For visualizing flow characteristics in wind tunnel tests, model surfaces are smeared with any common petroleum-base oils. These fluoresce under ultraviolet light and the flow patterns are readily visualized

Katzoff, S.

Loving, D. K.

English

NASA Technical Reports Server

April 1964
	 Brief 63-10311 
NASA TECH BRIEF 
This NASA Tech Brief is issued by the Technology Utilization Division to acquaint industry 
with the technical content of an innovation derived from the NASA space program. 
Oil-Smeared Models Aid Wind Tunnel Measurements 
The problem: Several methods have been used 
to aid in visualizing flow characteristics over the 
surfaces of test models in wind tunnels. The meth-
ods which have been commonly used have a num-
ber of deficiencies, however. One of these uses the 
ink flow technique, in which a colored liquid is 
injected into the surface flow through orifices in 
the surface of the model. For this method, the 
model must be specially built with a number of 
orifices precisely distributed over its surfaces and 
connected internally by means of tubing and reser-
voirs. Although this method provides for proper 
flow visualization under varying operating condi-
tions of the wind tunnel, it usually presents an 
extremely complicated construction problem. Other 
approaches use the sublimation method, in which 
the model surfaces are coated with a volatile solid 
that leaves a visible foglike trail conforming to 
the flow pattern, and the luminescent-lacquer 
method, in which the model surfaces are coated 
with a lacquer that becomes fluorescent as it dries 
in the wind tunnel. These two methods, however, 
can generally be used for only one operating con-
dition of the wind tunnel. For studying the flow 
under another operating condition, the wind tunnel 
must be shut down and the model recoated. For 
tests in a large high-speed wind tunnel, these 
methods are costly and time-consuming. 
The solution: The surfaces of the model to be 
tested are smeared with any of the common petro-
leum-base oils, which fluoresce under ultraviolet 
light. The flow patterns over the model illuminated 
with ultraviolet light in the wind tunnel can be 
seen as patches of varying brightness and orien-
tation caused by the variations in thickness of the
oil film. The shearing action of the boundary layer, 
regions of laminar and turbulent flow, regions of 
reversed flow, and regions of significant lateral 
flow are readily visualized. Other important infor-
mation is often apparent, such as the locations 
of shocks, pressure peaks, and appreciable lateral 
pressure gradients. 
The basic advantage of the method is that the 
coating of oil, even after it is thinned by wind 
shear during a long test, remains fluid enough to 
redistribute itself over the model when the operat-
ing conditions of the air flow are changed. It is 
thus not necessary to shut down the tunnel and 
apply a new coat of oil in order to study a new 
operating condition. Since only the oil film is 
rendered visible under the ultraviolet light, there 
is no background light in the wind tunnel to inter-
fere with the observation. 
How it's done: A typical, nonporous model needs 
no special preparation. Models using porous fill-
er material and models made of wood or with 
wooden additions should be sealed to prevent 
absorption of the oil and consequent disruption 
of the fluorescent patterns. Zinc chromate has 
been successfully used as a sealer. Flat-white lac-
quer also works well, and has the added advantage 
of enhancing the brightness of the oil fluorescence. 
No special technique is needed to apply the oil; 
it can be daubed or brushed on the surface, with-
out regard for even thickness. If it is desired to 
determine the direction of boundary-layer cross-
flow, it may help to apply the oil in small, discrete 
dabs which will, during the tests, stretch into 
lines which reveal the direction of surface shear. 
-	 (continued overleaf) 
This document was prepared under the sponsorship of the National	 this document, or that the use of any information, apparatus, method, 
Aeronautics and S pace Administration. Neither the United States Govern- 	 or process disclosed in this document may not infringe privately-owned 
ment, nor NASA, nor any person acting on behalf of NASA: A. Makes 	 rights; or B. Assumes any liabilities with respect to the use of, or for 
any warranty or representation, express or implied, with respect to the 	 damages resulting from the use of, any information, apparatus, method, 
accuracy, completeness, or usefulness of the information contained in	 or process disclosed in this document.
https://ntrs.nasa.gov/search.jsp?R=19630000049 2020-03-11T16:45:44+00:00Z
Notes: 
[. In general, selection of the proper hydrocarbon 
oil is not critical. Most of the common petro-
leum-base lubricating oils may be used without 
special preparation. The prime factor in selecting 
one, however, is viscosity, which determines the 
ease with which the oil forms patterns and the 
length of time it remains on the surface. For wind 
tunnel tests involving low airspeeds and low tem-
peratures, kerosene or other low-viscosity oils 
(of low vapor pressure) must be used. If these 
oils have low natural fluorescence, oil-soluble 
fluorescent dyes may be added to strengthen the 
fluorescence. Fluorescent dyes may be added 
also to the more viscous oils to increase the bright-
ness of the fluorescent patterns obtainable. 
2. The oil should, of course, be capable of wetting 
the model surfaces. If difficulty is encountered 
in this regard, a suitable wetting agent (e.g., par-
ticular organic acids) should be added to the oil.
3. Under repeated tests, the oil film may eventually 
become so thin that it can no longer redistribute 
itself properly and also becomes difficult to 
see under the ultraviolet illumination. Neverthe-
less, the oil stays on long enough to permit 
observing several different conditions before 
its mobility and visibility become too low for 
trustworthy observation. 
4. This method has been used with moderate suc-
cess for observing flow patterns over surfaces 
immersed in liquids, such as water, for example. 
5. A more detailed description of the method is 
available in NASA MEMO 3-17-59L. 
Patent status: NASA encourages commercial use 
of this innovation. No patent action is contemplated. 
Source: Donald K. Loving and Samuel Katzoff 
Langley Research Center (Langley-4) 
Brief 63-10311


Oil-smeared models aid wind tunnel measurements

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