Concerning the flow about ring-shaped cowlings

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Material Information

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

Subjects

Subjects / Keywords:
Aerodynamics   ( lcsh )
Radiators -- Thermodynamics   ( lcsh )
Genre:
federal government publication   ( marcgt )
bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

Notes

Abstract:
The measurements of part V (reference 1) of this series of reports, which concerned comparatively long ring profiles, are supplemented by measurements on shorter rings as they are used for shrouded propellers and cowlings of ring-shaped radiators. Mass-flow coefficients and profile drags are given. Furthermore, it has to be determined how far the potential theory describes the flow phenomenon with sufficient accuracy and whether the present theory for the calculation of thin annular profiles yields useful profile forms and is suitable for determination of the mass flow for thick profiles.
Bibliography:
Includes bibliographic references (p. 7).
Funding:
Sponsored by National Advisory Committee for Aeronautics
Statement of Responsibility:
by Dietrich Küchemann and Johanna Weber.
General Note:
"Report date February 1952."
General Note:
"Translation of Über die Strömung an ringförmigen Verkleidungen. VIII Mitteilung: Weitere messungen an ringprofilen. Zentrale für wissenschaftliches Berichtswesen der Luftfahrtforschung des Generalluftzeugmeisters (ZWB) Berlin-Adlershof, Forschungsbericht Nr. 1236/8, Göttingen, March 25, 1943."

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 003779491
oclc - 86221752
sobekcm - AA00006193_00001
System ID:
AA00006193:00001

Full Text
--CAC -









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NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS


TECHNICAL MEMORANDUM 1328


CONCERNING THE FLOW ABOUT RING-SHAPED COWLINGS


PART VIII FURTHER MEASUREMENTS ON ANNULAR PROFILES*

By Dietrich Kuchemann and Johanna Weber


ABSTRACT:

The measurements of part V (reference 1) of this series
of reports, which concerned comparatively long ring profiles, are
supplemented by measurements on shorter rings as they are used for
Shrouded propellers and cowlings of ring-shaped radiators. Mass-flow
coefficients and profile drags are given. Furthermore, it has to be
determined how far the potential theory describes the flow phenomenon
with sufficient accuracy and whether the present theory for the calcu-
Slation of thin annular profiles yields useful profile forms and is
i suitable for determination of the mass flow for thick profiles.

OUTLINE:

I. STATEMENT OF THE PROBLEM
II. THE ANNULAR PROFILES INVESTIGATED
III. THE METHOD OF MEASUREMENT
IV. RESULTS
V. APPLICATION OF THE RESULTS
VI. SYNOPSIS


I. STATEMENT OF THE PROBLEM


The measurements on annular profiles given in the present report
ierve as a supplement for part V (reference 1). However, whereas in
part V (reference 1) the annular profiles had a relatively great length I
.referred to the ring diameter 2ro, namely 1/2ro = 2, shorter profiles

*Uber die Str6mung an ringf6rmigen Verkleidungen. VIII Mitteilung:
::eitere Messungen an Ringprofilen. Zentrale fur wissenschaftliches
i Berichtswesen der Luftfahrtforschung des Generalluftzeugmeisters (ZWB)
Berlin-Adlershof, Forschungsbericht Nr. 1236/8, Gbttingen, March 25, 1943.




a:::.






2 NACA TM 1328


will be investigated now as they are used for shrouded propellers
(compare part VII (reference 2)) and as cowlings of annular radiators.
For both purposes of application, a profile form is desired for several
operating conditions, preferably for start and climb, with the smallest
possible profile drag and the greatest possible increase of the mass
flow. The annular profile alone was to be investigated first for a
preliminary clarification of these problems. In particular, the question
was to be treated as to how far the existing theory of thin annular
profiles (part III (reference 3)) enables the development of usable
profile forms and permits predetermination of the mass flow.


II. THE ANNULAR PROFILES INVESTIGATED


According to the calculation method given in part III (reference 3),
the mean camber lines of four profiles with different (negative) circu-
lation and of equal length (1/2ro = 0.5) were determined. The annular
profiles 5 and 5, as well as 7 and 8, have, in every case, the same
total circulation and are distinguished only by the facts that 5 and 7
have a curved mean camber line, while 6 and 8 have an additional
S-curvature. The calculation parameters cv of part III (reference 3)
have the values:

(Camber) (S-curvature)
Annular profile 5 c2 = -0.05 c3 = 0
Annular profile 6 c2 = -0.05 c3 = -0.05
Annular profile 7 c2 = -0.10 c3 = 0
Annular profile 8 c2 = -0.10 c3 = -0.10

The theoretical coefficients cr of the radial force which correspond
to ca for two-dimensional profiles are cr = -0.5 for the annular
profiles 5 and 6, and cr = -1.1 for the annular profiles 7 and 8.
The influence of the finite profile thickness has, so far, not been
treated theoretically. We were therefore obliged to assume a thickness
distribution and to superimpose it on the mean camber lines. The modi-
fication to the mass flow thereby produced may be calculated in first
approximation according to the continuity equation which proved to be
satisfactory for the profiles investigated in part V (reference 1),
however, a certain error is connected with this assumption which takes
effect particularly in the determination of the radial-force coeffi-
cient cr, as has been shown in part I (reference 4). The influence of
the thickness manifests itself in making the cr-values negatively larger
than those resulting for the mean camber line.








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Annular profile 9 c2 = -0.05 c3 =: -0.05


The theoretic


al coefficient of the radial force also is c, = -0.5.


III. THE METHOD OF MEASUREMENT


The rings were turned from wood and had the dimensions shown on
figure 2. They were attached with a sting to the drag balance as
described in part V (reference 1). The profile drag, W, was measured
for various free-stream velocities vo and plotted against a Reynolds

number, formed with the profile length Z, Re = v, in the form of a
coefficient


2 Vo2FM
cW = w/ V 2FM


:! with FM signifying the generating surface of the annular profile which
is identical with the surface area of the wing that is obtained by cutting
S open the annular profile and developing it into a plane. Thus, Re and
S cw may be compared directly with the values customary for two-dimensional
wings.

The mass flow was determined by a survey at the exit area of the
wing. The total-pressure measurement shows the regions where kinetic
energy of the flow is lost and thus forms a supplement of the drag
measurements.


1NACA TM 1328 3


We used the same thickness distribution for all annular profiles
(see fig. 1) as in part V (reference 1) with a maximum thickness of 20 per-
cent of the profile length at 35 percent of the length counted from the
leading edge. The profile forms produced by superposition of this
thickness distribution along the mean camber lines may also be seen
from figure 1.

Aside from these four annular profiles, a further shorter one
(1/2ro = 0.25) was investigated which has a curved mean camber line
with S-curvature and is related to the annular profile 6. The pertaining
data are:






4 WACA TM 1328


IV. RESULTS


The theoretical mean velocities vith in the narrowest inner cross

section Fi and the corresponding measured values vi are indicated
in the following table:


Annular profile FA/Fi vith/vo v /vlo VA/vo

5 1.34 1.44 1.35 1.01
6 1.39 1.44 1.37 0.99
7 1.42 1.70 1.46 1.03
8 1.54 1.70 1.4o 0.91
9 1.17 1.22 1.21 1.04


The deviations between the theoretical and the measured values are seen
to be slight in most cases. The theoretical presuppositions are
satisfied best for the annular profiles 5, 6, and 9 where insignificant
losses in mass flow occur. For the annular profiles 7 and 8, the theo-
retical value Df the circulation obviously is too large and does not
materialize in practice; this phenomenon was thoroughly discussed in
part V (reference 1).

The numerical table gives the ratio between the exit area FA
and the smallest inner area Fi and, additionally, the mean measured
velocity vA in the exit plane. It may be erroneously assumed that
approximately the undisturbed external pressure po and hence the
undisturbed free-stream velocity vo prevail in the plane of the exit
which would justify a calculation of the flow on annular profiles under
this presupposition in a simple one-dimensional manner. However, the
measurements of the velocity distribution in the exit plane do not
confirm this assumption, as is shown by the measured results indicated
in figure 3. First, one recognizes that the boundary layer on the inside
of the annular profile brings on a loss of flow. The limit of the range
where the total pressure does not reach the full undisturbed value is
characterized by a dashed line. In the entire adjoining inner space,
however, there prevails according to the measurements preeminently a
negative static pressure and therewith a velocity increased compared to
v This fact, also to be expected theoretically (see part 7 (refer-
ence 2)), is what causes the increase of the mass flow mentioned. The
fact that in some cases this velocity increase is on the average exactly






i. ACA TM 1328 5


cancelled by the reduction in velocity in the boundary layer, is
incidental.

The theoretical value of the increase of the mass flow was not
attained for annular profiles 7 and 8 (fig. 3). This discrepency is
caused by the relatively large regions with energy loss; one may speak
of distinct separation phenomena particularly for profile 8. The
measured drag coefficients for all profiles are plotted in figure 4.
The c, values lie, for the annular profiles 5, 6, and 9, in a range
which is usable for the practical application of such rings. Moreover,
a noticeable dependence of the cw value on the characteristic Reynolds
number appears so that one may assume even lower drags in practical
applications such as shrouded propellers because of the increase in Re.
In general, the drags are in a range which lies only slightly above the
one customary for two-dimensional profiles of corresponding thickness
and circulation. A certain increase of the profile drag due to the
influence of the ring is to be expected as was shown in part V (refer-
ence i).


V. APPLICATION OF THE RESULTS


The measurements, which are to be valued as spot checks for
clarification of the properties of annular profiles, show that a note-
worthy increase of the mass flow by a negative circulation is possible
for relatively short annular profiles without the profile drag becoming
excessive. One may expect, particularly for shrouded propellers, a
further increase of the effectiveness and an increase in static thrust
over those so far attained for short profiles since the additional
velocity o5 = vi/vo 1 caused by the present rings was considerably
increased compared to the value of 5o = 0.12 in part VII (reference 2).
For ring-shaped radiators it will in many cases be possible to design
the cowling of the radiator so that the mass flow in climb need not be
increased by more than 30 to 40 percent by auxiliary means such as small
additional profile drags on the ring. This is particularly conceivable
in the case of drum radiators. If one assumes, for instance, that the
mass-flow coefficient for such an arrangement (that is, the ratio between
the mean velocity vK at the radiator and the flight velocity vo) is
to be modified by the cowling between VK/vo = 0.1 (high-speed flight)
and 0.28 (climb), one would have to attempt, by suitable shaping of the
hub and the cooling block, to make vK/vo = 0.2 without cowling. The
cowling then has the function of either reducing this mass-flow coeffi-
cient to one-half (for high-speed flight) or to increase it to 1.4 times
its value (for climb) which, in an appropriate design, ought to be
possible by flaps without much additional drag. However, any increase






6 NACA TM 1328 ,;


in the mass-flow coefficient caused by the ring by more than about
40 percent (at the 1/2ro proportions considered here), is accompanied
by very considerable additional drags since the flow then certainly will
separate at the ring. (See measurements, particularly those of part V
(reference 1).

The measurements show further that a usable annular profile may
be designed according to the methods of part III (reference 3) where the
influence of the thickness of the ring on the mass flow is taken into
consideration with sufficient accuracy by the continuity condition.
The magnitude of the circulation up to which the flow at the profile
does not separate also may be estimated from the existing measurements.
For the design of a propeller shroud, one has to consider, additionally,
the slipstream and the influence of the propeller hub; this is discussed
in more detail in part VII (reference 2). Analogous requirements apply
to cowlings of ring-shaped radiators.


VI. SYNOPSIS


The profile properties of annular profiles as they are used for
shrouded propellers, cowlings of ring-shaped radiators, and similar
flow problems had been investigated for comparatively long profiles;
in the present report, the profile properties are clarified for shorter
profiles as well, in a first survey. All measurements are made on four
different annular profiles with 1/2ro = 0.5 and on one with Z/2ro = 0.25
with respect to the increase of the mass flow by the circulation about
the ring and to the profile drags appearing. It is found that the theory
yields useful profile forms and that, moreover, the air quantity flowing
through may, by means of the present approximation theory, be determined
beforehand with sufficient accuracy up to certain values of circulation,
the magnitude of which can be estimated. The profile drags in the
nonseparated flow region are insignificantly larger than the corresponding
values for two-dimensional wings. For the rings with Z/2ro = 0.5, it
was shown that the mean velocity in the narrowest inner cross section
can be about 30 to 40 percent higher than the free-stream velocity with-
out the profile drag becoming excessive. For the shorter profile with
Z/2ro = 0.25, the increase of the mass flow is correspondingly smaller
and amounts, at the most, to about 20 to 25 percent. The conclusions
to be drawn from these results as to the application of annular profiles
for shrouded propellers and ring-shaped radiators are briefly discussed.


Translated by Mary L. Mahler
National Advisory Committee
for Aeronautics






NACA TM 1328 7


REFERENCES


S 1. Kuichemann, Dietrich, and Weber, Johanna: Uber die Strimung an
ringformigen Verkleidungen. V. Mitteilung. Forschungsbericht
Nr. 1236/5, 1942. (Available as Tech. Intelligence Trans.
F-TS-620-RE, AAF, Air Materiel Command, Wright Field.)

2. KHchemann, Dietrich, and Weber, Johanna: Uber die Str5mung an
ringfbrmigen Verkleidungen. VII. Mitteilung. Forschungsbericht
Nr. 1236/7, 1942. (Available as ATI 27053, Air Materiel Command.)

3. Kichemann, Dietrich, and Weber, Johanna: Uber die Str'mung an
ringf'6rmigen Verkleidungen. III. Mitteilung. Forschungsbericht
Nr. 1236/3, 1941. (Available as Tech. Intelligence Trans.
F-TS-683, AAF, Air Materiel Command, Wright Field.)

4. Kichemann, D.: "Tber die Strmmung an ringformigen Verkleidungen
endlicher Dicke. I. Mitteilung. Forschungsbericht Nr. 1236, 1940.
(Available as NACA TM 1325.)







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