Solved: 100-year-old aerodynamic problem

September 26th, 2008 - 2:45 pm ICT by IANS  

Washington, Sep 26 (IANS) As a car accelerates up and down the hill then slows down to follow a hairpin bend, the airflow around it cannot keep up and detaches from the vehicle. This aerodynamic separation creates a drag that slows the car and forces the engine to work harder, using more fuel. The same phenomenon affects airplanes, boats, submarines, even your golf ball.

Now, in work that could lead to ways of controlling the impact on fuel efficiency and more, Massachussetts Institute of Technology (MIT) scientists have reported new mathematical and experimental work for predicting where that aerodynamic separation will occur, according to an MIT statement.

The findings were published in Thursday’s issue of the Journal of Fluid Mechanics.

The research solves “a century-old problem in the field of fluid mechanics”, or the study of how fluids - which for scientists include gases and liquids - move, said George Haller, a visiting professor at MIT’s department of mechanical engineering.

Haller’s group developed the new theory, while Thomas Peacock, an associate professor in the same department, led the experimental effort.

Fluid flows affect everything in our world, from blood flow to geophysical convection. As a result, engineers constantly seek ways of controlling separation in those flows to reduce losses and increase efficiency.

Controlling fluid flows lies at the heart of a wide range of scientific problems, including improving the performance of vehicles, Peacock said.

For example, 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,” he added.

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 idealised two-dimensional flows.

Ever since 1904 there have been intense efforts to extend Prandtl’s results to real-life problems, that is, to unsteady three-dimensional flows.

A century later, Haller published his first paper in the Journal of Fluid Mechanics explaining the mathematics behind unsteady separation in two dimensions. This month, his team reports completing the theory by extending it to three dimensions.

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