Page 282 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

years. They have created a 4-foot (ft) model “robot fish” that
swims down the Ocean Engineering Test Tank Facility. The robot
fish resembles a real fish. The skin is made of foam and Lycra. The
robot uses six external motors that are connected to pulleys and
tendons within the robot. The fish moves and swims like a real
bluefin tuna.
Swimming with foils

The tail of a fish is considered a hydrofoil. As the tail flaps side to
side, it pushes water backwards and propels the fish forward. As the
tail moves, vortexes are formed in the water behind it. It is believed
that the vortex formation is key to understanding the greater effi-
ciency of fish propulsion.

Dolphins are interesting; their hydrofoil tail lies horizontal. So
instead of moving their tails side to side like fish, they move their
tails up and down. This creates the same efficient thrust in water
propelling the dolphin forward.

Penguins swim by using the thrust generated by their wings. Pic-
tures of penguins swimming in water strongly resemble those of
flying birds. There is a difference though. With birds in flight, the 261
beating of their wings must supply lift as well as forward thrust.
The lift is necessary to counteract the force of gravity. With penguins
there is no necessity of lift. The density of water equals that of a pen-
guin’s body (neutral buoyancy), so the flapping of a penguin’s wings
simply needs to produce forward thrust.

Paddles and rows

Since we’re looking at methods of locomotion in water, we might
as well include paddles and rows. Ducks use their webbed feet as
paddles when swimming through water. Water beetles use their
legs as oars and row themselves along like tiny boats.

What have we learned so far?

Studies at MIT lead researchers to use a fluid dynamic parameter
known as the Strouhal number. For fish, the number is calculated
by multiplying the frequency of the tail flapping back and forth by
the width of the vortex created in the water divided by the fish’s
speed. A number of species of fish were studied. The results were
that maximum efficiency is found when the Strouhal number lies
between 0.25 and 0.35.
Team LRN Underwater bots