Researchers Create Rough Surfaces That Reduce Drag
Someone tell Michael Phelps to put down the razor: Smooth is not always better when it comes to reducing drag, researchers from the University of California, Los Angeles contend in their new study published in Physics of Fluids.
"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 teamed up with his colleagues to model the flow of fluids between two surfaces with tiny ridges, finding that even in turbulent conditions, the rough surfaces decreased drag created by the friction of flowing water.
The scientists are not the first to research the potential of rough surfaces in reducing drag; however, previous studies have had limited success. Recently, researchers have started looking at rough, superhydrophobic surfaces - or surfaces that resist water. The idea is that these surfaces can trap air bubbles, which provide a hydrodynamic cushion. The reality is a little more complicated, however, with air cushions often lost in chaotic flows.
The new study uses a superhydrophobic design other UCLA researchers had already found kept air pockets that survived even the wildest conditions. Kim and his colleagues covered the surface with tiny ridges that aligned with the direction of the flow and were surprised to find that drag-reduction was increased with turbulence.
According to Kim, the irregular fluctuations and vortices created under rougher conditions, although typically responsible for increasing drag on smooth surfaces, were altered near the surface by the air cushions created by the superhydrophobic ridges.
Such findings could one day be applied to much more than swimming, the researchers note, explaining that the undersides of cargo vessels or passenger ships could one day be equipped with surfaces designed using insight from their study.
"It could lead to significant energy savings and reduction of greenhouse gas emissions," Kim said.