Lateral lines in fish contain hundreds of tiny pressure and velocity sensors that enable them to navigate through currents and eddies as efficiently as possible. To mimic that ability, MIT researchers have developed inexpensive, sensitive MEMS-based pressure sensors and mounted them on a small experimental vessel in a pattern that replicates the distribution of the lateral lines. Image: MIT |
Anyone who has steered a boat knows how much effort is
needed to keep the boat on course when currents are pushing it in different
directions. Now, Massachusetts Institute of Technology (MIT) researchers have
developed sensors that can measure the pressure of flows around an oceangoing
vessel so that it can utilize rather than fight those flows, saving energy and
improving maneuverability. Other work aims to go a step further: to change
flows from patterns that impede progress to patterns that will help.
Flows
around autonomous underwater vehicles (AUVs) and other vessels can
significantly affect their performance. For example, when a vessel going 20 mph
turns sharply, it pushes into the current on one side and creates swirling
eddies on the other; as a result, its speed can drop suddenly to 7 mph. The
behavior of control surfaces such as rudders and propellers can also be
affected. A propeller operating in waves, for instance, can experience
cavitation, a phenomenon in which vapor layers form around the blades, impeding
performance. Preventing such phenomena could mean smoother, more
energy-efficient operation. Indeed, oceangoing vessels are now responsible for
8.6% of the world’s total annual oil consumption, so even a small increase in
efficiency could mean significant energy savings.
Natural
sea creatures do not experience such problems because they have special organs
that enable them to sense their environment. In many fish, dark-colored “lateral lines” running down their sides and around their heads contain
hundreds of tiny pressure and velocity sensors that perceive every minute
change in the water flowing by, enabling the fish to turn or take other
appropriate action. The effect can be astonishing. The Mexican cavefish, for example,
lives in absolute darkness. As a result, it has no eyes and must navigate using
only its lateral lines. In an experimental setting, a cavefish can dart among
obstacles, moving quickly along their edges and ducking through openings
between them.
“We
want to design sensors for our vessels that can do exactly what the lateral
lines do for fish,” says Michael Triantafyllou, the William I. Koch Professor
of Marine Technology and professor of mechanical and ocean engineering. “But
while we get ideas from fish, we needn’t use exactly the same design that they
do.” In fish, the lateral lines are made up of systems of fluid-filled canals
containing tiny hairs that monitor flows and send messages directly to the
fish’s brain.
“This
is an organ we don’t have, so we have no idea of how it really works, but it’s
good because it’s simple and doesn’t require the intense computation that
vision requires, for example,” Triantafyllou says. The engineered version, he
adds, should likewise generate “simple signals so that—without using a huge
computer—we know immediately what’s going on and can take action.”
To
design and fabricate his pressure sensors, Triantafyllou turned to the MIT
Microsystems Technology Laboratories (MTL). There, experts make various types
of inexpensive, high-performance sensors based on microelectromechanical
systems (MEMS). Led by Jeffrey Lang, a professor of electrical engineering, an
MTL team designed arrays of pressure sensors, each of which is a
two-millimeter-wide cavity covered by a 20-um-thick silicon membrane that bends
in response to pressure. A metal strain gauge on the surface of each membrane
senses that deflection and generates a signal that indicates pressure. Electronic
systems amplify and integrate the signals from all the sensors, producing
pressure information that can be displayed continuously online.
In
tests on small vessels and propellers, the sensor arrays proved robust and even
more sensitive than expected. In one set of experiments, Triantafyllou and his
colleagues in the Center for Ocean Engineering equipped a small vessel with
sensors in locations that mimic where they are on fish. They also installed
commercially available sensors that would generate reliable measurements for
comparison and guidance. Then they performed experiments in the 108-ft-long MIT
Towing Tank, a test facility equipped with a wave generator.
In
those experiments, they simulated a common situation: A vessel is traveling
straight ahead, but the oncoming current is approaching at an angle, so the
vessel must exert energy to offset that force. A more energy-efficient approach
would be to head straight into the current as long as possible and then turn,
much as a sailboat tacks in the wind. Pressure measurements could guide the
execution of such an energy-saving maneuver.
To
replicate that situation, the researchers propel their vessel directly into
oncoming flows from the wave generator and then at a gradually increasing
angle. As the angle increases, pressure asymmetries increase dramatically. The
combination of low pressure on one side and high pressure on the other creates
a drag force that must be overcome—a significant waste of energy.
“The
effect is very detectable,” Triantafyllou says. “These sharp pressure signals
can guide us as we develop techniques to navigate and maneuver more
efficiently.”
Other
work aims to detect eddies, swirling fluid structures that can also profoundly
affect navigation. Again, fish use their lateral lines to identify eddies—and
then take advantage of them. In one video, a trout swims in a tank as eddies
come toward it, first from one side and then from the other. The trout senses the
eddies and uses their suction force to stay in one place without swimming, thereby
expending little energy.
To
test their ability to identify eddies, the researchers again used the MIT
Towing Tank. For these tests, they seeded the water with small particles and
shone a laser beam from below so as to observe the patterns of flow without
disturbing them. Four sensors measured pressure as hand-generated eddies
swirled through the tank. Based on the pressure signals, a flow model estimated
the position and strength of the eddies. The model accurately tracked the
behavior of the eddies within the tank.
Triantafyllou
and his team are now developing methods of controlling flows that interfere
with propulsion and maneuverability. In one project, they designed a
torpedo-shaped submersible vehicle that has pressure sensors plus two small
rotating cylinders running down its sides. When the submersible heads at an
angle into the oncoming flow, the pressure sensors detect the formation of
eddies and start the small cylinders spinning. The cylinders spin in opposite
directions, creating suction that immediately prevents eddies from forming.
The
team is also looking at another possible animal model: the whisker of a seal.
This organ has a remarkable ability to sense the velocities of flows. In
experiments, a blindfolded harbor seal can detect the passage of a fish by
using its whiskers to sense changes in flow velocity—even 30 sec after its prey
has passed by.
The
researchers recently acquired whiskers shed by seals at the New England
Aquarium in Boston.
They have now developed large-scale models of these elaborate, undulating
structures and are developing computer simulations of how they behave. “We’re
trying to understand why these whiskers work so well,” Triantafyllou says. “Once again, we hope to emulate the ability of seagoing creatures to sense
flows around them—a prerequisite to developing ways to make our vessels more
energy efficient and maneuverable.”
