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An Experimental Study on theImprovement of Savonius TurbinePerformance Using Flexible SailsHakan Ersoy a & Saim Yalcindag aa Faculty of Engineering, Department of Mechanical Engineering ,Akdeniz University , Antalya , TurkeyAccepted author version posted online: 02 Aug 2013.Publishedonline: 17 Jan 2014.

To cite this article: Hakan Ersoy & Saim Yalcindag (2014) An Experimental Study on the Improvementof Savonius Turbine Performance Using Flexible Sails, International Journal of Green Energy, 11:8,796-807, DOI: 10.1080/15435075.2013.830121

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International Journal of Green Energy, 11: 796–807, 2014Copyright © Taylor & Francis Group, LLCISSN: 1543-5075 print / 1543-5083 onlineDOI: 10.1080/15435075.2013.830121

AN EXPERIMENTAL STUDY ON THE IMPROVEMENT OFSAVONIUS TURBINE PERFORMANCE USING FLEXIBLESAILS

Hakan Ersoy and Saim YalcindagFaculty of Engineering, Department of Mechanical Engineering, AkdenizUniversity, Antalya, Turkey

This study aimed to improve the mechanical productivity of the conventional Savonius rotorby using new type flexible and moving fabric curtains, termed “sails.” To this end, additionmoving-type sails were designed to be mounted on a single-stage, four-blade rotor. Afterthe theoretical positive effects of the new design on the mechanical productivity of therotor were represented mathematically, a prototype of the rotor was built and its practicalperformance was tested under field conditions. The test results were compared to the per-formance of the conventional design. The new design was found to outperform the classicaldesign in terms of starting and torque production, particularly at low wind speeds. Thus,the design changes reported in the present study contribute to improving the productivityof the conventional Savonius-type wind rotor, which is known to have a simple and easilyimplemented design but low mechanical efficiency.

Keywords: Savonius rotor; Experimental performance; Sail-type rotor design; Windenergy; Mechanical efficiency

INTRODUCTION

The Savonius-type wind rotor is a wind turbine consisting of two half-cylindersmounted on a vertical axis in such a way to face each other. Despite its low productiv-ity, it is widely used, especially in agricultural applications, due to its simple structure, lowcost, and its capacity to adapt to different wind directions and turbulent conditions. Studiesof this turbine type generally concentrate on improving the low rotor productivity, whichis deemed a disadvantage of this design. Significant improvements have been achieved,increasing the use of this wind turbine.

The Savonius-type turbine was invented by Sigurd Savonius in 1925; it has beenimproved by different researchers and various experimental and numerical results havebeen achieved in relation to different designs. Alexander and Holownia (1978) conductedwind tunnel tests on a number of Savonius rotor configurations at wind speeds of 6 to9 m/s. The variables tested were blade aspect ratio, blade overlap and gap, and the

Address correspondence to Hakan Ersoy, Faculty of Engineering, Department of Mechanical Engineering,Akdeniz University, 07058 Antalya, Turkey. E-mail: hakanersoy@akdeniz.edu.tr

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ljge.

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IMPROVEMENT OF SAVONIUS TURBINE PERFORMANCE 797

effects of adding end extensions, end plates, and shielding. Mojola (1985) reported perfor-mance characteristics of the Savonius windmill rotor under field conditions. Test data werecollected on the speed, torque, and power of the rotor at a large number of wind speedsfor each of seven rotor overlap ratios. Theoretical analyses of the behaviors of flexible-sail-type wind-turbines were presented by Fleming and Probert (1984). Aldoss and Najjar(1985) developed the swinging-blade for Savonius rotor. Fleming, Probert, and Tanton(1985) described the behaviors of the turbines’ sails and identified the optimal sail profilesfor corresponding to maximum wind harnessing capabilities. Fujisawa (1992) investigatedthe aerodynamic performance and the flow fields of Savonius rotors at various overlapratios, both with and without rotor rotation. Shaughnessy and Probert (1992) proposed aV-shaped deflector for augmenting the energy-harnessing effectiveness. The discrete vor-tex method was used to provide detailed information on the flow field and starting torqueon a stationary Savonius rotor by Aldoss and Kotb (1991). The aerodynamic performanceof a Savonius rotor has been studied by measuring the pressure distributions on the bladesurfaces at various rotor angles and tip-speed ratios (Fujisawa and Gotoh 1994). Altan andAtılgan (2008, 2010) introduced a curtain design to improve the low performance levelsof Savonius wind rotors, including experiments and simulations of their curtain design(Altan, Atilgan, and Ozdamar 2008). These improvements have been made in the form ofproportional changes in the rotor bucket gap, number of vanes, and rotor size. Such stud-ies have made considerable contributions to the improvement of S-type rotor productivity.These turbines have also started to be used for small-scale electricity generation (Menet2004). McWilliam and Johnston (2008) studied air flow around five different model verti-cal axis wind turbines. Mohamed, Janiga, and Pap (2010) improved an obstacle shielding areturning blade of the Savonius turbine and increased the output power and got a better self-starting capability. Ghosh et al. (2009) studied the suitability of modified Savonius windrotors for water-pumping application with a mathematical model developed for estimatingthe discharge over a period of time. Gupta and Biswas (2011) computationally analyzedthe performance of three-bucked Savonius and three-bladed Darrieus turbines. Irabu andRoy (2011) studied upon force measurement and characteristics on blades of Savoniusrotor at static state. Akwa, da Silva, and Petry (2012) discussed the influence of the buck-ets overlap ratio of a Savonius wind rotor on the averaged moment and power coefficients,over complete cycles of operations. Altan and Atilgan (2012) intended to increase the rotorperformance of the Savonius wind rotor with theoretical method by focusing on a curtainarrangement as a wind deflection. A helical Savonius rotor with a twist of 180 was pro-posed to increase the rotor performance by Damak, Driss, and Abid (2013). Yoon, Lim, andKim (2013) described the experimental results of a multi-blade vertical axis wind turbine.

As shown in Figure 1, the present study has developed a new Savonius-type rotorwith wind sails in addition to the vanes of the conventional design. After the theoreticaladvantages of the new design were presented, a prototype was produced and subjectedto field tests and measurements. The new design was compared to the classical design todetermine the conditions under which the former performs better.

NEW SAIL-TYPE ROTOR DESIGN

Design Concept

A sail generates force in two ways. First, according to Bernoulli’s principle, airflowspeeds differ across both sides of the surface depending on the load and form of the windangle on the sail. Thus, more air pressure is applied on one side of the sail. This pressure

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798 ERSOY AND YALCINDAG

Figure 1 Savonius-type wind rotor, designed as a with-sail, single-layer, four-blade turbine.

difference creates absorptive power and directs the sail to the lower-pressure side. Theresulting force moves the structure in the direction of the projections.

The second way in which a sail generates force is through the conservation ofmomentum. Wind penetrates from the front side of the sail and leaves the sail from itsside. During its motion, the wind transfers a part of its momentum over the sail to thestructure on which it is mounted and, thus, slows down. This momentum, proportional tothe own weight of the system, is turned into motion by the system. The forward componentof this motion is balanced when the system gains speed.

The rotor rotation of the system designed in this study enables the sails to position atdifferent angles according to the wind direction. Thus, both the momentum conservationand Bernoulli’s principle are observed to contribute to the rotor rotation.

In the design shown in Figure 1, the system was developed in such a way to increasethe difference between the positive and negative pressures on the rotor and contribute totorque by benefitting from the flexibility and mobility of the sails. The field tests deter-mined the performance of the design in terms of speed and torque under moving andnonmoving conditions.

Design Procedure for the Prototype

Designed as a single-layer, four-blade turbine, the turbine rotor was fixed from belowby embedding into concrete via anchor rods and, from above, by a steel-wire tension mech-anism. To prevent any problems during the disassembly and reassembly of the turbine, asteel base (1×1 meter in size and 5 mm in thickness) was produced and the turbine wasfixed to the ground by the weight of this steel base. Blades were placed at angles of 0◦,90◦, 180◦, and 270◦ on the main axis. Blade diameter was 250 mm and blade length was1800 mm. Accordingly, rotor diameter was 565 mm, including the spindle. Taking into

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IMPROVEMENT OF SAVONIUS TURBINE PERFORMANCE 799

Figure 2 Conventional Savonius rotor, without-sail.

consideration the effects of the diameter height ratio on the power and ease of handling ofSavonius-type rotors, the design used a diameter height ratio of 3.6:1 (Figure 2).

A tensioning system was developed to ensure high-productivity of the sails. To pre-vent the sails from opening and closing due to this tension, the sails were tightened fromtheir lower ends with the help of tension bars, and from their middle and upper partswith the help of frames made of fine wires and fabrics (as shown in Figure 3). Since thesails were required to be flexible and resistant, parachute fabric was used for this pur-pose, as it is the best fabric to meet these requirements. Sails were produced in a special,three-dimensional form.

EXPECTED PERFORMANCES OF SAVONIUS-TYPE WIND ROTORS

Performance Analysis of Conventional Savonius-Type Wind Rotors

Calculation of the forces applied on the scoop is a widely accepted method of exam-ining the productivity of Savonius-type wind rotors (Figure 4). Power is expressed asfollows in the scoop model (Robert and McDonald 1994).

P = 1/2 · cp · ρ · A · υ2 · u, (1)

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800 ERSOY AND YALCINDAG

Figure 3 Sail tension bar.

Figure 4 Scoop model.

where; Cp: Power coefficient (dimensionless); ρ: the density of air (kg/m3); A: the sweep-ing area of the scoop (m2); υ: wind velocity (m/s); u: peripheral velocity of the blade tipof the Savonius-type wind rotor (m/s).

Due to the pressure difference between its blades, any Savonius-type wind rotor iscapable of converting the kinetic energy of the wind into torque. The power expression“P+” can be used in the concave surface and “P−“ in the convex surface of the rotor. In suchcase, the following equations are obtained for the power expressions of the concave andconvex blade surfaces, respectively,

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IMPROVEMENT OF SAVONIUS TURBINE PERFORMANCE 801

P+ = 1

2ρA

[cp,1(υ − u)2] u, (2)

P− = 1

2ρA

[cp,2(υ + u)2

]u. (3)

A rotation effect is generated by the difference between these two surfaces:

Pnet = P+ − P−. (4)

Equations (2) and (3) are placed in (4); optimum rotor speed required for optimum poweris set at uopt = 1/3υ; Equation (5), below, is obtained after the expressions are abbreviated(Kolaçan 1995; Yalçindag 2008)

Pnet = 1

2ρA

[cp,1(υ − u)2

]u − 1

2ρA

[cp,2(υ + u)2

]u, (5)

Popt = 1

2ρA

[cp,1(υ − 1

3υ)

2]

1

3υ − 1

2ρA

[cp,2(υ + 1

3υ)

2]

1

3υ, (6)

Popt = 2

27ρAυ3(cp,1 − 4cp,2). (7)

Performance Analysis of the New With-Sail Type Rotor

The scoop model can be applied to the with-sail Savonius rotor case in order tocompare it with the conventional without-sail design. In this case, the power expression“P+” can be used for the concave and “P−“ for the convex surfaces, respectively, as shownbelow:

P+ = Pwing + Psail = 1

2ρA

[cp,1(υ − u)2

]u + 1

2ρAsail

[cp,sail(υ − u)2

]u, (8)

P− = 1

2ρA

[cp,2(υ + u)2

]u. (9)

Sails are mounted to the rotor with hinged folding arms therefore at the negative sidesails are folded around the rotor so that the negative effect of the sails to torque can beignored due to folding arms and flexible sails.

Rotation effect is caused by the difference between these two surfaces:

Pnet = P+ − P−, (10)

psum,sail =[

1

2ρA

[cp,1(υ − u)2

]u + 1

2ρAsail

[cp,sail(υ − u)2

]u

]−

[1

2ρA

[cp,2(υ + u)2

]u

].

(11)

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802 ERSOY AND YALCINDAG

A design made in such a way to set the optimum rotor speed required for optimumpower at “uopt = 1/3υ” and the construction at “A≈Asail” gives the following equation:

Popt,sail = 2

27ρAV3(cp,1 + cd,sail − 4cp,2). (12)

Division of the with-sail (12), by the without sail (7), gives the following nettheoretical power difference:

Popt,w.sail

Popt,w.o.sail=

227ρAV3(cp,1 + cp,sail − 4cp,2)

227ρAV3(cp,1 − 4cp,2)

. (13)

The following equation is obtained after simplification:

Popt,w,sail

Popt,w.o,sail= 1 + cp,sail

cp,1 − 4cp,2. (14)

Here, Popt,w,sail refers to the optimum power in the with-sail case; Popt,.w.o,sail to theoptimum power in the without-sail case; cp,1 and cp,2 to the air resistance coefficients of theconcave and convex surfaces of the blades, respectively. As can be understood from (14),a theoretical power increase can be achieved.

EXPERIMENTAL APPARATUS AND PROCEDURES

Tests were made at two stages. In the first stage, in order to see the effects of thesail on the Savonius-type wind rotor, a conventional Savonius-type wind rotor was used(Figure 5) to measure the rotation and torque values at wind velocity of 0 to 4.9 m/sinterval.

Figure 5 Without-sail rotor measurements.

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IMPROVEMENT OF SAVONIUS TURBINE PERFORMANCE 803

Figure 6 With-sail rotor measurements.

While horizontal axis rotors must be positioned a minimum of 20 to 30 m aboveground level, vertical axis rotors can be used at ground level. Wind speeds are lower atthe ground level, due to roughness and related turbulences. Therefore, study measurementswere limited to 0 to 4.9 m/s interval and the sail affect on the Savonius-type rotor wasexamined at low speeds. In the second stage, the tests were repeated at the same windvelocity intervals with the proposed sail design attached (Figure 6).

Measurements of rotor torque have been measured by a torque meter (Lutron TQ-8800) with Accuracy of +/− 1.5% and resolution of 0.001 Nm. Tachometer: DT-2268 hasbeen used with the resolution of 0.1 rpm (<1000 rpm) and 0.01 m/min (<100 m/min)and the accuracy of +/− 0.05%. Anemometer: Davis Vantage Pro2 multifunctionalanemometer has been used with the resolution of 0.1 m/s. Uncertainty analysis has beenmade for the experimental system and it has been found +/− 2.5%.

EXPERIMENTAL RESULTS AND DISCUSSION

During initial field tests, sails were observed to fall behind and fail to capture thewind when not supported by an appropriate tensioning mechanism. Therefore, the designdescribed above was developed and an onboard tension mechanism was incorporated. Afterthe tension mechanism was installed, the sails were observed to capture the wind better, toopen easily and to close completely in case of a reverse direction motion.

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804 ERSOY AND YALCINDAG

Table 1 Wind Velocity-Based Mean Rotor Tangential Speed Values (m/s)

Wind Speed (m/s) 0.9 1.3 1.8 2.2 2.7 3.1 3.6 4.0 4.5 4.9

Without-Sail 0.0 0.0 0.0 0.0 0.0 0.705 0.867 1.07 1.43 1.74With-Sail 0.0 0.183 0.272 0.347 0.417 0.462 0.490 0.523 0.600 0.723

Table 2 Wind Velocity-Based Mean Torque Values (Nm)

Wind Speed (m/s) 0.9 1.3 1.8 2.2 2.7 3.1 3.6 4.0 4.5 4.9

Without-Sail 0.0 0.0 0.129 0.211 0.279 0.371 0.543 0.682 0.817 0.950With-Sail 0.350 0.520 0.588 0.816 1.04 1.34 1.61 1.87 2.16 2.44

As emphasized above, one of the most important advantages of Savonius-type rotorsis that the design is capable of operating irrespective of the wind direction. This advantagewas preserved after mounting the sails to the rotor; the wind direction was observed not toaffect the rotor.

Measurements were made on the basis of the speed and momentum values for thewith-sail and without-sail rotor cases. The essential task was to compare both cases and todetermine the advantages and disadvantages of the new design. The results are presentedin tables and graphics. Table 1 lists the blade tip speeds for the with- and without-sailrotor case. Table 2 shows rotor torque values for both cases. Measurements were made onthe rotor via optical and contact tachometer. As can be concluded from Tables 1 and 2,the classical Savonius-type wind rotor started rotation at a wind speed of 3.1 m/s whilethe with-sail Savonius-type wind rotor started at 1.3 m/s. Savonius-type wind rotors havebetter starting characteristics than vertical axis wind turbines; the results show that the

0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Wind velocity (m/s)

Tagetial ro

tor

tip s

peed (

m/s

)

w/o. sail

w. sail

Figure 7 Rotor speeds versus wind velocity.

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IMPROVEMENT OF SAVONIUS TURBINE PERFORMANCE 805

0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.5

1

1.5

2

2.5

Wind velocity (m/s)

Roto

r to

rque (

Nm

)

w/o. sail

w. sail

Figure 8 Rotor torques versus wind velocity.

addition of sails further improved these starting characteristics by enabling the rotor tostart at lower speeds. Figure 7 compares the results obtained from the two rotor designs.Examination of Figure 7 shows that the with-sail rotor started at lower wind speeds, butremained at lower speeds compared to the without-sail rotors at higher wind speeds. Thedifference between the speeds of the two rotors increased with the increase in wind speed.The reason for this situation is that the sails have a high friction coefficient and cause windresistance during rotation and they need some time to open and close.

Table 2 lists and compares the mean torque values measured during the nonmovingstate of the without-sail and with-sail rotor cases, respectively. The first torque values forthe classical Savonius-type rotor were obtained at a wind speed of 1.8 m/s, while those forthe with-sail rotor were obtained at a wind speed of 0.9 m/s. Considering that the with-saildesign started to rotate at a wind speed of 1.3 m/s and the without-sail design started ata wind speed of 3.1 m/s, it can be concluded that the with-sail design started generatingpower earlier than the without-sail design.

Figure 8 shows that higher torque values were obtained from the with-sail rotorcase at all wind speeds and that this advantage was conserved at increased wind speeds.Mean tangential speed and torque values of the rotors are presented in Tables 1 and 2,respectively, for the without-sail and with-sail cases.

CONCLUSIONS

This study reviewed previous studies on the Savonius-type wind rotors and exam-ined the working principles, blade theory, and design parameters of this type of rotor inorder to create a rotor design with appropriate performance characteristics. A sail was thenmounted on the revised design to make rotor speed and torque comparisons with the clas-sical Savonius-type wind rotor. These comparisons revealed that it is possible to improvethe mechanical values and productivity of the conventional Savonius-type rotor by usingsail-induced design.

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806 ERSOY AND YALCINDAG

The results of the field tests showed that the newly-developed with-sail Savonius-type rotor is capable of starting at lower wind speeds and operating at higher torque valuesthan the conventional rotor design. An important factor to be considered in this scope isthe assembly and maintenance costs of the sail. However, it should be noted that thesesail-related costs account only for 5 to 10% of the total rotor costs. Another advantage ofthis new design is the mountability/demountability of the sails, which means the reviseddesign is adaptable to differing climatic and regional conditions.

FUNDING

The financial support of the Scientific Research Projects Unit of Akdeniz Universityis gratefully acknowledged.

NOMENCLATURE

A Sweeping area of the blade (m2)Cp1,2 power coefficient (subscript 1 for concave side, subscript 2 for convex side)PBetz Power according to Betz’s law (W)r measurement radius (m)υ Wind velocity (m/s)u Blade-tip speed (m/s)P+ Blade Concave Power (W)P− Blade Convex Power (W)ρ Density of air (kg/m3)

REFERENCES

Akwa, J.V., G.A. da Silva, and A.P. Petry. 2012. Discussion on the verification of the overlap ratioinfluence on the performance coefficients of a Savonius wind rotor using computational fluiddynamics. Renewable Energy 38(1):141–49.

Aldoss, T.K., and M.A. Kotb. 1991. Aerodynamic loads on a stationary Savonius rotor. JSMEInternational Journal Series II-Fluids Engineering Heat Transfer Power CombustionThermophysical Properties 34(1):52–55.

Aldoss T.K., and Y.S.H. Najjar. 1985. Further development of the swinging-blade Savonius rotor.Wind Engineering 9(3):165–70.

Alexander, A.J., and B.P. Holownia. 1978. Wind-tunnel tests on a Savonius rotor. Journal ofIndustrial Aerodynamics 3(4):343–51.

Altan, B.D., and M. Atılgan. 2008. An experimental and numerical study on the improvement of theperformance of Savonius wind rotor. Energy Conversation Management 49:3425–32.

Altan, B.D., and M. Atılgan. 2010. The use of curtain design to increase the performance level ofSavonius wind rotor. Renewable Energy 35:821–29.

Altan, B.D., and M. Atilgan. 2012. A study on increasing the performance of Savonius wind rotors.Journal of Mechanical Science and Technology 26(5):1493–99.

Altan, B.D., M. Atilgan, and A. Ozdamar. 2008. An experimental study on improvement ofa Savonius rotor performance with curtaining. Experimental Thermal and Fluid Science32:(8):1673–78.

Damak, A., Z. Driss, and M.S. Abid. 2013. Experimental investigation of helical Savonius rotor witha twist of 180 degrees. Renewable Energy 52:136–52.

Dow

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ded

by [

Mos

kow

Sta

te U

niv

Bib

liote

] at

03:

42 1

8 Fe

brua

ry 2

014

IMPROVEMENT OF SAVONIUS TURBINE PERFORMANCE 807

Fleming, F.D., and S.D. Probert. 1984. Flexible sail wind turbines review of pertinent theoreticalanalyses. Applied Energy 18(2):89–99.

Fleming, P.D., S.D. Probert, and D. Tanton. 1985. Designs and performances of flexible and taut sailSavonius-type wind-turbines. Applied Energy 19(2):97–110.

Fujisawa, N. 1992. On the torque mechanism of Savonius rotors. Journal of Wind Engineering andIndustrial Aerodynamics 40:277–92.

Fujisawa, N., and F. Gotoh. 1994. Experimental-study on the aerodynamic performance of a Savoniusrotor. Journal of Solar Energy Engineering-Transactions of The ASME 116(3):148–52.

Ghosh, P., M.A. Kamoji, S.B. Kedare, and S.V. Prabhu. 2009. Model testing of single- and three-stage modified Savonius rotors and viability study of modified Savonius pump rotor systems.International Journal of Green Energy 6(1):22–41.

Gupta, R., and A. Biswas. 2011. CFD analysis of flow physics and aerodynamic performance of acombined three-bucket Savonius and three-bladed Darrieus turbine. International Journal ofGreen Energy 8(2):209–33.

Irabu, K., and J.N. Roy. 2011. Study of direct force measurement and characteristics on blades ofSavonius rotor at static state. Experimental Thermal and Fluid Scienced 35(4):653–59.

Kolaçan, A.F. 1995. Wind Energy and Savonius Turbines MSc Thesis in mechanical engineer-ing. Istanbul Turkey: Graduate School of Natural and Applied Sciences, Istanbul TechnicalUniversity, [in Turkish].

McWilliam, M., and D.A. Johnson. 2008. Velocity measurement of flow around model vertical axiswind turbines. International Journal of Green Energy 5(1–2):55–68.

Menet, J.L. 2004. A double-step Savonius rotor for local production of electricity: A design studyRenewable Energy 29:1843–62.

Mohamed, M.H., G. Janiga, and E. Pap. 2010. Thevenin D., Optimization of Savonius turbines usingan obstacle shielding the returning blade. Renewable Energy 35:2618–26.

Mojola, O.O. 1985. On the aerodynamic design of the Savonius windmill rotor. Journal of WindEngineering and Industrial Aerodynamics 21:(2):223–31.

Robert, W.F., andA.T. McDonald. 1994. Introduction to fluid mechanics. 4 th ed. SI version. NewYork: John Wiley & Sons INC.

Shaughnessy, B.M., and S.D. Probert. 1992. Partially-blocked Savonius rotor. Applied Energy43(4):239–49.

Yalcındag, S. 2008. A research on the improvment of efficiency of the Savonius type wind rotors. MScthesis in mechanical engineering. Antalya Turkey: Graduate School of Natural and AppliedSciences, Akdeniz University, [in Turkish].

Yoon, S.H., H.C. Lim, and D.K. Kim. 2013. Study of several design parameters on multi-bladevertical axis wind turbine. International Journal of Precision Engineering Manufacturing14(5):831–37.

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