The visualization of laminar to turbulent boundary layer transition plays an important role in flight and wind tunnel aerodynamic testing of aircraft wing and body surfaces. Visualization can help provide a more complete understanding of both transition location as well as transition modes; without visualization, the transition process can be very difficult to understand. In the past, the most valuable transition visualization methods for fight applications included sublimating chemicals and oil flows. Each method has advantages and limitations. In particular, sublimating chemicals are impractical to use in subsonic applications much above 20,000 feet because of the greatly reduced rates of sublimation at lower temperatures (less than -4 degrees Fahrenheit). Both oil flow and sublimating chemicals have the disadvantage of providing only one good data point per flight. Thus, for many important flight conditions, transition visualization has not been readily available. This paper discusses a new method for visualizing transition in fight by the use of liquid crystals. The new method overcomes the limitations of past techniques, and provides transition visualization capability throughout almost the entire altitude and speed ranges of virtually all subsonic aircraft flight envelopes. The method also has wide applicability for supersonic transition visualization in flight and for general use in wind tunnel research over wide subsonic and supersonic speed ranges

Croom, Cynthia C.

Gall, Peter D.

Holmes, Bruce J.

Kelliher, Warren C.

Manuel, Gregory S.

English

NASA Technical Reports Server

NASA Technical Memorandum 87666 
A NEW METHOD FOR LAMINAR BOUNDARY LAYER 
TRANSITION VISUALIZATION IN FLIGHT - COLOR 
CHANGES IN LIQUID CRYSTAL COATINGS 
Bruce J. Holmes, Peter D. Gall, Cynthia C. Croom, 
Gregory S. Manuel, and Warren C. Kelliher 
January 1986 
NI\SI\ 
National Aeronautics and 
Space Administration 
Langley Research Center 
Hampton, Virginia 23665 
111111111111111111111111111111111111111111111 
NF01248 
NASA-TM-8766619860012047 
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https://ntrs.nasa.gov/search.jsp?R=19860012047 2020-03-20T15:01:57+00:00Z
NASA TM-87666 
A NEW METHOD FOR LAMINAR BOUNDARY LAYER TRANSITION VISUALIZATION IN FLIGHT 
- COLOR CHANGES IN LIQUID CRYSTAL COATINGS -
Bruce J. Holmes; Peter D. Gall; Cynthia C. Croom; Gregory S. Manuel; 
Warren C. Kelliher 
NASA Langley Research Center 
INTRODUCTION 
The vlsuallzatlon of laminar to turbulent boundary layer transltlon plays an 
important role ln fllght and wlnd-tunnel aerodynamic testlng of alrcraft wlng 
and body surfaces. Visualizatlon can help provlde a more complete 
understandlng of both transitlon locatlon as well as transition modes; without 
vlsuallzatlon, the transltion process can be very dIfflcult to understand. 
In the past, the most valuable transition visualization methods for fllght 
appllcatlons lncluded subllmatlng chemlcals and 011 flows. Each method has 
advantages and Ilmltations. In partlcular, subllmating chemicals are 
lmpractical to use ln subsonic appllcatlons much above 20,000 feet because of 
the greatly reduced rates of subllmatlon at lower temperatures (less than -4 
degrees Farenhelt). Both 011 flow and subllmatlng chemlcals have the 
dlsadvantage of provldlng only one good data pOlnt per fllght. Thus, for many 
lmportant fllght conditions, transltion vlsualizatlon has not been readily 
avallable. 
ThlS paper dlscusses a new method for vlsuallzlng transltlon ln fllght by the 
use of llqUld crystals. The new method overcomes the llmitatlons of past 
technlques, and provldes transltlon vlsuallzatlon capablllty throughout almost 
the entlre altltude and speed ranges of vlrtually all subsonlc alrcraft fllght 
envelopes. The method also has wlde appllcabllity for supersonlC transltlon 
vlsuallzatlon ln fllght and for general use ln wind tunnel research over WIde 
subsonlc and supersonlc speed ranges. 
LIQUID CRYSTALS 
Orlglnally dlscovered ln 1888, llqUld crystals have been applled ln 
aerodynamlc research only wlthin the past twenty years (refs. 1-6). Those 
prevlous efforts lnvolved use of llquid crystals for aerodynamlc 
lnvestlgations in wlnd tunnels for supersonic and hypersonic heat transfer and 
boundary-layer transltlon measurements. 
LlqUld crystals are a pecullar state of matter between SOlld and llquid, ln 
WhlCh the molecules are elongated in one dlrectlon. Although they appear as 
oily llquids, they have certaln mechanlcal propertles WhlCh are Slmllar to 
SOlld crystals. In particular, liquld crystals scatter light very 
selectlvely. The molecules possess a helical structure; fortuitously, the 
pltch length of the hellxes can occur at the same wavelengths as vlslble 
llght. Thus when subjected to certaln physlcal lnfluences, the hellx pitch 
length changes and the wavelength of reflected llght changes accord1ngly. In 
th1S fashion liqU1d crystals change colors 1n response to changes 1n shear 
stress, temperature, pressure, ferromagnetism, and 1n the presence of chemIca 
vapors. Slnce the fundamental chemical structure IS unaffected by these 
changes, a llqU1d crystal coating wIll respond repeatably to the same physlca 
changes. Thus, the color changes are reversable virtually 1ndefin1tely. 
LIquid crystals WIll also reflect other non-vlsable wavelengths In the 
electromagnet1c spectrum, such as X-ray and 1nfrared (ref. 7). Liquid 
crystals have been widely used for temperature measurements, and have the 
capability to resolve temperature by color changes to w1thln 0.18 degrees F 
(ref. 8). These mater1als have also been used 1n medical applicat1ons, as 
well as for electron1C displays (dig1tal watch displays being the most 
common). Other appl1cations include non-destructIve testing for flaws or 
fatlque In materIals, and malfunction detectIon In electrIcal clrcu1ts. In 
nature, lIqUId crystals are present as fatty acids 1n anImals and provide 
colors In some Insect wIngs and shells (ref. 9). 
USE IN AERODYNAMIC TESTING 
Previous wInd-tunnel work on the applications of lIqUId crystals 1n 
aerodynamIc testIng IS reported In references 4-6 and 10-14. Many of the pas 
appl1catlons, espec1ally those at hIgh speeds, utilIzed encapsulated lIqUId 
crystals to provIde coatIngs WhICh would respond to temperature and not to 
shear. For flIght appl1cat1ons, the liquid crystals must operate In aircraft 
skIn surface temperature ranges between near -30 degrees F and +300 degrees 
F. The lower surface temperatures occur at stratospher1c altitudes at 
compressIble subsonic speeds. For example, for flIght at Mach = 0.8 at 45,00 
feet pressure altItude, the total (I.e. skIn surface) temperature is about -3 
degrees F even though the statIc temperature is near -60 degrees F. At 
supersonIc speeds, where compressible heating of the skin surfaces IS even 
more signIficant, surface temperatures can approach +300 degrees F at Mach 
2.7 in the stratosphere. The temperature range over which known lIquid 
crystals operate is from -22 degrees F to +480 degrees F. Thus, 1n theory 
lIqUId crystals are avaIlable which wIll operate at VIrtually all altItudes 
and speeds of Interest for subsonic and supersonic flight applications. Two 
types of lIqUId crystals were used In the present flIght experiments, 
cholester1c and ch1ral nematIC. 
Although much IS known about lIqUId crystals as thermolndlcators, relatIvely 
Ilttle 1nformat1on eX1sts 1n the Ilterature about their use for quant1tat1ve 
shear-stress or boundary layer transItIon indIcators. PrevIous studIes of 
lIqUId crystals for quantItatIve shear stress measurements are described In 
references 5 and 6. For use in aerodynamIc flIght testing, unencapsulated 
liqUId crystal formulatIons were sought WhICh would provIde a color change 1n 
response to skin frIction d1fferences between the laminar and the turbulent 
boundary layers. LIqUId crystals can be formulated to change colors across 
the entire vls1ble spectrum, from red to violet, over a temperature range as 
small as 1.8 degrees F or as large as 90 degrees F (see ref. 8). For flIght 
applications it 1S usually deSIrable to use formulatIons wIth as wide a 
temperature band as possible, so that transItIon measurements may be made OVE 
a large range of alt1tudes or Mach numbers dur1ng one flight. The w1de 
temperture band formulations are chosen to m1nim1ze sens1tivity to 
temperatures. In order for a liquid crystal coatIng to Indicate lamInar 
2 
boundary layer transItIon, the operatIng temperature of the coatIng must be 
wIthIn the temperature range In which color IS present. The coating then 
develops dIfferent colors In the laminar and turbulent boundary layers or 
develops a color pattern at the transition locatIon, dependIng on the 
temperature band of the formulation used. OutsIde of the temperature band for 
the coatIng, no color may be present. SelectIon of the specIfIc liqUId 
crystal formulation for a gIven test condit1on requires "a pr1or1" knowledge 
of the range of the test surface temperatures expected. The best test surface 
IS an adIabatIc one; that IS, one WhIch IS well Insulated. If the test 
surface has substructure which acts as a heat sInk, these locatIons may be 
visIble in color changes or as colorless areas; a knowledge of the structures 
IS therefore important to avoId mIsInterpretatIon of coatIng patterns In such 
areas. With care, the liqUId crystals can also be used to Indicate the 
locatIons of such aerodynamIc phenomena as shock locatIons and lamInar 
separation bubbles. 
The tIme response of a liquId crystal coatIng can be better than 0.2 second 
(ref. 3). Dur1ng the recent flight test1ng, very rapid time response of the 
liqU1d crystal coat1ngs was observed by watching turbulent wedges emmanat1ng 
from art1f1c1al roughness (gr1t) 1nstalled near 11fting surface lead1ng 
edges. As the roughness passed between being subcrit1cal and subcritical, the 
result1ng turbulent wedge was observed to change from laminar to turbulent 
colors very rap1dly. 
APPLICATION TECHNIQUES 
Various appl1cat1on techn1ques are effective. The best visual and 
photograph1c results have been obtained by using the Ilqu1d crystal coat1ngs 
appl1ed over a flat black surface, but good trans1t1on patterns have been 
observed on other dark-colored backgrounds as well. The 11quid crystal 
mater1al can be applIed by hand, w1th a brush, or 1t can be sprayed on. It 1S 
1mportant that the coat1ng be un1form 1n thickness for max1mum clar1ty in 
trans1t10n pattern development. For brush1ng or spraying, th1nn1ng 1S 
accompl1shed uS1ng such solvents as 1,1,1 tr1chloroethane, acetone, or Freon 
(TMC). When uSIng compressed aIr spray equIpment, a SolutIon of one part 
liquId crystal to 8 parts solvent can be sprayed on uS1ng 25' psi aIr 
pressure. When applIed in this ratIo, one quart of SolutIon will cover about 
100 square feet of surface area. UndIluted lIqUId crystal can be applIed at a 
rate of about one mIllIlIter per square foot of surface area. Th1S coverage 
WIll provIde adequate thIckness for good color response; coatIngs WhICh are 
too thIn will not develop color responses to shear. CoatIngs which are too 
thICk will run under aerodynamIc loading, and the thIckness of the coating can 
affect the transItIon location. A sIngle coatIng can be used repeatedly from 
flIght to flIght for trans1t10n measurements. L1qu1d crystals are sens1tive 
to ultraviolet light, however, and after extended exposure (days or weeks) can 
cease to operate. For health and safety reasons, the spray applIcatIon 
procedure should only be conducted In a well-ventIlated area uSIng a half-face 
mask carbon-filter type respIrator. 
3 
RESULTS 
The photograph ln flgure 1 lS an example of transitlon vlsualization in fllgr 
using a chiral nematic llqUld crystal coatlng on a flat-black background. 
ThlS particular liqUld crystal formulatlon had a temperature response range 
beginning at -22 degrees F. The flight conditlon is Mach = 0.8 at an altituc 
of 48,000 feet. The coatlng lS on the rlght wlnglet of a Gates LearJet Mode] 
28/29 business transport airplane. Percent-chord locatlon markings are at tt 
lower edge of the black reglon, and transitlon lS lndlcated by a color changE 
near the 60-percent chord locatlon. Thus in the lamlnar flow region on the 
forward part of the alrfoll where the skln frlctlon lS very low, no color lS 
present; ln the aft turbulent flow region where the skin frlction is several 
times hlgher, colors appear in the liqUld crystal coatlng. The crisp Ilne oj 
demarcatlon between the two reglons lS where transitlon occurs. A wedge-
shaped turbulent reglon appears near the wlnglet tlP, caused by an lntentlon, 
roughness trlp. Excellent correlatlons have been observed between transltiol 
lndlcated by liquid crystals and transltlon lndicated by other methods, 
including sublimating chemlcals, oil flow and hot fllms. Results can be 
recorded elther photographlcally, or on video tape, but the quallty of the 
recorded lmage will depend on both the quantity of light avallable for 
lllumlnatlng the coatlng, and the angle at which the llght strikes the surfal 
under examinatlon. The best results were obtalned wlth lightlng emanatlng 
from behlnd the Vlewer and camera onto the coated surface ln front of the 
eye. The use of a polarlzing fllter on the camera lens provldes improved 
color rendltlon and minimum spectral reflectlons under many Ilghtlng 
conditlons. Even wlth less than optimum Ilghtlng angles for the camera, the 
human eye can stlll dlscern color changes assoclated wlth transltlon 10catlOJ 
CONCLUDING REMARKS 
Some recent exploratory NASA flight experlments have shown that liqUld 
crystals offer a Solutlon to the Ilmitatlons of prevlous transitlon 
vlsuallzatlon method for fllght appllcations. The advantages of the liqUld 
crystals include reverslbility, rapid tlme response, nontoxlcity, ease of 
application, and low cost. The prlmary dlsadvantages are that the data are 
real tlme only - that is some means of photographlc recording is necessary -
and that the coatings are sensltlve to temperature dlscontinulties ln the 
surface. Care must be taken to avold the effects of heat slnks ln aerodynam 
surfaces such as occur over fuel cells or major structural components such a 
spars. 
Use in thlS report of names of manufacturers does not constltute an offlclal 
endorsement of products or manufacturers, elther expressed or lmplled, by th 
National Aeronautlcs and Space Admlnistratlon. A partlal IlSt of candldate 
suppliers lS glven ln the Appendix; additional companles unknown to the 
authors may also supply the liquid crystal materlals. 
4 
REFERENCES 
1. Fergason, J. L.: "LiqU1d Crystals". SC1ent1f1c American, August, 1964. 
2. Fergason. J. L.: "Liquid Crystals - I, OptIcal PropertIes". LIqUId 
Crystals, Gordon and Breach, SC1ence Pub., 1966, pp 89-103. 
3. Fergason, J. L.; et.al.: "Cholesteric Structure - II, Chemical 
Signif1cance". LIqU1d Crystals, Gordon and Breach, Science PublIshers, 1966, 
pp 105-119. 
4. Klein, E. J.: ApplicatIon of LIquid Crystals to Boundary-Layer Flow 
VIsualIzation. AIAA Paper No. 68-376, 1968. 
5. Kle1n, E. J.; and Margozzl, A. P.: Exploratory InvestigatIon on the 
Measurement of Sk1n Fr1ct1on by Means of LiqUId Crystals. NASA TM X-1774, 
1969. 
6. Klein, E. J.; and Margozzi, A. P.: Apparatus for the CalibratIon of Shear 
SensitIve LIqU1d Crystals. The RevIew of ScientIfIc Instruments, Vol. 41, No. 
2, 1970, pp 238-239. 
7. Hugenschmidt, M.; and Vollrath, K.: Infrared Diagnos1s USIng LIquid 
Crystal Detectors. Inst1tut Franco - Allemand de Recherches, St. LOUIS, 
France, (AvaIlable NTIS 73N32564). 
8. DaVls, F.: "L1qUld Crystals, A New Tool for NDT". Research/Development, 
June, 1967. 
9. Cotter111, R: Strangeness 1n Proport1on: L1qU1d Crystals. The Cambr1dge 
GU1de to the Mater1al World, Cambr1dge Univers1ty Press, Cambr1dge, 1985, pp 
203-214. 
10. Lemberg, R.: L1quid Crystals for the V1sual1zat1on of Unsteady Boundary 
Layers. Paper presented at the Th1rd Can. Congo of Applied Mechan1cs, 
Calgary, May, 1971 (Available from NTIS as 77N81664). 
11. Rhee, H. S.: Koseff, J. R.: Street, R. L.: Flow Visual1zat1on of a 
Recirculating Flow by Rheoscopic Liquid and L1quid Crystal Techniques. 
ExperIments 1n Fluids, Vol. 2, No.2, 1984, pp 57-64. 
12. Scholer, H.; and Baner J 1, A.: "V Isual ization of Boundary Layer 
Transition on a Cone w1th L1quid Crystals. ICIASE, September 1983, pp 15-22. 
13. Zharkova, V. M.: The Application of Liquid Crystals 1n Experimental 
Aerodynam1cs. Fluid Mechan1cs - Soviet Research, Vol. 9, May - June, 1980, pp 
51-56. 
14. Zharkova, V. M.; Khachaturyan, V. M.; Vostokov, L. A.; and Alekseev, 
N. M.: Study of L1qU1d Thermoind1cators, Advances in L1qU1d Crystal Research 
and Applicat1ons, Ed1ted by L. Bata, Pergammon Press, Oxford, Akademia1 K1ado, 
Budapest, 1980, pp 1221-1239. 
5 
APPENDIX 
LIQUID CRYSTAL SUPPLIERS 
1. Atomerglc Chemetals Corp. 
100-T Fairchild Avenue 
Plalnview, NY 
2. Damert Co. Dept. T 
900 75th Street 
Oakland, CA 
3. DaV1S Llquid Crystals, Inc. 
14692-T Wicks Blvd. 
San Leandro, CA 94577 
415 351-2295 
4. Eurand America, Inc. 
1464-A Mlamisburg-Centerville Rd. 
Dayton, OH 
5. General Electric Co. 
Falrfield, Cr 06431 
518 438-6500 
6. Hall Crest Products/Llquld Crystal Technology 
1820-T Plckwick Lane 
Glenview, IL 60025 
312 998-8580 
7. Har Chemlcals Inc. 
15840-T Joleen Way 
Morgan Hill, CA 
8. Llquld Crystal Appllcatlons, Inc. 
990 Rarltan Road 
Clark, NJ 07066 
201 574-1496 
9. Liquid Crystal Industries, Inc. 
Dept. TR 
P. O. Box 8124 
Pl ttsburgh, PA 
1 o. PCI, Inc. 
2455 Augustine Drlve, P. O. Box 58097 
Santa Clara, CA 
11. Practlcal Products Co. 
7820-T Concord Hllls Lane 
Clnclnnatl,OH 
12. Qmax Technology Group, Inc. 
P. O. Box 1509 
125 Bacon Street 
Dayton, OH 45402 
513 228-2817 
6 
'-J 
Figure 1. - Transition visualization using liquid crystals on winglet of Lear 28/29 
Airplane. Pressure altitude = 48,000 ft . , M = 0.8; leading edge 
sweep = 3le • 
L-86-1 401 
1 Report No ~ 2 Government AccessIon No 3 RecIpIent's Catalog No 
NASA TM-87666 
4 TItle and SubtItle 5 Report Date 
A NEW METHOD FOR LAMINAR BOUNDARY LAYER TRANSITION January 1986 
VISUALIZATION IN FLIGHT - COLOR CHANGES IN LIQUID 6 Performmg OrganIzation Code 
CRYSTAL COATINGS 505-61 41-02 1 Author(s) Bruce J. Holmes, Peter D. Gall, Cynthia C. 8 Performmg OrganIzatIOn Report No 
Croom, Gregory S. Manuel, and, Warren C. Kelliher 
9 Performmg OrganIzation Name and Address 10 Work Unit No 
NASA Langley Research Center 
Hampton, VA 23665 11 Contract or Grant No 
12 Sponsormg Agency Name and Address 13 Type of Report and Penod Covered 
National Aeronautics and Space Admin~strat~on Technical Memorandum 
Wash~ngton, DC 20546 14 Sponsormg Agency Code 
15 Supplementary Notes 
16 Abstract 
The vlsuahzation of laminar to turbulent boundary layer transition plays an Important 
role in flight and wind-tunnel aerodynamic testing of aircraft wing and body surfaces. 
Visualization can help provide a more complete understanding of both transition location as 
well as transition modes; without visualization, the transition process can be very difficult 
to understand. 
In the past, the most valuable transition visualization methods for fhght applications 
included sublimating chemicals and oil flows. Each method has advantages and 
limitations. In particular, sublImating chemicals are impractical to use In subsonic 
applications much above 2.0,000 feet because of the greatly reduced rates of sublimation at 
lower temperatures (less than -4 degrees Farenheit). Both 011 flow and sublimating 
chemicals have the disadvantage of providing only one good data pomt per flight. Thus, for 
many important flight conditions, transition visualization has not been readily available. 
This paper discusses a new method for VisualiZing transition in fhght by the use of 
liquid crystals. The new method overcomes the hmitations of past techmques, and prOVIdes 
transition visualization capability throughout almost the entire altitude and speed ranges of 
Virtually all subsonic aircraft flight envelopes. The method also has Wide applicability for 
supersonic transition visualization In flight and for general use in wind tunnel research over 
wide subsonic and supersomc speed ranges. 
17 Key Words (Suggested by Authors(s» 18 DlstnbutlOn Statement 
Laminar Flow Flight Research Unclass~fied - Unlimited 
Flow Visualization 
LiqUl.d Crystals 
Transition 
Boundary Layer Subject Category 02 
19 Secunty Classlf (of thIS report) 120 Secunty Classlf (of thIS page) 21 No of Pages 122 Pnce 
Unclassified Unclassified 8 A02 
For sale by the NatIOnal Technical Information ServIce, Sprmgfield, VIrginia 22161 NASA-Langley, 1986 
End of Document 


A New Method for Laminar Boundary Layer Transition Visualization in Flight: Color Changes in Liquid Crystal Coatings

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