MIT researchers discovered the solution to a problem put 100 years ago by Ludwig Prandtl. It refers to the situation when, for example, a car accelerates up and down a hill, then slows to follow a turn, and the airflow around it cannot keep up and separates from it. This aerodynamic phenomena creates an additional drag, slowing the car, and forcing you to push harder on the pedal, consuming more gas. The same applies to airplanes, boats, subs, and generally, any moving object following the rule mentioned above.
The MIT scientists report a new mathematical explanation for this phenomena, predicting where the separation will occur. George Haller, a visiting professor in the Department of Mechanical Engineering, with a group of other researchers, developed a new mathematical theory, while Thomas Peacock, the Atlantic Richfield Career Development Associate Professor in the same department, made experiments based on Haller’s theory.
Papers on the experiments and theory are being published in the Sept. 25 issue of the Journal of Fluid Mechanics and in the September issue of Physics of Fluids, respectively.
For example, picture air flowing around, over and past an object. “Instead of flowing smoothly past the object, the air tends to dramatically part from the surface, or separate,” said Peacock. Like the wake behind a boat, the water doesn’t automatically reconfigure into a single stream. Rather, the region is quite turbulent. “And that adversely affects the lift [or vertical forces] and drag [or horizontal forces] of the object.”
In 1904, Ludwig Prandtl derived the exact mathematical conditions for flow separation to occur. But his work had two major restrictions: first, it applied only to steady flows, such as those around a car moving at a constant low speed. Second, it only applied to idealized two-dimensional flows.
“Most engineering systems, however, are unsteady. Conditions are constantly changing,” Haller said. “For example, cars accelerate and decelerate, as do planes during maneuvers, takeoff and landing. Furthermore, fluids of technological interest really flow in our three-dimensional world,” he added.
As a result, ever since 1904 there have been intense efforts to extend Prandtl’s results to real-life problems, i.e., to unsteady three-dimensional flows.
The research was initially supported by an internal source, the MIT Ferry Fund. Currently the work is supported by the Air Force Office of Scientific Research and the National Science Foundation.
The researchers say that it’s too early to make an estimation of how much this discovery will affect the performance of the vehicles, nevertheless we’ll wait for other experiments and theory developments to emerge. This research field could be applied to electric cars, in the future, so it’s not only for gas-powered ones. “This is the tip of the iceberg, but we’ve shown that this theory works,” Peacock said.