Techniques for ship frictional resistance measurements

12 DEC. 1972

ìRCi IIEF.

D

NAVAL SHIP RESEARCH AND DEVELOPMENT CENTER

DOCUMENTATIE

D AT U ti:

-vo

DOCU N ENTA I E

Washlngton.D.C. 20007

TECHNIQUES FOR SHIP FRICTIONAL RESISTANCE

MEASUREMENTS

by

T.T. Huang and D. Lysy.

This document has been approved for

public release and sale; its distri-bution is unlimited.

DEPARTMENT OF HYDROMECHANICS

RESEARCH AND DEVELOPMENT REPORT

Lab,

v.

Scheepsbouwkunde

Technisché Ho

May 1970 Report 3307

L

Onderafde

.si

ische HogesCh00 iotheek van 1 cuwku"

The Naval Ship Research and Development Center is a directed at achieving improved sea and air vehicles. It was David Taylor Model Basin at Carderock, Maryland and the Naval Ship R & D Laboratory) at Annapolis, Maryland. The Ship R & D Laboratory) Panama City, Floridà becamepart of

U.S. Navy center for laboratory effort formed in March 1967 by merging the Marine Engineering Laboratory (now Mine Defense Laboratory (flow Naval the Center in November 1967. Naval Ship Research and Development Center

Washington, D.C. 20007 SHIP CONCEPT RESEARCH OFFICE 0H10 DEPARTMENT OF ELECTRICAL ENGINEERING ASSO DEPARTMENT OF MAcHIMERY TECHNOI.00Y *700 DEPARTMENT OF MATERIALS TECHNOLOGY ASSO DEPARTMENT 0F APPLIED SCIENCE SYSTEMS DEVELOPMENT OFFICE 0H01 ERROL ANNAPOLIS CANOING OFFICER, TECHNICAL DIRECTOR

F

H

J-MAJOR NSRDC ORGANIZATIONAL COMPONENTS

DEVELOPMENT PROJECT OFFICES OIl 50. 80. 90 NSRDC CARD EROCK C0ANOER TECHNICAL DIRECTOR

I

DEPARTMENT OF HYDRONECHANI 500 DEPARTMENT OP AERODYNROIICS 600 DEPARTMENT OF STRUCTURAL MECHANICS 700 DEPARTMENT 0F APPLIED MAThEMATICS 000 DEPARTMENT OF ACOUSTICS AND VIBRATION

000 NSRDL PANAMA CITY cDEBOANDING OFFICER TECHNICAL DIRECTOR DEPARTMENT 0F OCEAN TECHNOLOGY pilo DEPARTMENT OF MINE COUNTERMEASURES P730 DEPARTMENT OF INSHORE I WARFARE AND TORPEDO

DEFENSE I P740 DEPARTMENT OF AIRBORNE MINE COUNTERMEASURES P730 I * REPORT _ORIGINATOR

DEPARTMENT OF THE NAVY

NAVAL SHIP RESEARCH AND DEVELOPMENT CENTER WASHINGTON, D. C. 20007

TECHNIQUES FOR SHIP FRICTIONAL RESISTANCE MEASUREMENTS

by

T.T. Huang and D. Lysy

This document has been approved for

public release and sale; its distri-bution is unlimited.

TABLE OF CONTENTS

Page

ABSTRACT i

ADMINISTRATIVE INFORÌ1ATION 1

INTRODUCTION i

METHODS FORMEASURINGSHÏPRESISTANCE COMPOÑENTS ...4

ANALYSTS OF PRESSUE AND SHEAR STRESS MEASUREMENTS

_s

EXPERIMENTAL TECHNIQUES FOR MEASURING MAGNITUDE AND DIRECTION OF

Si-fEAR STRESS

HOT-FILM SHEAR PROBES ...9

DIRECTIONAL PRESTON PROBES

lo

RECOÌvffvNDATÏONS FOR SHEAR STRESS MEASUREMENT 14

CONCLUSION 15

ACKNOWLEDGMENTS 15

REFERENCES 26

LIST OF FIGURES

Page

Figure 1 - Coordinate System - 16

Figure 2 - Friction Plane Mounted to Towing Carriage and the

Locations of Hot.Fj1m.Shear Probe and Preston Tube 17

Figure 3 - Typical Hot-Film versus Preston Tube Calibration

Curves 17

Figure 4 - Typical Directional Response of Hot-Film Probe 18

Figure 5 - Detail and Mounting of Directional Preston PObe 19

Figure 6 - Plate - Details and Probe Locations 19

Figure 7 - DPP in Position on the Plate 20

Figure 8 - Turning Mechanism with all Plugs in Position, on

thePlate 20

Figure 9 - Details of Interference and Static Pressure Taps 20

Figure 10 - Response of Directional Preston Tube (Nose Angle

a = 45 Degrees) ₂₁

Figure 11 - Directional Response of Center Tube (Nose Angle

c= 45 Degrees) 22

Figure 12 - DirectiOnal Response of th Two Side Tubes

(Nose Angle = 45 Degrees)

...23

Figure 13 - Directional Response of the Single Preston Tube 24

Figure 14 - Iñterference Effect 25

K o

Kp

KTM TL - TM = O). (e)

TM

= TU (0) TM (e = O) TL (0) NOTAT I ON

d Outer diameter of the Preston tube

ds Hull area element

ds' Project of ds on the midship section

ds" Project of ds on the half-girth plane Pressure force on the surface element

-p

Ff Frictional force on the surface element

i,j,k Unit vctors in x,y,z directions

TU - TM

111

(e

= O)

9., m, n Direction 'cosines othe inward normal of the surface êlement

N Unit inward-normal vector of the surface element -;

P Pressure rotation

Dynamic pressure

P5 Statiò pressure

Difference between a Preston tube pressure reading p. and the

static pressure at the boundary

F Skin-friction resstànce

Rpv Pressure resistance.of viscous origin

R

Pressure resistance

p

-Wave resistance Free-stream velocity Cartesian coordinates I APd log 4pv 2

t

y*

10

4pv A Displacement weight.

Nose angle of the directional Preston probe

O pirection between shear stress

t0

and the Preston tube or

hot-film shear probe

e,1,

Trim angle

y Kinematic viscosity of the fluid

Orthogonal coordinates on the surface element

Unit vectors in directions

p Mass density of the fluid

t0

Shear stress at the ship hull

Angle between the direction and the direction of shear stress

Subscripts .

DPP Directional Preston probe

SPT Single Preston tube

SL Lower static pressure tape

SU Upper static pressure tape

TL Lower. tube of the directional-Preston

probe--TM Center tube of the directional Preston probé

TU Upper tUbe

of

the directional Prèston probê

ABSTRAÇT

Totäl ship resistance consists of two components,

fric-tional resistance arid pressure (residual) resistance.

Fric-tional resistance can be obtained by the integration of the

measured shear stress distribution over the hull, and pressure

resistance can be obtained from the integration òf the

measured pressure distribution on the hull Pressure resistance

is further divided into a wave component and a viscous

com-ponent (form drag). Wave resistance can be measured by a

method which takes a longitudinal or a transverse cut of the

wave pattern. The expressions for computing the frictional

and the pressure resistançe from the measured shear stress

and normal pressure distributions are derived in this report.

Experimental techniques for measuing the magnitude as well as the direction of shear stress have been explored iñ

detail by hot-film shear probes. These probes are recommended for measuring the shear stress distribution on small models tested in towing tanks; hot-film shear probes with ambient temperature compensation and directional Preston probes are recommended for use.in tests of full-scale ships and

large-scale models.

ADMINISTRATIVE INFORMATION

This work was sponsored by the Naval Ship Systems Command (NAVSHIPS)

and funded under Subproject SF 35.421.006, Task 01713.

INTRODUCTION

Since the contribution of Froude nearl.y a century ago, total ship resistance has been assumed to be composed of two separate and. independent

parts: (1) frictional tesistance equal to te resistance of an equivalent plank or flat plate of the same wetted area and length as the ship and

(2) the remainder called ttresidual" resistance. I,t has been. a practical

engineering solution to extrapolate the resistance measured on a modelto that of a full-scale ship either (1) by assuming that the fricional

resist nce follows the Reynolds scaling law and that the residual resistance

follows t1e Froude scaling law .(Froude) or (2)b.y using a form factor to

distribute part of the.resiuài resistance Into viscous form drag (Hughes). These assumptions have never been satisfactory from a scientific

point of view. For instance, the. boundary layer on the ship is different from that on a flat p1ate, and the flow around..a ship hull is

three-dimensional in nature. Becäuse of the augmented fluid velocities around a

three-dimensional form, the skin friction of a ship hull is somewhat greater

than the corresponding friction of a flat plate. In addition, the gradual

energy loss resulting from viscous dissipation along tIe ship hull prevents

inviscid pressure rèstoration at the stern and produces form drag. If

eparâtiori occurs, there will be an additional form drag.. A combination of

skin friction drag an form drag is called viscous drag. When the body is

traveling on or near the suface, a wave system on the surface will be

created, and energy thát must be stipplied continuously by the ship to the wave system is called wave resistance. This résistance, in turn, manifests

itself in a pêsure distribution, over the hull, and the resulting net

fore-and aft-force is wave resistancé. It is known that the development of a boundary layer depends on the pressure distribution along the streamlines

around the hull. Since wave resistance is one of the parameters contributing to the total pressure distribution over a hull, it is apparent that wave resistance should play a role in the development of a boundary layer on the

hull,. The growth of a boundary layer, in turn, alters the flow lines near

the ship stern. _{This viscous effect will, in turn, modify the wave}

resistance. At present the interdependence of the above-mentioned resist-ance components and the scale effects on these interactions are unsolved

problems.

Ländweber and Wu1 have reported that the viscous resistance measured by wake survey for various speeds was a sinuous line rather than the smooth

curvé that IS usually assumed. Townsin2 determined frictional resistance

indirectly by subtracting the total resistance from the measured _pressure

resistance obtained by intgrating the measured pressure distribution over

the hull. Again he found that the frictional resistance oscillated with

speed. In addition, Shearer3 found interdependence of shear distribution

and Fröudènurnber. _{Although these recently measured interactioñs oi model}

scale between the resistance components show that the basic Froude _scaling

techni4ües need tO be reevaluated,-these findings do not imp'ove the

pre-diction of ship resistance from model testS since the scaling laws for

these interactions are not known. It has long been. recognized that a better

understanding of the real nature of the components of ship resistance and their interdependence may not only improve methods of prediction but may also lead to better procedures for minimizing full-scale resistance

The complicated nature of the flow arothid a ship hull has prevented

a successful treatment of thê ship resitance problem by ááömplêtely

analytical approach.. FOr inStánce, the accurate potntïaI flow field

around the ship at finite Froude number is not available. _{Conputatiôn of}

three-dimensional ±urbulent boundary layers aloi hip fOrms has to rly on

the approxithated potential flow field as the computat.on schemi

fdrthü-lated by Webster and Huán'g.4 Th resUlts oLt

önutaioñs proide

only a qualithtive assessment fpr the real situátion, ànd thir procedure is not acceptabiê for predicting frictional resistance.

The increasing availability of modern instruméñtation and _automatic

data processing has improved, experimental capability _{to such an extent that}

direct measurements of individuai resistance _{cOmponents can now be ahieved.}

It is important to select and to develop experimental techniquès which can

be applied to the measurements of resistance _{cOmponents of small ship}

models (10 to 20 ft), large-scale models (80 _{to 100 ft), and prototype}

shjps.,*

The results of such experimental series cOuld provide insight into

the physics of the individual resistance _{components which make up total}

resistance and could determine the scaling _{laws of thêse interactions.}

This report briefly discusses _{a number of experimental method for}

measuring the resistance components of various _{scale ship models, describes}

two techniques for measuring shear stress, presents experimental results

from these two techniques, and recommends further _{studies. The aim of the}

present study is to advance experimental _{techniques for deriving ship}

resistance components. _{These techniques will be applied t-o study resistance} components of various scaled ship models at NSRDC.

*

METHODS FOR MEASURING SHIP RESISTANCE COMPONENTS

With the present. capability of instrumentation and automatic data

processing, it is possible to measur the nOrmal pressure and shear Stress

distributions over a ship huU through the use of a sufficient number of

flush-mounted pressure transducers and shear probes. The integration of a

shear stress distribution over a hull is the ship frjctiona]. resistance

consisting of the traditional flat plate frictiOn plus the additional skin friction due to augmented velocity resulting from flOw over thé

three-dimensional ship form. The integration of the fore-and-aft components öf the normal stress (pressure) over the hull surface is called the pressure

resistance and it contains the wave and form resistance çomnponents. The

wave resistance can be determined independently from the wave pattern by

such methods as the, "longitudinal or transverse" cüt.5 Form drag is then obtained by subtracting the measured wave resistance from the p±essure

resistance. Of course, the sumfi of thé integrated skin friction and the

integrated pressure resistance make up the total ship resistance which again

may be checked by he total 4rag measurements.

The. scheme shown belöw indicates the dervatiori of the components of ship resistance.

TOTAL SHIP. RESISTANCE (total resistance measUrement)

Frictional Resistance. (obtained by the inte-gration of shear stréss over the. hull surface)

Pressure Resistance 1.

(obtáined by the

inte-2

gration of normal.

(

stress (pressure))

Flat plate friction Form resistancê due to additional skin friction originated

from velocity

aug-mentation Form resistance Wave resistance measured by methods of lòngitudinal or transverse cut

The measurement of total resistance is _{a routine procedure for a}

model, and it can also be achieved for a ship. _{A balance beam or modular}

force gage is commonly used in the case of a model, whereas towing or jet

propulsion is required in the case of a ship. _{Pressure distribution on the}

hull can be obtained by using a sufficient number of flush-mounted _pressure

the longitudinal or transverse cut of the wave pattern behind the hull.5

However a reliable wave height sensor for measuring wave patterns is yet to

be developed, especially for use with large models and ships. Shear stress

distribution is relatively difficult to measure and involves a determination

of its magnitude and direction. Two promising devices available for this measurement are the hot-film shear probe and the directional Preston probe; both of these are discussed in detail inthe following sections.

ANALYSIS OF PRESSURE AND SHEAR STRESS MEASUREMENTS

Since skin friction and pressure resistance must be obtained from the integrations of measured shear stress ând pressure distributions over the ship hull, it is necessary to outline the computation procedure which

is simple but is necessaiy for planning data analysis.

For a stationary ship, a set of cartesian coordinates, x, y, and z fixed in the ship, is chosen such that the x-axis lies in the ship center-plane and parallel to the keel line, the x-y center-plane corresponds to the

undisturbed free surface, and the z-axis is positive upward. As the ship

moves in the positive x-direction at a given speed U0 on the calm surface,

it usually trims slightly down by the stern as shown in Figure 1. The

trim angle between the undisturbed water level and the direction of the

keel line is denoted by eT. On a hull area element ds (with unit inward

normal vector N whose directional cosinès are 1, m, n), there is stress

t0

acting tangentially to the surfaceand a pressure p normal to the surface. The pressure force acting on the surface elemént dis

F=pds(Zi+mj+nk)

where i, j, and k are unït vectors in the x, y-, and z-directions,

respectively. The òothponents of the pressure force in the x- and

z-directions are AF i p ds Z and tF . k = p ds r.

A set of orthogonal coordinates (, , n) with unit vectors (, ,

n) may be usêd to computethe drag from the shear stress vector r0 on the

ship hull. 1f the course of the ship is straight, the shear stress

dis-tribution is- synunetric with respect to the ship centerplane; then it i

necessary to consider only one side of the ship. We may let and be

tangential, to the surface element and be paia.11el to the y-z plane.

Furthermore, let n be parallel to the inward, normal of the surface element.

Thus, , , and n are -related tó i, j, k as

- a . b + b2 Va2 + b2 1/2 2 2 1/2 .2 2 -

112

2 2 Vc + d + e + d + e p/c + d + e c . d .. e

3+

k and and

1f

2.

+ m + = + mj +

By orthogonality, i.e.,

n =

C C n = O, we have

.2 .2

n

m +n

b, e=d, c=-

-- d

m m 9.m

Then and are reduced to

n . m

-

3+

1/2

2 1/2 2

The angle between and T0 may be denoted by 2; then the frictional fOrce

on the surface element can be written as

= ds [sin + cos.. 2

The components of the frictional force in the x- and z-directions are

''2 2

LF i=-t dscosc1Vn +m

_f

and

The total resistancè can then be reréented. as

RT=

cos

ds

Jn

sin 0T ds

ccs

c os O ds

J

O I

iÍ2

L 2 2 2 I T

-'r isin Q:.

...

+

cos

-- .-.-

isin O

ds

(1)

v"

+m

n

The total dïspl-aceméntweight òf theship is equal to

$

''

0T ds

_+J

Pn COS 0T ds -

Jr/

cos 2 sin

+ i t i sin 2

T.

+ COS 1CO5 O

J

01

/2

1/2

₂ I T C I . m Zn I

(2)

L

+ m - n + m

In these expressions, p is the total pressure on any element of

hull surface, but i,t is more convenient to measure only the change in pressure. Hence, p may be set equal to

d + where is the static

pressure on the element

at

rest

_and

d is the measured change in pressure

on the element dring motion. Two çonditions of the integrated static

pressure on the hull at rest are:

and

J-p Z ds = sin

Jp

n ds

= _{cos 0T}

By substituting

Pd ₊ and the static condItions into Equations (1)

and (2),

one obtains

L1,

=-

JPd Z cos 0T

ds+

J

Pd n sin 8T ds

+JT

/ÇÇ2

cos 2 cos

0T

ds

+

It

. -.

_lin

O ds J °

_[2

2 _{1/2. 2 i} T I/n + pin

+m.J

7

R-JPd

where R

= R

+R-p. -w- -PV (10)

f

J

d + Ç T [sien m ds = 0 (5)

JO[

1,/2 2

1/2

2]

n +m

n +m

Subtracting (5) from (4) produces

cos + sin 0T

T

+

J T

(cos

+ (sin 0T - 0T tan °T (sin

i. .12 2 245 cos 2 yn + in ds d cos 0T +

j

T cos

where 9ds is equal to the projêctiön of area element ds on the midship

section denoted by ds', i.e., y-z plane, and Vn2 + m2 . ds is the

pro-jectioI of the area element on the half-girth plane and denoted ds."

The skin-friction resistance and the pressure resistance may then

be defined respectively as:

rT. COS

I O R =1 ds" F j

coseT

r ds' cos

The measured pressure resistance R- Consists of wave resistance R and

- p...

w.

pressure fòrm drag Rp. Thus the total ship resistance Ft,. is made up by

the following measurable components:

8

12 2

Vn + m cos 2 sin O ds

o- T

+$in o,., -tan O,)1/.m2ds sin O

+5

d

n cos ds

It is obvious that if the distribution p,

t,

and 2 ar measured..

directy, the çQE1pation of Rf and R in Equations (7) and (8) can be

performed either by a high-speed computet or by a simple graphical

inte-gration. The wave resistance may he measured by th.e wave pattern behind

the ship ineperdçntly (i.e., by the method of longitudinal or transverse

5

cut). The pressu'e form rag is then obtained by Rpv = R

_R.

Record-ing of the total resistance. is routine practice in towRecord-ing tank tests..

Thus, an independent check of + RF is a]nipst always available.

EXPERI?vENTAL TECHNIQUES FOR MEASU.ING MAGNITUbE AND DIRECTION OF SHEAR STRESS

As shown in Equation (7), it is necessary to measure the

distri-bution of the magnitude of the shear stress and it angìé on the

half.-girths plane with respect to the x-4irectiQn (Figure lb) to obtain the

total skin friction resistance of the ship Two promising measuring

devices are considered in th.is repott.: the flus]i-mounted.hot-film shear

probe and the direçtiona1 Preston probç.

HOT-FILM SHEAR PROBES

The principle of hot-film shear probes is that. skin.friction. is a

function of electrical current required to mantaìn a platform film at a

constant temperature when placed ön the hull surface.6'7 .Linearization

öf the output from the hot-film anemometer with espect to shear stress

7 .

has been developed by L.ng. Nevertheless, the functioial relationship between the output of the anemometer and the shear stress is obtained through calibration, and slight nonlinear response is tolerable.

-, ...

-.

. . 6,7

A flush-mounted,'hotfilm.shear probe (designed and built by Ling)

was used to dethonstrate the capability of measuring shear frèss in the.

NSRDC towing tafiks. .A strip of platinum film about 0.2mm wide and 0.8mm long was fuséd. uder-. high temératiire oñt'o th _{polishèd èfid of à pyrex rod} 0.11 in. in diameter and 1 in. long. _{This hot-film shèar pÏobe was flüsh_T} mounted on a 21-ft-long friction plane. A Preston tube was also attached

to'the plane adjacent to the hot.film sheatpobé (Figure 2). _The

friction plane was towed at speeds frm 2 to 14 knots in the NSRDC.deep:

through the calibration curve of Patel.8 Figure 3 shows the outputs of the

hot-film anemometer versus the shear stresses. Note that the response of

the hot-film probe to the shear was linear.

The directional response of the hot-film shear probe was calibrated

by rotating the direction of thehot-fiim element with respect _{to the flow}

direction. Typical results are shown in Figure 4. The directional

response is proportional to cosine O up to O = 50 deg.

Hot-film shear probes mounted flush with the hull surface do not

disturb the flow, and their response is independent of the pressure

gradient. _{However, the hot-film probes should be calibrated before and} after test.

DIRECTIONAL PRESTON PROBES

The Preston method of measuring skin friction in the turbulent

boundary layer makes use of a circular Pitot tube resting on the wall.

The Preston tube pressure, together with the static pressure at the same

point, permits the computation of the skin friction at that point. The

use of the Preston tube is based on the assumption that there is _{a region}

of dynamic similarity close to the wall where the flow is dependent _only

on the local shear stress r, the mass density of the fluid _{p, its}

kine-matic viscosity y, and a typical length parameter, i.e., the _{outer diameter}

d of the Preston tube. _{The calibrations of a Preston tube reported by}

Landweber and Siao,9 by Patel,8 and by many others _{are in good agreement.}

The recent results of Patel8 will be used in the _{present work. The}

cali-bration curves are:

10 x* = y* + 2 log10 (l.95y* _{+ 4.10)} (11) if 3.5 < y* < y* 5.3, = 0.8287 - 0.1381 x + 0.1437

x2

- 0.0060 x*3 ₍₁₂₎ if 1.5 < y* < 3.5, and y* = 1/2 x* + 0.037 - ₍₁₃₎ if y* < 1.5.

Here x = log10 ( 2 T

d2

Lp.d\

_*=

_l('CT

/0

_I 4p\)2

)1J

-

bl0L

₂ \ 4p \) TU -- f (e)

K=

o

TLN

), and

p is the difference between the Preston tube pressure and the static

pressure.

Patel8 also found that a Preston tube can be used with acceptable accuracy (maximum error 3 percent) if the pressure gradient parameter is

limited to the range -0.005 < v/(pU) dp/dx < 0.01, where dp/dx is the

pressure gradient along the flow direction and _{UT =}

(T0/p)'2

In order to obtain the direction of the shear stress, two three-tube

directional Preston probes were built and calibrated. Similar experiments

were done by Rajaratnam and Muralidhar,1° but their data were quite

scattered. _{In order to obtain a reliable calibration curve for the}

directional response of the Preston probes, it is necessary to perform

certain experiments.

If the three-tube direction Preston probe is placed on the boundary

at an angle e with a directional shear stress

T,

_{a ratio of pressure}

differences among the three tubes, i.e.,

is a function of e only where the center tube is designated

two side tubes by TU and TL (Figure 5). _{The main objective}

to determine the functional relationship between e and K0.

the pressure differences between pressure results from each

static pressure are functions of

t

and e, i.e.,

i

P.d2

_T

d2

) KTM(e)

TM'

=

f1( °

2 e =

[

4 2 j

4pv2

4pv

Pud

Td2

) 2

_f2(

218

pv

\4pv

= K(e)

11

{,

PTh.d2]

4pv

8=0 (14) by TM and the of the test is Furthermore, tube and the

-(T0.d =

[

pmd21

₍₁₅₎ 2 y _/ 4p V 0=0 Cont'd If [

p.d2/4 y2]00 and

SPT'4

v2]0 of the single Preston tube

(hereafter designated by SPT) have the same response with respect to T0 at

0=0 (as is found to be the case), calibration curves (11) through (13) can

be used here. If calibrations of KTM(0),

_ru0'

and KTL(0) are provided,

then the magnitude of shear

t0

can be obtained from Equation (15) with the

aid of Equation (14) for determining

t0.

The aluminum plate shown in Figure 6 was mounted in the NSRDC 24-in.

water tunnel at a slight angle of attack (1/4 to 1/2 deg). Two directional

Preston probes (hereafter abbreviated DPP) shown in Figure 5 with nose

angles of 35 and 45 deg were calibrated against the shear stress angle B

at velocities of 10, 15, and 20 ft/sec. e is defined as positive for the DPP titled above the horizontal and negative for the DPP titled below the

horizontal. The DPP was pivoted in such a manner that readings at

different angles were taken at essentially the same location on the plate. A turning mechanism for the rotation of the DPP is shown in Figures 7 and

8. The number of turns of the threaded rod was calibrated against the

angle of the DPP to the flow so that the angles could be set from outside the water tunnel and all readings taken without having to reset the

velocity. During these experiments, the interference taps Il-14 and the static pressure taps Sl-S3 forward of the DPP were plugged (see Figures

5-9).

The upper, middle, and lower tubes of the DPP, as mounted on the

plate, were designated TU, TM, and TL, respectively. A four-channel

system of Pace differential pressure gages connected to digital voltage

meters (DVM's) was used to record data. The common side of the four

gages was connected t the lower static pressure tap SL, which was used as

the reference throughout the investigation. Lines from TU, TM, IL, and SU were connected to the other sides of the four gages and the pressure

differentials recorded.

A similar calibration of angular response was carried out for an

SPT. The interference taps shown in Figures 5 to 9 were used to determine

the effect of a similar probe located 6 to 24 in. forward of the DPP, with

the DPP aligned with the shear stress (O = O deg). Readings for the 35-deg

nose DPP were taken at all three velocities with the DPP dummy plug shown

in Figure 9 located at four upstream positions and for the SPT at the three

velocities with the SPT dunmiy at Location 14 only. The static pressure taps Sl-S3 were used to determine the approximate pressure gradient in the

immediate vicinity of the DPP. It was found that the velocity gradient

on the plate was negligibly small.

Calibrations of the turning mechanism for setting the angle O and the Pace gages were performed before and after the experiments in the

water tunnel and were identical. Because of difficulties with the turning

mechanism, each run was made with the DPP moving from the lowest (-50 deg)

to the highest (+50 deg) position. Four or five readings of the DVM were

taken for each data point. These were corrected for the instrument zero

drift (generally negligible) and record zero drift (generally < 0.1 in. of

water) before conversion to pressures. Because of the good linearity of

the calibration, this procedure introduced negligible error.

Readings for the SPT were taken simultaneously on two channels to

check the accuracy of the gage calibration and response. These were

treated as independent data points for purposes of analysis.

Examination of the data for SU indicated some interaction between

the DPP and SU and SL. It reached a maximum at the extreme angles when

the middle portion of the DPP was closest to SU or SL and caused the static

pressure to increase by a maximum of about 3 in. of water at 20 ft/sec. Consequently the reference static pressure was obtained at either SU or

SL, depending on the angular position of the _{DPP. For positive O (DPP}

nearer to SU) it was assumed that the static pressure at SL was unaffected

by the DPP.

To nondimensiona].jze the data for the different _{velocities in the}

most accurate manner, the

p reading at the zero angle to the shear stress

was used for reference pressure (not

SPT

Figures lO through 12 are

the nondimensional experimental results _{of the response of the directional}

Preston probe with nose angle = 45 deg. _{Figure 13 is the directional}

response of the single Preston tube. The present data from the SPT lie in

the middle of the data of Rajaratnam and Muralidhar1° and are slightly

different from those of Sigalla.11 Readings of the center tube of the DPP

with o. = 45 deg and the SPT were identical for a given T0 with O = O deg. Thus the single Preston tube calibration curves can be used for the DDP

with o. = 45 deg. However the reading of the center tube of the DPP with

o. = 35 deg was slightly less than that of SPT. No further application of

the DPP with o. = 35 deg is recommended.

The interference effects of a similar probe located at various

locations upstream of the DPP are shown in Figure 14. An upstream probe

tends to increase the shear stress downstream. On the basis of the results in Figure 14, it is suggested that if a large distribution of Preston probes is needed to determine the shear stress distribution on a ship hull, downstream probes should be placed at least 4 ft away from

up-stream probes to avoid interference effects.

RECOMMENDATIONS FOR SHEAR STRESS MEASUREMENT

Hot-film shear probes mounted flush with the hull surface are recommended for the measurement of shear distribution on the smooth ship

hull because (1) they do not disturb the flow, (2) their response is

inde-pendent of the pressure gradient, and (3) their directional response is

found to be approximately a cosine function of the angle. The hot-film technique can presently be used for measuring shear stress distribution on ship models in towing tanks where the temperature variation is expected to

be small. However, for tests where varying ambient water temperature is a significant parameter, suitable techniques for the ambient temperature

compensation of hot-film shear probes are yet to be developed. A dummy probe connected to the opposite arm of the bridge to the measuring probe

should provide the feature of temperature compensation. _Further

investi-gation into this technique will be undertaken.

At present the directional Preston probe and/or the single Preston

tube are the alternatives for measuring the shear stress on large and

full-scale ships since they are relatively insensitive to _tempexature

ambient variation.

Before actual measurements öf skin frictiö resistahce of a ship can be performed, techniques must be deve.pped for handling a large number

of probes and for processing 1-arge amounts of datä by modern data-processing methöds.

CONCLUSION

It is recommended that measured totaiship resistance be determined

through jts measurable components. Frictional resistance can be obtained

from the integration f shear stress over a

hull,

and pressureresistánce

can be obtained fröm the integration of the nôrmal press acting on a

hull. Wave resistance may be directly measured by the longitudinal or

transverse cut of the wave pattern. The results of the recommended studies would provide insight into the physics of the itidividual resistance com-ponents, their interactions, atid the scale effect of their interactions;

they might also lead to procedures for minimizing full-scale resistance. Experimental techniques for measüring the magnitude and direction

of a shear stress distribution havé. been exlorèd in detail. Hot-film shear probes are recommended for use in towing tank tests of small models

but at present hot-film probes. with ambient.temeratùre ¿ompensation or

directional Preston probes should be used for tests where varying ambient

water temperatureis a significant parameter. .

ACKNOWLEDGMENTS

This wörk was done ufìde± the úérvi-ion of Mr.: G.G. Cóx;

his

encouragement and advice are gtatefully acknowledged. _{The hot-film shear}

probe and the anemometer were supplied by Professor S C _{Ling of the}

Catholic University of America.

Figure la - Pressuré and Shear Stress Acting

oi the Ship Hull

ACTUAL

WL-lO 9 8 7 6 5 4

AP

Figure lb Shear Stress Acting. Qn:'the.Half-Grth Plane

Figure 1 Coordinate System

16

LWL

D

2I-0 20- O SIDE PLATES-ALUMINUM 20' 4x 1!4' 5.0 4.0 LO

TOWING CARRIAGE DYNAMOMETER

14-5'

HOT-FILM SHEAR PROBE-.4

PRESTON TUBE

Figure 2 - Friction Plane Mounted to Towing Carriage and the Locations of Hot-Film Shear Probe and Preston Tube

17 V2" .

o

. O 0.001 0.002 0003 0.004 0.005 0.006 0.007

r(PSI), BY PRESTON TUBE

1.4 1.2 LO 0.8 0.6 0.4 0.2 o CURVE OF OS O

o

o

18 O IN DEGREES

Fig.ire 4 - Typiçal Directional Response of Hot-Film Probe

21"

7MA MiA

_WASHERS

i

FLOW

FLOW

NUT

Figure 5 Détail and Mounting of Directional Preston Probe

X

X

XX:

SANDPAPER STRiP (LOST DURING TESTING)

1/4" RADIUS LEANG EDGE+ SIDES

O.072"O.D., O.040"I.D. STAINLESS STEEL TUBING 3 TUBES SOLDERED TOGETHER

78'

6" OPP SOLDERED TO 8-32x 11/2 BOLT

14 13 12

Q-- ,

o

XX MOUNTING POINTS

Il-14 INTERFERENCETAPS,6"APARTAND6"FORWARDOFDpp

Si-53 STATIC PRESSURE TAPS, 1'APART AND 1" FORWARD OF DP.P

SU.SL STATIC PRESSURE TAPS, 1" ABOVE AND BELOW DSP

11F HTFILMPROBE

OPP DIRECTIONALPRESTONPROBE

STATIC PRESSURE TAPS ARE V16" DIAMETER

Figure 6 - Plate - Details and Probe Locations

1 9 a NOSE ANGLE PLAT E TURNING BAR Xx TU XX

H_'i"

- 1/2" ALUMIÑUM PLATE

'

&.

(

t'

Figure 7 - DPP in Position on the Plate

INTERFERENCE TAPS

Figure 8 - Turning Mechanism with all Plugs

in Position on the Plate _{STATIC PRESSURE TAPS}

Figure 9 - Details of Interference and Static Pressure Taps

PLUG

DPP DUMMY

1.2 0.8 0.4 O 0.4 0.8 1.2 TL TM K0 n n rTu rTM 450

_-TU DPP : U0 ( F P S)

Qio

015.

TU TL

- TM

- TM

1.6 -50 -40 -30 -20 -10 0 10 O IN DEGREES

Figure 10 - Response of Directional Preston Tube (Nose Angle a = 45 Degrees)

20

50

30

40

KTM Ap(0 0)

111

ii

07450

U0(FPS)

0

15

020

DPP -50 -40 -30 -20 -10 0 10 20 30 40 50 60 O IN DEGREES

Figure 11 - Directional Response of Center Tube (Nose Angle a = 45 Degrees)

1.1 1.0 0.8 0.6

I-0.4 0.2 O

1.2 O. 8 0.4. - 0.4 - O. 8 - 1.2

-50

-40

-30

-20

-10

0 10 20 30 40 50 60 O IN DEGREES

Figure 12 - Directional Response of the Two Side Tubés (Nose Angle

1.2 1.0 0.8 0.6 02 -0.2 -50 -40 -20

-lo

O 10 O IN DEGREES 30

Figure 13 - Directional Response of the Single Preston

Tube 40 50 60 L KsP=1\PSp,.(00) SPT()

(

1L

H

e

\)

'

\

\ RAJARATNAM

MURALIDHAR DATA REGIONOF AND, 10

SIGALLA1\%\

U0 c4> I

Q

o

020

UQ(FPS) 10 15 : I H L 1 T0 -I I I

1.20 1.15 'o 110 1.05 1.O iill K K. NO PLUG 1OFPS 20 FPS

.

Qio FPS (KTU + NO PLUG

I

4. o 6" 12" 18" 24"

DISTANCE OF DUMMY DPP UPSTREAM OF THE DPP

2

3

4

PLUG LOCATION

REFERENCES

Landweber, L and Wu, J., "Formal Contribution on Resistance,"

Tenth International Towing Tank Conference, London (1963)

Townsin, R.L., "The Frictional and Pressure kesistance of Two

'Lucy Ashton' eosims," Quart. Trans. Roy. Inst. Nay. Arch., Vol. 109, No. 3 (Jul 1967).

Shearêr,:J.R., "The -Experimental -Determination-of the Components of ship Resistance for a Mathematical Model," Quart. Trans. Roy. Inst.

Nay. Arch., Vol. 107, No. 4 (Oct 1965).

Wbster, W.C. an Huang, T.T.,"Study of the Boundary Layer on

Ship Forms," Hydronautfcs, Inc. TechPical Repon 608-1 (Jan 1968).

Eggers, K.W.H. et al., "An Assessment of Söme Experimental

Methods for Determining the Wavemaking Characteristics of _{a Ship Form,"}

trans. Soc. Nay. Arch. and Mar. Eng., SNA!t, Voi. 75 (.1967).

Ling, S.C., "Heat transfer Characteristics of Hot-Film _Sensing

Elements Used iti Flow Measurethent," Trans. Am. Soc. Mech.. Eng., ASME, J. Basic Eng, Vol. 82, Nô. 629 (1960).

Ling, S.0 _{et al.., "Application of Heated-Film Velocity afld}

Shear Probes to Hemodynamic StudIes," Circulation _{Res., Vol. XXlll No. 789}

(Dèc 1968).

Fatel, V.C., "Calibration of the Preston Tube and Limitation on

Its Use. in Pressure Gradients," J. Fluid Mech., _{Vol. 23, (1965), pp.} 185.-208.

Landweber, L. and Siao, T.T., "Comparison of two Analyses of

Boundary-Layer Data ona Flat Plate," J. Ship _{Res. (Mar 1958).}

lo. Rajaratnam, N. and Muralidhar, D., _{"Yaw Probe Used as Preston}

Tube," Aeron _{J. Roy. Aeron. Soc., Vcl. 72, No. 1060 (Dec}

1968).

li. Sigalla, A., 'Experiments with Pitöt tubes _{Used for Skin}

Friction Measurement," British Iron afld _{Steel Research Msociation Report} (Mar 1958).

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UNCLASSIFIED Seèurit

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FORM 1473

(PAGE I) I NOV 651 S/N Ò1Ô1.807.6801 UNCLASSIFIED Security Classification

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(Scç_urity c!assifiration oftitle, bodyof absr,oct and indcsing annotation must be entered s'ben the overa!! report is cJaSsi(led)

(ORIGINA TINGACTIVITV

Naval Ship Research and Development Center Washington, D.C. 20007

Za. REPORT SECURITY CLASSIFICATION

Unclassified.

2b. GROUP

3.REPORT TITLE .

-TECHNIQUES FOR SHIP RICTIONAL RESISTANCE MEASUP.EMENTS

4. DESCRIPTIVE NOTES(Type ofrepon and inclusive da(es) -

-5. AU THOR(S) (FiI'.st name middle ini(ial,last name) ...-

-Thomas T. Huang and Dusan Lysy

6. REPORT DATE

May 1970

76. TOTAL NO. OF PAGES

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b.PROJECTNO.

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3307

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

Il. SUPPLEMENTARY NOTES (2. SPONSORING MILITARV ACTIVITY

NAVSHIPS

13. ABSTRACT .

Total ship resistance consists of two components, frictional resistance and pressure (residual) resistance. _{Frictional resistance can be obtained by} the integration of the measured shear stress distribution over the hull, and pressure resistance casi be obtained from the integration of the measured pressure distribution on the hull. _{Pressure resistance is further dIvided} into a wave component and a viscous component (form drag). _{Wave resistance}

can be measured by a method which takes a longitudinal _{or a transverse} cut-of the wave pattern. _{The expressions for computing the frictional and the} pressure resistance from the measured shear stress and normal pressure dis-tributions are derived in this report.

Experimental techniques for measuring the magnitude as well as the direction of shear stress have been explored in detail by hot-film shear

probes. _{These probes are recommended for measuring the shear stress}

dis-tribution on small models for measuring the shear stress disdis-tribution on

small models tested in towing tanks; hot-film shear probes with ambient temperature compensation and directional Preston _{probes are recommended for} use in tests of full-scale ships and large-scale _models.

hi

Unclassified,

DD

1 NOV 45 IFORM 1473 (BACK)

(PAGE 2)

Securtty Classification

UNCLASSIFIED

Seéuritj Classification

14. - -

-KEY WORDS - LIN,( A- -- LiNK Ô LINK C

ROLE WT ROLE WT FOLE Wr

-Ship Resistance Components Shear Stress Measurement

Magnitude and Direction

Hot-Film Shear Probes Directional Preston Probes