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• If 52250

TECHN I CA L NOTES

NATIONAL ADVI SORY CON11.'i I TTEE F OR AE RONAU'l' l CS

No . 250 -.... ~

INFLUE NCE OF THE ORI FICE ON ~EASURED PRESSURES

By Paul E. He!'!1k e Langley Hemoria l Aeronaut i cal Lab orat o r y

Washi ngt on Novemb er, 192 6

FI E Cl:?Y' To be returned to

the fibs of the r' ational Ac"h.ory C{)rrmittee

for f ,ronautics 1:1~ ',' tun n,

https://ntrs.nasa.gov/search.jsp?R=19930081065 2018-07-10T03:14:45+00:00Z

NAT IONAL ADVISORY COMlJlITTEE FOR AERONAUTICS.

TECHNICAL NOTE NO. 250.

INFLUENCE OF THE ORIFI CE ON MEASURED PRESSURES.

By Paul E. Hemke.

Summary

The influence of different orifices on the result of meas­

uring the same pressure distributions is the subject of this

note. A circular cylinder is exposed to an air stream perpen­

dicular to its axis and its pressure distribution is repeatedly

determined. The pressure on the greater part of the upstream

half of the cylinder apparently increases when the orifice size

increases. The pressures measured on the downstream half of

the cylinder do no t change for t h e , orifice sizes used in the

tests. Rounding the edge of an orifice has the same effect as

increasing its siz e.

The max imum value of the ratio of orifice diameter to radi­

us of curvature of the surface in the plane of motion, for which

no measurable error was found, is given. Values of this ratio

for orifices as used in aircraf t and model airfoils were found

to be much less than the maximum ratio.

I ntroduction

The a,ir pressure acting on an aircraft in fli ght is usually

measured by providing an orifice at the investigated point and

N.A.C.A. Technical Note No. 250 2

connecting this orifice to a manometer by means of a tube. When

measuring pressure distributions over the sur:f"aces of model air-

foils in the wind tunnel the same method is used. In the latter

case the ratio of orifice diameter to the ra~ius of curvature of

the surface at the point under inv estiga tion may b e quite dif-

ferent than in the full-size airplane. In either case no stand-

ard size of orifice is used nor is any information available

which will tell the investigator what errors he may expect when

h e varies the size of the orifice or in some other way changes

its shape.

The present investigation deal s wi th the effect of orifice

size and shape on the pressure recorded at a designated point

on a body placed in an air stream. This effect was measured by

making pressure distributions over a circular cylinder placed

in an air stream so that its axis is p erpendicular to the direc­

tion OI the stream.

Apparatus

1. The Six-Inch Wind Tunnel.

The tests were made in the six-inch wind tunnel of the

Langley Memorial Aeronautical Laboratory.

A photograph of this tunnel is shown in Fi g . 1. Fig. 2 is

Cl diag rammatic sketch showing the essential features of the

tunnel.

A Sturtevant multivane blower :3, driven by a 3i HP. varia­

bl e speed, D. C. motor M, forces the air through the pas sages

N.A.O.A. Technical Note No. 250 3

P. Three sets of vaneS V, turn the ~ir stream through 90 0

the requisite number of times u~til at 01

, the entrance cone,

the nir moves horizontally from left to right as seen in Fig. 2.

After traversing the working section A, the air enters the

exit cone 02' and thence passes into the blower B.

The ~assage s P, are of wood cons truction and rectangular

in cross section. The horizontal passage is constructed so

that its cross sections graciually i ncreas e in a.r ea from right

to left. The entrance cone C, is an a luminum casting. The

section at one end is a sqUAre. This is gr adually changed until

it becomes a circle of much smaller cross section. The cone

terminates in a cylindrical piece 8 inches long , having a circu­

lar cross section 6 inches inside diameter.

The exit cone, also 2..n alumin~n casting, has a bell-shaped

mouth and for the remainder of its length is a circular cylinder

of constant cross section. It is fastened to the blower by means

of a sheet metal sleeve.

A honeycomb H, is placed just in front of the entrance

cone. The air velocity is fairly constant and with proper

alignment of the entrance and exit cones, there is very little

"spilling over l1 a.t the mouth of the exit cone. The ".ir veloci­

ty can be varied from 20 M. .P.H. to 118 H.P.H.

2. Apparatus for the Orifice Tests.

A photograph of the apparatus in place in the tunnel is

shown in Fig. 3. Fig. 4 is a diagrammatic sketch of the apparctus~

N.A.C . A. Technical Note No . 250 4

It consists of a brass cylinder 1 inch in diameter, and 8

inches long. The cylinder is placed symmetrically in the ai r

st re~~ so that its axis is at right angles to the latter. A

hol e coaxial with the cylinder extends f rom one end to the center

of the cylinder. This hole is connected at one end by means of

a rubber tube to a manometer. At the other end it is connected

by means of a r adial hole to an orifice on the surrace of the

cyl i nder.

A c ircular disc graduated in degrees is fastened to one end

of the cylinder. The angular p osition of the radial hole termi­

nating in the orifice is then known .

M:ethod 0 f Making Te st s

The tests consisted in making pressure distributions around

t he cylinder for each type of orifice u sed. Pressures were re­

corded at intervals of 2tO to 100• The dynamic pressure Was

held as constant as possible. It s magn itude Was measured by

means of a static plate located on the side of the tunnel just

ahead of the entrance cone . This static plate had been previ­

ously cal ibrated against a Pitot static tube placed in the work­

ing or test section of the tunnel.

Each orifice Was tested for two dynamic pressures corre­

sponding to 4 in. and 12 in. of water, respectively. The ori­

fices varied in diameter from . 008 in. to . 250 in. There were

two series of these orif i ces , one having sharp edges and the

other rounded edges .

I

N.A.C·A· Technical Note No. 250 5

Results

1. Orifices with Sharp Edges.

The tests show that the measured pressure varies chiefly

over the forward or upstream side of the cylinder as the orifice

size is changed. If the stagnation point of the flow coincides

'Ni th the zero po~i tion of the orifice, then the changes observed

ere limited to the 00 - 90 0 ra.nge of orifice positions . No aIr­

preciable change occurs in the interval 90 0 - 180°, even for

the greatest change made in the orifice dicmeter . This is shown

in Figs 6 and 7, where the pressure distributions are plotted

for the smallest and largest orifices. The minimum values of

the pressure recorded differed in this instance by 17 per cent

from the dynamic pressure.

In Figs. 8 and 9 t he pressures recorded at certain stations

on the cylinder are plotted a gainst orifice diameters. The

pressures change slowly up to diameters of .06 in. Thereafter

the changes are more rapid, particularly at the 700 station .

Let R = ratio of orifice diameter to radius of curvature

of the surface at the point where the orifice is located. The

air flow i s assumed to be two dimensional. We assume further

that the p:ressures mea.sured wi th t h e s;naller or ifices are prac­

tically correct . I t appears then from the curves in Figs. 8

and 9 that practically no error in t he pressures measured occurs

when R = .06. The experimental error was about 2 p er cent of

the dynamic pressure. The error increases proportionally up to

N .A.C.A. Technical Note No . 250 6

a value of R = .12 . where it is 5 pcr cent of the dynamic pres-

sure. Thereafter the errors increase rapidlY.

The tests for the two Reynolds Numbers corresponding to the

two vel ocities used show a close agreement.

2. Orifices with Rounded Ed~es.

The series of rounded orifices used were constructed as

shown in Fig . 5. A sharp edge orifice (a), of diameter d

was first test ed. The edges were then rounded to a radius r,

as shown in (b), and this new orifice was then tested . The

next orifice in the series (c), was then made having a diame-

t er d + 2r and sharp edges.

In all cases tested the difference between the pressure dis­

tributions obtained v(Then orifices of type (b) and the next in

the series of type (c) were used, did not exceed the experi-

m ente"l error. Fig. 10 shoV'rs a typical curve comparing the pres-

sure distributions ob tained when using a rounded and sharp edge

• of" OrL-lce.

Conclusions

The tests show that with the cylinder of 1 in. d iameter,

orifices of diameter .06 in. may be used vrithout seriouslyaf-

fecting the pressure dist ribution over the cylinder.

If the pressure distribution obtained for the smallest ori­

fic e is considere~ as standard, then it is possible to make cer-

IT •. ~l • C • A • T e c ~·1.l,,} i cr> 1 Not e K () . 2 50 7

tain conclusions fo r the larger onos · I f the rQtio of orifice

dia~eter to radius of curvature of the surface in the pl ane of

the motion (tIJO d imensional) is . 1)5, the -pressures are not in

eTror by more than 2 per cent of the dynamic pressure. For

values of this ratio equQl to .12 the errors do not exceed 5

per cent of the dynrrmic pressure. With larger values of the

sa.me ratio the var iations from the standard <:tre irregular and

more rapid as 'Nell as larger.

On this basis of comparison tests made on models arc fairly

accur2,t e as far as orifice diarneter is concerned. For the preB­

sure distribution tests l:1D.de at the La.ngley Uer,;orial ~e"!:'nn"" _Lti..c-­

al Laboratory the largest value of the ratio of orif ice dian eter

to radius of curvature of the wing section 'Nas found to be about

.005. On full- sized airplanes this ratio is considerably less .

N.A.C.A. Technical Note ~o.250 Figs. 1 & 3

Fig. 1 Six - inch ind tunnel

Fi~ . 3 Apparatus in place

I

~ .A.C.A. Techni cal No te No .250

C\l o

. c::x:

o c::x: .

C\l .

Fi g . 2

N.A.C.A . Technical N6te No.250 Figs.4 & 5

--------------~.To manometer

Entrance cone

Fig.4

-d -

( a)

II I' I I ,I II I' II I'

I Orifice- -'_I

/Brass cylinder

{Graduated disk

Exit cone .

Cylinder and or ifice in place.

rd+2r -1 ,----~ ~--~

.... -d+2

(c)

Fig.5 Orifice sections.

N.A.O.A. Technical Note No.250 Fig.6

1.0 ~ ~'}.,

~\\ . 8 ~ \

~ \ \ ~ • 7 ~

~

1\ \. 0 6 .r-!

• c ,,~

\\ ~ » · 5 'C

0

\1= Or i f ice 1/64 11 iameteI .p } q = 4 7 in . . '.~ ater

Or i f i ce 1/4 11 iameteI . 4 ~

~ \ \ r.a . 3 Q.; •

P< \ ~ Q)

.2 0

ct-i

\ 1\ .r-! F-i

.1 0

ct-i

\ ~ 0

o 0

.p ~O 4\ \ EO 80 l( 0 1~0 11 0 l~O 180 ccJ

ex: Angul 3.r posi ion of pr i f ic e -.1

-.2 \\ \\ . 3 x"-- xl2::® - c ~ -'@,-l ~~- ~

-=- ~ ,-;:;r-

\~ / rl'/64 II - .4 /

~/ x~

-. 5 \

- 6 ~ Fig.6 Pressure distribution wi th orifices of different diamete rs.

N.A.C.A. Tochnical. Note No .250

.7 \\

. 6 \\

. 5 \\

.4

\

\ f'<;.a - 1. 6 in . vv~ter \ - 0 l' i fie ~ 1/,1" d i amo tor lJ • -

.1

o

o +'

(D H ;j CD CD Q)

H '») p..<:.>

-.1 ~ ·rl ct-i ·rl

-. 2 ~

\\ 4 )\x\ 6 j 8 ) Ie 0 l~ 0

\ AngulaI positi )n of 0 ifice 140

\\

Fig .7

IE 0 180

'8 x\\ .L::"""O- r.. -. 3 _ "><7 )(_"-\--0~ ,., -n--O- x _ x_ X [) / )( f X - X=';t Q--)k _ x-

~ '1 / f/ I -. 4 ~ ~ r-~---+------~------~9~----T+-------r------~------+-------+-----~

- 5 0\

\'0 ~.~6~--+---__ --r------4-------+-------+------~------~------+------~

- . 7

Fi g .7 Pre ssure distributi on with orifi ces of different diame t ers.

Pressure for or i f i ge posit i ons 9·00-.J..80o. plot as hor i zontal l i nes located bet'lIJeen

CD H ::J1.0 (f) (f) (')

H P, . 8 o

• ..-1

e ~ ~ . 0 :>. 'tj

o 4 ~ . ill H

~ . 2 w ill H P,

CD o

• ..-1

o

'+-1 _ 2 -..-I • H o

'8 - .4

o -..-I

t wo do t ted l ines . Dyna mic p ressure 5 i nches of wB,t er . -

, ! 0 0 I

J'. -1.Qp_ r-.J--!--,f-+--t--r---, r--- ~' r=:- I I 20P ----r -t-1

-<--<.J" I

1- --- - . - +--r-J J _ _ 1-;---- , I 30 f-- I

I I _ r-< . I ! r--

~ -o-1---Io-~ _ ---j

, c--t--q i : I '±u 1--1-1-

-0- ~ 1--- - - :----r--- I

! _c.-->--L--\.--c+--t-r-

50P -~ r -aOfJ -= _ __

----v _

-~ ~o - -- 1 "'- 1 I 1 - ~ r----- I J , _ i---Ir--r---- --. 70p-o. I 1 1

~ - . 6 p:: 0 .02 . 04 . 06 . 08 .10 . 12 .14 . 16 . 18 . 20 . 22 . 24 . 26

Orif i ce d i ameter (in . )

Fi g.8 Pressure c,t vc,rious ori f i ce posit i ons plotted against orif i ce d i ame t er.

!2:

> o . > . I-CJ CD o ::r' ::;i 1--'­o p.l f-'

z o c+ CD

~ o /)J ()l o

t:rj 1--'­

(Jq

en

-, -Pressure for orifice positions 900 -1800 plot as horizontal lines located between

0) ~ ;j wI. w (J)

~ p...

C) • .-1 S cD s:: :>, 'd

o -P

(j)

~ ;j w w (j)

~ p...

(j) C)

• .-1 CH -.-1 - • ~ o 'H 0- .

o • .-1

two dott e d lines. Dynamic pressure 2 inches of water.

[~~~;;~~~1~~t=~~~0~0~f=~~~=+~~~~~~~~~~~ J"l. ..t1.

o -"l. _ __ _ .lC~ r-- -- '-r--r--r---+--j---r~ tlm 8 1::-

~~~~==~~==f==t~~~~==i--+--+--t-t--t-r--t--l-I--j-~r-'---,---- --r--6 ~.

J:::~~tti~~'~=~~+3=O~Pt=~r-~t--~~~t+ii~~FFt: 1- I 4 L 0 J

I ~--2 I 40i-fl-I--r-r- I I 1~:~~=F9Fi~~~i'Fl~~'----r-' -~+-+-~~--+~--II-~l-II

C _

! ~~~~l-~~-+--t~~1--t-jt=1==t:jt=~~ s ( 80 _ _ t::.- to-....

~ ~ 60 J I - = L1 La. v ra---r--- 70P

-P 6' cD -. ~ 0 .02 .04 .06 .08 .10 .12 .14 .16 .18 .20 .22

OrificG diaDGtor 'Cia!) .24 .26

....

Fig.9 Pressure at various orifice positions plotted against orifice diameter~'

z > o

>

f-3 CD o P' ~ f-'-o P' f-'

~ o c+ CD

~ o !:I.) 01 o

'Tj ....,­

(Jq

(0

M.A.C.A. Technical Note No . 250 Fig.lO

It ----c----'

.9

. 8 \ \

I . 7 '" I ~ f...j

\ XOri o Ori rice l/~ 2" diam p.ter} ;j

4.7 i UJ ice 1/6 ~II diamE ter,rou pded q = n . wate . 6 UJ

'" f...j

\ p.,

5 0

s 'k I cO ~

4 ?-> '\ I 0 \ +' I

.3 '"

I f...j

\ i I

;j UJ

.2 UJ 1---

f...j \ I p.,

r .1 ~ I x I ·rl

~ 'H I ·rl

.0 f...j r-.

.'H 2O 4 p\ Sp EO 1 pO l~ 0 140 lE 0 0 Angular positi pn of OI ifice -.1 r-.

·rl \ +' cU

-.2 p::

'x

-. 3 \ /: {---~-- --~. ~ )

\ ~

-. 4 ) :i./

-.5 Fig .10 Pressure d i st~~ i but i on with sharp and round edge ori fices.