Influence of tank width on model tests in waves

Anru'rF

.

tita

Stevens Institute of Technology

Hoboken, New Jersey

Lab. v. Sut eirii::)3uoikun Technische Hogesthool

NOLO. 551

August 1959

INFLUENCE OF TANK WIDTH ON MODEL TESTS IN WAVES

by

EDWARD NUMATA

FOR PRESENTATION AT TWELFTH MEETING OF AMERICAN TOWING TANK CONFERENCE

SESSION ON SEAGOING QUALITIES AT UNIVERSITY OF CALIFORNIA

BERKELEY, CALIFORNIA AUGUST 1959

Note No. 551

August 1959

DAVIDSON LABORATORY

STEVENS INSTITUTE OF TECHNOLOGY HOBOKEN. NEW JERSEY

INFLUENCE OF TANK WIDTH

ON MODEL TESTS IN WAVES

by Edward Numata

FOR PRESENTATION AT TWELFTH MEETING OF AMERICAN TOWING TANK CONFERENCE

SESSION ON SEAGOING QUALITIES

AT UNIVERSITY OF CALIFORNIA. BERKELEY. CALIFORNIA

Approved by

Edward V. Lewis

An experimental program to evaluate the influence oftank width on the motions of a ship

model in -regUlar..head,WaVell, is discussed. The . results of the -wide tank phase of the program at.

ADDENDUM

Lab. v. ScheepsbouwkunUd55 Technische Hopschool

Delft

Subsequent to the original printing of this note, further experimental

data were obtained, analyzed, and included in the writer's oral presentation

of this note at the ATTC meeting in Berkeley on 31 August 1959. The purpose

of this addendum is to summarize the oral presentation of the additional data.

Series 60, 0. 60 block model 2108 was towed in DL Tank No. 1

(100 ft. x 9.0 ft.) using the same instrumentation and carriage speeds as in

the square Tank No. 2 tests (see page 2). The wave size was 1. 50L by L/48.

The pitch and heave amplitude results have been added to Figure 6 (revised). It is seen that except in the region of w v/g = 1/4, the results in

the wide and narrow tanks are in close agreement. In the region of wv/g = 1/4, however, there is a pronounced difference between the two sets of data.

Although a small irregularity in the trend of the wide tank results is discern-able in this region, the corresponding narrow tank results exhibit a very large irregularity in trend. Repeat runs were taken in order to verify this

effect and close agreement was obtained in all cases.

At model speeds in the region of wv/g = 1/4, large damping waves from the model were observed to travel laterally from the model and with a forward component of velocity approximately equal to the model velocity. In the narrow tank these damping waves were observed to reflect from the tank side walls back to the model. In the wide tank with the nearer tank side being approximately five times as far from the model as in the narrow.

tank, this reflection effect was either absent or greatly reduced.

Tests of the model in 0. 75L, 1. OOL and 1. 25L waves in the narrow

tank will be completed in the near future and the results can then be com-pared to the corresponding wide tank data in the hope of obtaining definite conclusions regarding wall effect.

PITCHING AND HEAVING MOTIONS 5-FOOT SERIES 60, 0. 60 BLOCK MODEL

1. 50L x L/48 WAVES 3 2 1 Degrees HEAVE Double Amplitude 1.5 PITCH Double Amplitude FIG, 6 Revised A LEGEO

--.31.-.Mode1 .2108, DL Tank No. 1

Model 2108, DL Tank No. 2

Model 11045, DL Tank No. 1

. Model 1).4/1.5, MIT

Model 1/15, DTMB

1

Vg

:I Predicted Speed for

Motion Peak Knots I I I Knots .5 1.0 1.5 2.0 I I I II I ap Calculated, KorvineJacobs SHAME; 1957 2.5 3.0

1.111111

odel and Test Apparatus -Discussion of Results TABLE OF CONTENTS Concluding Remarks Iteferences . . . e eeee550 . 000 S

,,.--.Observed and '.Predicted Speeds at MbtiOn'Peaks.,...

Motion's in Region of Abrupt Change in Hydrodynamic_{, ,}

Damping.

Comparison of Wide and Narrow Ta.nk Results at LOW Speeds 4

INTRODUCTION

In 1956 the Series 60 Task Group of the Seakeeping Characteristics Panel, Society of Naval Architects and Marine Engineers, completed a

correlation of seakeeping test results for a five-foot model of the Series 60,

0. 60 block hull in three different towing tanks. The tests were conducted in the 140-foot basin at David Taylor Model Basir the 108-foot ship model towing tank at Massachusetts InsLitute 1; Technology, and the 100-foot Tank No. 1 at Davidson Laboratory. Professor Abkowitz, Chairman of the Task Group, presented these results at the 1956 meeting of the ATTC.

An important conclusion of this study was that the motion amplitude

results obtained in the three tanks agreed within an accuracywhich was

considered acceptable at speeds above one knot for a five-foot model; Below one knot, however, there was a marked dispersion of results which was attributed to the effect of model generated waves which were reflected from the side walls of the tank back to the model. This effect was

accen-tuated by the relatively narrow widths of all three basins -- 10. 0 feet at DTMB, 9.0 feet at DL and 8. 6 feet at MIT.

Accordingly, the Task Group recommended an experimental investi-gation to evaluate the effect of wave reflection in narrow tanks on the sea-keeping behavior of models. The Davidson Laboratory was suggested as

the facility best suited to carrying out such a study since, at the

time, it

was operating both a narrow basin (100 feet by 9 feet) and a wide basin 75 feet square and had provisions for using the same towing and recording

apparatus in either basin. Thus a model could be tested in each basin under almost identical conditions with the important exception that the un-desirable effect of side Wall reflections would be appreciably reduced in the wide tank. A comparison of test results obtained in the two tanks was expected to show the extent to which reflection affected the narrow tank

results.

The Seakeepin.g Characteristics Panel approved the recommendation of the Task Group, and late in 1958 the SNAME provided funds to enable the Davidson Laboratory to proceed with the investigation. Due to schedul-ing difficulties, only the wide tank phase of the test program inTank No. 2 has been completed to date, and only these results are presented in this

note. They are compared with the original Task Group results from DTMB, MIT and DL.

N-5'51

-1-MODEL AND TEST APPARATUS

A. Series 60, 0. 60 block, Live-foot _{varnished wood model (DL No} 2108) having "standard" sheer and "mean" forebody flare was used. To

avoid water being shipped _{on the foredeck, a forecastle with a length of} 15 percent of the model length and a uniform height of 1.25 inches was

faired into thebasic hull form. _{The model was ballasted to a total weight}

of 32. 27 pounds corresponding _{to an even keel draft of 3. 20 inches with the}

center Of kravity,O. 9 inch aft of the rnidstation.. _{The ballast was distributed} so as to yield a longitudinal radius of gyration of 0. 25L as determined by a

compound pendulum method.

-The DL motions apparatus was used to tow the model with freedom

to pitch, heave and surge, as shown in Fig. 1. Transducers mounted on the apparatus permitted simultaneous _{records to be taken on an oscillograph} of pitch angle, heave at CG, and drift of the Model with respect to the

con-stant-speed Main carriage. " A signal from _{a resistance-type wave probe} suspended from the main carriage ahead of the model _{was recorded simulta} neously with the motions.

The Tank No. 2 bridge was positioned 21. 25 feet from the north wall

of the tank, as shown in Fig. 2. A

have been desirable. _{However, Fig. 4 shows that it was not possible to}

move it any closer to thecenter of the tank because of the

restrictions

posed by the geometry, of the bridge-stabilizing _{arm-rotating arm linkage} and the building walls..

Wave lengths: 0. 75, 1. 00, 1.25 and 1. 50times model length.

Wave height: 1/48 times model length.

Speeds: _{maximum increments of 0. 1 knot between}

zero and 1.0 knot, and 0. 3 knot

between

1...0 knot and 2. 75 knots. Note that the lowest available carriage speed was 0. 33

knot.

:TEST 'PROCEDURE..

TEST RESULTS

Double amplitudes of pitch angle and heave at the CG versus model

speed are presented in Fig. 3 to 6. The results of the original Task Group comparative tests were taken from Abkowitzl and added to these figures for comparison.

Except as noted below, each amplitude represents the average of ten complete cycles recorded over a fixed interval of model travel equal to 20 feet or four model lengths. Exceptions were made at the higher

speeds in the longer waves where less than ten cycles were recorded

during the fixed interval. In such cases, the actual number of cycles in

the interval were averaged, the minimum number being six cycles at

2.75 knots in 1. 5L waves. The average amplitudes were thencorrected by direct proportion for deviations of the average recorded wave height

from the desired height of L/48.

DISCUSSION OF RESULTS

Observed and Predicted Speeds at Motion Peaks

There is reasonable agreement between the speed at which an

amplitude peak occurs and the predicted speed for such a peak. The

predicted speeds were calculated by using the natural periods of pitching and heaving as determined by manually oscillating the model in calm

water. These periods were 0.725 and 0. 70 seconds, respectively. Motions in Region of Abrupt Change in Hydrodynamic Damping

Several theoretical contributions have established the speed-dependent character of damping coefficients for mathematical hull forms moving forward and oscillating on a water surface. It has been shown --most recently by Newman for a polynomial hull form -- that damping becomes large in an abrupt manner when the non-dimensional frequency and speed parameter wv/g has a value of 1/4.

For the case of a ship model test in waves, this concept suggests that, in the region of the critical wv/g value, the model motions should

be extremely sensitive to small changes in wave frequency of encounter, w , and model speed, v. Examination of the present test results in Fig. 3 through 6 shows some scatter of data points in the region of

v/g = 1/4 as well as apparent discontinuities in the pitch curves at the two longer walse lengths.

N-551

-3-N-551

4

-Comparison of Wide and Narrow Tank Results at Low Speeds

The original Task Group report by Professor Abkowitz1 _pointed

out the dispersion of results at speeds below _{one knot and attributed this} scatter to wave reflection effects caused by the relatively _{narrow widths}

of the three tanks involved. _{Other experimenters have since reported on} this phenomenon and have suggested criteria for determining the speed

below which it would occur. _{The pattern of waves generated byan}

oscilla-ting model at low speed is illustrated in Fig. 1.

Gerritsma3, in discussing the results of a geosim series of sea-keeping model tests, suggested that the critical speed for inception of tank wall reflection effect occurs when c,,v/g = 1/4. He referred to the

theoreti-cal work of Brard4 who defined the patterns of surface waves generated by a moving, pulsating source. Briefly, such waves proceed both upstream and downstream from the source when w v/ g < 1/4; they trail downstream

only when cov/g > 1/4. _{Applying the former condition to the case a an}

oscillating model, the waves within a sector forward of the beam would

travel outward to the tank sides and be reflected back _{to the model. Under}

such circumstances, Gerritsma _{noted a scatter of experimental data obtained}

in the 14-foot wide Delft tank with models ranging in length from 6. 5 to 10

feet.

Recently Van Manen5 presented a partial summary of results obtained in a seakeeping wall effect study at Wageningen in which a ship model was tested in basin widths of 1.75 and 5. 4 times the model length. _{In the low} speed range, the trends of pitching amplitudes obtained in the narrow basin were strikingly different than the wide basin results. In each of the four wave lengths used it was found that the speed of inception of this apparent wall effect could be accurately predicted through _{use of a theoretical} contri-bution by Hanaoka6. The calculation procedure for Hanaoka's method i not

presently available to the writer. By analyzing Van Manen's data, however,-it appears that inception of wall effect can be defined by a function of cov/g and the ratio of tank width to model length.

It is difficult to draw any conclusions regarding tank wall reflection effects from a comparison of the present wide tank results and the original Task Group narrow tank data. (See Fig. 3-6) In the speed region below one knot where such effects are expected, the Task Group data is too sparse,

particularly in the longer wave lengths. The narsrow tank phase of the present investigation, when completed, should provide a firm basis for evaluating

the speed at inception and the magnitude of such wall effects:

CONCLUDING REMARKS

Since the test program of this investigation has not been completed,

conclusions relative to its basic objective cannot be made at this time. The results of the wide tank tests appear satisfactory, however, and it is. planned that, in the near future, the narrow tank data will be obtained and

made available, thus bringing the project to its conclusion.

REFERENCES

Abkowitz, M, A.: "Correlation of Model Tests in Waves -- Report of

Panel", Trans. ATTC 1957, DTMB Report 1099, September 1957. Newman, J. N.: "The Damping and Wave Resistance of a Pitching

and Heaving Ship", Journal of Ship Research, June 1959.

Gerritsiria, J.: "Seaworthiness Tests with Three Geometrically

Similar Ship Models", Proceedings of Symposium on Behavior of Ships in a Seaway, Wageningen, Holland, September 1957.

Brard, R.: "Introduction to the Theoretical Study of Pitching at

Forward Speed", Association Technique Maritime et Aero-nautique, 1948. (In French)

5, Van Manen, J. D.: "Research Program of the Netherlands Ship

Model Basin", International Shipbuilding Progress, June 1959.

6. Ha.naoka, T.: "Theoretical Investigation Concerning Ship Motion

In Regular Waves", Proceedings of Symposium on Behavior of Ships in a Seaway, Wageningen, Holland, September 1957.

N-551

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.lik' '

021=10 Gilo II

0 60 BLOCK MODEL INSQUABE TANK

93 fps IN: 0 75L 3'c L/48 *AYES:, ANN. 4 , 141 14.41 . ' t.

N-551

16a.

PLAN VIEW OF TANK NO. 2

FIG. 3

0

Degrees

PITCHING AND HEAVING _MOTIONS

5-FOOT SERIES 60, 0.60 _{BLOCK MODEL}

0. 75L x L/48 WAVES 1.0 PITCH Double Amplitude 1.5 Knots LEGEND

Model 2108, DL Tank No. 2

0 _{Model 1445, DL Tank No. 1}

0 Model 1445, MIT

A Model 1445, DTME

wy 1

Predicted Speed for Peak Motion

2.0 2.5

N-551

3.0

N-551 PITCHING AND HEAVING MOTIONS 5-FOOT SERIES 60, 0. 60 BLOCK MODEL

1. OOL x L/48 WAVES 1.5 1.0 .5 IMP Ile 0 HEAVE

Double Amplitude \Lai 'k?.!1

A Predicted Speed for Peak Motion

Ai, Model 1445,102MB

FIG. 4

Knots

.5 1.0 1.5 2,0 3,0

iii iiii 1

1.1

till

2.0 Inches

.

LEGEND

Model 2108, DL Tank NO. 2 o

O

Model 1445, DL Tank No. 1.

Model 1445, MIT 1 1 1 1 1 I

II It

Iiii

II IIIIIIIIIIIIII

FIG. 5 2.5

r

PITCHING AND HEAVING _MOTIONS

5-FOOT SERIES 60, 0.60 BLOCK _MODEL

1. 25L x L/48 WAVES

1.0

attba A

LEGEND

.Model 2108, DL Tank No. 2

Model 114145, DL Tank No. 1 Model 1115, MIT

Model 1145, DT/a3

w v1

A Predicted Speed for

Peak Motions IC ta 1.5

0

1.0 HEAVE Double Amplitude 0 Knots 1.0 1.5 PITCH Double Amplitude A LEGEND

Model 2108, DL Tank No. 2 Model 1145, DL Tank No. 1

Model 11451 NIT A Model 1445, DTMB

w v 1

vg T

A Predicted Speed for

Motion Peak

2.0 2.5 3.0

N,-551 PITCHING AND HEAVING MOTIONS _{FIG. 6}

5-FOOT SERIES 60, 0.60 BLOCK MODEL

1. 50L x L/48 WAVES

Inches 2.

2.0