Journal of Industrial Aerodynamics, 3 (1978) 343--351 343 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

WIND TUNNEL TESTS ON A SAVONIUS ROTOB

A.J. ALEXANDER and B.P. HOLOWNIA

Department of Mechanical Engineering, Loughborough University of Technology, Loughborough, Leicestershire (Gt. Britain)

(Received January 19, 1976; in revised form March 22, 1978)

Summary

Tests have been made in a wind tunnel on a number of Savonius rotor configurations in wind speeds of 6--9 m/s. The variables tested were blade aspect ratio, blade overlap and gap and the effects of adding end extensions, end plates and shielding.

For low aspect ratios (~ 1 ) with no additions the efficiency was low ( -0 .065) but for higher aspect ratios ( - 5 ) with optimum blade configuration and shielding a maximum value of efficiency of 0.25 was obtained. Tests with three and four bladed configurations gave appreciably lower values of efficiency.

A special study was made of wind tunnel corrections for blockage ratios up to 0.3.

List of symbols

a , b , d AR C c

CD CDc h 1 S

V Yc

x , y

¢

P

width and position of shield (Fig. 2d) blade aspect ratio, h/c tunnel cross-sectional area rotor chord, m (Fig. 2a) uncorrected drag coefficient (--- measured drag/1/~ p 172 ~) corrected drag coefficient (-= measured drag/1/~ p V~c S) rotor height, m (Fig. 2c) length of end extensions (Fig. 2c) maximum projected frontal area of windmill (including shield if

present) wind velocity in empty tunnel, m/s wind velocity in wind tunnel corrected for wake blockage effects,

m/s rotor overlap and gap distances (Fig. 2a) tip speed/Vc angle of shield relative to wind (Fig. 2d) air density kg/m 3 efficiency = measured power output/(1/~ p Vc 3 X S' X 0.593), where

S' is windmill frontal area excluding shield

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Introduct ion

Although windmills of all types, especially the vertical axis machines, are being studied fairly intensively as a result of the energy crisis, the amount of factual information available is relatively small. As part of an overall Alternative Energy Sources programme a study has been made of vertical axis machines and wind tunnel tests have been made on Savonius and Darrius rotors. This report is on the Savonius rotor tests where there is very little available inform- ation [1--4].

The Savonius rotor is a very simple concept which has been constructed, and used successfully, from oil drums. Simplicity, ease of construction and maintenance are of major importance particularly in under-developed countries and although the efficiency of this machine is low it is capital cost which mainly determines power costs.

Although the efficiency of the basic Savonius rotor is relatively low there are a number of variables which affect the efficiency and do not appear to have been studied systematically. For example, the aspect ratio of the rotor blades is important and high aspect ratios should improve efficiency whereas most existing machines appear to have low aspect ratios. If it is not possible for, say, structural reasons to have a high aspect ratio it is possible to increase efficiency by the aerodynamic device of using end plates and this has been studied with encouraging results. The effect of the relative position of one blade with another and also the number of blades has been tested. Finally it was clear that since the drag of the advancing rotor blade reduces the torque and hence the power output some form of shielding would improve matters while at the same time deflecting more air into the retreating (and power producing) rotor. This modification has also given large increases in efficiency.

In order to obtain reasonable values of power output some of the models were fairly large compared with the tunnel cross-sectional area. This raised the problem of correcting the windspeed to allow for blockage effects and led to a further programme of research as no suitable theory or data were available.

Further work with a larger Savonius rotor is being undertaken on an out- door site for correlation with the wind tunnel results.

Experimental method

The tests were carried out in the department 's low speed wind tunnel (Fig. 1). The working section is 1.20 m square with velocity variations in the empty tunnel up to -+ 2%. Test velocities ranged between 6 m/s and 9 m/s, these being typical of actual windspeeds encountered in practice. At speeds lower than 6 m/s the power measured was too low to give reasonable accuracy with the smaller windmills tested.

The rotors were constructed from 16 gauge aluminium mounted on a 12.5 mm diameter steel rod. The cross-section of the rotors was semi-circular

345

Fig. 1. Model mounted in wind tunnel.

with chords of 0 .19 m and 0.38 m and lengths f rom 0.46 m to 0.91 m giving aspect ratios, fo r individual r o t o r blades ranging f rom 1.2 to 4.8. Overlap of, and gap be tween , ro tors , Fig. 2a, were varied to see if there was an o p t i m u m posi t ion . A n u m b e r o f tests were made in the s tandard or basic conf igura t ion , Fig. 2b with no gap be tween the blades, y = 0, b u t a small negative overlap, x = - 1 2 . 5 mm , due to the vert ical m o u n t i n g shaft . Also, due to the presence o f this shaf t no tests with values o f y less than 0.07 were possible fo r x positive. The ef fec ts o f end plates and small f lat hor izon ta l end ex tens ions f rom the ends o f the blades were s tudied, Fig. 2c. In o the r tests the e f fec t o f shielding the advancing r o t o r blade was shown to be highly significant. A n u m b e r of f lat shield widths and posi t ions were tes ted to find the o p t i m u m , i.e. a, b, d and ~ were varied, Fig. 2d. T w o lengths of the circular shield, Fig. 2d, equal in length to one-quar te r and one-hal f o f the c i r cumfe rence with ap p ro x im a te ly 12 m m clearance f rom the blades, were tes ted in a n u m b e r o f d i f f e ren t angular pos i t ions bu t as very l i t t le i m p r o v e m e n t was ob ta ined ef for ts were concen t r a t ed on the flat shield. This gave cons iderable increases in p o w er o u t p u t par t icular ly in the o p t i m u m posi t ion , def ined by ¢ = 90 °, a = 1.25c, b = 0.5c, d = 0.2c. All

346

c

Ca)

J

[b) ]

wind

J / \

/ \

/ \

\ / . . #

Fig. 2. (a) Def ini t ion of overlap, +x, gap, +y, and chord c. (b) Standard or basic configurat ion, x/c = - 0 . 0 7 , v/c = 0.0. (c) Def ini t ion of height, h, and end extens ion length, I. (d) Defini t ion of shield sizes and positions.

tests made with a shield quoted in this report were made with this configuration. Torques on the vertical shaft ranged up to 2 N m and were measured and

recorded on a torque transducer/indicator uni t manufactured by Westland Aircraft Ltd. The rotational speeds, up to 800 rpm, were measured on a Smith's Revolution Counter. Both the torque meter and revolution counter were calibrated.

The efficiency ~ was defined as the measured power output divided by the theoretical maximum available energy which is 59.3% of 1/~ p V~c × maximum projected frontal area of any particular configuration but excluding the shield.

Wind tunnel corrections

These corrections are always important especially when they are comparatively large. A 1% inaccuracy on windspeed leads to 3% inaccuracy on wind power and hence on efficiency 7.

Because of their nature in extracting power from an airflow all windmills have large wakes and any velocity corrections must be made on the basis of wake blockage rather than solid blockage.

347

So far as is known no other work has been carried out with regard to wake blockage on windmill tests in wind tunnels, but complementary work on square flat plates normal to the airstream giving large wake blockage is due to Maskell [5 ] , Gould [6] and Cowdrey [7] .

The blockage correction is of the form

CD ~ c 1

CDc V 2 mS 1 - - -

C

where m is an experimentally determined factor and is equal to wake area/ tunnel C.S.A. for small values of S/C see Maskell [5 ] . Clearly for very large values of S/C, m -~ 1 and the wake must be distorted by the presence of the tunnel walls.

In order to obtain values of m measurements of drag were made both on square flat plates and on Savonius rotors. Maskell [5] argues that the drag corrections obtained on non-lifting flat plates can be applied to lifting but stalled wings and similarly the lifting component of the Savonius rotor is not likely to invalidate the drag corrections made on the same basis as for flat plates.

Further details of this work will be published shortly but the velocity correction for both flat plates and Savonius rotors is given in Fig. 3.

This gives velocity corrections of more than 50% for SIC values greater than 0.32 and it must be questioned whether such large corrections are valid.

In order to check the application of these corrections a further series of tests was carried out on several different rotor configurations in the 8 ft X 4 ft tunnel at Cranfield which has twice the cross-sectional area of the Loughborough tunnel.

Figure 4 shows a comparison for a Savonius rotor having a value of S/C =

1.6

V c 1,4 V

/

15 / / /

q's o,o< 1.3 , ,,,~<,

1.2

I.I

kO

0 0.05 O.lO 0.15 0.20 0-25 0-30 0"35 S C

0.:

0.;

0.I

I I I 0 04 0.2

2 blades

h=O'92rn c=O'lgm

X : - o ' 0 7 ~ = 0 ' 0

• Cranf ie ld t e s t -~=.121 - × L o u g h b o r o u g h test cS-:.24~

0"3 0"4 0-5 0"6 0-7 0"8 0"9

Fig. 3. Variation of velocity correction factor with SIC. Fig. 4. Comparison of results in Cranfield and Loughborough wind tunnels for AI~ = 4.8.

348

0.249 in the Loughborough tunnel and a value of S / C = 0.121 in the Cranfield tunnel. Considering the size of the corrections (a factor of 1.37 on velocity and 2.57 on power for the larger blockage case) the agreement is good and gives confidence that the corrections are meaningful. Other comparisons up to the maximum value of S / C = 0.334 show similar agreement.

Test results

For all the tests reported here the cross-sectional shape of the blades is semi-circular. Two basic sizes were tested, 0.19 m and 0.38 m chord, with a number of different lengths giving blade aspect ratios of 1.2, 2.4, 3.6 and 4.8.

A number of tests were made on a basic configuration of 2 blades, 0.46 m long and 0.19 m chord, Fig. 2b, with the blades having no gap and a slight negative overlap, x / c = -0 .07 and y / c = 0.0, but having end extension pieces of various lengths I fi t ted to the ends of the blades, Fig. 2c. These have the same effect as end plates fi t ted to aircraft wings and increase the effective aspect ratio and hence the efficiency. Figure 5 shows the effect on efficiency

0'3

0"2

0'1

0 0

2 blades ~ symbol h=O'46m c:O.19m 0 ®

0.27 x ~:-oo7 ~ ; o o 040 •

0.53 •

0 - 6 7 •

0.1 0.2 0"3 0 .4 0,5 0"6 0.7 0 ' 8 0 '9 I '0 k

Fig. 5. Va r i a t i on o f e f f ic iency w i th t ip speed parameter and end extension l eng th , A R = 2.4.

~? of fitting end extensions of various lengths. Maximum efficiency increases from 0.092 for ~ = 0.40 with no end extensions, l / c = O, to a value of 0.137 for l / c = 0.67 at a value of ~ = 0.52. Higher values of l / c do not appear to be beneficial. Two further tests were made with the extension pieces bent outwards to give a funnelling effect but these showed only a very small im- provement in 77. However, because of the greater projected area of the windmill the power output was increased by about 5%.

Figure 6 shows the effect of end extensions on the performance of a similar configuration but with twice the chord, 0.40 m. In this case maximum efficiency rises from 0.065 at X -- 0.25 and l / c = 0, to 0.102 for ~ = 0.42 and l / c = 0.67.

Reference 3 quotes a theoretical maximum efficiency due to Betz (not clearly defined) of 20% for the Savonius rotor and indicates that some over-

349

0 .3 2 blades ~ symbol

h = 0 . 4 6 m ¢ = 0 .38m 0 e

~ - : - 0 . 0 3 Y : 0 0 0 ' 2 7 × ) 0 . 4 0 •

0 . 5 3 • 0 '2 0 " 6 7 •

04

L ' I 0 0 0'1 0"2 0"3 0"4 0 6 0"6 0"7 0 ' 8 0 9 I '0

),

Fig. 6. V a r i a t i o n o f e f f i c i e n c y w i t h t ip s p e e d p a r a m e t e r a n d e n d e x t e n s i o n l eng th , A R = 1.2.

lapping of the blades is desirable. Fig. 7 shows the results of a number of tests with overlapping blades covering a wide range of x / c and y / c values. These tests were made with two blades, 0.46 m long and 0.19 m chord and show a clear maximum for ~ of 0.147 f o r x / c = 0.22 and y / c = 0.07. Compared with the standard configuration of x / c = -0 .07, y / c = 0 this represents an increase of efficiency of nearly 60% and an increase in power output of 50%.

r

0.6}- 2 blades h : 0 . 4 6 m c :O.Igm

'067 0

0.41- .094 -I00 .097 -083

m.=.12 \ ,o~ . / . - - ~.,z2 \

412 426 "129 "102 • ~o "1 9 •

4 0 ~

'0176 "0 II J shaft 0 "°2' L

- 0 . 4 -012 0 0i-4 0,2 0.6 x

Fig. 7. V a r i a t i o n o f m a x i m u m e f f i c i e n c y w i t h re la t ive b l a d e p o s i t i o n , A R = 2.4.

Figure 8 shows how the performance of a 2 bladed stendard configuration ( x / c = 0.07, v/c = 0.0) is enhanced by the addition of circular end plates, 0.40 m diameter, and a 0.23 m wide shield placed ahead and to one side of the windmill in the opt imum position, see Fig. 1. The effect of adding the end plates only is very similar to that of the largest end extensions as might be expected. The effect of adding the shield gives a further increase in maximum efficiency to 0.193 representing an increase in power output of a factor of 2.1 over the basic configuration.

The most efficient configuration, ~Tmax = 0.142 for x / c = 0.2, y / c = 0.07, Fig. 7, was also tested with circular end plates and shield to see if the various

350

0.3 2 blades h=O,46m c .O.19m x ~Y-O.O =-0.07 _ _

0.2

"q,

0.1

0 I I l r I _ I ~ [b ] ~ J 0 0.1 0.2 0"3 0 .4 0"5 0"6 0"7 0-8 0"9 I '0

with end plates and 0 2 3 m width flat

~ eld

Fig. 8. E f f e c t o f circular end plates and f lat shie ld on e f f i c i ency , A R = 2.4.

improvements had an additive effect, Fig. 9. The percentage increase in efficiency was not so large as on the basic configuration but did produce a higher maximum value of 77, with the shield, of 0.243.

The tests quoted so far have been with blades of low aspect ratio, 1.2 and 2.4. Similar tests performed on higher aspect ratio basic configurations ( x / c = -0 .07 , y / c = 0.0) give higher values of ~max and these are shown in Fig. 10. Efficiency ~ increases steadily with blade aspect ratio up to the maximum value of five. The beneficial effects of aspect ratio and shielding are clear and the difference in power output between a low aspect ratio un- shielded rotor and a high aspect ratio shielded one of the same area can be as high as four.

Brief tests were made on three and four blade configurations but these proved to be less efficient than the two bladed case either with or without end plates and shielding. The three bladed case was approximately 30% less efficient than

0"3

0 '2

~L

0.1

/ 2blades with end plates and

h=O.46m c=O.19m 0.23rn width flat shieldl ~:o.o7 X:o2 • i 0.3

0.2

O.I

0"2 0 '3 0"4 0"5 0"6 0 . 7 0 '8 0 "c) / '0 H- k

2 blades with end plates and X = - o . 0 7 0 .23m width flat

= 0 '0 s h i e l d ~

with 0.4m dia.

i i O 0 2 4. 6

A~

Fig. 9. E f f e c t o f circular end plates and f lat shield on e f f i c i ency , A R = 2.4. O p t i m u m blade conf igurat ion .

Fig. 10. Var iat ion o f m a x i m u m e f f i c i e n c y w i th aspect ratio.

351

wi th t w o blades while the fou r b lade was 50% less p r e s u m a b l y due to m u t u a l in t e r fe rence b e t w e e n the blades. No a t t e m p t was m a d e to f ind an o p t i m u m b lade conf igura t ion . Tes ts wi th a th ree b laded r o t o r did, however , c o n f i r m tha t the i r s ta r t ing charac ter i s t ics were b e t t e r t han for t w o blades [ 4 ] .

Conc lus ions

Tests have been m a d e on a series of Savonius ro to r s cover ing a n u m b e r of d i f f e ren t conf igura t ions . The var iables inc luded the gap and over lap b e t w e e n r o t o r blades, b lade cho rd and he ight (and hence aspec t ra t io) , n u m b e r of blades (2, 3 and 4) and the add i t ion of ex t ens ion pieces, end pla tes and shielding e i ther singly or in c o m b i n a t i o n .

The values fo r e f f ic iency were general ly r a the r low for low aspec t rat io, un- shielded ro to r s w i t h o u t end pla tes (~max = 0 .065 fo r AR = 1,2). H o w e v e r at the h ighes t a spec t ra t io tes ted , A R = 4.8 and wi th the o p t i m u m b lade and shielding con f igu ra t i on the m a x i m u m value of ef f ic iency, 77 -- 0 .243 a p p r o a c h e d the q u o t e d value [4] o f 31%. The ra t io of t ip speed to windspeed , X, was s o m e w h a t lower in the p resen t tests, however .

Genera l ly , it m a y be conc luded t h a t op t imi sa t i on o f the p e r f o r m a n c e of Savonius ro to r s b y the devices descr ibed in this r e p o r t can raise m a x i m u m ef f ic iency to values a p p r o a c h i n g the m o r e sophis t i ca ted wind energy devices w i t h o u t de t r ac t i ng f r o m the i r i nhe ren t s impl ic i ty and general ruggedness.

Careful a t t e n t i o n was paid to wind- tunne l co r rec t ions and the i r va l id i ty for high b lockage ra t ios c o n f i r m e d b y tests carr ied ou t in two wind tunnels o f d i f f e ren t sizes.

A c k n o w l e d g e m e n t s

The au tho r s are i n d e b t e d to Mr. R.I . Harris fo r al lowing s o m e tes ts to be carr ied o u t in the 8 f t X 4f t wind tunne l at the E n v i r o n m e n t a l Sciences Research Uni t at Cranf ie ld Ins t i tu t e o f T e c h n o l o g y .

Refe rences

1 J. Sladky and P. Kliman, Aerodynamics of the Savonius rotor, Proceedings of 2nd U.S. National Conference on Wind Engineering Research, Colorado, June 1975, paper V3.

2 J.L. Loth, West Virginia University Wind Energy Concentrators, Paper E2, (September 1976). International Symposium of Wind Energy Systems, Cambridge, September 1976, paper E2.

3 S.J. Savonius, The S-rotor and its applications, Mech. Eng., 53, (5) (1931) 333--337. 4 W. Vance, Vertical axis wind rotors -- status and potential. Conference on Wind Energy

Conversion Systems, Washington, 11--13 June 1973. 5 E.C. Maskell, A theory of the blockage effects on bluff bodies and stalled wings in a closed

wind tunnel, Aero. Res. Council, Repts. and Memoranda 3400, 1965. 6 R.F.W. Gould, Wake blockage corrections in a closed wind tunnel for one or two wall-

mounted models subject to separated flow, Aero. Res. Council, Repts. and Memorandum 3649, 1970.

7 C.F. Cowdrey, Application of Maskell's theory of wind-tunnel blockage to some large solid models, Proc. Symp. of Wind Effects on Buildings and Structures, Loughborough University, April 2--4, 1968, paper 29.