An experimental study on shear stress variation on a series 60 ship model

AN EXPERIMENTAL STUDY

OF SHEAR-STRESS VARIATION

ON A SERIES-60 SHIP MODEL

by

K. T. S. Tzou

This Research Was Carried Out Under The Naval Ship Systems Command General Hydromechanics

Research Program Administered by the Naval Ship Research and Development Center.

Prepared Under the Office of Naval Research

Contract Nonr-1611 (05)

uHR Report No. 108

Iowa Institute of Hydraulic Research

The University of Iowa

Iowa City, Iowa

February 1968

This document has been approved for public release and sale; its distribution is unlimited.

AN EXPERIMENTAL STUDY OF SHEAR-STRESS VARIATION ON A SERIES-60 SHIP MODEL

by

K. T. S. Tzou

ABSTRACT

A study of the change in water-surface profile with Froude number near the stern of a Series-60 model, the shear-stress distribution on the after body, and a discussion of the possible laminar boundary-layer

ef-fects are presented. The results suggest that the sinuous trend of the

viscous-drag curve is attributable to the change in surface

configura-tion near the stern and the shear-stress distribuconfigura-tion on the ship hull, and cannot be attributed to the presence of a laminar boundary-layer.

LIST OF SYMBOLS

C. coefficient from 1957 ITTC correlation formula coefficient of total resistance

C coefficient of viscous resistance

V

F Froude rnnnToer V/V

g gravitational acceleration

L ship length at the waterline

Pt total Dressure-hea reading of the stagnation tube

P total pressure-head reading in the free stream R Reynolds number = VL/v

V velocity of a uniform stream kinematic viscosity

An Experimental Study of Shear-Stress Variation on a Series-60 Ship Model

Introduction

This study, a continuation of the work reported in [l]*, was under-taken to determine the cause of the observed sinuous variation of the viscous resistance coefficient with Froude number. Two series of experiments were

con-ducted: In the first the water-surface profile along the shin hull was photo-graphed, in the second the shear-stress distribution on the after body was

measured. Furthermore, in response to a suggestion that the observed variation of the viscous drag may be due to the presence of a laminar boundary layer,

this possibility is also discussed.

Equjment and Procedure

The uHR towing tank (lO feet wide, lO feet deep and 315 feet long) is euippd with a cable-driven carriage which can be operated at constant speeds

up to 21 feet per second. _{A digital tachometer indicates the carriage speed to} an accuracy of 0.01 fps. A 10-foot Series-60 ship model, the parent form of 0.60-block coefficient, was attached to the carriage for both experiments in

this study.

An automatic photographing system was set just above the water surface on the channel wall and was located at about 100 feet from the north end, where

the carriage had attained a constant speed. The photographing system consisted of a 35-mm camera, an electronic flash, a cable release, and an L-shaped crank

which, when contacted by a rod projecting from the passing carriage, acturated. the photographic system.

Since the camera was set at a distance of only four feet from the ship hull, it was not possible to include the entire model in _{one photograph. All}

the photographs in Figs. 1 and 2 were obtained by matching two sets of _photographs, each pair of which was taken at the same ship-model speed. _{For these the carriage} speeds were varied from 3.0 to 6.0 feet per second, in increments of _{0.50 fps.}

A stagnation tube with 0.125-inch OD and 0.096-inch ID was used as a

-2-Preston tube for measuring the shear-stress distribution along the ship hull. As indicated in Fig. 3, the set-up was so designed that the stagnation tube

could be adjusted to reach any measuring point on the ship model. Measurements were taken at 32 points on the hull for the speeds of 3.0, 4.0, 4.5, 5.0 and

6.0

feet per second. The resultant contours of constant shear stress are

pre-sented in Fig.

4.

Analysis and Discussion

1. Surface-profile study

The photographs in Figs. 1 and 2 show the water-surface configuration

along the ship hull for seven different speeds. These show no appreciable change in the surface configuration for Froude numbers less than 0.22, but a large change in surface profile for Froude numbers T = 0.25 to 0.28.

At F =

0.25 the wave crest was located at

0.6

foot, and the wave trough at 2.0 feet

downstream from the centerline of the ship model; the wave height from trough

to crest was 0.5 inch. At 1F = 0.28, the crest occurred at 1.1 feet, the trough at 2.5 feet downstream from the centerline of the ship model, and the

wave height was 1.1 inches. At F = 0.30, the trough near the stern of the ship model became negligibly small, and vanished completely when the Froude

number reached 0.33.

Flow behavior near a curved boundary under a surface wave is associ-ated with a complicassoci-ated three-dimensional boundary-layer problem. The

genera-tion of a secondary-flow within the boundary-layer near the free surface,

in-duced by the curvature of the surface wave, has been demonstrated in [2] and [3]. On the other hand, when a streamline is climbing up the crest of a wave, this must be accompanied by an adverse piezometric pressure gradient. One would ex-pect that this adverse gradient would result in a thickening of the boundary layer and, as a consequence, an increase in the pressure defect at the stern of

a ship model.

On the basis of the foregoing remarks, the surface profiles shown in Figs. 1 and 2 suggest the follbwing explanation of the rising trend of the

vis-cous drag curve shown in Fig.

4.

Between F = 0.25 to 0.28, the wave

ampli-tude doubled in magniampli-tude and the positions of the crest and trough moved 0.5

foot toward the stern. Hence one would expect an increase in the pressure defect at the stern and a corresponding increase in the viscous resistance coefficient.

-3-Shear-stress measurement

In his study of the frictional and pressure resistance of "Lucy

Ash-ton' models [)4], Townsin obtained the frictional resistance by subtracting the integrated pressure resistance from the total. His results showed that the frictional resistance is not a monotonic function of Froude number, and that the variations were within reasonable bounds, but at F >

0.36,

the frictional

resistance was unexpectedly low. In the present study, a single stagnation tube served as a Preston tube for measuring the shear-stress distribution on the ship

hull: see Fig. 3. The calibration curves presented by Landweber and Siao [5, Fig. 2] were applied to obtain the shear stress on the ship hull from the total

pres-sure-head readings

Pt of the stagnation tube. The frictional resistance was

then obtained by integrating the shear-stress distribution over the body. p

Figure 1 shows the contours of ¡P (where P is the total pressure-head reading of the stagnation tube in the free stream) for five different speeds. A decreasing trend in Pt1p is observed from F = 0.11 to = 0.25;

from F

0.25 to 0.28 an increase of approximately 5 percent occurs, followed by a de-creasing trend from F = 0.28 to F = 0.33. The frictional resistance curve, ob-tained by integrating the shear stress on the body, also shows a sinuous trend, sim-ilar to that of the viscous drag curve, in the neighborhood of F 0.25, as is seen in Fig. 5.

Turbulence stimulation

The tests of this study were conducted at Reynolds numbers

VL/,

from about

3 x

106 to

6 x

106. A row of pins was used as a turbulence stimulator.

Figure 6 shows the total drag coefficient curve

[6]

and two curves of residuary resistance coefficients, one of which was calculated from Wu's data

[6]

for the

present model, and the other from Todd's data [1] for a 20-foot model with studs

used for turbulence stimulation. The nature of the total drag coefficient for the 10-foot model and the good agreement between the two curves of residuary resistance coefficient indicate that the turbulence stimulators were effective in preventing an appreciable extent of laminar boundary-layer.

Conclusions

1. There is a strong correlation between the characteristics of the wave profile near the stern of the ship model and the viscous resistance coeffi-cient.

The frictional resistance coefficient shows the saine sinuous trend

as the viscous resistance coefficient at Froude nunfbers in the neighborhood of

F = 0.25.

The sinuous trend of the variation of viscous drag with Froude number is attributable to both the nature of the pressure defect near the stern and the shear-stress distribution on the ship hull, and cannot be attributedto

References

-5-[i] K. T. S. Tzou and L. Landweber, "Determination of the Viscous Drag of a Ship Model", uHR Report No. 101. March

_1967.

S. K. Chow, "Free-Surface Effects on Boundary-Layer Separation on Vertical Struts", Ph.D. Dissertation, The University of Iowa, June

_1961.

K. T. S. Tzou, "Secondary Flow Near a Simulated Free Surface", M.S. Thesis, The University of Iowa, June

_1966.

[1.] R. L. Townsin, "The Frictional and Pressure Resistance of Two 'Lucy Ashton' Geosims', R.I.N.A. Quarterly Transactions, Vol.

109, No. 3,

July

_1967.

L. Landweber and T. T. Siao, "Comparison of Two Analyses of Boundary-Layer Data on a Flat Plate', Journal of Ship Research, Vol. 1, No. , March

_1958.

J. Wu, "Technique of Determining the Viscous Drag of a Ship Model by Means of a Wake Survey", Ph.D. Dissertation, The University of Iowa, August

1963.

[1] F. H. Todd, "Some Further Experiments on SingleScrew Merchant Ship Forms

-Series

60',

Transaction of the Soc. of Naval Arch. & Marine Eng., Vol.

61,

Fig. 1.

r

-Fig. 2.

Surface Profile Along the Ship Hull for

1F

0.25, 0.28,

5.0

:i

-Stagnation tube (O.126u 0.D., 0.O96! i.n.)

1 t

Ship hu1l

Series-60 Ship Model

Fig. 3. Sketch of Equipment for

0.6 0.1 0.3 5.0 4.O

-9-3.0 2.0 o.6 5.0

Fig. 1.

Contours of

Pt/p(%)

_{on the After Body}

1. 0.1 0.3 0.5 0.6 0.1 0.3 0.5 o.6 0.1 0.3 0.1 0.3 V = .5 fts F = 0.25

1_.__.

V 5.0 fs F 0.28

T

-IVA

V 6.0 fts F = 0.33 1.0 0.0 0.1 0.3 0.5 0.6 0.1 0.3 0.5 0.5 0.6 0.1 0.3 0.5 0.6 0.1 0.3 0.5 0.6 0.5 0.6 0.1 0.3 0.5 _0.5 0.6 3.0 2.0

0. 004 0.003 0,002 0.001 0.000 0.10

-10-0.15

0.20

i'=

Fi.. 5.

Variation of Frictional Resistance

and Viscous Resistance with Freude number for a 10-foot Series-60 Ship Model

0.25

0.30

0.35

iscous Resistance

1957 ITTC

Correlation Coef. C Formula

FrLctionesistanceCoef.

0.006 0.005 0.00)4 0.003 0.002 0.001 0.000 0 15 0.20 0.25 F = V/v'

Fig. 6 Total Drag and Residuary Resistance

Coefficients for a Series - 60 Ship Model 0.30 0.35 tal Drag

Ct_Cf

from Wu's for 10

--

--data ft. model I F

I

/

/

I

I

-

Cf from Todd's data for 20 ft model

Uncias si fi ed

DOCUMENT CONTROL DATA - R & D

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,llI,.INATINI ACTIVITy I,,rf,I,.,t,-.lIlI1IIr)

Iowa Institute of Hydraulic Research

.U,.RLÍ'OHT SI-:Cu,iiIY CLAI;SIIICAIION

Unclassified

The University of Iowa 2h.

Iowa City, Iowa

o r t'oli f irT L L

AN E)ERIIVENTAL STUDY OF SHEAR-STRESS VARIATION ON A SERIES-6O SHIP MODEL

4 LDITSC i/I P ir yE NOTES (Type of report and inclusive dates)

Interim Report

s AU THOR/SI (First name, middle ,nitial. last rci.,mc) Kent T. S. Tzou

RI PO,i DAlL 711. TOTAL NO. OF PAGE5 7h. NO. OF NEFS

February 1968 11 pages 7 references

tia. CON TIiAC T OR GRAN T NO 90. ORIGINATORS REPORT NUMBER/SI

Nonr-l611( 05)

h. PROJFCTNO. IIHR Report No. 108

SROO9O1O1

- C. 95. 0TH ER REPORT NO(S) (Any other vnrnibers that may be assigned

this report)

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This document has been approved for public release and sale; its distribution

is unlimited.

II. ,UPPLI MENTARY NOTI C 2. SPONSORING MILITARY ACTIVITY

Naval Ship Research and Development Center

Washington, D. C. 20007

AL1StHSCT

A study of the change in water-surface profile with Froude number near the stern of a Series-60 model, the shear-stress distribution on the after body, and a discussion of the possible laminar boundary-layer effects

are presented. _{The results suggest that the sinuous trend of the}

viscous-drag curve is attributable to the change in surface configuration near the

stern and the shear-stress distribution on the ship hull, and cannot be

at-tributed to the presence of a laminar boundary-layer.

DD FORM 1473

(PALE 1) _Unclassified

Unclassified

S((urity Classifieation

D D FORM 1473 (BACK)

_{i NOV65 I}

_Unclassified

4

KEY WORDS LINK A LINK B LINK C

ROLE WT ROLE WT NOLE WT

Ship Resistance

Viscous Drag

Frictional Resistance

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