Fluid flow technology takes a cue from fast-swimming sharks

A study of airflow in pipes may help solve a mystery concerning the ears of fast-swimming sharks. The results could also lead to new audio technologies, according to an engineer at Ohio State University.

Konrad Koeltzsch, a postdoctoral researcher in chemical engineering and the Alexander von Humboldt Fellow at Ohio State, and his colleagues investigated grooves in sharkskin called riblets.

Koeltzsch began to study sharkskin while he was a postdoctoral researcher at the Dresden University of Technology in Germany. He worked with Albrecht Dinkelacker, a German researcher who pioneered the study of riblets, and Dresden professor Roger Grundmann. The three published their results in a recent issue of the journal Experiments in Fluids.

Some 20 years ago, engineers found that lining a pipe with riblet-like grooves could speed flow through a pipe by as much as 10 percent.

The very idea that a textured surface could speed fluid flow appears counterintuitive at first, Koeltzsch said. “We normally think that smooth surfaces cause the least drag,” he explained. A fundamental point in fluid mechanics is that rough surfaces increase drag, and sharkskin is considered rough. If such a rough surface reduces drag, that doesnt seem to make sense.

The answer still lies in fluid mechanics, Koeltzsch said, in a phenomenon called “wall-bounded turbulence” that hasn’t been well understood until now.

The reason that riblets work the way they do is complex, Koeltzsch said. The most promising explanation comes from researchers at Seoul National University in Korea, who suggested that the size of the riblets and the rotation of spiraling areas of fluid known as vortices are both important factors. The optimal arrangement, Koeltzsch said, is when the distance between the peaks of the riblets is half the diameter of the vortices. In this case, the fluid caught by the vortices only contacts the peaks of the riblets and not the walls of the pipe, so friction is reduced and the fluid moves faster.

Other researchers previously determined that riblets could be used to speed the flow of fluids such as water and air. NASA, for instance, has been developing riblet technology for airplanes, sea vessels, and even swimsuits since the 1980s.

Koeltzsch said it is now well understood that the riblets running along a shark’s body from head to tail help the shark swim faster. But very fast-swimming sharks, such as the silky shark and blue shark, also have special arrangements of riblets, the function of which is a mystery.

The riblets converge or diverge in a “V” pattern on the skin surrounding the shark’s sensory organs. One set angles in toward the shark’s pit organ, and other angles away from the lateral-line organ. The function of the pit organ is a matter of controversy in biology, Koeltzsch said, but scientists believe the lateral-line organ functions similarly to the human ear. Scientists have debated the exact purpose of the converging and diverging riblets for a decade.

Koeltzsch and his coauthors suspected that the diverging riblets drew water away from a shark’s “ears” to prevent the noisy sound of rushing water, which inhibits the sharks hearing. If that were true, the riblets could enable sharks to better hear signs of prey.

To simulate riblets in the lab, the engineers lined their pipe with the textured film by technology company 3M so that the ridges made an angle of 45 degrees with the length of the pipe. The ridges on one side of the pipe formed a converging pattern, and the other side formed a diverging pattern.

In tests, the converging riblets slowed airflow near the pipe wall by as much as 15 percent, and the diverging riblets sped up airflow by the same amount. This means diverging riblets reduce turbulence, making water flow more quietly past the shark’s ear.

“Everybody might have had that experience, when on a windy day we hear the noisy sound of air rushing past our ears,” Koeltzsch said. “A fast-swimming shark listens to that noise constantly, only the fluid rushing past its ears is water, not air. The faster it swims, the louder the noise. This study suggests that fast-swimming sharks evolved diverging riblets to speed water past their lateral-line organ and reduce background noise in just the right way to aid their sense of hearing.

“Since the function of the pit organ is still not clear, these findings could help biologists solve that mystery, too,” he added.

Koeltzsch suspects that diverging riblets could also be used to improve the performance of microphones, where the flow of air or water over the equipment affects audio quality.

This work was funded by Deutsche Forschungsgemeinschaft, the German equivalent to the U.S. National Science Foundation. Koeltzsch is continuing his investigations at Ohio State, using the funds from his von Humboldt fellowship.

With Robert Brodkey, professor of chemical engineering, Koeltzsch has now turned his attention away from sharks, to penguins and seals. He hopes to determine whether hair makes these aquatic mammals more hydrodynamic. Initial studies by other scientists have shown that natural and artificial fibers can reduce drag by amounts that vary from 1.5 to 50 percent.

Continued research could show whether hair would improve the design of boat hulls and even airplanes, Koeltzsch said. Airplanes wouldnt need to use as much high-cost fuel if they experienced less friction as they cut through the air.

“Wouldn’t it be something if, in the future, airplanes had hairy surfaces?” he asked.

Contact: Konrad Koeltzsch, (614) 688-3210; Koeltzsch.1@osu.edu
Written by Pam Frost Gorder, (614) 292-9475; Gorder.1@osu.edu