Concerning the flow about ring-shaped cowlings

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Material Information

Title:
Concerning the flow about ring-shaped cowlings
Series Title:
TM
Physical Description:
19 p. : ill ; 27 cm.
Language:
English
Creator:
Küchemann, Dietrich, 1911-1976
United States -- National Advisory Committee for Aeronautics
Publisher:
NACA
Place of Publication:
Washington, D.C
Publication Date:

Subjects

Subjects / Keywords:
Aerodynamics   ( lcsh )
Jet engines -- Air intakes   ( lcsh )
Genre:
federal government publication   ( marcgt )
bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

Notes

Abstract:
The influence of thickness and length of a hub projecting from an inlet opening was investigated on one of the two new classes of circular cowls reported in NACA TM 1360.
Bibliography:
Includes bibliographic references (p. 7).
Funding:
Sponsored by National Advisory Committee for Aeronautics
Statement of Responsibility:
by D. Küchemann.
General Note:
"Report No. NACA TM 1361."
General Note:
"Report date October 1953."
General Note:
"Translation of "Über die Strömung an ringförmigen Verkleidungen. XIII Mitteilung: Der Einfluss einer vorgezogenen Nabe." Untersuchungen und Mitteilungen No. 3144. (ZWB)."

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 003769590
oclc - 85869702
sobekcm - AA00006168_00001
System ID:
AA00006168:00001

Full Text
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NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS


TECHNICAL MEMORANDUM 1361



CONCERNING THE FLOW ON RING-SHAPED COWLINGS

Part XIII

THE INFLUENCE OF A PROJECTING HUB*

By D. Kuchemann


Abstract: The influence of thickness and length of a hub projecting
from an inlet opening was investigated on one of the two new
classes of circular cowls reported in NACA TM 1360.

Outline: I. STATEMENT OF THE PROBLEM
II. RESULTS
III. SUMMARY
IV. REFERENCES


I. STATEMENT OF THE PROBLEM


In some applications, there arises the problem of scooping a given
air quantity out of the free stream, as, for example, through a circular
opening in the case of an annular radiator. It could also be visualized,
however, that such an encircling inlet opening might be provided in the
installation of special propulsion units, for instance in the fuselage.
One may regard such an annular inlet as an inlet with projecting hub for
which the flow phenomena to be expected are already known to a great
extent. One knows, moreover, from various model tests that, with such
a projecting hub, one can reduce the excess velocities on the outside of
the circular cowl but has to accept on the other hand a decreased total
pressure in the entrance opening. (See for instance ref. 1.) More accu-
rate data for evaluation of the usability of this inlet arrangement (from
which the actual numerical amount of the separate phenomena in various
arrangements could be estimated) are still lacking, however.

The properties of two new classes of circular cowls have been investi-
gated in reference 2. We shall in the present report observe more closely
the influence of the hub on one of these circular cowls. We select for
this purpose the least contracted cowl of class IV with FE/Fa = 0.6 and

"Ueber die Str'dmung an ringfSrmigen Verkleidungen. XIII. Mitteilung:
Der Einfluss einer vorgezogenen Nabe." Untersuchungen und Mitteilungen Nr.
3144 (ZwB).






NACA TM 1361


provide it with six different hub bodies which differ in thickness and
length.1 (Compare figs. 1 and 2.) The hubs obstructed 50, 65, and 80 per-
cent of the entrance cross section FE' of the circular cowl and were,
beginning from the foremost point of the cowl, circular-cylindrical in
its interior. For the short hubs, a nose in the form of a semiellipsoid
of an axis ratio 1:2 was affixed so that the length IN of the projecting
part of the first hub was exactly equal to the diameter 2 RN. In a sec-
ond series, the hub was lengthened frontward by a cylindrical piece of
the length of one diameter so that IN/2 RN became 2. The investigations
are limited to pressure distribution and mass-flow measurements.


II. RESULTS


In the following figures, we indicated only the most significant
results which show the important phenomena; the detailed measuring results
may be had from the AVA.

From figures 3 and 4, one can see how the suction peaks at the out-
side of the cowl are lowered by the guiding action of the hub which shifts
the stagnation point farther outward and reduces or eliminates separation
(or welling-over) of the cowl boundary layer. For the case without hub
the pressure distributions show the unfavorable properties of cowlings
with slight rounding of the lip; the cowl lip appears, from the pressure
distributions, to become increasingly rounded out with growing hub size.
Accordingly, the excess velocities decrease sharply (figs. 5 and 6). The
magnitude of this reduction depends on the inlet velocity ratio; it is
highest when the entering flow is completely throttled and can, naturally,
hardly be ascertained at all when free-stream velocity prevails in the
entrance cross section. How far this drop in excess velocity increases
the critical drag-break Mach number cannot be determined, since the
character of the pressure distribution also is changed entirely by the
hub. Furthermore, the reduction of the excess velocities is particularly
noticeable in case of oblique approach flow due to the hub as can be seen
from figures 7 and 8.

The decrease of the excess velocities may be explained partly on the
basis of potential theory; we recall that thrust forces must act on the
outside of the cowl (compare refs. 1 and 3) and that a certain area must
be put at their disposal if the negative pressures are not to drop below
certain values. If the hub projects very far from the fairing, it cannot
be put to use for the application of the thrust forces so that only the
frontal area of the cowl Fa FE' could be considered as thrust area.
As is discussed in detail in reference 1, a decisive factor for the

1As no symbol list was included in the German text, a list has been
compiled by the NACA reviewer and is given in an appendix.






NACA TM 1361


magnitude of the excess velocity is in addition to the inlet velocity
ratio, vE/vo, the ratio between the entrance area and the thrust area,
that is, the quantity (FE' FN)/(Fa FN), which replaces the con-
traction, FE/Fa, in the hubless inlet. This quantity decreases more
and more with increasing thickness of the hub, that is, the contraction
of the cowl increases and the cowl becomes more favorable. The maximum
excess velocity vmax/vo which would be obtained for vE = 0 if the
cowl had a constant pressure distribution (Ruden's entrance cone of
minimum contraction) is calculated from

FN FE'
v 1 -,--
max FE Fa
Vo 1 FEl
Fa

For the arrangements investigated we obtain the values:


Hub ratio Contraction Excess velocity
FN/FE' 1 FN/FE' m/vo for vE = 0

Fa/FE' FN/FE'

0 0.60 1.58
.50 .43 1.33
.65 .34 1.24
.80 .23 1.14

These values of excess velocity are not attained with the cowl used
since it is not a Ruden minimum form; for the long hub with a larger
radius of curvature at the lip where the presuppositions probably come
true, the minimum values are exceeded by approximately 10 percent.

There is a second reason, however, which is responsible for the
reduction of the excess velocities: (that is) the flow separation at
the hub. This phenomenon is, moreover, expressed in the fact that at
the entrance the full total pressure is not attained. For two different
flow quantities, the total pressure distributions at the entrance (begin-
ning of the cylindrical part) for the various arrangements in rectilinear
flow are shown in figure 9; figures 10 and 11 show the values averaged
over the entrance cross section referred to the stagnation pressure of
the approach flow which is designated as inlet efficiency nE. One can
see that the losses rise with increasing hub thickness and length and
reach noticeable amounts in case of the thickest hub. The numerical






4 NACA TM 1361


values themselves, however, can give only a clue, since they are cer-
tainly caused, to a high extent, by the small model size (maximum outer
diameter of the hub 200 mm, free-stream velocity about 40 m/s). It
should further be pointed out that the inlet efficiencies depend also
very considerably on the shape of the hub and are, for instance, essen-
tially less favorable in forms which are thickened ahead of the inlet
and are again contracted at the entrance. (Compare, for instance ref. 1.)
In case of oblique approach flow, the separation phenomena become still
less clear. In figure 12, one can see measurements for the most extreme
arrangement in a vertical center section which shows the greatest vari-
ations. The entrance losses for static condition for the hub FN/FE' = 0.5
may be taken from reference 2. For the thicker hubs, the inflow for
static conditions takes place practically without losses.


III. SUMMARY


In a circular cowl, the influence of a hub body projecting from the
inlet opening was investigated; the reduction of excess velocities on the
outside of the cowl and the amount of total pressure losses in the inlet
as a function of thickness and length of the hub were ascertained by
measurements. The tests are visualized for the application in annular
radiators and the installation of special propulsion units; in the latter
case, such an arrangement would have to show many constructive and other
advantages in order to prevail over the customary forms of installation
since one does not gain anything in frontal area and always has to accept
noticeable entrance losses.


Translated by Mary L. Mahler
National Advisory Committee
for Aeronautics.






NACA TM 1361


APPENDIX


SYMBOLS1


Fa = gRa2

FS = 1RE2 RN2

FE/Fa

FE'/Fa = RE2/a2

FE' = RE2

FN itRN2

p

pges

PO

qo = pV02/2

Ra
a

RE





vE

VE/'o

Ev /
VE/Vo

VEO/x

Vmax


contraction of cowl

contraction of circular cowl without hub






local static pressure

local total pressure

static pressure in the undisturbed flow

dynamic pressure of approach flow

maximum outer radius of cowl

radius of cowl in entrance cross section narrowestt
cross section in the inlet part)

radius of hub in entrance cross section

mean value of velocity in the entrance cross section

inlet-velocity ratio

inlet-velocity ratio for a = 00 for the same
position of sliding throttle valve

maximum velocity on the outside of the circular cowl


1This appendix was added by the NACA reviewer.






6 NACA TM 1361


vo undisturbed free-stream velocity

x, r rectangular coordinates; x in direction of the axis
of rotation

a angle of attack

IN length of hub ahead of inlet
Pges -P
TE inlet efficiency; mean value of over the
entrance cross section qo






NACA TM 1361 7

IV. REFERENCES

1. Kiichemann, D., and Weber, J.: Das Einlaufproblem bei Triebwerks-
verkleidungen. Mtt. d. Dt. Akad. d. Luftfahrtforschg, 1943.

2. Kuchemann, D., and Weber, J.: Ueber die Stromung an ringfo'rmigen
Verkleidungen. XII. Mitteilung: Zwei neue Klassen von Ringhauben.
UM 3111, 1944. (Available as NACA TM 1360.)

3. Kuchemann, D., and Weber, J.: Zur Fraget Kreierunde oder ovale
Hutzenl. UM 3058, 1944.







NACA TM 1361

















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Figure 3.- Circular cowl IV/0.6; a = 00; vE = 0;
SN/2RN = 1; pressure distribution outside.


with short hub


NACA TM 1561


-x/R,
0.1 0.2 03 04 0.5 0.7 O0


F N/F=0.80 0.65 0.50



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Figure 4.- Circular cowl IV/0.6; a = 00; vE = 0;
IN/24N = 2; pressure distribution outside.


with long hub


NACA IM 1561


).5 --- --- --x/Ra-

0 OJ 0.2 03 04 0.5 0.6 07 0.8

F F/F, '=0.80 0.65 0.50
.5


.0
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NACA TM 1561


^ox~ ~ ~ v -^ -- -- h -
Umax ---
Uo I.7






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15







1.2 0.8 -1.0


1.1 -
1.0 3-- --



0 0.5 0.6 0.7 0.8 0.9 1.0
wo FN/FE


Figure 5.- Circular cowl IV/0.6; a = 00; with short hub 1N/2RN = 1;
excess velocities outside.






NACA TM 1361


2.0


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1. -


1.2 A -




4.0
0 0.5 0.6 Q7 0.8 Q9 t.0
FN/FE
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Figure 6.- Circular cowl IV/0.6; a = 00; with long hub N/2RN = 2;
excess velocities outside.

























































-5.


Figure 7.- Circular cowl IV/0.6; with short hub 1 N/2RN = 1; excess
velocities outside.


NACA TM 1561






NACA TM 1561


-50 00 50 10 a


Figure 8.- Circular cowl IV/0.6; with long hub IN/2RN = 2; excess
velocities outside.







NACA TM 1361









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o-





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NACA TM 1561


--,F /FE
"N E


Figure 10.- Circular cowl IV/0.6; a = 00; with short hub I N/2RN = 1;
inlet efficiency.





NACA TM 1561


--FN /FEI


Figure 11.- Circular cowl IV/0.6; a = 00; with long hub IN/2RN = 2;
inlet efficiency.





NACA TM 1561
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