Michael Phelps has silky-smooth legs because he wants to reduce drag has he moves through the water; but new research suggests rough objects could be even faster.
"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 said in an Amercian Institute of Physics (AIP) news release.
A UCLA research team observed how fluid flowed between two surfaces that were covered in tiny ridges, they found the rough surface effectively reduced drag (caused by friction imposed by the water's movement) even when the water was turbulent.
Researchers have had "limited sucess" in displaying drag reduction using rough surfaces. Scientists are now looking at rough surfaces that also posses superhydrophobicity (water resistance). These surfaces are believed to capture air bubbles which form "hydrodynamic cushions"; the problem is these cushions are often lost in rough conditions.
The team waas able to create a superhydrophobic surface that could hold onto their hydrodynamic cushions even in the most turbulent conditions. The surface's ridges all pointed in the same direction as the water flow.
The team analyzed both laminar and turbulent flows and found the drag reduction was most prevalent in the turbulent conditions.
"The irregular fluctuations and swirling vortices in turbulent flows on smooth surfaces generally increase drag," the news release reported. The ridges influence the turbulent patterns near the water's surface which reduces its effect.
This type of surface could be used in the future to make more efficient ship hulls.
"It could lead to significant energy savings and reduction of greenhouse gas emissions," Kim said.
