Experimental Investigation of the area ratio of double-layer

blades as obstacle blades on swirling savonius rotor

performance
Indra Herlamba Siregar 1,*, Moch Effendy1, and Akhmad Hafizh A R1
1 Mechanical Engineering Department, State University of Surabaya
Kampus Unesa Ketintang, Ketintang Surabaya 60231, Indonesia
Email: indrasiregar@unesa.ac.id

Abstract. The effect of the area ratio of the double layer obstacle blades to the main blade to the
performance of the swirling savonius rotor has been identified. There are four studied ratios of 1:
6,2: 6, 3: 6 and 4: 6 which are equivalent to 30 0, 600, 900 and 1200 of the double layer obstacle blades
arc angles in wind tunnels with wind speeds between 3 m / s to 6 m / s which is equivalent to
Reynolds number 56323 to 112645. The results of the study show that a small ratio of 30 0 and 600
blades arc angles can improve the performance of swirling savonius rotor, especially at Reynolds
number 112645. Besides that, the existence of double-layer obstacle blades can increase the
resistance of swirling savonius rotor toward the load.

Keywords: area ratio, double layer obstacle blades, arc angels, Reynolds number, swirling savonius
rotor

1. Introduction
In recent times the worlds energy supply Today is dominated by non-renewable fossil energy.
Apart from this, the use of fossil energy also impacts the environment and human health. A severe
impact on the environment is the occurrence of weather changes caused when fossil energy is
converted into energy that is useful for humans will release carbon dioxide into the atmosphere
which causes the formation of the greenhouse effect which will ultimately change the weather [1].
Emissions released by fossil energy also pollute the air, which has a severe impact on human
health [2].
To overcome the impact of the use and non-renewable nature of fossil energy, researchers
have recently looking for alternative energy supplies that are environmentally friendly and
renewable. Wind energy is one of the most promising renewable energy sources. Many countries
have explored and used wind energy because of pollution-free and the availability of abundant
sources of wind energy for conversion [3].
The technology that can convert wind energy into energy that can be utilized by humans is a
wind turbine, where the wind turbine is divided into two kinds of vertical type wind turbines and
horizontal type wind turbines. In general, vertical axis wind turbines have simple construction, are
low cost, have better start capability than horizontal axis wind turbines, and do not require yaws
and the turbine orientation always faces the wind direction [4].
 Vertical axis wind turbines are divided into two kinds of Savonius wind turbines that work due
to differences in drag forces that arise on the sides of concave and convex blades, while Darrieus
wind turbines work based on lift forces acting on the blades [5]. Savonius wind turbines have the
ability to self-starting at low wind speeds better than darrieus wind turbines, but in terms of
efficiency lower than darrieus wind turbines [6].
Indonesia geographical position at the equator causes its wind characteristics to vary, and the
average speed is also low with an average of 3 m / s to 6.3 m / s. However, Indonesia based on
PEU has a wind energy potential of 9 GW, so that with a large enough energy potential and low
average wind speed, the development of savonius vertical axis wind turbines is very appropriate.
There are many experimental studies concerning to improve the performance of Savonius wind
turbines has been carried out, starting from the influence of the number of blades [7-9], the
results show that the number of blades in Savonius wind turbines has an impact on turbine
performance and the number of blades 2 which gives the best performance.
Researches related to the geometry of many blades carried out by researchers include Modi
and Fernando [10] modifying the blades developed by Savonius, where the new blade was able to
increase the value of Cp from the turbine. Kamoji et al. [11] modified the savonius blades with J-
shaped blades in which the returning advancing blades are separated, this modification gets a Cp
value of 0.2 while Kacprzak [12] modifies the blades developed by Kamoji by reducing the flat
plane of the geometric blades the resulting Cp value is better than the blade developed by Kamoji
et al. [11] and Tartuferi [13] develops a new blade for wind turbines based on drag forces named
SR 3345 and SR 5050, but the resulting Cp value is no better than the blade developed by kamoji
however, the maximum Cp value is achieved at a lower tip speed ratio.
Besides, to improve the performance of the savonius wind turbine, the researchers also
conducted research related to controlling the flow on the surface of the blade by adding horizontal
plate [14,15] and vertical plate [16] on the surface of the main blade and adding multiple quarter
blades [17,18].
The recent study is focusing on the effect ratio of the double-layer blade as an obstacle to the
main blade to improve the performance of the savonius wind turbine in the wind tunnel.

2. Research Methods
In this study, the wind turbine model is made of PVC with a height of H = 300 mm, D = 300 mm so
that the aspect ratio (H / D) = 1, and overlapping ratio (m / D) = 20%, the number of blades is 2
blades, shaft diameter = 15 mm made of PE for more details see figure 1.
Tests of the swirling Savonius rotor wind turbine model were carried out in an open-type
subsonic wind tunnel with an area of 2025 cm 2 with a 16” blades diameter blower specification
integrated with speed inverters can produce wind speeds ranging from 0 to 15 m / s. The setup of
wind turbine models in the wind tunnel can be seen in figure 2.
The instrument used in this study to measure the rotation of the wind turbine model is a laser
tachometer DT-1236L with a measurement range of 10-99999 rpm and accuracy: ± (0.05% + 1 digit),
wind speed measurement with a flexible anemometer KW0600562 with a measurement range of 0.6
- 30 m / s.
 Figure 1. Dimensions of model Savonius wind turbine.

Figure 2. Scheme set up of wind turbine model in the wind tunnel.

Torque measured by the Prony brake method proposed by kamoji et.al [11] with load every 250
gr till the wind turbine model stops, the scheme can be seen in Figure 3.
 Figure 3. Prony brake scheme for torque measurement

The main variable of the study is the ratio of the area of the double blade as an obstacle (
/1800 x circumference of the blade x height of the blade) to the area of the main blade
(1800/3600 x circumference of the blade x height of the blade). The ratio is  = 00 (only main
blades),  = 300 (1:6),  = 600 (2:6),  = 900 (3:6) and  = 600 (4:6). While the control variable is the
speed of the wind coming out of the wind tunnel 3 to 6 m/s adjusting the range of average wind
speeds in Indonesia.

Figure 4. Design of obstacle blade

The performance of wind turbines described in a dimension parameter in the form of power
density and 3 non-dimensional parameters in the form of tip speed ratio, power coefficient, and
Reynolds number. Power density is defined by the results of measurements of torque and angular
velocity  divided by the sweep of turbine A, such as the following equation
Tω
Power Density=
A (1)

The tip speed ratio is defined based on angular velocity measurements , turbine shaft radius
R and velocity of wind Vw like the following equation
ωR
λ=
Vw (2)
 While the torque measurement T is used to calculate the power coefficient
Tω
C p=
0 . 5 ρ A V 3w   (3)

Reynolds number is the ratio between inertial force and viscous force what is used to
determine whether the fluid flow is laminar or turbulent.
ρVD
ℜ= (4 )
μ

3. Results and discussion

The swirling savonius rotor type wind turbine model is made of PVC sheet and tested in an open
wind tunnel with performance parameters such as power density and power coefficient. Figures 5
and 6 show that the maximum power density of the swirling savonius rotor is obtained at a ratio of
2: 6 or at the arc angle of the double layer blades as an obstacle 60 0 at Reynolds 112645 at 30.62 W
/m2, under this condition the resulting power coefficient is 25.25 %, while the maximum power
coefficient obtained at the Reynolds number 93871 is 27.73% with the power density under this
condition 19.46 W / m2.

Figure 5. Power Density max in the variation of the ratio of obstacle blades to main blades and
different Reynolds number

The existence of the double layer blades as an obstacle in front of the main blade can improve
the performance of the swirling savonius rotor both the power density and the power coefficient
produced by the swirling savonius rotor. This is suspected by the existence of the double-layer
blades as an obstacle at a certain position the obstacle blade can increase the flow thrust in the
overlap area so that it pushes the convex blades in the direction of the concave blades which
results in the negative torque produced by the convex blades decreasing (see figure 7) which
causes the positive torque difference generated by the concave blade with the negative torque
produced by the convex blade, the fact that there is an increase in torque from the swirling
savonius rotor with the obstacle blade than without the obstacle blades see Figure 8.
Figure 6. power coefficient max in the variation of the ratio of obstacle blades to main blades
and different Reynolds number

Figure 7. Illustration of flow in an overlap area

 Figure 8. Static torque max in the variation of the ratio of obstacle blades to main blades and
different Reynolds number

Figure 8 also shows that at the arc angle of the double layer blades as an obstacle of 300 and
600 the presence of the disturbing blade can increase the static torque capability of the swirling
savonius rotor, where the increase in static torque is correlated with the increased self-starting
capability of the swirling savonius rotor [7].
Figures 9 to 12 show that the addition of the double-layer blades as an obstacle is able to
increase the power coefficient of the swirling savonius rotor in the small ratio of the area of the
double-layer blades as an obstacle to the area of the main blade, this is represented by the arc
angle of the double-layer blades as an obstacle 30 0 and 600. The greater area ratios represented
by arc angles 900 and 1200 have an impact on the decreasing power coefficient of the swirling
savonius rotor. This is because the presence of the double-layer blades as an obstacle in this
condition causes the flow that will enter the overlap area to be distortion by the free stream but
also causes the focus point on the concave side main blade (f) to be obstructed so that both of
these causes the thrust produced by the concave side blade to decrease which in finally the
positive torque produced is also reduced see figure 13.
In addition, figures 9 to 12 also show that the existence of the double-layer blades as an
obstacle increases the resistance of swirling savonius rotors to the load, which is shown by the
extent of the graph produced by swirling savonius rotors with the double-layer blades as an
obstacle which are wider than swirling savonius rotors without the double-layer blades as an
obstacle.
 30.00

25.00

20.00

15.00
Cp (%)

10.00

5.00

0.00
19 64 09 69 94 00 18 89 79 00 97 70
1. 0. 1. 0. 0. 0. 1. 0. 0. 0. 0. 0.

TSR

0 30 60 90 120

Figure 9. power coefficient in the variation of the ratio of obstacle blades to main blades
at Re = 56323

30.00

25.00

20.00

15.00
Cp (%)

10.00

5.00

0.00
20 92 00 10 90 79 10 91 72 51 17 85 70 00 93 72 00
1. 0. 0. 1. 0. 0. 1. 0. 0. 0. 1. 0. 0. 0. 0. 0. 0.

TSR

0 30 60 90 120

Figure 10. power coefficient in the variation of the ratio of obstacle blades to main blades
at Re = 75097
 30.00

25.00

20.00

15.00
Cp (%)

10.00

5.00

0.00
28 99 80 69 25 05 99 78 46 24 02 87 82 59 29 04 88 74 61 11 90 75
1. 0. 0. 0. 1. 1. 0. 0. 0. 1. 1. 0. 0. 0. 1. 1. 0. 0. 0. 1. 0. 0.

TSR

0,00 30,00 60,00 90,00 120,00

Figure 11. power coefficient in the variation of the ratio of obstacle blades to main blades
at Re = 93871

30.00

25.00

20.00
Cp (%)

15.00

10.00

5.00

0.00
1.31 1.12 0.86 1.25 1.11 0.96 0.67 1.17 1.00 0.92 0.00 1.08 0.93 0.74 1.18 0.98 0.77
TSR

0 30 60 90 120

Figure 12. power coefficient in the variation of the ratio of obstacle blades to main blades
at Re = 112645
 Figure 13. Illustration of flow over overlap area for 900 and 1200 the double-layer blades as an
obstacle arc angles

4. Conclusions
The addition of the double-layer blades as an obstacle can increase the durability of the
swirling savonius rotor wind turbine model against loading and is able to improve the
performance and ability of the self-starting model in the small ratio of the area the double-
layer blades as an obstacle with the main blade.

Acknowledgments
The author would like to thank the ministry of education and culture which funded this research in
basic research schemes.

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