Flow observations on flat plate

NATIONAL PHYSICAL LABORATORY

,

In the reoent paper(1)on friction resistance the leading part of each of the submerged sheets was. a flat metal plate cf thicmess

_0.0345"

with square-cut lead.ing edge. This type of leading edge was used. to stimulate turbulent flow. The correction for the resistance of this edge was obtairied from a series of tests with similar plates of five different thicknesses.

The argument developed in the paper shows that turbulent fl'ew was obtained in all these tests from within about 001 to .0.2 in. from the

leading edge, and that this short length of poss.ble laminar flow regime had negligible effect on the values of the resistance coefficients for the longer -sheets and did not affect the deduced basic friction, curve above log R 6.2. However, various interested persons have expressed the desire to see further evidence that turbulent flow was obtained. The present note gives the results of an attempt to obtain this evidence by visual flow observation.

The tinest plates were not suitable for this work. Thö plate used in test No.

1141)

_{was chosen.} This plate was

_0.0656

in. thick and

4. in

long in the direction of flow. The trailing edge was tapered to 0.0014. in. thickness, the taper being 1 in. long., The plate was 214. in. wide across the flow direction and in the original rnthod of' test was 'supported by three legs at 12 in. immersion.'

For the flow observation tests a chaxme]. was cut 'in the underside of the plate running from the centre leg position close to and parallel to the square leading

edge

for abit 2 in., then diagonally tow&nds the trailing ed.ge for' about ₅ in. This channel was 1/8 in. wide and. as deep as possible, leaving a topside wall of about 0.015 in. thickness. Th.is was pierced with' a number of holes of 0.02 in. diameter. The channel was widened to 1/14. in. for a very small depth only, to take a cover strip of lead which was pressed in and provided a go'id. water-tight seal.

Dye (potassium permanganate) was led to this channel through a tube secured close g0 the c'ontre leg so that there was no interference with the flow in the region of the holes. The observations were taken exactly as for the resistanc tests, i.e. at 12 in. immersioi.

The results are shown in the attached photographs. Tests were made in each direction of motion, i.e. with the square edge leading and with the tapered edge leading. In all the photographs the square edge is at the top. The peculiar patch running across the plate near the tapered edge is due to a marking on the surface of the plate and has no significance.

The fIrst two pages of prints 'are for zero incidence, which was the condition in the resistance tests. With this setting and with 'the tapered edge leading it was possible to obtain very' f ile threads of dye for all speeds up to

₇

f.p.s. This was done by reducing the flow of dye to an absolute minimum so that t did nbt penetrate the boundary layer. These observations

clearly indicate that the flow was laminar in these tests. At 8aiñ 9 f.p.. the threads showed indications of widening with blurring of the edges', presumably due to transition to turbulent flow c.,nditions.

With zero incidence and with the square edge leading similar fine threads were obtained at 0.5 f.p.s., but at 1.0 f.p.s. these began to break up and for all higher speeds it was quite impossible to produce any fine

threads at all. 'Instead, 'the dye immediately diffused in conical fashion from

Lab. y.

Technische

Uc

_chooJ

De ¡fi

A

_RCHIEF

FLOi7 OBSERVATIONS ON FLAT PLATE

-2-each hole with a rapid diminution of .th

intensity o.f the colour so that at a

short distance downstream from each hole the flow was barely visible.

These

observations clearly indicate turbulent flow conditions

ri interesting po.nt

in these tes ts is that at low speed the dye was observed to move uistream from

the tvfo holes iiearést the léading edgè (abáut 0.25 in, from the edge).

This

also occurred occasiònally at'3 and ¿. f.p.s. but not at hig1er speed.

This

upstr&am flow indicates the presence of a small eddy caused by the corner of

the leading edge.

Tests were also tnade with the plate inclined at 5°..pos.tive angle

of incidence. with the square edge leading, and

50 negative angle of incidence

with the tapered edge leading.

AsSeën from the photographs the flow cond.itioris

were then reversed, fine thxeads indicating laminar flbw being obtained in the

former case, and. a very disturbed flow indidating turbulence in the latter.

The resistariàe results (.gs. 1, 2 and 3 of the paper())

show

arâllei lines for the five thic1esses of each set (3 sets of lengths , 2

and i.. in. respectively).

The. analysis speed was about 9 f.p.s. in each case.

it is certain, therefore, that similar flow Jbnditions weie obtained for each

thiclthess, and. that the above observations may be considered equally applicable

to the other 'thicIiesses and lengths of plate.

These obseÎ'rations completely endorse the coxiciusons in the paper

which were made from the resistance results onJ3r.

These combined results and.

observations prove beyond question that turbulent flow conditions were

obtained

in all, the submerged sheet tests from close to the leading edge, and. that the

conclusion reached in the paper that the pröposed. basic fric.tion line

represents

complêtelr turbulerit flow above log R

62is justified.

(i')

Hughes G.

.'.'priction' and Form Resistance in Turbulent Flow, and. a

Proposed Formulatioíi for, Use in Model and. Ship Correlation".

Trans. I.N.A., l95.

r'

NATiONAL PHYSICAL LABORATORY

FLOW OBSERVATIONS ON FLAT PLATE

PLATE

4" LONG IN

_{DIRECTIOt'J OF FLOW.}

THICKNESS OOE56",

IMMERSION

12g.

5QUAE - CUT EDGE d'AT TOP .0F PHOTOGRAPHS.

TAPERED EDGE AT BOTTOM OF PHOTOGRAPHS.

SPEED

05

10

3O

SQUARE EDGE

LEADING

oC =

-TAPERED EDGE

LEADING

OC =

00

Da Reference - Drg. No Passeiby (40838) WL16)8l7 500 8150 A.& R.W.Ltd. Cp.68$

NATiONAL PHYSICAL

LABÖRATORY

FLOW OBSERVATIÓNS'

ON FLAT PLATE

PLATE 4' LONG 1W DIRECTIÒN OF

FLOW.

THICKNESS O0656',

IMMERSION lad.

5QUAE-CLJT EDGE AT TOP OF' PHOTOGRAPHS.

TAPEREÓ EDGE Al' BOTTOM QF' PHOTOGRAPHS.

SPEW

7O

9O

SUARC EDGE

TAPERED EDGE

LEADING

LEADIÑG

.4

5O

(40838) Wt.16208/7 500 6150 A& LW.Ltd. Op.68$ 4 j

o(= O

o(=00

Date

Reference

Dig. No.

10

hlIh 1

tít IL

30

5.0

NATIONAL PHYSICAL LABORATORY

FLOW OBSERVATIONS ON FLAT PLATE

PLA1'!

4" LONG IN bIRcTIOÑ 0

FLOW.

THICKNESS

00656",.

IMMERSION

1!"

SQUARE - CUT EDGE AT TOP OF PHOTOGRAPHS

TAPERED EDGE AT 80T10M OF PH0TOGAPHS

r, 0.5:?

30

4.0

fi

'J Date Reference

No.

Passed by t4088) Wt.1620817 soo 6150 A.&R.W.Ltd. Cp.688

SQUARE

EDGE.

TAPERED EDGE

SPEED

LEADING

SPEED

LEADING