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AEROLAB
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The fluorescent oil film method of subsonic boundary layer transition visualization in wind
tunnels was investigated. This technique was found to be effective, inexpensive, and fast for
low speed testing with relatively small models. However, less volatile, more viscous carrier
oil solutions are necessary for larger surfaces and higher tunnel speeds. Methods of
ultraviolet illumination in the University of California, Davis Son of BART wind tunnel test
section were examined. The details of an actual installation are included. China clay and an
inexpensive infrared method of transition visualization were also cursorily investigated.
Neither of these methods could be readily implemented although recommendations for
future investigations are made. The following is a brief summary and the full report is
available.
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- Background
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Like sublimating, evaporative, and infrared techniques, the fluorescent oil film method is
based on shear stress variation. Because shear stress dramatically varies between laminar,
turbulent, and transition regions, transition location can be readily deduced from
visualization of patterns of shear stress.
Visualization is accomplished via a solution of fluorescent dye and a carrier oil.
Historically, oil coatings have been used repeatedly in flow visualization applications. A thin
film of the fluorescent oil solution is applied to the model surface. In areas of relatively high
shear stress, oil is swept away from the surface. At areas of low shear stress, such as a
laminar separation bubble, the oil collects and pools.
- Test Rig
- The configuration chosen for this study was a NACA 0012 halfspan. The dye used in this method is called A-680 Plus and is the product of a company called
Ultra-Violet Products (UVP). It is commonly used in automotive repair applications for
the detection of oil leakage and is priced at approximately $25 for six one ounce bottles.
Kerosene, as recommended by Loving and Katzoff
for low speeds and temperatures, was
used as a carrier fluid.
- Still photography was chosen as the method of recording data of the fluorescent oil flow
visualization experiments. Recording on video tape was also considered, but the low lighting
conditions would have required special equipment to produce good quality results. The
reduced lighting conditions somewhat restricted the photography since exposure times of up
to 5 seconds precluded point and shoot 35 mm cameras as well as most digital still
cameras. However, a 35mm SLR camera was found to be suitable for the job. The use of a
tripod for these exposures was a necessity.
- Results
- The effectiveness
of this technique was evident by mostly favorable correlation of boundary layer transition
areas with CFD calculations. Kerosene was chosen as the carrier oil for the experiments at
low speeds as recommended by Loving and Katzoff. Shortcomings of the kerosene due to
its low viscosity and high volatility were evident at higher Reynolds numbers. However,
valuable data was acquired and a useful visualization methodology was verified. The following table
presents the measured transition regions and the predicted transition locations for an angle of attack of 4 degrees.
| Velocity (mph) |
Forward location of region (x/c) |
Rearward location of region (x/c) |
CFD location prediction (x/c) |
| 50 |
0.250 |
0.464 |
0.390 |
| 100 |
0.099 |
0.446 |
0.300 |
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