From the sleek¹（圆滑的） hulls² of racing³ yachts to Michael Phelps' shaved legs, most objects that move through the water quickly are also smooth. But researchers from UCLA have found that bumpiness⁴（崎岖不平） can sometimes be better. "A properly designed rough surface, contrary to our intuition（直觉）, can reduce skin-friction⁵ drag," said John Kim, a professor in the mechanical and aerospace⁶ engineering department at UCLA. Kim and his colleagues modeled the fluid flow between two surfaces covered with tiny ridges⁷. 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 rough surfaces that are also extremely difficult to wet, a property called superhydrophobicity（超疏水）. 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 chaotic⁸ 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 entrapped⁹, even in turbulent conditions. The surface was covered with small ridges aligned¹⁰ 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 fluctuations¹¹ and swirling¹² 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 surface, reducing their effect, he said.

 

The team expects insights gleaned¹³ 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 cargo¹⁴ vessels¹⁵ and passenger ships. "It could lead to significant energy savings¹⁶ and reduction of greenhouse gas emissions," Kim said.

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