Smooth sailing: Rough surfaces that can reduce drag

Jan 17, 2014

From the sleek hulls of racing yachts to Michael Phelps' shaved legs, most objects that move through the water quickly are also smooth. But researchers from UCLA have found that bumpiness can sometimes be better.

"A properly designed rough surface, contrary to our intuition, can reduce skin-friction drag," said John Kim, a professor in the mechanical and aerospace engineering department at UCLA. Kim and his colleagues modeled the fluid flow between two surfaces covered with tiny ridges. They found that even in turbulent conditions the rough surface reduced the drag created by the friction of flowing water. The researchers report their findings in the journal Physics of Fluids.

The idea of using a rough surface for reduced drag had been explored before, but resulted in limited success. More recently scientists have begun experimenting with that are also extremely difficult to wet, a property called superhydrophobicity. In theory this means that the surfaces can trap air bubbles, creating a hydrodynamic cushion, but in practice they often lose their air cushions in chaotic flows.

The ULCA team chose to model a superhydrophobic surface design that another group of researchers at UCLA had already observed could keep air pockets entrapped, even in turbulent conditions. The surface was covered with small ridges aligned in the direction of flow.

The researchers modeled both laminar and turbulent flows, and unexpectedly found that the drag-reduction was larger in turbulent conditions. The irregular fluctuations and swirling vortices in turbulent flows on smooth surfaces generally increase drag, Kim explained. However, the air cushion created by the superhydrophobic ridges altered the turbulent patterns near the , reducing their effect, he said.

The team expects insights gleaned from their numerical simulations to help further refine the design of rough, drag-reducing surfaces. Further down the line, such surfaces might cover the undersides of cargo vessels and passenger ships. "It could lead to significant energy savings and reduction of ," Kim said.

Explore further: How slippery are water-repellent surfaces? (w/ Video)

More information: The paper, "A numerical study of the effects of superhydrophobic surface on skin-friction drag in turbulent channel flow," authored by Hyunwook Park, Hyungmin Park, and John Kim, appears in the journal Physics of Fluids: dx.doi.org/10.1063/1.4819144

Related Stories

MASSIVE advances in aerodynamics

Aug 30, 2012

Engineers are closer to understanding, and therefore manipulating, invisible aerodynamic drag forces, that cause an estimated 50 per cent of transportation fuel to be lost before we can use it.

Not a drag: breakthrough will create cleaner, faster planes

Jul 09, 2010

(PhysOrg.com) -- A world first model for predicting fluid flows close to surfaces will enable engineers to reduce drag in vehicles, and in turn, lead to more efficient and greener planes, cars and boats, according to a University ...

Recommended for you

IHEP in China has ambitions for Higgs factory

14 minutes ago

Who will lay claim to having the world's largest particle smasher?. Could China become the collider capital of the world? Questions tease answers, following a news story in Nature on Tuesday. Proposals for ...

The physics of lead guitar playing

1 hour ago

String bends, tapping, vibrato and whammy bars are all techniques that add to the distinctiveness of a lead guitarist's sound, whether it's Clapton, Hendrix, or BB King.

The birth of topological spintronics

2 hours ago

The discovery of a new material combination that could lead to a more efficient approach to computer memory and logic will be described in the journal Nature on July 24, 2014. The research, led by Penn S ...

The electric slide dance of DNA knots

6 hours ago

DNA has the nasty habit of getting tangled and forming knots. Scientists study these knots to understand their function and learn how to disentangle them (e.g. useful for gene sequencing techniques). Cristian ...

User comments : 0