Reducing drag – particularly in the transportation and energy sectors – could lead to economic and environmental benefits. A new method reduces drag in high-turbulence conditions by counteracting large eddies in the flow of air or water.
Researchers have developed an energy-efficient method to reduce drag in high-turbulence conditions. When a fluid – such as air or water – flows over an object, the resulting friction slows the object’s movement. The drag reduction method works by using oscillations on the object’s surface to counteract eddies in the flow.
Previous studies predicted that the effectiveness of such a method would decrease as turbulence increased. An international research team led by Professor Ivan Marusic modified the method to make it effective at high levels of turbulence – particularly the levels found in transportation and energy systems.
Drag caused by turbulence hinders the speed and fuel efficiency of large vehicles such as airplanes and ships. It also decreases the capacity of wind turbines to generate power and of long-distance pipelines to transport materials. Reducing drag could therefore yield significant economic and environmental benefits.
The researchers looked at two approaches for using oscillations to reduce drag along the surface of an object. They installed a machine that controls flow, called the surface-actuation test bed (SATB), in the High Reynolds Number Boundary Layer Wind Tunnel at the University of Melbourne. The SATB is made up of 48 slats lying across the width of the tunnel. They oscillate at frequencies up to 25 Hz – essentially sliding back and forth, perpendicular to the direction of flow.
In the first approach, which has been studied before, the researchers oscillated the SATB at frequencies matching those of nearby small eddying motions in the flow. This reduced drag by up to 25 per cent, but only when turbulence was relatively low. Also, the cost of powering the oscillations exceeded any savings produced by reducing drag.
In the second approach, which had not been studied before, the researchers matched the frequency of the SATB to those of the largest eddies farther from the object’s surface. This approach reduced drag by up to 13 per cent, even when turbulence was as high as the levels found in transportation and energy systems. And, unlike the first approach, the drag reduction increased as turbulence increased. This approach also required much less power.
Introducing oscillations to the surface of vehicles, wind turbines or pipelines would require significant innovation. But the discovery of a method to reduce drag by disturbing flow creates new opportunities for tackling drag in high-turbulence conditions.
The researchers plan to use the insights gained from this study to devise practical methods that target the same physical mechanisms but don’t require direct surface movement.
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Marusic I et al (2021) An energy-efficient pathway to turbulent drag reduction. Nature Communications 12: 5805. doi: 10.1038/s41467-021-26128-8
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First published on 31 March 2022.
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