120 kW. Behind the fan, flow
is decelerated in a settling chamber, which is equipped with honeycomb straighteners and wire
mesh in order to smooth the flow. The tunnel nozzle accelerates the wind, from settling chamber to
test section, up to 40 m/s. The nozzle has contraction ratio of 12.5, which ensures a uniform
velocity profile with a turbulence ratio less than 0.25%. The semi-guided test section has a cross-
section of 1.35 m x1.65 m and a length of 2 m. The static pressure in the test section is equal to
atmospheric pressure. Hence, the upstream velocity depends on the stagnation pressure in the
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 3 -
settling chamber which is measured by pressure transducer Furness Control FC20.
The tested wind turbine has a two-blade rotor with a diameter D=219.5 mm and the height
H=200 mm. As result, the rotor aspect ratio AR = H/D is low, in order of 0.91. The blade has
circular arc form of 180° and a thickness of 1mm. The blade radius r is 57.5 mm and the gap width
s is 11.5 mm. Hence, the gap width ratio s/2r is 0.1. The endplates with diameter of 300 mm reduce
the flow leakage from pressure to suction blade sides. Also, the endplates permit to strengthen the
rotor structure. The blades and endplates are fabricated from thin polymethylmethacrylat sheet in
order to ensure the transparency, which is needed for PIV investigation, fig 2.
Fig. 1 Test bench Fig. 2 Flow field around the rotor
The rotor is mounted on a shaft coupled with a DC generator. The rotor load is controlled with a
rheostat connected to the generator. The coupling between the rotor shaft and the generator is made
via a contactless torque transducer HBM T20WN. This transducer also emits 360 pulses per
revolution. Because the measured power is quite low, the seals of the rotor ball bearings are
removed, and the grease is replaced by thin silicon oil. A fiber optic sensor Keyence FS20V, which
detects a reflective target on the shaft, permits to locate the passage of the blade considered to be
the reference. Thus, by counting the number of square signals delivered by the torque-meter, after
the passage of the reference signal, it is possible to obtain the rotor's angular position with an
accuracy of 1°. The acquisition of data from the sensors is carried out by data acquisition card,
which emits a TTL signal for triggering the PIV measurements at a desired rotor angular position.
During the test, the wind turbine rotational speed varies from 800 rpm to 1000 rpm and the
upstream flow velocity V∞ varies from 9 m/s to 15 m/s. Thus, Reynolds number calculated with
rotor diameter and rotor peripheral velocities during the tests is greater than 140,000. Data
acquisition is carried out by computer equipped with acquisition card. At each operating point, all
the torque T, rotational velocity n and upstream wind velocity V∞ are measured continually with a
rate of 10 samples per second. To minimize the torque fluctuations, before to calculate the rotor
power, the results are averaged for a period of 1 minute. Then power coefficient Cp, which
represented the ratio of obtained and available power is calculated:
AV
PCp
2
3
∞
=ρ
(1)
The test permits to obtain the variation of the power coefficient Cp when the tip speed ratio TSR
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
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varies. The TSR is calculated as the ratio between peripheral velocity and upstream wind velocity:
∞
=V
UTSR (2)
The variation of power coefficient with respect to tip speed ratio is presented on fig. 3. The curve
presents typical characteristics of Savonius wind turbine, for similar Reynolds number, and for
similar aspect ratio. The curve is quite flat and the power coefficient has a maximum of 0.18 for a
TSR equal approximately of 0.8, which is chosen to perform PIV exploration.
Fig. 3 Power performance Fig. 4 Coordinate system and azimuth angles
3. PIV investigation
The PIV system is managed by the Dantec DynamicsStudio 2.30. The taking of images is done by
implementing a Litron Nano-L 200-15 laser, with an impulse power of 2x200 mJ, a two cameras of
2048x2048px Dantec FlowSense 4M, equipped with the lens Micro-Nikkor AF 60 mm f/2.8D, a
frame grabber cards and a synchronization system. The last synchronizes images and laser flashes
with the blade angular position. The seeding of flow is made with micro droplets of olive oil created
by a mist generator 10F03 Dantec. The droplets diameter is supposed to be 2-5µm.
The measuring of flow field inside and downstream of the rotor is carried out in the case of low
rotation velocity n=500 rev/min. The upstream velocity is 7.2 m/s, which corresponds to a
TSR=0.8.
The study is carried out for 6 different azimuth positions of the rotor θ = 0°, 30°, 60°, 90°, 120° and
150°, and for each point a sequence of 200 double images is taken. The image capture is
synchronized with the rotation of the rotor and the PIV system is triggered when the blade is
positioned at the desired angular position.
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
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Fig. 5 Raw PIV image
Fig. 6 Raw image and instantaneous flow field
The flipped vertically and horizontally image, taken by first camera is presented in fig. 5. The
results show a good cross-correlation, except in the region near the blade surface. Despite image
saturation near the blades due to reflections and geometric perceptive, the velocity field is
exploitable and can be used for comparison with CFD simulation, fig. 6. Before the tests, the
calibration images are taken in order to calculate the scale and to synchronize spatially the two
cameras. Here the first camera takes the images around the rotor and the second with slight
overlapping takes in the downstream. Finally, the treatment of all PIV images has permitted to
establish a database containing the instantaneous velocity fields, and the average velocity field for
each of the 6 positions of the rotor, fig. 7.
PIV 0°/180° CFD 0°/180°
PIV 30°/210° CFD 30°/210°
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
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PIV 60°/240° CFD 60°/240°
PIV 90°/270° CFD 90°/270°
PIV 120°/300° CFD 120°/300°
PIV 150°/330° CFD 150°/330°
Fig. 7 Averaged velocities fields around the rotor
4. Numerical modeling
Because the aspect ratio AR=0.91 of the rotor is low, there exists a flow leakage near the blade tip,
between the suction and pressure surfaces. The blade circulation variation during rotation creates a
periodical vortex structure downstream of the rotor. This vortex structure is similar to vortex
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
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structure of an oscillating wing with low aspect ratio. As results a three dimensional modeling is
carried out Dobrev&Massouh (2011).
The grid is created by means of ANSYS Gambit 2.4.6. Because the rotor changes its position with
respect to upstream wind direction, the concept of sliding mesh is applied. The grid has two distinct
parts: an external stationary, which represents the flow around the turbine and an internal, which
rotates in order to represent the rotor blades.
The external part of the grid is composed by multiply structured blocks with 5.7 million cells. The
internal part is composed by 2 unstructured blocks with more than 2.3 million parallelepiped cells.
In order to have the parameter y+ <10, 10 layers of brick cells are created around the blades. All the
computations are performed using the Navier-Stokes solver ANSYS Fluent 12.1. Detached eddy
simulation (DES) technique is used in order to improve the flow simulation. In this study the DES
treats near-wall region by means of k-ω turbulence model. Because the flow field does not have
such periodicity like those obtained by other kinds of turbulence modeling, the averaged flow fields
and rotor torque are obtained from 100 revolutions. The numerical results presented on Tab. 1 are
closer to the experimental data for all TSR and confirm the capability of DES to represent well the
detached flow.
Table 1. Experimental and numerical results for power coefficient, Dobrev&Massouh (2011)