Underwater photographs of flow patterns

ÁRCHEF

A.

0

________

Experimento! Towing Tank Stevens Institute of Technology

Hoboken, New Jersey

1_ab. y. Schep5boiwkun

Technische Hogchoo

De1t

Experimental Towing Tank Stevens Institute of Technology

Hoboken, New Jersey

UNI'WATER PHOTtXIRAPHS

OF FLOW PATTERNS

by

William H. Sutherland

UNDATER PHOTO&RAPHS OF FLOW PATTERNS

Underwater photographs revealing the flow patterns about sur-Lace vessels have been taken recently at the Experimental Towing Tank,

and show promise of providing a valuable aid to the understanding of flow phenomena around a body9 The method, similar to the "tuft"

tech-nique used in wind-tunnel and flight testing of aircraft, utilizes limp threads attached to the model itself or to fine wires extending

from it0

Pbr the initial experiments, the threads, or tufts, were

short lengths of black woolen yarn glued into shallow holes in the

surface of the model. The pictures shown in this note represent the

first trials of the technique0 VÇnile they are scarcely examples

of photographic perfection, they seem sufficiently provocative to

warrant publication at this time; they hold some promise of

lead-ing to an understanding of flow conditions not easily obtained in av other fashion.

A decided advantage of this method over other possible ones is that it yields a picture of the simultaneous flow pattern over a wide area.

TM.8G

-1-MODEL

The photograph on the facing page is an "oyster-eye view", 4 an oblique angle, of the

stern of a self-propelled ship model in a tight turn,.

The photograph shows clearly the flow across the bottom of the skeg as the model turns.

This flow le accompanied by a flow in the other direction near the sur-

face of the hull itself, strongly suggesting a vortex with its axis

right-SKEG

JVORTEX

hand screw thread.

As far as we know, this is the first time such a vort.ex has been shown to

exist.

The fine wires near the rudders are used to hold threads at a distance

from

the surface

of the model.

The setup for taking the

photograph

is shown in the sketch below.

Kodatron flash unit

lamps (not shown) were placed under the water in boxes similar to the one for

the mirror.

This

particular setup

has an interesting property; near the model the angles made by the rays of light

entering the camera are such that there is total reflection from

the under side of the water

eux'-face.

The camera cannot "seo" objecta above the

water through the srÎace, and it sees some parts

of the model twice - once directly and once reflected in the under

side of the

water

surface, as

la shown by the dashed line in the sketch below.

Reflections of a rudder, propeller, and shafting

may be distinguished in

the upper part of

the photograph on the facing pafle.

/'N

PLATE

//

/ '

MINESWEEpER

D/L3

V//E: 0.8

FLOV LINES DR BflGE KES

-The thread method was tried ecperitnental1y to test

_its

suita.-bility for determining the proper location for bilge _keels. The

photograph below shows the bow of a model proceeding

_{to the right.}

Stations and buttock linos are marked on the model. The threads show

the ftrection of flow clearly0

It is of interest to note the reflection of the forefoot

the bow wave.

MODEL 1032

DECK OF MODEL NOT _VI8

IN PHOTOGRAPH

v//[

REFLECTION

The photograph on

with a step, photographed the photograph identifies of the mode]. is reflected on the previous page.

'ruYrs ON A FLANI SURFACE

the opposite page shows a planing surface

ob1ique) from below. The diagram below

the various parts 'of the model. The bottom

in the water surface, ch as in the example Theoretical treatment of planing surface flow patterns has

indicated that the nearer the spray root, the closer the direction

of flow should be to the direction of the spray root ite1f0 The

theozy is borne out by examination of the threads in the picture

most striking)y b' the single thread at the forward part of the top row0

ABOVE WATER VIEW FEFL Ecr, CHtN SPRAY CHINE STEP (EEL

VIEW LOOKING OBLIQUELY UPWARD

WATER

PART OF BODY NOT

vISBL AC E

j

TANK BOTTOM

/ / / 7/ / / / / 7_

MODES IN TIGHT 1JRN. AS SEEN FROM

DIRECTLY BIlLOW

On the fol1wing pagss are two pictures of the model previously

shown on page 3, but thia tine the pictures were taken looking

vertí-cUy upward. _{The previous pictures, being oblique, were difficult}

to measure; _{"vertical&' cm be used for reasonably accurate} quantI-tative measures of the flow direction resulting from, for _instance,

chances in the appendages of a model in turning.

CAMERA

/

MIRROR

SPEED LIGHT

///// / / /////

The next two pIcturos

_{in fact, illustrate the change in flow}

a L'aired-.hi skeg, the second shows the eae model with a small

exten-sion to the kog, in the fo

of a flat plato.

as rich as

might be eectod0 _{Both pictures show clearly the vortex discussed} In the previous photograph.

1ost of the threads were glued to the surface of the bottom,

but since the flow conditions away from the surface _{are also of}

in.-terest in turning studies, soue were held at a distance fron the

sur-face with thin w-..ro. There are five such w-ires on the model shown

in the next two photographs. Provision can be made for identifying

the separate threads on the sane wires. A a supplement to the vertical

NOTE: A SECOND SPEED

LICHT ON LEFT GLASS SIDE NOT SHOWN.

MINESWEEPER AS DESIGNED

DIAMETER

MINESWEEPER

WITH ADDED SKEG AREA

DIAMETER

photograph, a vieic of the same mode]. taken directly from the side may be desirable, and appears quite feasible.

*

This technique appears to have many advantages over other methods heretofore used. To our knowledge it has not previously been applied to ship forms and other comparable bodies and it should serve to increase the general understaMing of flow condition, and

inter-ference effects. It can also be enployed in locating the optinum

position of bilge keels, scoops, appendages, etc. In addition, its application may also effect a reduction in the aiunt of repetitive testing required in steering and turning investigations. In these

applications, its virtue of simplicity, and of simultaneously

record-ig the differing flow conditions found over a fairly wide area,

makes this technique seem a very worth-while addition to our research