A recently proposed method of flow visualization was investigated at the National Aeronautics and Space Administration's Langley Research Center. This method of flow visualization is particularly applicable to the study of lift-induced wing tip vortices through which it is possible to record the entire life span of the vortex. To accomplish this, a vertical screen of smoke was produced perpendicular to the flight path and allowed to become stationary. A model was then driven through the screen of smoke producing the circular vortex motion made visible as the smoke was induced along the path taken by the flow and was recorded by highspeed motion pictures

Jordan, F. L., Jr.

Patterson, J. C., Jr.

English

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TECHNICAL	 NASA TM X- 72769
MEMORANDUM
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A STATIC-AIR FLOW VISUALIZATION METHOD
^-	 TO OBTAIN A TIIm E HISTORY OF THE
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v+	 LIFT-INDUCED VORTEX AND CIRCULATION42
By James C. Patterson, Jr. and Frank L. Jordan, Jr.
( y ASA-T m -X-72709)	 A FT!'TIC AIR FLOW
VIS"ALI2ATION MrTHrE TO CETAIN 4 TIMF
H1S'TCPY OF T HE LI FT-IKD"CFD VC°TFX AN[
CIACUTATICN ( y ASA)	 11 p HC $3.25	 CSCL )1A
N75-32025
Unclas
G3/02 41130
This documentation medium is used to provide accelerated or
special release of technical Information to selected users. The contents
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NATIONAL AERONAUTICS AND SPACE ADMINISTPATION
LANGLEY RESEARCH CENTER, HAMPTON, VIRGINIA 13US
}s The National. Techniul Information Service, Springfield, Virg
T't 'Available from (j
1 !	 ISTIF/NASA Sclentifk and Technical. Information Facility, Pd
1
1, Report No. 2. Oawnnrnt Aocseeion No 3, Recipient's catalog No,
NASA TM X-72769
/. Title and Subtitle S. R"on oau
A Static-Air Flow Visualization Method to Cbtaic. September 1975
a Time History of the Lift-Induced Vortex and e, hdotmWq organization Coda
Circulation
7, Auttwa) g. 110101	 kq Organization Report No,
James C. Patterson, Jr, and Frank L. Jordan, Jr.
10, Work Unit No,
545-C6-22-02g. Performing OrWinikln Name end Addrw
NASA Langley Research Center
Hampton, VA	 23665
11, Contact or Grant No,
17. Type of Report and Period Covered
Technical Memorandum12 Sponsoring Agency Nar a and AddressNational Aeronautics and Space Administration
14. Sponsoring Agency Cade
1
Washington, DC	 20546
15, Supplementary Notes
16, Abstract
An investigation of a recently proposed method of flow visualization
has been conducted at the National Aeronautics and Space Administration's
Langley Research Center.	 This method of flow visualization is particularly
applicable to the study of lift-induced wing tip vortices through which it
is possible to record the entire life span of the vortex. 	 To accomplish
this, a vertical screen of smoke is produced perpendicular to the flight	 j
path and allowed to become stationary.
	
A model is then driven through the
screen of smoke producing the circular vortex motion made visible as the 	 1
smoke is induced along the path taken by the flow and is recorded by high- 	 I
speed motion pictures.
17. Key Words (Suppwtad by Autitor4l)
	
(STAR catagary, underlined) 10. Distribution Seterrent 	 I
Flow' Visualization !I
Lift-Induced vortex Flow Unclassified - Unclassified
Vortex Attenuation
19. Swurlw Caw(f. (of this report) 20. Scully classic lol thh POP) 21. No, of Papas 22 Price'
Unclassified Unclassified 11 $3.25
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7A STATIC-AIR FLOW VISUALIZATION METHOD
TO OBTAIN A TIME HISTORY OF THE
LIFT-INDUCED VORTEX AND 01MLATION
By James C. Patterson, Jr. and Frank L. Jordan, Jr.
Langley Research Center
SUMMARY
An investigation of a recently proposed method of flow visualization has
been conducted at the National Aeronautics and Space Administration's Langley
Research Center. This method of flow visualization is particularly applicable
to the study of lift-induced wing tip vortices through which it is possible to
record the entire life span of the vortex. To ,iccomplish this, a vertical
screen of smoke is produced perpendicular to the flight path and allowed to
become stationazy. A model is then driven through the screen of smoke pro-
ducing the circular vortex motion made visible as the smoke is induced along
the path taken by the flow and is recorded by high-speed motion pictures.
As a result of a more precise examination of the motion of the wing tip
vortex afforded by this flow visualization method, it is suggested that by in-
troducing a positive pressure gradient downstream of the wing tip it may be
possible to cause the vortex to break down. This has been tested by employing
a parachute attached to the wing tip and located approximately one chord length
behind the wing trailing edge. The visual data obtained of the vortex resulting
froin the parachute configuration indicates that the vortex is virtually elimi-
nated in this manner.
INTRODUCTION
There are a number of ways available today that may be used to make the
flow in a test facility visible as it moves over various shaped objects such
I	 I	 I	 I	 I	 I
s
as spheres, cylinders, airfoil sections, or any object designed to move at
such a speed that air resistance is a concern. This has been accomplished by
introducing smoke or other light materials into the air stream (dye in the
case of water tunnel) upstream of the model and observing the path traced by
the visible material as it is carried by the flow over the model. This is
very informative of the various flow phenomena such as flow separation, regions
of stagnant flow, reverse flows, and the effect of mutual interference between
bodies and has been a tremendous aid to the aerodynamicist in his research.
When employing the established flow visualization methods for the lift-
inc°,uced wing tip vortex studies in the wind tunnel one observes only an in-
stant in the life of the wing tip vortex and may even form the false impression
that the vortex is being emitted from the wing. The air molecules affected by
the wing tip in actual flight arE set in a circular motion as the wing passes
with little or no longitudinal motion with the exception of the air molecules
in the vorter core itself. The air molecules are affected in a manner similar
to water molecules in the wave action of the sea where there is vertical motion
of 'lie  molecules with no horizontal motion. This'mcvement of air molecules
only in the vertical plane is quite apparent in actual flight tests where the
airplane is flown upwind of a tower supporting a smoke bomb. The wing tip
vortex produced by the aircraft drifts through the smoke produced by the smoke
bomb and is made visible.
Based on observations made in the wind tunnel and in flight tests, a new
flow visualization method has been suggested such that the true motion of the
vortex may be readily obserred and recorded under controlled conditions from
the moment the vortex is created throughout its life span to the point that it
dissipater. To accomplish this, a vertical screen of smoke is produced
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perpendicular to theflight path and allowed to become stationary. A model is
then passed through the smoke screen producing the circular vortex motion
which may be recorded throughout its entire life span by high-speed motion
picture cameras. The results in this paper were originally reported in a 1971
Langley working paper. These results are also included in reference 1.
APPARATUS AND PROCEDURES
Test Facility
This investigation was conducted in the Langley Towing Tank Facility
using the 54.86 meter monorail catapult system of this facility. This catapult
is normally used to launch model aircraft into the tank to study seaplane land-
ing performance or the water dite;r3ng characteristics of land based aircraft.
The catapult is composed of a dolly propelled by e, cable along a single rail
at speeds up to 33.52 meters per second. The rail is located approximately
1.22 meters above the water level in the 5.49 meter wide tank.
Model
A dimensioned drawing of the semispan model wing is shown in figure 1.
The wing panel was built-up of wood using an NACA 64 1-AO12 airfoil section and
covered with aircraft fabric in an attempt to keep the model as light as
possible. A reflection plane was installed at the root of the wing as part
of the semispan configuration. The span of th 6. wing was 0.92 meters Giving an
aspect ratio of three for the complete wing. The dimensions and position of
the parachute is a mean size of the parachutes tested. Nylon cloth and light
chord were used to construct these parachutes. The end plate was constructed
of plywood 0,64 centimeters thick. The leading edge was rounded while the
trailing edge was tapered. The end plate was designed to form two vortices,
3
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one from each trailing tip, and to be variable such that the mutual inter-
ference of the two vortices could be maximized by increasing or decreasing
the tip separation. The distance between the end plate tips was fixed for
this test.
Smoke Generation
A commercial hand-held fogging gun was used to produce the screen of
smoke required by the method of flow visualization. Kerosene was supplied to
the smoke generator under a pressure of 103.421 kN/m2 where it was heated and
sprayed from a nozzle forming the required screen of smoke.
Measurements
i	 Motion pictures and still pictures were taken of the vortex formation as
l
}	 the wing panel moved through the screen of smoke. The cameras were located
f	 approximately 12 meters upstream from the smoke screen position. The motion1.
Ii picture camera was operated by 64 frames per second or approximately two- and
two-thirds times the normal frame speeds while the still photographs were
taken at 1/10 second intervals.
DISCUSSION OF RESULTS
Vortex Visualization
An investigation has been conducted to establish the merits of a new
static-air flow visualization method. A series of still photographs taken of
the vortex flow patterns induced by the lifting wing panel are shown in
figures 2, 3 and 4. Although the individual photographs in each figure are
small, upon close examination of each picture it is possible to obtain quanti-
tative as well as qualitative information about the vortex flow.
The first photograph in figure 2 shows the model as it had ,just passed
through the smoke screen causing the vortex motion shown in the following
4
,J	 1	 _I	 I	 I	 I	
,,_
views.	 The wing panel seen on the left side of this photograph is moving
toward the viewer at approximately 17 meters per second at a lift coefficient
of approximately 0.80. 	 A reflection of the smoke screen and vortex may be
seen in the water in the tank below the model.
	
The lift-induced vortex formed
i
rotates in a counter
-
clockwise direction indicated by the smoke entrained from
the right side of each picture toward the left by the circulatory field
t
around the vortex. 	 The vortex moves downward and inward toward the plane of
symmetry under the influence of the wing downwash.
	
It is possible through
this flow visualization method to measure these changes in vortex position
'i
with time as well as determine the growth rate of the vortex diameter in the
l
plane of the smoke.	 Some particles of smoke are induced upstream - along the
periphery of the vortex core resulting possibly from the lower pressure 1
associated with the more recently created vortex forward of the screen. 	 This
feature of the flow visualization system affords a three-dimensional view of
the aircraft wake revealing a time history of the vortex core diameter, the
meander of the tubular image of the vortex core, and the interaction of the
vortices where a multi-vortex system exists. 	 The.actual value of the induced;
rolling moment at a particular position along the flight path must be detec-
mined by a measuring device introduced into the core of the vortex itself su h
as a small instrumented wing panel. j
It was observed in the motio;l pictures during these te pts that there is
a definite acceleration to the vortex core motion shortly after the model had i
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passed.	 The reason for this apparent energy addition is uncertain but detailed
s^
observation such as this would be difficult if not impossible in other flow
visualization methods.
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1	 1	 1	 1	 1	 I.
Vortex Reduction
Tests were made with the end plate shown in figure 1 installed on the j:
wing tip.	 A comparison of the visual data (fig. 3) obtained for the end plate
li
configuration with that of the plain tip configuration indicate that the span f
of the vortex tends to be greater and there is a delay in the complete roll-up {
of the vortex caused by the end plate. 	 These effects reduce the effect of the
tip vortex on the downwash behind the wing panel itself, resulting in a reduc-
tion in induced drag usually associated with end plating but do not effect, j
the key problem of vortex persistence. 	 It may be conjectured that other tip
4
treatments such as blowing tips, baffles placed perpendicular to the stream,
and various end plate configurations will only effect the flow in the immedi-
ate region of the wing but will have only small effect on the development of
the vortex in the far-field. 	 Based on this assumption it was proposed that a
positive pressure gradient employed downstream of the wing, at approximately
the point that the wing tip vortex core starts to develop, might cause the
vortex to dissipate. 	 To test this possibility, the parachute shown in figure 1 {
g
was attached to the wing tip and propelled through the stationary smoke screen.
In comparing the visual data with and without the parachute, it was noted
there is a strong tip vortex formed for the plain wing panel configuration
while little or no vortex motion is observed with the parachute installed.
Quantitative data obtained in later tests have verified these visual results.
As a more practical method of forming an unfavorable pressure gradient,
tests were conducted with a metal disk probe-mounted downstream of the wing .y
tip.	 The disk frontal area was comparable to that of the parachute, including
;a
the effect of the porosity of the parachute, with similar vortex attenuating'
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results. 1
6
CONCLUDING REMARKS
	 ^. f
The results of an exploratory investigation to determine the possibility
of using a newly proposed static-air flow visualization method in the study of
lift-induced vortices indicate the following:
1. It is possible through utilization of this static-air flow
visualization method to observe under controlled conditions
t:<q entire life span of the lift-induced vortex from its
initial formation through its dissipation.
2. Through this particular method of flow visualization, the
effect of any wing tip treatment on the vortex development
in the region of the wing as well as its lasting effect on
the vortex development with time may be determined.
3. The visual results obtained for the various wing-tip con-
figurations tested during this investigation suggested that
a positive pressure gradient, produced by a tip mounted
parachute, would force the vortex to break down.
REFERENCE
1. Patterson, James C. % Jr.: Lift Induced Wing-Tip Vortex Attenuation.
AIAA Paper No. 74-38. Presented at the AIAA 12th Aerospace Sciences
Meeting, Washington,, D. C., January 30 - February 1, 1974.
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A static air flow visualization method to obtain a time history of the lift-induced vortex and circulation

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