A team of researchers from the UCLA has recently published their findings indicating that a rough surface may reduce the amount of drag in certain objects moving on water. While the idea of a roughened surface moving faster through fluids has been around for a while now, the new study sheds more light on using specially designed surfaces to reduce drag.
Previous experiments with rough surfaces moving through water have been tried before but scientists often come across a number of problems that limit the effect on drag reduction. The UCLA team found that the using a specially designed rough superhydrophobic surface can have a profound effect on reducing drag caused by friction. A superhyrdrophobic surface is very effective at repelling water and is impossible to wet. Due to the combination of these two factors, the drag experienced by the specially designed surface is lowered due the ability of the surface to hold pockets of air.
The rough surface has little ridges that can direct the flow of water allowing the air pockets to remain intact even if it comes in contact with turbulence. In fact, the drag reducing effects was found to be more effective under turbulent conditions. The effect of turbulent flow was minimized because of the specially designed superhydrophobic ridges.
The team's findings, published in a paper called "A numerical study on the effects of superhydrophobic surface on skin-friction drag in turbulent channel flow," could have a profound effect on boat hull design and manufacturing. With a hull that experiences less drag, boats can move faster and more efficiently through the water. This can reduce both travel time as well as make a boat more fuel efficient, further reducing costs as well as gas emissions.
When thinking about objects moving faster due to rough surfaces, golf balls are often the first things that come to mind. While the principles are generally similar, the dimples on a golf ball are only effective due to the spherical, and spinning nature of a ball in flight. Moreover, a roughened surface using dimples on a streamlined body such as a boat's hull will do little to reduce drag.
Aside from golf balls, dolphins have also been known to have special rough surfaces on their skin that can help reduce drag. This theory was first described in what is called Gray's paradox, which was put forward in the 1930's by a scientist named James Gray after observing that dolphins could swim far faster than what should have been possible due to their muscle mass. However, the theory has since been disproved due to new evidence regarding dolphin musculature and physiology. To date, the UCLA project is the first time a rough surface with superhydrophobic properties has been proven to reduce drag.
With the UCLA team's findings, a new generation of boats with specially designed hulls using superhydrophobic ridges may bring about faster, more fuel-efficient boats and marine craft.
