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UW’s Mavriplis Goes With the Flow When Researching Aerodynamic Designs in Computer Models


June 7, 2012 — As a boy, Dimitri Mavriplis built model airplanes and became knowledgeable about the industry through his father, Fotis, who was manager of the aerodynamics department at Bombardier Aerospace in Montreal, Quebec.

So, it comes as no surprise that the University of Wyoming professor of mechanical engineering now simulates the aerodynamics of aircraft wings, helicopter propellers and wind turbines -- on a computer. Mavriplis works with computational fluid dynamics (CFD), a branch of fluid mechanics that uses numerical methods and algorithms to analyze and solve fluid flow challenges.

Traditionally, the aerospace industry has designed and tested aircraft using wind tunnels. For example, in the 1960s, the development of the Boeing 747 required more than 20,000 hours of wind tunnel testing, which was both very time consuming and costly, Mavriplis says.

"Today, we simulate most of that on the computer," he says.

His scientific simulations and computer calculations will scale up this fall, when he uses the National Center for Atmospheric Research (NCAR)  Wyoming Supercomputing Center (NWSC).

Mavriplis's research goals include obtaining more accurate aerodynamic effects by resolving increasingly fine details of turbulence; being able to run simulations with more realistic effects; being able to run such simulations faster and eventually make it so engineers can do such computations on desktop computers or tablets; and perhaps, eventually, have the Federal Aviation Administration (FAA) be given the ability to certify aircraft through results from computer models.

"One thing more powerful computers can do is resolve more details of the turbulent flow that occurs over aircraft configurations, thus providing more realistic simulations of the aerodynamics which, in turn, lead to more effective and efficient designs," says Mavriplis, who received grant funding from the Army, Navy, Air Force and NASA for his research. "If we do our job well, we can get at least a factor of 100 more from the NWSC than we can with currently available hardware."

On a wing and a computer

Wind tunnel testing of airplane aerodynamics under various conditions started with the Wright Brothers and will likely continue for the foreseeable future, Mavriplis says. But computer modeling -- using computational fluid dynamics -- is gradually replacing and reducing wind tunnel test time and expense, he says.

Mavriplis expects the supercomputer's parallel computing power will allow for more complex simulations, which are already quite intricate.

Mavriplis takes information obtained from a CAD (computer-assisted design) and builds, on the computer, a grid or computational mesh around this geometry. The mesh consists of a large collection of small cells surrounding the aircraft body. In each cell, the computer stores a value or air density, pressure and velocity.

"What flows out of one cell flows into another. For each little cell, you have an equation, which depends on the values in the neighboring cells," Mavriplis explains. "The grid can contain up to 100 million cells, which results in 100 million coupled equations that need to be solved simultaneously. When these equations are solved on a powerful computer, the entire flow field is obtained, showing how (air) flow goes over the body of the aircraft."

One of the more significant problems with computer models at the desktop level is having enough resolution or mesh cells to capture all of the fine details of eddies caused by turbulence, Mavriplis says. Some eddies can be as large as the airplane wing itself, but they cascade down to smaller and smaller eddies, which can be tens of thousands of times smaller than the largest eddies. They are all important in determining the aerodynamic performance of an aircraft, he says.

The ability to incorporate additional effects, such as the "flexing of the wing due to aerodynamic loads," on a computer model is a more recent development that will "assist in improving the accuracy of our simulations and making them more realistic," he says.

"There is no definite answer. Computer simulations will probably never replace (wind tunnel) testing completely," says Mavriplis, who has been at UW since 2003 after 16 years at NASA's Langley Research Center. "But it can replace more and more of the testing, and really reduce the expense and design cycle time for new aircraft development. It's continuous improvement."

Helicopters and wind turbines

In addition to airplanes, Mavriplis's research extends to helicopters and wind turbines. UW has a partnership with the University of Maryland's Vertical Lift Research Center of Excellence to study aerodynamics of helicopters.

For helicopter aerodynamics, a time-varying simulation must be undertaken because, unlike the stationary wings of an airplane, the rotors on a helicopter are turning, he says. This makes computer modeling for a helicopter much more time consuming.

"It's the difference between a still picture and a movie," Mavriplis says.

While studying airplane wings and helicopter rotors are two different things, wind turbine action is similar to helicopters - although other challenges arise in the case of wind turbines, Mavriplis says.

Because wind currents are not constant and there are intermittent gusts, aerodynamics of wind turbines must take into account the variations in the atmospheric wind flow, he says. Couple that with strong winds bending and vibrating turbine blades, as well as stressing the gear boxes, and this creates another area of aerodynamics ripe for computer modeling.

"If you can simulate that, you can understand the weak spots," Mavriplis says. "You can improve the design so it does not fail."

The NWSC is the result of a partnership among the National Center for Atmospheric Research (NCAR); the University of Wyoming; the state of Wyoming; Cheyenne LEADS; the Wyoming Business Council; Cheyenne Light, Fuel and Power; and the University Corporation for Atmospheric Research. NCAR is sponsored by the National Science Foundation (NSF).

The NWSC will contain some of the world's most powerful supercomputers (1.5 petaflops, which is equal to 1.5 quadrillion computer operations per second) dedicated to improving scientific understanding of climate change, severe weather, air quality and other vital atmospheric science and geo-science topics. The center also will house a premier data storage (11 petabytes) and archival facility that holds irreplaceable historical climate records and other information.

Photo:
Dimitri Mavriplis, a UW professor of mechanical engineering, conducts computational science that simulates the aerodynamics of aircraft wings, helicopter propellers and wind turbines.

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