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BBQs inspire scientists to reduce drag


A sphere falling through liquid has less drag when it is heated beyond the boiling point of the liquid (Source: I.U. Vakarelski/KAUST)

Hot slide Sizzling barbeque hotplates have triggered scientists to uncover a new mechanism to reduce drag.

The findings could lead to more efficient pumping of fluids or help give underwater vehicles a temporary boost, says researcher Dr Derek Chan, a professor of mathematics from the University of Melbourne.

Chan and colleagues' research appears in a recent issue of Physical Review Letters.

"The genesis [of the idea] came from having a drink at the Intercontinental Hotel in Singapore," says Chan.

He and co-author Dr Ivan Vakarelski of King Abdullah University of Science and Technology in Saudi Arabia, were mulling over what happens when you throw a drop of water onto a barbeque plate.

"On a very hot plate the water does not evaporate instantaneously but stays around for a very long time," says Chan, adding the water drops appear to slide around on the hot plate.

This effect was first noticed 225 years ago by a German doctor and theologian Johann Gottlob Leidenfrost.

Leidenfrost noted that when a drop of liquid falls on a solid surface it will slide around without evaporating when the solid surface is much hotter than the liquid's boiling point.

Later English physicist John Tyndall identified a vapour layer between the water and hot solid. This provided insulation stopping the water drop from evaporating immediately, and also provided a lubricating interface between the water and the hot surface, allowing the drop to slide.

Chan and colleagues decided to turn the Leidenfrost effect "inside out".

Instead of dropping water onto a hot surface, they dropped a hot object into a liquid to see if the vapour layer that formed around it would reduce its drag as it moved through the water.


Chan and colleagues heated up ball bearings and dropped them into cylinders of liquid.

Instead of water they used fluorocarbon liquid, which has a relatively low boiling point of 56°C, making it easier to heat the solid to the desired temperature.

The researchers took movies of the falling ball bearings and measured their terminal velocity - the constant speed they reach after an initial acceleration.

This movie [1 MB] shows the different velocity at which the three spheres fall.

The one on the left is at room temperature, the one in the middle is heated to 110°C and the one on the right 200°C.

The researchers also repeated the experiment by pulling the balls upwards from the bottom of the tank using a counterweight.

"We found that it doesn't matter if the sphere rises or falls, the drag reduction is the same," says Chan.

Drag coefficient

Chan says the drag coefficient of the hot vapour-covered sphere reduced from 0.4 to 0.07. By comparison a sports car has a drag coefficient of 0.3, he says.

While the theoretical calculation of the limit to drag reduction was made in the 1960s, this is the first time it has been actually demonstrated.

Chan says the findings could lead to the development of techniques to reduce drag and save energy when pumping fluid.

And it could also help reduce drag for vehicles that travel in water, although with some practical limitations.

"Practically for a ship it is difficult. If you heat the hull of the ship to 200-odd degrees, corrosion and energy cost will be an issue," says Chan.

However, he says, reducing drag in this way could be useful for accelerating an underwater vehicle for short periods of time.

Exactly what kind of underwater vehicle, Chan did not want to elaborate on.

Tags: energy, engineering, physics