Seeing biology through an engineer’s eyes

Rhythmic contractions jellyfish use to propel themselves through water inspired a UAA engineering assistant professor to apply for-and win-a $174,957 National Science Foundation grant that's helping him and a graduate student learn more about propulsion efficiency.

Dr. Jifeng Peng and Zachary Garcia, a mechanical engineering graduate student, have, since receiving that 2013-2015 NSF grant, spent their days studying vortex rings-also called vortices-doughnut-shaped rings formed when fluid flows back on itself and spins, looking like a watery version of smoke rings.

Jellyfish drift. They can also move, however, gathering water into their bells and releasing it in pulsing movements forming a series of these vortices. Peng and Garcia have been creating similar vortices with a tank and computer-controlled piston, and studying them using a laser and digital particle image velocimetry (DPIV).

"This is a novel propulsion technique," said Peng, who uses this project as an instructional demonstration and outreach for summer engineering camp sessions and high school students' tours. "If you think about engineering a propulsion system in water, propellers are pretty much the only way. Propellers generate a large amount of thrust, but their relative energy efficiency is not as high as [the use of vortices]. That's why we see in nature a lot of animals use this technique."

Squid accelerate by quickly ejecting a pulse of fluid at high velocity. French scientists in 1938 succeeded in photographing "tourbillons"-what appear to be vortex rings-behind a pigeon in slow flight, using tobacco smoke to make them visible. Other researchers used talcum powder to highlight and photograph vortex rings in the wakes of hovering insects, and scientists have also claimed to observe vortex elements in the wakes of flying finches.

'Like making bubble rings in a pool'

Peng and Garcia use a laser to spotlight particles that follow the pulses of water from the piston. They photograph the pulses and use DPIV to track the movement of each particle, which measures 14 microns-about the length of three red blood cells laid end to end, in a row.

"It's like making bubble rings in a pool," Garcia said, "but what we're doing, instead of air in the water, we're making a vortex ring of water in water."

The piston moves inside the cylinder, generating a vortex ring at the end which, when the piston stops moving, separates and moves out into the tank.

"It's kind of hard to see in the water and we want to be able to record the vortex ring so we can analyze it, so what we've done is add silver-covered glass particles," Garcia said. "They're hollow so they don't sink; they just kind of stay buoyant. They're silver coated so we can reflect a laser off of them and see them a lot better."

When Peng and Garcia first started their experiments, they used a raised tank of water and a valve-the valve opened, water flowed down and they used that rush of water to create a pulse. Then, they used a pump and a valve. The pump pressurized the water behind the valve, so opening and closing the valve created a pulse of water.

"Mostly what we've been trying to do is generate pressure, refine the method," Garcia said. "It's been trial and error." Then, joking, "That's what grad students are for!"

One of the valves they were using opened and closed quickly, but Peng and Garcia found a computer-controlled actuator could accomplish that task even faster-a computer regulates up to 12 volts of electricity that can be supplied to the actuator-like adjusting a valve on a hose to affect flow.

"At the end is the piston," Garcia said. "You can turn the valve a little or open it up to allow the maximum amount of power."

They shine a laser through a lens that splits it into a sheet of light. That sheet bisects the tank and also bisects the vortex rings, allowing the researchers to see their cross sections, the swirling rings, and record them with a camera at 30 frames per second.

"We can control the pulse length, which then corresponds to the size of the vortex ring, and we can also do multiple vortex rings with gaps in between them," Garcia said.

That's key to what they're studying-the interaction between the vortex rings as well as keeping an eye on how they interact with the surrounding fluid.

"We look at how the particles change, how their positions change between each frame," Garcia said. "We don't do it by hand; computer programs look at that. We can look at the velocity, how it's moving. You would expect the center to be spinning faster than the outside, things like that."

Opening new vistas in science

Peng's bioengineering project is important-figuring out the optimal combination of consecutive vortex size and frequency could someday help engineers devise more efficient submarines and other watercraft, wind turbines and eventually advance research into Lilliputian vehicles that could be employed in microsurgery.

"The NSF wanted to cultivate interdisciplinary research," Peng said. "As a researcher, you're always looking for a new field of study."

Written by Tracy Kalytiak, UAA Office of University Advancement.