Conventional wind tunnel flow visualization techniques which consist of surface flow methods, tracers, and optical methods are presented. Different surface flow methods are outlined: (1) liquid films (oil and fluorescent dye and UV lighting, renewable film via porous dispenser in model, volatile carrier fluid, cryogenic colored oil dots, oil film interferometry); (2) reactive surface treatment (reactive gas injection, reversible dye); (3) transition and heat transfer detectors (evaporation, sublimation, liquid crystals, phase change paints, IR thermography); and (4) tufts (fluorescent mini tufts, cryogenic suitability). Other methods are smoke wire techniques, vapor screens, and optical methods

Settles, G. S.

English

NASA Technical Reports Server

THE STATE OF THE ART OF CONVENTIONAL 
FLOW VISUALIZATION TECHNIQUES 
FOR WIND TUNNEL TESTING 
Gary S. Settles 
Princeton University 
Princeton, NJ 
https://ntrs.nasa.gov/search.jsp?R=19820024789 2020-03-21T06:14:33+00:00Z
CONVENTIONAL WIND TUNNEL 
'FLOW VISUALIZATION TECHNIQUES' 
1) SURFACE-FLOW METHODS 
2) TRACERS 
3) OPTICAL METHODS 
PHOTOGRAPHY,AND VIDEOGRAPHY 
SURFhCk-FLOW MEi'tiODS 
1) LIQUID FILMS 
. OIL + FLUORESCENT DYE, UV LIGHTING 
l RENEWABLE FILM VIA POROUS DISPENSER IN MODEL 
l VOLATILE CARRIER FLUID: DRY TRACE 
0 CRYOGENIC : LIQUID PROPANE + PIGMENT 
l COLORED OIL DOTS 
. OIL FILM INTERFEROMETRY YIELDS C, 
2) REACTIVE SURFACE TREATMENTS 
l REACTIVE GAS INJECTION 
l REVERSIBLE DYE 
3) TRANSITION & HEAT TRANSFER DETECTORS 
l EVAPORATION 
l SUBLIMATION 
l LIQUID CRYSTALS 
l PHASE-CHANGE PAINTS 
l IR THERMOGRAPHY 
4) TUFTS 
l FLUORESCENT MINI-TUFTS 
l CRYOGENIC SUITABILITY 
10 
KEROSENE-LAMPBLACK SURFACE STREAK TRACES 
,A mixture of kerosene and lampblack has been useful for 
many years in producing dry traces of surface flow patterns, 
which are not disturbed when the wind is stopped. Rather than 
photographing these patterns, a novel method of preserving 
them is to press 8"-wide "Scotch Magic Transparent Tape" onto 
the model and actually peel the pattern off. The result is a 
full-scale, highly-detailed trace from which quantitative 
measurements can be made of surface streak features and angles. 
This example is from a series of 3D shock/turbulent boundary 
layer experiments done by the author. 
Kerosene-Lampblack Surface Streak Trace 
for X = 60°; A = apex, C = corner, 
R = reattachment, S = separation. 
Source: Settles, .G. S. and Teng, H. Y., AIAA Paper 82-0229., 
Jan. 1982. 
11 
SUBLIMATION AND LIQUID CRYSTALS 
FOR TRANSITION DETECTION 
Liquid crystals are by far the most optically active 
substances available. They respond with a perceived color 
change to minute differences in temperature. Thus, as in 
this example, they have been proved useful for detecting the 
temperature change at transition. Liquid.crystals have some 
advantages over traditional sublimation techniques in that 
the transition pattern is displayed with color contrast and 
is able to indicate changes due to, say, a change in models 
attitude during a single wind tunnel run. 
Source: Klein, E. J., "Liquid Crystals in Aerodynamic 
Testing," Astronautics & Aeronautics, July 1968, 
PP. 70-73. 
12 
FLUORESCENT MINI-TUFTS 
Fluorescent mini-tufts are a recent advance in the state- 
of-the-art of tuft visualization for flow near a solid surface. 
Extremely thin fluorescent nylon monofilament is glued to the 
model surface in short lengths according to a regular grid 
pattern. Under UV lighting these mini-tufts can be photographed 
despite their small size. The results, as in this example, give 
a graphic indication of surface flow angles and separation. 
Further, the inventor of this technique (J. P.Crowder of Boeing) 
has shown by rigorous tests that the mini-tufts do not produce 
a significant disturbance to the flow. 
Source: Crowder, J. P., Astronautics & Aeronautics, Nov. 1980, 
PP. 54-56. 
13 
TRACERS 
1) SMOKE 
l SMOKE-WIRE TECHNIQUE 
l SMOKE FILAMENTS AT HIGH SPEEDS 
2) "VAPOR SCREEN" 
l GLOBAL SEEDING W/LASER LIGHT SHEET 
l LOCAL SEEDING W/LASER OR STROBE 
l CRYOGENIC: LIQUID HE YIELDS N, "FOG"? 
3) OTHER METHODS 
l FLUORESCENT AEROSOLS, PARTICLES, OR ATOMS 
l NEUTRALLY BOUYANT BUBBLES 
l SPARK TRACING 
l PROBE SURVEYS + GRAPHICS 
SMOKE-WIRE TECHNIQUE 
For low-speed flows the smoke-wire technique is a simple 
and,elegant means to produce high-quality smoke filaments for 
flow visualization. A fine resistive wire on the order of 
O.lmm diameter is coated with oil and heated by electrical 
current, whereupon the oil forms beads and ejects filaments 
of aerosol particles. Given proper lighting and a low level 
of free-stream turbulence, 
reveal, 
these filaments can dramatically 
for example, boundary layer transition on a wing. 
PROFILE ViEW 
PLANFORM VIEW 
Smoke-wire visualization for a = 8 deg and Re, =.55,000. 
Source: Batill, S. M. and Mueller, T. J., AIAA Journal, 
March 1981, pp. 340-345. 
15 
HIGH-SPEED SMOKE 
Recent efforts by various investigators - notably 
including the group at Notre Dame University - have extended 
the range of smoke visualization techniques to include tran- 
sonic and supersonic speeds. While extra precautions may be 
necessary in assuming a large contraction ratio and flow 
smoothing, the results are quite good. This photograph, for 
example, clearly shows the leading-edge vortices of a delta 
wing at angle of attack and Mach 0.9. It was obtained by 
introducing a single smoke filament along the flow centerline 
in the wind tunnel stilling chamber. 
- 
70 A 45 
Source: Stahl, W., "Zur Sichtbarmachung von Strijmungen urn 
schlanke Deltaflcgel bei hohen Unterschallgeswindig- 
keiten," DFVLR-AVA-70-A-45, Oct. 1970. 
16 
LOCALIZED VAPOR SCREEN 
The vapor screen method can be highly useful for inves- 
tigating complex flows and obtaining flowfield "cuts" which 
are impossible with standard optical methods. The usual vapor 
screen technique involves global flow seeding with an aerosol 
such as water fog. However, vapor screen results can also be 
had with localized seeding, as shown in this example. Here, 
a small quantity of acetone was injected through a pressure 
tap upstream of a 3D shock/turbulent boundary layer interac- 
tion. The acetone boiled and atomized, filling the incoming 
turbulent layer with fog. A streamwise light sheet was gen- 
erated by spreading the beam of a l-watt argon-ion laser with 
a cylindrical lens. The resulting vapor screen photo clearly 
shows the turbulent boundary layer lifting off the surface. 
(For more details of the technique, see AIAA Paper 82-0229 by 
Settles and Teng.) 
Source: G. S. Settles and F. Lu, Princeton University. 
17 
SPARK TRACING 
Profiles of flow velocities from incompressible to 
supersonic speeds can be obtained by the spark tracing 
method. Electrical pulses at up to 250kV and 100kHz 
generate periodically luminous spark plasma trails which 
are convected with the flow. This example shows the flow 
pattern over a transonic airfoil (spark frequency: 10kHz). 
The equipment is made by IMPULSPHYSIK GmbH, Hamburg FRG. 
Source: Sales literature, IMPULSPHYSIK GmbH. 
18 
OPTICAL METHODS 
11 SHADOWGRAPH 
l CONVENTIONAL PARALLEL BEAM 
l CONICAL SHADOWGRAPH 
l RETROREFLECTIVE MODEL SURFACE 
l SPECULAR MODEL.SURFACE 
l WIDE-FIELD MOIRE-GRID PROJECTION 
2) SCHLIEREN 
l CONVENTIONAL PARALLEL BEAM, KNIFE-EDGE 
l COLOR-CODING 
l MULTIPLE-SOURCE, SHARP-FOCUSING 
l STEREOSCOPIC 
l SIMPLE GRID BACKGROUND 
l SPECULAR MODEL SURFACE 
l QUALITATIVE vs, UJANTITATIVE EVALUATION 
l RELATED METHODS: PHASE CONTRAST, 
RESONANT REFRACTIVITY, LASER DEFLECTOMETRY, 
HOLOGRAPHY 
3) INTERFEROMETER 
19 
CONICAL SHADOWGRAPH 
A variation of the traditional parallel-beam shadowgraph 
can be used to investigate quasi-conical flowfields. The light 
source (in this case a spark generated at the end of a coaxial 
cable inserted into the flow) is positioned near the apex of 
the conical flow. The shadowgram is cast upon the tunnel wall 
downstream, where it may be photographed. 
CABLE 
/ 
TUNNEL FLOOR I 
TUNNELhOE WALL 
SE,=ARATlON INDUCED SHOCK 
Sketch of Experimental Arrangement for 
Conical Shadowgraph, Showing Shock 
Wave Shape for X = 40°. 
Conical Shadowgram 
Source: Settles, G. S. and Teng, H. Y., AIAA Paper 82-0229, 
Jan. 1982. 
20 
SCHLIEREN: COLOR-CODING 
A variety of modifications (usually simple) to existing 
schlieren equipment can convey useful flow visualization data 
by color-coding the schlieren image. This example shows a 
1970-vintage Shuttle Orbiter model in the NASA-Ames 6x6-foot 
supersonic tunnel. Here the color-coding serves to provide 
contrast between the flowfield and the model, and to combine 
both vertical (yellow-blue) and horizontal (red-green) knife- 
edge settings in a single photograph. Additionally, the color 
schlieren arrangement used here has sensitivity equal to that 
of black-and-white schlieren using the same optics. At least 
a dozen different color-coding schemes are available to high- 
light different types of information in the schlieren image. 
(See Settles, G. S., "Color Schlieren Optics - A Review of 
Techniques and Applications," in Flow Visualization 11,ed. W. 
Merzkirch, Hemisphere Pub. Corp., 1982, pp. 749-763. 
L-82-5674 
Source: G. S. Settles/NASA Ames Research Center. 
21 
. . _ .---.-- 
MULTIPLE-SOURCE SCHLIEREN 
While conventional schlieren methods generally use small 
single light sources, there are reasons to use multiple sources 
for some applications. An extended light source gives a sharp- 
focusing capability, allowing one to concentrate, say, on the 
centerline of a complex 3D flowfield. Perhaps even more impor- 
tant for transonic testing is the ability to configure a 
multiple-source schlieren system to look through slotted or 
perforated tunnel walls, as in this example. 
j. 
, 
BLUFF BODY (0.60 fNCH DIAMETER) AT M = f.30 ^ 
L SANOIA CORPORATfON 12 x 12 JNCH.TRANSONIG WIN0 TUNNEL 
Source: R. C. Maydew, Sandia Corp. 
22 
SIMPLE BACKGROUND-GRID SCHLIEREN 
Shock wave locations and shapes can often be found by the 
simple expedient of viewing through the wind tunnel test sec- 
tion toward a background grid. This technique lacks the sensi- 
tivity of more sophisticated optical methods, but requires no 
equipment and is adaptable to a wide range of viewing angles 
and to tunnels not originally designed for flow visualization. 
Here, an airfoil is tested at Mach 0.8 in the Boeing Model 
Transonic Wind Tunnel. A sheet of ordinary drafting grid was 
placed a few feet in back of the test section. Stroboscopic 
backlighting was used to visualize the shock, which appears 
clearly at about 60% chord. 
Source: G. S. Settles/Boeing Aerodynamic Laboratory, Boeing 
Commercial Airplane Company. 
23 
STEREOSCOPIC SCHLIEREN 
Various arrangements are possible for taking stereoscopic 
schlieren views of 3D flow phenomena. These methods overcome 
the inherent disadvantage of conventional schlieren visuali- 
zations: the integration of 3D flow information into a 2D 
image. The simplest stereo schlieren method involves taking 
stereo photos with the grid background described earlier. 
As shown in this sketch, the method can be used, for example, 
to visualize the 3D transonic shock pattern on the wing of a 
commercial air transport model. Such visualizations were 
carried out by the author and J. P. Crowder of Boeing's Aero- 
dynamic Laboratory during 1981 tests in the Boeing Transonic 
Wind Tunnel. 
Source: G. S. Settles/Boeing Aerodynamic Laboratory, Boeing 
Commercial Airplane Company. 
24 
SCHLIEREN AND SHADOWGRAPH USING 
REFLECTING MODEL SURFACES 
Test models appear only in silhouette in most optical 
flow visualizations. This is a disadvantage in observing 
complex 3D flows, where the model may obscure important parts 
of the flowfield. Specular model surfaces can sometimes be 
used to overcome this problem by allowing one to collect the 
reflected light and form an image of the normally-obscured 
flow. In this example a reflecting splitter-plate was used 
to visualize an inlet shock pattern that would otherwise have 
been obscured. Such techniques have the potential to visualize 
transition on polished wings by planform schlieren or shadow- 
wwhy . Further, in NTF the model surfaces may need to be 
specular for other reasons as well. 
tomprife rhadwgraph and scMirren d Lhellav. Spliltec plats I: high pylon: p = P: 6$ira, = 0.72. c-m-lm3 
Source: Moseley, G. W., et al., NASA TN D-3385, April 1966. -- 
25 
PHOTOGRAPHY AND VIDEOGRAPHY, 
1) LIGHT SOURCES 
l CONVENTIONAL, STROBOSCOPIC, 5, LASER 
2) CAMERAS 
l HIGH-SPEED: > 1 MHz 
l STANDARD VIDEO: 30 Hz 
l HIGH-SPEED VIDEO: 12 KHZ B&W, 200 Hz COLOR 
II 


The state of the art of conventional flow visualization techniques for wind tunnel testing

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