The cavitation tunnel at the Swedish State Building Experimental Tank and some aspects of tunnel tests principles

BROOAIISKI IIISTITVT SHIPBUlLflIN6JESER(ff INSTITUTE ZAREB

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2dJ2/3

The Cavitation Tunnel at the Swedish State Shipbuilding

Experimental Tank and some Aspects

of Tunnel Test Principles

by

Dr. Hans Edstrand.

Superintendent Swedish State Shipbuilding Experimental Tank, Göteborg and

C. A. Johnsson

Swedish State Shipbuilding Experimental Tank, Göteborg

Iaper to 1e presented at the Symposium on the

Towing Tank Facilities, Instrumentation and

Measuring Technique Zagreb

22-25

September 1959.

by

Dr. Hans Edstrand and Mr. C.-A. Johnsson

Introduction

The cavitation tunnel in operation since 1957 at the Swedish State Shipbuilding Experimental Tank, SSPA, is in-tended to be used for tests on normal self-propulsion propel-lers (diameter about 9 in.) and for flow investigations on different types of submerged bodies. A detailed description of this cavitation tunnel and its equipment is given in [i)

The tunnel has a more or less conventional shape but differs to some extent from older designs in that it is comparatively long and also that the test propeller is driven by an upstream

shaft. Fig.l.

The length was dictated partly by the need for a long test section in which long bodies could be investigated and partly by a desire for a long diffuser ahead of the upper downstream bend. The long diffuser serves to increase the pressure arid

re-duce the velocity immediately ahead of the bend and thus mini-mises the risk of cavitation in the bend itself, which is a

critical region.

In order to increase the availability of the tunnel two in-terchangeable test sections are used, Fig.l. In the case of the

1) The numbers within brackets refer to the list of references at the end of this paper.

small test section, which has the dimension, length = 7.2 ft., area = 20 in. x 20 in., the area is approximately doubled bet-ween this section and the first downstream bend, i.e. the cavi-tation number in the bend is up to 16 times that in the test.

section. The smalle.* section is suitable for tests of pro-peller models in homogeneous flow at low cavitation numbers.

p -e

Cavitation numbers as low as 0

= 0.15

and even lower

p/2

_VB

can be used in this section.

The large test section has the dimensions, length =

7.9

ft., area =

27.5

in. x

27.5

in. which is approximately the same area as at the downstream bend, so that the cavitation number remains

virtually constant. This means that tests at very low cavitation numbers cannot be carried out in the large test section without cavitation occurring in the downstream bend. In practice, =

1.5

forms a reasonable lower limit for tests in this section. The large test section is suitable for tests of propeller models in non-homogeneous wake fields and for tests of submerged bodies.

In order to minimise longitudinal pressure and speed varia-tions in the test secvaria-tions due to the increasing thickness of the boundary layer, the walls of the test sections have been made slightly divergent. Preliminary calibrations have indicated

that the flow quality is good in both the small and the large test sections, in spite of the fact that the contraction in the nozzle of the latter is as low as 4:1.

As for all model tests it is naturally of great Importance for investigations in a cavitation tunnel to imitate as far as possible the conditions in full scale. Further it is important to know the influence on the conversion to full scale of such

values and factors, which necessarily have to differ In model scale. Unfortunately our knowledge of the laws which govern a correct conversion is very restricted.

One of' the difficulties is to define the load condition of the propeller in the cavitation tunnel, i.e. the connection between advance coefficient, thrust, torque and cavitation num-bér. It is standard practice at many establishments when deter-mining the associatedvalues of thrust, torque and revolutions to use the characteristics from the open water tests with the same model propeller. However, at comparative tests in the ca-vitation tunnel at atmospheric pressure and undeopen condi-tion in the towing tank one very often finds that the above mentioned components are not identical. It is the aim of this paper to try to collect different reasons for such discrepan-cies and to discuss investigationsin this field in the cavita-tion tunnel at SSPA.

Reasons for Discrepancies in Test Results in the

Cavitation Tunnel and in the open Condition

Influences can be obtained for the folLowing reasons, al-though the importance of the influence is very difficult to de-termine for some of

them:-1. Differences in Reynolds Number and the Degree of Turbulence The tests in the cavitation tunnel are usually carried out at higher water velocities and revolutions than in the open con-dition. Further the degree of turbulence may be entirely diffe-rent in the cavitation tunnel than in the towing tank. As a

and the characteristics are influenced accordingly.

Theoretical investigations of the influence on the charac-teristics can be made using the analysis with the equivalent pro-file according to Lerbs [2]. The results from such an analysis for a typical cargo ship. propeller are shown in Fig.2. From Fig.2 it is evident, that an increase in the profile drag of about 170 % in the neighbourhood of the peak efficiency, which can be assumed to be an extremely high value in this

tunnel-tank comparison, causes an increase in the torque characteristic of about 10% while the thrust characteristic is decreased by

on-ly about 2%. These tendencies are verified by experimental in-vestigations, see also [3]

From this point of view the thrust characteristics would be most suitable for comparisons between open water tests in the

towing tank arid tests in the cavitation tunnel. This is so at

moderate values of the pitch ratio. At very great values of the pitch ratio the influence on the thrust coefficient will increase.

2. The Wall Influence

The differences in 'flow conditions between open water and cavitation tests are schematically illustrated in _Fig.3.

In open water tests both the velocity, yE, and the static pressure, p, are constant for the flow surrounding the propeller, while ir the cavitation tunnel these quantities vary as the re-suit of the velocities induced by the model propeller itself.

The wall influence adopts different values depending upon the method of measuring the water velocity in the cavitation tunnel. If the velocity is measured by the Venturi meter in the contraction nozzle, the wall interference is obtained as the

ratio Vl/VE, see Fig.3b. Calculations for this case have been carried out by Wood and Harris [4,5]. The results from their calculations are given in Fig.4a.

If the velocity is measured by a Bitot tube placed in the same cross section as the propeller, the wall, interference is

obtained as the ratio VO/VE, see Fig.3b. Calculations for this case have been carried out by Lerbs [6], who used the same basis as Wood and Harris. The results from his calculations are given

in Fig.4b.

It is evident from Figs.4a and 4b that the wall influence is of different direction arid magnitude in the two cases.

The following assumptions and approximations were used in

the' calculations:-.

As the equivalent open velocity, 11E' that velocity has been chosen which for a certain number of revolutions gives the

same thrust, T, and velocity, yE + - , as in the tunnel,

see also Fig.3.

The axial component of the velocity is assumed constant over the propeller disc.

Both the pressures p and p1 are assumed constant over the tunnel cross section.

The propeller thrust has been calculated according to the simple momentum theory, i.e. the tangential component of. the induced velocity is not taken into consideration.

Velocity fluctuations in the cross sectipn due to viscosity have not been taken into consideration.

3.

The Influence from the Pressure Distribution on the Propeller Jet In the calculations in point 2. no corrections have been

made for influences on the thrust coefficient from the static pressure fluctuations along the curved contraction boundary of

the propeller jet, see curve A B. C in Pig.3b.

Mime-Thomson 7] has shown, that the sum of these varia-tions is zero for a propeller in the open condition, while a force, X, is obtained for a propeller working in a tunnel with circular cross section. This force has opposite direction compared with the propeller thrust and has the magnitude, see Pig.3b.

(R22

-(1)

h2-R12

Errors in the Measurement of the Tunnel Velocity

Errors can appear when determining the water velocity in the tunnel when either the Venturi Meter or a Pitot tube is used. In both cases calibrations ought to be made by measuring

the velocity distribution over the test section for different velocities. As the contraction, , usually is rather small,

h

different velocity distributions may be obtained at different velocities in the tunnel. Measurements with orifices have shown that up to a critical value of Reynolds number the ratio effec-tive area to geometrical area of the orifice is very dependent

upon the velocity. This ratio is independent of the velocity above the critical value.

Further,

in

tunnels with square test sections the Pitot tube measurements can be disturbed by secondary flow.

The Influence of the Propeller Shaft Arrangement

placed on the forward end of the driving shaft and is thus meet-ing undisturbed water while the drivmeet-ing shaft is arranged up-stream i'n the cavitation tunnel at SSPA, see Fig.l. In the lat-ter case the velocity will differ at different radii due to the boundary layer flow around the shaft. Further in the case of open water tests the shaft behind the propeller will influence the pressure distribution. Both these circumstances may cause dif-ferences in the propeller characteristics.

6. Errors from the Measuring Instruments

In order to avoid errors from the test instruments affecting the test results, it is desirable that the same measuring instru-ments are used in both types of tests. This, however, which has also been pointed out in [8], is not always achievable from a practical point of view.

Theoretical and Experimental Investigations of Discrepancies in Propeller Characteristics from Towing Tank and Cavitation Tunnel

A research program has been planned at SSPA with the pur-pose of investigating the discrepancies in the ecperimental re-suits from open water tests in the towing tank and the cavita-tion tests in the tunnel. The first part of this program contains the following

points:-1. Theoretical and experimental investigation of the wall influence. The experiments include comparative investi-gations of propeller characteristics obtained in towing tank and cavitation tunnel. (Four geometrically uniform

propellers with different diameters and four uniform pro-pellers with equal diameters but different pitch ratios have been investigated.)

Investigations of the velocity distribution in both the test sections of the cavitation tunnel.

Comparative investigations of characteristics obtained with propellers with different radial distributions of the

cir-culation but otherwise calculated with the vortex theory from the same basic assumptions.

This research program is still in progress and only preli-minary results can be given in this paper.

1. Theoretical and Experimental Investigation of the Wall Influence In [9] a comparison is given showing propeller characteris-tics obtained in towing tanks and cavitation tunnels of diffe-rent establishments. The results from the cavitation tunnels are corrected for the wall influence according to the method

de-scribed in [4 and 5].

It is evident from [9] that these corrections bring the

re-suits

from

the cavitation tunnels into better agreement with those from the towing tanks. The applied corrections are, how-ever, in most cases insufficient and discrepancies still remain.

The remaining differences can depend upon reasons which have been discussed above.

An attempt has been made at SSPA to approximately calcu-late the wall influence in the case when the velocity is measured by the Venturi meter in the nozzle. The calculation was based on

the vortex theory and was accomplished as

in-duced velocity has been calculated with the aid of the equation given below and obtained from the law of continuity

R

= 1 + 2a 1k Ca/2 dx (2)

yE

1m v1

where x = = ratio of the radius for a particular blade element to the propeller radius

Ca _{= axial component of the induced velocity} at

the propeller

km = a factor for calculating the mean axial in-ducea velocity for a particular radius

a = ratio of propeller disc area to tunnel test section area

See also Fig.3.

Equation (2) presupposes v0 VE , see Fig.3b. The equa-tion is much simpler than those which have been used for the calculations in Fig.4.

If the axial induced velocity can be assumed constant over the propeller disc the following is obtained

= 1 + a (Ca12)

VE yE

(2a)

If the simple momentum theory is used (as earlier in this paper) to calculate the connection between propeller thrust and induced velocity, the following is

obtained:-Cal2

_V

1 + C - 1

yE - 2

and where = 1 +

[Vi

+CT_lI

= T T PL 2itD2 VE

T

A comparison between values calculated according to (3) and values from Fig.4a gives only negligible differences.

If the induced velocities are calculated for every b1de section according to the vortex theory, the following is ob-tained for a propeller with the optimum Betz

distribution:-(°a/2)m = k Ca/2 l_.fl VE In _yE = k _x2 + ?.. 2

The mean factors, km for the axial components of the in-duced velocities are not available in the literature and would require extensive calculations. As a temporary approximation the Goldstein -value for the mean of the circulation at different radii can be used and the integration over the propeller disc

give s:

-dx

1 + 2a

fl_ni

1

yE _'j

x+1

Although equation (4) assumes an optimum distribution of the circulation over the propeller blade, it may be used as a good approximation (when calculating the wall influence) also for propellers with distributions of the circulation differing from the optimum one. Such propellers are used very often

nowa-days.

In order to verify the different equations for the

calcula lo calcula

-(3)

tions of the wall influence, comparisons have been made at SSPA between calculated and experimental values for two series of

pro-pellers.

In Fig.5 the results of such a comparison for a series of three-bladed propellers are shown. The propellers had the dia-meter Dm = 10 in. The pitch ratio was varied in four steps

= 1.000, 1.152, 1.309 and 1.454. Tests were carried out with the four propellers in both the tunnel sections with a constant water velocity of 20 ft./sec. The comparison was made at two values of the advance coefficient,

nmax corresponding to the

peak efficiency in open condition and 0.75

In Fig.6 a similar comparison is shown for a series of three-bladed geometrically uniform propellers with the dia-meters 6, 8, 10 and 12 in. The water velocity in the tunnel

sections was 7.5 ft./sec. in this case.

The experimental test results concerning the wall influence

are given in Figs.5 and 6 asJ/J, where LJ is the deviation

in the advance coefficient for or KQ-idelltitY and this

iden-tity is based on the characteristics obtained under open water tests in the towing tank and those obtained under atmospheric pressure in the cavitation tunnel.

The calculated values were obtained, firstly from the dia-gram in Fig.4a (Wood and Harris), secondly using equation (4). The Influence from the propeller hub has been taken into consi-deration in 'the calculations according to equation (4) by using values of from [io] and values of the ideal efficiency n1 from iii].

The points in Figs.5 and 6 representing experimental re-sults show a considerable scattering. However, the calculated

values can be said to be reasonably close to the experimental ones except in the case of the large test section at the lower water velocity, v = 7.5 ft. sec, Fig.6. This would probably in-dicate, 'that the above mentioned critical value of Reynolds number was not exceeded in this test section with an according-ly unfavourable velocity distribution. This case will be further investigated by de±ailed examinations of the velocity ñistribu-tions in the two test secñistribu-tions.

In a comparison between values from the open water condi-tion and the cavitacondi-tion tunnel based on RT_ KQ-identitY as in Figs.5 and 6 the total influence of all the .ctors mentioned

earlier in this paper is obtained. In order to separate approxi-mately the wall influence from the other factors the following experiment was carried out in the cavitation tunnel with some of the propellers.

At a certain water velocity the number of revolutions was chosen, which gave the advance coefficient for the thrust T = 0. This advance coefficient gives no wall influence. The number of revolutions was then changed to other (lower) advance coeffi-cients. The alterations in the Venturi meter reading were ob-served. The number of revolutions on the impeller was kept

con-stant the whole time. If the influence on the impeller from the changed number of revolutions on the model propeller can be neg-lected, the alterations in the Venturi meter readings can be

as-sumed to be a measure of the magnitude of the w311 influence. The results obtained in these experiments are shown in Fig.7. From the diagram it can be seen that the tendendy of the

curves representing the calculated values is similar to that of

-the points representing -the expritnenta1 results although the absolute values are too low. This holds good even for the case, illustrated in .Fig.7. (upper diagram), where. the ComParison.

Fig.6 gave divergeficeS. The discrepancy in Fig.7 is probably caused partly' by' lack 'f ,accuracy in. the calcu'lati3Ofl of the fac

tor, 'km (q) in.eguation (4) for cal 4t.i9n

o,'.:th.w.l .!ffect

and paitly by fluctuations in the impe1er'.efficie.flCY caiased .by the influence of the model propeller, The latter cause can be assumed to be rather small.

Final comments

The results from the investigations descrIbed' in.tTiS pa-per show that the influence from the tunnel walls js the most

important reason for discrepancies in 'the propeller

charè.cteriS-tic's from open tests in the towing tank compared witi tests in:

the cavitation tunnel. When stating this it is assumed that th

influence from' the' Venturi meter reading is eliminated through' calihiatiofl of the velocity distributOfl by Pitot tube measure-ments. '

The scattering of the points in Pigs.5 and 6, representflg experimental results, illustrates the difficulties of obtaining

an 'accurate estimation of the' wail influence by comparing. KT

and KQ-Characteristics from th. towing tank and the cavitation

tunnel. ne reason, for the scattering of the' points is,, Of ôourse,

that the inaccuracies of the two tests compared are added., This raises the ôuéstion if thrust identity is the most suitable base'

for tests in the cavtation tunnel. May be it is sound to accep.t

one of the aforementioned theoretical cbrrections as a

provisiO-nal correction until,,'a better one is obtained.

-Experiments of the type illustrated in Fig.7 may, if care-fully performed, be a good way of checking the results of new calculations of the wall influence.

When the Venturi meter is used for the velocity determi-nation, the vortex theory in its simplest form can be expected

to give a good approximation for the calculation of the wall influence if correct values of the mean factor, km in equation

(4) can be obtained.

Calculation of the wall Influence by the vortex theory

gives a certain advantage. Through modifications of the equations it is possible to approximately investigate how velocity

altera-tions due to influences from the propeller are distributed over the cross section. With acctirate enough correction factors, this might give the possibility of treating each blade section sepa-rately by using a variable correction.

-1k-"References

]' Lindgren, Has: "The Cavitation. .Laborátory of. the edish State

Shipbuilding Experimental Tank", SSPA Publ.No.43,

Goteborg,

1958.

[2] Lerbs, H. "On the Effects of Scale and Roughness on Free runnmg Propellers", Journal ASNE, Vol.63,No.1, New York, 1951. Nordstrom, H.P., Ed5trand,Hans Lindgren,Hans: "On Propeller

Scale Effects", SSPA Publ.Nó.28, GOteborg,

1954.

[4]

Wood,'R.McK.., Harris, R.G'.:.'!Some Notes Onthe Théory;of an

AirscrewWorkingina Wind Channel", A.R.C.,,R&M.

No.662, London, 1920. .

f'5]

Gláuert, Ii.,: "The Elements of Aerofoll & Airsrew Theory.",

Cambridge University Press,

1947.

[6] Ler.b, H.: "Untersuchung der Kavitation an Schraubenpropellern", Mittelung 131 der Hamburgis.chen Schlffbau-Versuchsanstalt,

.1936. -, r .

[

] Mime - Thomson, L.M.,: "Theoretical Aerodynamics",' Macmillan

and Co., Loñdon,,.1948..

Van Manen, J,D.: "Tests, in Cavitation Tunnels and their Comparison with Open - Water Tests", mt. Shipbuilding Progress, Vol.1,No.3,. Rotterdam, 1954.

Burrill, L.C. "Alternative Method of Presentation of Results", Contribution to Comparative Propeller Tests, Seventh. mt.

-Conf.on Ship Hydrodynamics, SSPAPubl.No.34, GOtebórg,

1955. ','

rio] Tachmindji, A.J.,' Milam, A.B.: "The Calculation of the

Circula-tion DistribuCircula-tion for Propellers with Finite Hub Having Three., Pour, Five and Six Blades", DTMB Report No.1141, Washington, 1957.

-].5-[u]

Schultz Jr.,J.W.: "The Ideal Efficiency of Optimum Propellers Having Finite Hubs and Finite Numbers of Blades",

DTMB Report No.1148, Washington, 1957.

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