May 22, 2001 Five years in the making, the nation's largest Matched Index of Refraction facility can make airplane wings, heating pipes and waste containers disappear. Oddly enough, scientists use this disappearance to study how fluids such as air and liquids flow around and through objects, allowing for improved designs that are safer and work better.
Built at the Department of Energy's Idaho National Engineering and Environmental Laboratory through an international collaboration with researchers from the Institute of Fluid Mechanics (LSTM) at the University of Erlangen-Nuremberg, Germany, INEEL's MIR uses baby oil and lasers to investigate the flow around quartz models of complex structures.
In a demonstration of the facility's capabilities, INEEL researchers examined fluid flow over a surface rib on a flat quartz plate, modeling how air would flow over bumps on an airplane wing. They reported their results in the April 2001 issue of the journal Experiments in Fluids. "We wanted to introduce the facility to the world, since outside researchers are welcome to perform experiments here at the INEEL MIR facility," said lead author Carl Stoots, an INEEL chemical engineer.
Stoots, now working at the LSTM, said this MIR's size and choice of a nontoxic fluid make it unique. The facility is 5-10 times larger and eminently safer than other Matched Index of Refraction facilities. "This facility is huge. Its size allows experiments that previously were impossible to do," he said.
The most desired type of airflow over a level-flying airplane wing is smooth and flat, also called laminar. Bumps and divots on the wing's surface, though, cause turbulence. Turbulent air flows chaotically and moves in all three dimensions. In a level-flying plane, chaotic airflow causes drag and decreases the fuel efficiency of the plane. When a plane is climbing steeply, however, chaotic airflow reduces drag. The switch between laminar and turbulent flow occurs near the surface of the wing.
According to Stoots, understanding how a planned bump influences the laminar-to-turbulent flow transition will allow airplane manufacturers to improve aircraft fuel efficiency and safety. "If a flow is going to go turbulent on you, it'd be nice to control where it goes turbulent," said Stoots.
In addition to studying air flow over airplane wing models, the INEEL MIR facility has been used to study convective heat transfer from surfaces. The MIR facility can be used in two ways. Researchers can study flow through something, such as examining the confined movement of gas through a pipe. Or, they can look at channel flow -- fluid flowing on the outside of something -- such as water past a submarine.
MIR takes advantage of the same property that causes a spoon to appear bent sticking out of water -- bending light rays. A model of what is to be studied -- in this case a flat plate with an attached wire that resembles the surface of a ribbed wing -- is crafted from quartz and placed in the oil.
Normally, light bends as it passes from the oil to the quartz -- or in the case of the spoon, from air to water -- due to an optical property called the index of refraction. The oil's index of refraction, however, changes as the oil's temperature changes. The researchers raise the temperature of the oil until its refractive index matches the quartz. The light rays then travel straight through both, making the quartz model disappear in the clear oil.
To visualize the flow in the test section of the MIR, the researchers float tiny particles that reflect laser light in the oil. Using laser doppler anemometry (LDA, which measures velocity by bouncing laser light off a moving object, similar to a traffic radar gun), the particles' velocities and positions are measured. The information is compiled in a computer program to get a graphical representation of what is invisible to the eye.
The MIR holds about 2,700 gallons of mineral oil, which loops around through honeycombs and screens to remove bubbles and turbulence. The smoothly flowing oil then travels through the test section, which is 2 feet square by 8 feet long. The large-scale test section allows increased resolution of flow at surfaces, and more room for bigger objects.
Stoots said the increased resolution generates more accurate data. "Say you want to study a submarine. No matter what, you have to use a scaled model. The bigger the test section, the bigger the model can be. The bigger the model, the bigger the flow patterns around it. LDAs are basically a fixed-sized measurement instrument. The ratio of LDA size to flow pattern size becomes better and better (smaller and smaller) as you increase the size of the model."
Another novel characteristic of the INEEL MIR facility is its choice of fluid. Most MIRs use hazardous fluids such as diesel fuel. Since the volume of the loop is so large, the team wanted to find a safer fluid. It turns out the index of refraction of mineral oil, the nonperfumed base for baby oil, can be matched to quartz if the oil is carefully kept at 86 degrees Fahrenheit.
"Other MIR loops use fluids that are volatile or toxic. There's another loop we work near that uses diesel fuel, and you can smell it all the time," Stoots said. "Mineral oil is so safe you can almost eat it. It's a pleasant fluid to work with."
One drawback of using mineral oil is that quartz is more expensive and harder to work with than plastic, which is commonly used in smaller MIR loops. "Quartz is not as flexible as plastic," said Stoots. "If you have an object made out of tubes, spheres or plates, quartz is not a problem. Even a ship propeller can be crafted relatively easily from quartz. But if it requires complex internal machining, that could be a problem."
Although mineral oil is safe for human hands and more pleasant to work around, the team was surprised by how many materials were incompatible with the oil, including Tygon tubing, fluorescent lights (which degrade the oil over time), and silicon glue, which swelled in the oil. Also, in order to maintain the oil at the correct temperature -- key to matching the refraction indices -- the researchers had to design a better temperature control system, Stoots said. The improved temperature control system allowed the team to tailor the oil's index of refraction for different colors of laser light.
Stoots and his colleagues hope the INEEL MIR facility will become an internationally recognized user facility. Its early success has inspired Stoots' LSTM collaborators to build one of their own. Armed with his INEEL experience, Stoots assisted in the design of the LSTM facility and is now conducting similar transition experiments in that facility.
Their paper, "A large scale matched index of refraction flow facility for LDA studies around complex geometries," by C. Stoots, S. Becker, K. Condie, F. Durst and D. McEligot, appeared in Volume 30 Issue 4 (2001) pp 391-398 of Experiments in Fluids.
The INEEL is a science-based, applied engineering national laboratory dedicated to supporting the U.S. Department of Energy's missions in national security, environment, energy and science. Bechtel BWXT Idaho, LLC, in partnership with the Inland Northwest Research Alliance, operates the INEEL for the DOE.
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