New biomimetic research inspired by lamprey eels and jellyfish indicates that underwater propulsion methods based on “pushing off” against the water, such as using propellers or kicking feet, may not be the most efficient way to move through water.
A team of mechanical engineers working with biologists at Stanford University have collaborated to study the movement of jellyfish and lamprey to determine how they swim through the water with unmatched efficiency.
What they discovered is that rather than pushing against the water to move forward, lampreys and jellyfish create pockets of low pressure around their bodies which pulls them forward through the water.
The team believes this pulling propulsion is what makes these creatures such efficient swimmers.
According to the team’s research, as a lamprey swims with an undulating motion, it creates pockets of low pressure inside the bends of its body.
As the water ahead of the lamprey rushes into the pockets of low pressure, the flow of this water pulls the lamprey forward.
Jellyfish propulsion is similar. As the umbrella-shaped plume collapses, water ahead of the jellyfish is pulled behind it, propelling it forward.
“It confounds all of our assumptions," said John Dabiri, a professor of civil and environmental engineering and of mechanical engineering at Stanford. "But our experiments show that jellyfish and lampreys actually suck water toward themselves to move forward instead of pushing against the water behind them, as had been previously supposed."
The team used an equation to describe the theoretical behavior of fluids moving around solid objects that examines the interplay of three main variables: time, the rate of flow, and the pressure exerted by each fluid molecule on its neighbors.
"Interactions between solid objects are usually straightforward, like two billiard balls bouncing off each other, and therefore you can calculate the forces without much difficulty," Dabiri said. "But in a fluid every molecule is like a billiard ball and they are practically innumerable. There isn't a simple way to calculate all those interactions."
The measurements for time and flow were easy to determine but pressure is much more difficult to gauge, particularly as an animal swims through it.
The Stanford researchers devised an experimental system that enabled them to approximate this pressure variable.
They began with a rectangular acrylic tank measuring approximately one foot wide, four feet long and six inches deep. The tank was filled with water and millions of tiny hollow glass beads that act as proxies for water molecules.
Two laser and digital camera setups were positioned opposite each other on the thin sides of the rectangle.
As small jellyfish and lampreys swam through the tank, their motions perturbed the glass beads. The lasers tracked the positions of the glass beads and the digital cameras recorded it all in fractions of a second.
Lamprey swimming in the corner of a water tank is illuminated by green and red lasers. Reflections in the corner of the water tank give the appearance of multiple lampreys. (Image courtesy of Stanford/Sean Colin.)
The experiment then became an exercise in computation.
Time and flow at each moment could be precisely measured for the beads. Making thousands upon thousands of calculations, the team used these two precise variables to solve for the third, elusive variable of pressure.
The researchers fed these results into a computer that transformed the movement data into a visual representation of the pressure forces involved.
It became clear that the low-pressure pockets created on the inside edge of each undulating animal movement were the dominant driver of propulsion by pulling water toward the animal to move it forward.
As additional support to their findings, the researchers compared two batches of lampreys of the same species.
The control batch moved in natural, undulating fashion. The experimental batch were modified so that only their tail ends flicked, using a less efficient kicking motion similar to that of human swimmers.
"The body undulations of the normal lampreys set them apart as much better swimmers than you and me," Dabiri said. "Human swimmers generate high pressure instead of suction. That's good enough to get you across the pool but requires much more energy than the suction action of lampreys and jellyfish."
The team sees these new biomimetic models of propulsion as having applications for designing advanced underwater craft, such as submarines and other deep sea exploratory vehicles.
As the distance-for-effort in lamprey and jellyfish swimming is so efficient, using a similar style of flexible propulsion design could enable much more efficient and maneuverable vehicles.
The complete paper describing their suction-based propulsion method is published in the journal Nature Communications and is available to read here.