The results of the velocity measurements in the boundary layer described in NACA-TM 411 are here discussed in greater detail. The measurements made were of the velocity distribution in the vicinity of an airfoil model fitted with a rotary cylinder and were undertaken for the purpose of obtaining a closer insight into the phenomena observed in experimenting with this model

Koning, C

Wolff, E B

English

NASA Technical Reports Server

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‘“ TECHNICAL lJFJJO.RAN.DUMSl!.
NAT IONAL ADVISCRY COMMITTEE FOR AERONAUTICS
No. 424
DISCUSSION OF THE .FOZSULTS OF THE
OF AN AIRFOIL FITTED WITH A
\
BCXJ1iDARY-L~YERTESTS
ROTARY CYLlNDER
By E. B. Wolff and C. Koniilg
Report A 130
of the Rijks-Studiedienst voor de Luchtvaart, “ Amsterdam
“ December 25,“De Zngenieur,Reprint frow. 1926
.,, ...;,
,,’,
Washington
AuOgust, 1927
.
https://ntrs.nasa.gov/search.jsp?R=19930090790 2020-06-17T03:54:57+00:00Z
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NAfiOiTAL ADVISORY COMMITTEE
TECHNICAL MEMORANDUM
.,, .. ,.
DISCUSSION OF THE RESULTS OF THE
FOR-AERONAUTICS. .
NO. 424.
BOUNDARY-LAYER TESTS
OF AllAIRFOIL FITTED WITH A ROTARY CYLINDER.*
By E. B. Wolff and C. Koning.
The results of the velocity measurements in the boundary ,
layer described in Report A 129** are here discussed ii~greater
detail, ai~da program is developed for fl~rther research.
The measurements made by Van der Hegge Zijnen of the ve-
locity distribution in the vicinity of an airfoil model fitted
with a rotary cylinder were undertaken for the purpose of ob-
taining a closer insight into the phenomena observed in experi–
menting with this model. In Report A 129 the results of these
experiments were briefly discussed from this standpoint. It
is desirable, however, to consider some aspects in greater de–
tail, especially because some of the results have a wider ap-
plication aside from the special problem here investigated, in
that they @so explain the phenomena which occur in the flow’
around an ordinary airfoil in immediate proximity to the sur-
face and therefore in the boundary layer.
*1’Beschouwingen naar aanleiding van de grenslaagmetingen aan het
model met draaiende rol~” From Report A 130 of the !’Rijks-Stud–
iedienst voor de Ltichtvaart,’rAmsterdam. (Reprint from “L?e In-
genieur, “ ~CC= 25, 1926=)
**For the translation of Report A 129, see N.A.C.A. Technical
Memorandum No. 411: Determining the Velocity Distribution in
the Boundary Layer of an Airfoil Fitted with a Rotary Cylinder.
N.A.C.A. Technical Memorandum No. 424 2
-..
With the establishment, by weans of the boundaty~~ayer
. ...
measur&nents, of the previously assumed explanation of the ef-
fect of the rotary cylinder in the wing, the preliminary in-
vestigation of this p~oble~ may, in a certain sense, be con-
sidered closed: The time has now come to develop.a program
for the eventual continuation of the tests. For a more syste-
matic investigation, a division of the problem seer,sdesirable.
Flow Around an Ordinary Airfoil
Of the groups of measurements with the cylinder at rest
(B and C), only the latter group will here be considered, be-
cause in group B the slot or gap between the cylinder and the
after portion was not closed a-ridproduced so great a distu~b-
a.nce in the flow, that the model in this condition could hardly
be re~rded as an ‘ordinary airfoil.” In group C, on
trary, .the gap was carefully closed, thus producing a
surface.
The results of the tests (Figs. 3 and 4) confirm
the con-
smooth
the CUS-
tomary assumption in the boundary-layer theory that the flow
around an obstacle can be divided into two overlapping regions.*
The first rpgion, where the viscosity or inter-nalfriction pre-
vails, embraces only a thin boundary layer. Here the relative
velocity falls off rapidly and becomes zero at t’hewall. The
*For the literature on this point, see footnote 4 in “Rapport
A I05° on p. 1$31of “De Ingenieur” for March 6, 1926 (or N.A.C.A-
Technical Memorandw No. 354, p. 11).
N.A.C.A. Technical Memorandurt hro.424 3
second region embraces the rest of the field of flow,”in which
there is generally a small loss in velocity. At the foreiiost
mbasuring.points (I - V) the separation of the two regions is
very sharp, the thickness of the boundary layer being here .
about 1-1.5 mm (0.04 to 0.06 id. Farther aft, the separation
becomes less distinct, while the region in which the veloci-
ties are small (i.e., the boundary layer) becomes thicke~. At
the point VI its thickness is about 5 mm (0.2 in.); at VII,
abotit 12.5 mm (0.5 in.). Probably a separation of the.flow oc–
curs oilthe aftermost portion of the surfacej even at the #mall
angle of attack at which the measurements are made, whereby a
turbulent region with a small mean velocity is developed, which
increases in thickness toward the rear. The possibility of
this phenomenon has already been demonstrated by Betz in his
explanation of the difference between the actual and the theo-
retical circulation around an airfoil.*
‘~he same phenomenon occurs at a large angle of attack, but
.
the separation point is then so far forward that the turbulent
region is very great and the whole charactez of the flow is
,:
changed.
*Betz “Unters~chung einer Schukowskyschen Tragflache.” From
“Zeit&chrift fur Flugtechnik und Motorluftschiffahrt, “ 1915,
P“ 173.
I ——
1,’,
N.A. C.A. Technical Menorazxlum No. 424 4
Effect of
Although in the tests
the Rotary Cylinder
m~th the rotating cylinder (A),
the gap between the cylinder and the ~Lfterpiece was open, the
general character of the fl.o~?still conformed cldsely to that
in the case of the model with the cylinder at rest anclthe @p
closed (C) (Figs. 1
that the cylinder, as
can offset the effect
and 4). This confi~,s the conjecture
regards the general character of the flow,
of the gap or at least greatly reduce it.
The gap does not prevent Ideal disturbances, however, which
here have no effect at the small angle of attack, but may have
det~lment~l consequences at la?ger angles of attack.
On comparing the velocity distribution in cases A and C
(Fig. 4) , it appears that the !C;rliilderconsiderably increases
the velocity in the boundary layer. At the foremost measuring
points (111 - V) the velocity increase is only local and is
confined mostly to the portion of the flow lying withip.2.5 mm
(0.1 in.) of the su~face. These differences increase toward the
rear. Behind point V t’nerp is developed a slight difference
in the opposite direction in th-ezegion between ij = 2.5 mm
(.1 in.) and ij = 10 mm (0.4 in.), which has not yet been
explair.ed.
The neasuring points VI and VII give the impression that
the original character of the boundary-layer flow is maintained
farther back; that the separation of the flow here occurs later;
—
?—.
-— ——. . ..
j
5
and that the turbulent region is smaller. The thickness of the
disturbe@ region is about 3 mm (0.12 in.)at the p“”o-imtVI, but
can be estimated at about 10 mm (0.4 in.) at point VII, where
the line of separation is v“ery indefiilite.
This is completely confirmed by the explanation of the ac-
tion of the cylinder in Report A 105. The momentum of the air
in the boundary layer is increased by the cylinder. This in-
crease is noticeable along the entire surface and opposes the
separation of the flow at the rear. In cases where the separa–
tion occurs so far forvard that a considerable turbulerit region
is developed and the whole flow is thus disturbed (at large
angles of attack), the cylinder reduces the disturbance and
thus increases the lift ~hile diminishing the drag.
At the points II and III, the curves exhibit a yet unex-
plained phenomenon. Since these points lie on the cylinder, it
is to be expected that, for y = O, the velocity of the air
will be the same as that of the cylinder, 18.6m (61 ft.) per
second, just as in the remaining points the velocity for y=()
seems to be near zero. This is not the case, however, above
y = 1 mm (0.04 in.) when t’nevelocity diminishes, which at
this point is 9.9 m (32.5 ft.) or, 10.4 m (34.1 ft.) per sec.,
although less rapidly than in case C (cylinder at rest). At
the point VIII (Fig- 6), on the contraryj which is located on
the lower side of the cylinder, the velocity increases with
rotating cylinder on approaching surface.
I7~j/!:
,’ N.A. C.A. Technical Memorandum No. 424
The results obtained at the points
lower surface make it verj probable that
6
VIII and IX on the
a contrary flbw is de-
.—.—
veloped here in the vicinity of the model, in that the air is
taken along with the cylii~der in a direction opposite to the
general direction (Fig. 6). In determining the velocity dis-
tribution at point VII~, the hot–wire anemometer gives the ve-
locity, but not” the.direction of the flow. The portion of the
curve between y = O hnd y = 2.5 mm (0.1 in.) must therefo~e
be here given the negative sign, to indicate the
the flow. There is therefore a turbulent region
direction of
between the
two regions of
gion gives the
ing account of
opposite flow. Since the
absolute magnitude of the
the direction, no turning
anemometer in this re-
velocity, without tak-
point can here be
quickly indicated. At point IX, which is located on the fixed
part behind the cylinder, no contrary flow is noticeable, but
there is a strongly retarded layer of about 10 wm (0.4 in.) in
thickness.
Effect of Gap with Cylinder at Rest
The results of group B (Figs. 2 and 5), on comparison
with group C, clearly show the considerable effect of t
when the cylinder is at”rest. Only a,tthe point III in
le gap
front of
the gap is there any retardation noticeable. On the con~rary,
..
at both the following points, IV and V, the velocity for
ij c 3 mm (0.12 in.) is considerably diminished, and at point
.
/!~, .
J/
JI,,’ ,
,’
~ N.A.C-A. Technical Memorandum No. 424
IV it is ccmbined with
outer region. It thus
away from the surface,
some increase in the
seems as if the flow
pfobably by’a S1ight
by the sharp edge. The points VI and VII
7
velocity in the
mere here foroed
turbulence developed
show what effect
this disturbance has on fhe flow dver the remaining p6rtion of
the surface. At both points the disturbed regior.is considera-
bly greater than for the model without gq. .(C). For the case
B the.thickness at the points VI “and VII can be estimated at
10 and 20 mm
on the other
spectively.
(0.4 and 0.8 in.), respectively, while for case C,
hand, it is 5 and 12.5 mm (0.2 and 0.5 in.), re
These results confirm the conclusion reached in previous
experiments ,* that a small and seemingly tin~mportam.tirregular-
ity on the surface of an airfoil, which causes a local disturb-
ance of the flow, may have a
ably affect the whole course
great indirect effect and consider-
of the flow.
9
*See Report A 51, “Experiments with a Device for Shortening the
Glide and Landing Run’of an Airplane” (N.AGC.A- Technical Merflo-
ra.ndumNo. 272) . Report A 29, “InvestiO~tion of the Effect on.
the Aerod~amic Properties of Cutting Away Part of the Leading
Edge of a Fokker FIII,Wingll (N.A.C.A* Technical Memorandum NO.
103) . From lfVerslagen en Ve~handelingen van den Ri”ks-Studied-
ienst voor de Luchtvaart,f’ Amsterdam. $Part 11, 192 .
Iq
,,,,,,,,,,,,,,,,,.,,,,,.,,,,,.,.,,,,,,, ,,,,,,,,,,,,,,, , , ,,,,-,,,,,,,,, , ,,..-,,.,.,,,,,.,..,, ,.- .-— .,.
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~
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N.A.C.A. Technical Memorandu~ No. 424 8
Program for Furthex Research
.The,theoretical, side of this problem ha~ been considerably
.=.
advanced by the explana~ion of the effect of the rotating cyl-
ind~t in the,airfoil; which
,.
~~as dete~ined through boundary-
layer measurements. The making of further boundary-layer meas-
,’
urements on the existing or on a sinilaT model, e.g. , at other
angles of attack or other ratios between the peripheral veloc–
ity of the cylinder and the ve~ocity of the wind, is at present
of very little value, since the results would probably be very
similar. These would repay the trouble of the very tedious ex-
periments, only if a correspondingly quantitative treatment
were possib’le. The investigation method hitherto followed, in
which the combination of airfoil and cylinder is treated as a
unit, makes the problem too complex, hoveverj for such a treat-
ment. It is therefore desirable to divide the investigation
in sucha my that each part offers the possibility of quanti-
tative results and of drawing conclusions which can be applied
to the combination of airfoil and cylinder. Lastly, an exper-
imental ,confirmation of the correctness of this application is
necessary. Such a division also has the advantage that differ–
ent parts of
interest for
the investigation can give results, which may be of
more general problms.
,,
1$KWI
)
N.A.C.A. Technical Memorandum No. 424 9
The following is a possible program embodying these ideas:
a,) Boundary layer of an ordinary airfoil, i.e., fiithout
..... .,.!.-
cylinder;
,’.
b) Separate cylinder;
c) Boundary laydr of a fixed body behind the cylindeyt
1. Transition of the accelerated %Gund=.ry layer to
the fixed body;
2. Behavior of the accelerated boundary layer far-
ther along this body.
d) Iiodel withr~tary cylinder, designed on the basis of
the results of a - c.
Since the purpose is to influence the flow ir.the boundary
layer of the airfo~l with the aid of the rotary cylinder; the
first question regards what ha,ppenswith an ordinary airfoil
(point a). An impression of this is obtained from the ~fleasure-
ments already made, b~t the available data are far from suffic-
ient for obtair.ing a complete picture of the phenomena produced.
Moreover, the airfoil here used is not suitable for comparing
an actual flow witklthe one obtained by theoretical calculations,
since these calculations are very complicated for “an arbitrary
airfoil section. The investigation should therefore be carried
out with an airfoil suitable for these calculations, whereby
the flow at different angles of attack must be investigated :_?,nd
special attention given to the circumstances under which the
separation of the flow occurs at large angles of attack.
,,
N.A.C.A. Technical ile~.ormndunNo. 424 10
Another point of great importance is the question as to
how the cylinder affects the surrounding flow, and what factors
m ,.
,—–’ ..-
are here impo’rta”nt~’‘“such as the degree of rouglmess, the ratio
of
of
be
in
the peripheral velocity to the wind velocity and the value
Reynolds Number (point ”’c). ~ereby an explanation must also
sought for the phenomena mentioned in point c, as now observed
the neighborhood of the rotating cylinder.
After the cylinder has accelerated the air in Its vicinity,
this air must be brought over the fixed part behind the.cylin-
der with as little loss in velocity as possible (point
this transition, the <ap between the two parts is very
tant, especially as regards its width, since a portion
c“:) . In
imp or-
of the
boundary layer passes through it unused. Attention must also
be given the shape of and any irreg~larities on the edge of the
rear portion. The part placed behind the cylinder may be given
as simple a shape as possible, e.g., it may be a flat plate.
For determining tb..effect of the accelerated boundary
layer on the fixed rear part, it is desirable to know how *such
a layer generally behaves, when flowi~.galong a stationary wall
(point c2). Hence a flat surface can be chosen for the e~eri-
ments, in order to make them as simple a.spossible.
The last point of the program necessitates the canbining of
the results and their adaptation to the combined airfoil and
cylinder.
Such a division of the investi~tion requires a comprehen-
,..-... ....,. .. .,.,..-,..-.-,,,.----,_.,,
N.A.C.A. Technical Memorandum No. 424 11
sive program, if the experiments are not to be made at random.
It remains an op,enquestion, however, as to how far the given
.
program can be carried out with the limited a%ilable time and
means.
(a) iS to ‘OChandledthrough the kindly coop-The first .point
eration-of Professor Burgers, since, on the one hand, a suitable
model is available and, on the other hand, this part of the in-
vestigation promises important results of more general interest.
The model, which had already been designed by the R-SOL- (’~Rijks-
Studiedienst voor de LuchtvaartfI), was made for the purpose of
more thoroughly investi~ting the flow around an airfoil by de–
termining the pressure and velocity distribution in the boundary
layer. The cross section or profile of this airfoil was so
chosen as to enable the calculation of the theoretical flow and
its comparison with the actual flow. This investigation is
deemed very important, because the boundary-layer phenomena appar-
ently have a momentous effect on the principal properties of an
airfoil, such as the profile or wing-section drag, the influence
of Reynolds Number and all the phenomena connected with the crit-
ical angle of attack. For lack of time, this investigation can
not now be carried out by
again becme prominent as
gram, it will now be made
tory of Professor Burgers
the ROSCLO Since, however, it ~s
the basis of the above-mentioned pro-
by Van der Hegge Zijnen in the labora-
at Delft in cooperation with the R.S=LO
Translation by Dwight iti.Miner,
National Advisory Committee for Aeronautics.
.Vi Velocity
ij, Distance from surface
II-WI, Kaasuring points
~ (See section 3)
!a
L
V, Velocity h
ij, Distance from surface ~
IU-WI, Measuring points
L
.
(See section 3) g
II / “1P I I i I b’Y\l w
I
V. J.)
d- VI.
ml
o 2 4
c1
@
P
.-
V,ill/S V,ill/S . -P
w
Velocity distribution with cylinder Velocity distribution with
Fig,l, rotating (A)* Fig.2 cylinder at rest (B). “m
(Reproduction of Figs,3 & 4 of T.M.411)’ .
JN.A.C.A. Technical
9-...
,,
1.0
0.8
0.6
ij,cn
0.4’
0.2
0
.
Memorandum
,,,.
NO.424
.—
—
0 2 ~ 6 8
v,m/s
TJ, v~lociljy
ij, Distance from surfe.ce
IIT-WI,Iieasuring points
(See section 3)
Velocity distribution with cylindsr
Fig.3 gap closed”(C).
(Reproduction
>~.,..,... -- .,. ..-,-,’ f
of Fig.5 of T.M.411)
,,.! ,,
at
10
rest anti
.
i ,.
1I
,,j
;
5,0
4.5
4.0
3.5
3.0
‘j’cy’~’2.5
,2.0
1.5
1.0
0.5
0
(Reproducti
V, Velocity ij, Dist~ilcefrom purfs,co
U-WI, Measuring points (Sea section 3)
A,--- – c,------:--
The observations are indicated by cross marks
.
V,m/s
~f T.Mo&H)
Comparison of the velocity distri-uutionoilt;n surf~t.c::.of
the model with cylinder”rctztin~ (A) and th?ti~~;ithcylil~?,~r
L
Foig.4
at rest and gap closed (C).
I
— —.
.;
1
N.A.C.A. Technical
,,.
Memorandum No.424
~, lJ~locj.ty
ij, ,,3+st2mce from surfr.ce
III-WI,Keasuring pnints
(See section 3)
B,
c, ---------
The observ?.t.ionsare indicated
Fig.5
5.0
‘1.5
3.5
3.0
ij,cm
2.5
2.0
1.5
1.0
0.5
n
Coiuparison of the vslocity distribution on the
Fi ‘.5
5
surface of the model with cylindsr at r3st(E)
an that with cyliild:rat rest and ~ap closed(C).
(Reproduction of Fig.7 of T.M.411)
,,, ... ,.- . ..— -....-— —
N.A.C.A. TeblnnicalMemorandum NO.424
V, Velocity
ij, Distance fr’om
surface
VII-x Measuring
points -
~ (~_e section 3)
B;-----.---– ‘
The ohscrvations
are indicated by
cros~ marks
Fig. 6 Co!flparison
of t-hevelocity
distributioil on the
surface of the model
;l~ithcylind[er
rotating (A) and
with cylinder at
TeFt (!3)
5.0
‘: . 5
-+
2.5
*
2.0
1.5
1.0
0.5
Fig.6
..
..,-
?znIIx
(Reprod~~ction of Fig.S of T .M.411)
,,.


Discussion of the results of the boundary-layer tests of an airfoil fitted with a rotary cylinder

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