by David Geer
Contact the author at geercom@windstream.net
Knifefish Research Robot Models
Undulating Fin Wave Propulsion
By studying the waves the knifefish makes with its undulating, body-length
fin, Malcolm A. MacIver, PhD, associate professor and other researchers at
Northwestern University plan to learn more about this novel form of fish
propulsion so they can develop comparable fins for water-based robots to
enhance their mobility, hovering, and station-keeping capabilities.
Robotic fins would make for better propulsion in hovering and station keeping
because the current use of propellers offers poor thrust performance at low
speeds. “In addition, they are prone to entanglement in weeds and damage
due to the need for high spin rates for smooth performance,” stated MacIver.
Why and How They Studied
the Black Ghost Fish
The specific fish under the researcher’s thoughtful
attention is the black ghost fish — a type of knifefish from
South American regions. The fish was selected due to its
unique sensing, navigation, and propulsion skills. The fish
does object sensing using an electrical discharge it sends
out in all directions. The fish can change direction very
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quickly. When simply moving ahead or back, the fish moves
the ribbon fin in waves either front to back or back to front.
“The black ghost fish are low speed, high agility specialists.
I have shown that they reverse direction in about half a
second when swimming at around 10 cm/s,” commented
MacIver.
The curious navigational wizardry of the black ghost
fish occurs when it is hovering or station keeping. The fish
can remain in one place and keep its station against the
water currents by use of what the researchers call inward
counter-propagating waves. The fish creates two recurring,
simultaneous waves along its fin. One starts at the front
and moves to the tail; the other starts at the tail and moves
toward its head; and the two waves meet in the middle.
Because the amplitude of each of the two waves tapers to
zero when they meet at the middle of the fin, both waves
stop at the center of the fin when they meet, according to
MacIver.
To demonstrate and study the fish’s keen station-keeping capacity, the researchers apply computational fluid
dynamics to a computer model of the actual fish while
using digital particle image velocimetry (DPIV) to observe
and clock the speed of the flow around the robotic fish.
Malcolm MacIver holding his knifefish robot used to
research waves created by the real knifefish’s body-length
underbelly fin. Photo by Andrew Campbell.
