Shark Skin Research Could Reduce Airplane Drag By
30 Percent
by
Mary Grady, News Writer,
Editor
It may seem obvious that the
surface of an airplane should be as smooth as possible to
minimize aerodynamic drag, but that's not really the case. A
bit of roughness can break up the boundary layer and improve
efficiency. Sharks, with skin formed of rough scales called
denticles
(http://www.elasmo-research.org/education/white_shark/scales.htm),
can slip through the water at speeds of up to 60 mph with
minimal drag. This week, The Lindbergh Foundation
(http://www.lindberghfoundation.org) awarded a grant to Dr.
Amy Lang, at the University of Alabama
(http://uanews.ua.edu/anews2007/nov07/shark112907.htm), to
study whether the surface texture on the skin of fast-swimming
sharks, capable of bristling their scales when in pursuit of
prey, could be mimicked and used to reduce the drag on
aircraft. "If we can successfully show there is a significant
effect, future applications to reduce drag of aircraft and
underwater vehicles could be possible," said Lang. The
technology has the potential to increase aerodynamic
efficiency up to 30 percent, with savings of billions of
dollars and substantial reductions in fuel burn and emissions.
Engineering Project Explores Energy Conservation
Through Shark Research
by
Allison Bridges
TUSCALOOSA, Ala. – The stars of
the “Jaws” films–sharks–have recently become the subject of a
University of Alabama engineering research project. Conducted
by Dr. Amy Lang, assistant professor of aerospace engineering
and mechanics, the project explores energy conservation and
boundary layer control in regard to a shark’s surface.
The project findings will allow
researchers to explore natural solutions for the reduction of
skin friction over solid surfaces, which could result in new
innovations and applications concerning energy conservation.
This research will not only provide a greater understanding of
the evolutionary development of sharks, but it will also
investigate methods of flow control and drag reduction that
can be easily applied to mobile vehicles.
Research has shown the issue of
reducing drag over solid surfaces can save thousands of
dollars. For example, it is estimated that even a 1 percent
reduction in drag can save an airline company up to $200,000
and at least 25,000 gallons of fuel per year per aircraft. The
resulting reduction in emissions into the air is equally
impressive.
Funded through a National Science
Foundation Small Grant, the project is investigating the
boundary layer flow over a surface that mimics the skin of a
fast-swimming shark. The boundary layer is the area closest to
the surface where viscous conditions cause drag–in this
instance a shark’s skin.
Lang hopes to explain why fast
sharks that swim upwards of 60 mph have smaller denticles, or
scales, than slower shark species. Evidence suggests that
sharks with smaller denticles have the ability to stick out
their scales when they swim, allowing them to swim faster and
creating a unique surface pattern on the skin that results in
various mechanisms of boundary layer control.
“We hope to explain how a shark’s
skin controls the boundary layer to decrease drag and swim
faster,” said Lang. “If we can successfully show there is a
significant effect, future applications to reduce drag of
aircraft and underwater vehicles could be possible.”
Lang’s research is being conducted using a water tunnel
facility in Hardaway Hall. The water tunnel lab can increase
the shark skin geometry by 100 times with a corresponding
decrease in flow over the model. This makes the flow over the
skin observable, and it allows for the visualization and
measurement of flow using modern experimental techniques.
In addition to the National Science Foundation Small Grant,
Lang recently received a Lindbergh Grant for this research
project. Lindbergh Grants are made in amounts up to $10,580, a
symbolic amount representing the cost of building Charles
Lindbergh’s plane, the Spirit of St. Louis.
In 1837, The University of Alabama became one of the first
five universities in the nation to offer engineering classes.
Today, UA’s fully accredited College of Engineering has about
1,900 students and nearly 100 faculty. In the last seven
years, students in the College have been named USA Today
All-USA College Academic Team members, Goldwater scholars,
Hollings scholars and Portz scholars.
The University of Alabama, a student-centered research
university, is in the midst of a planned, steady enrollment
growth with a goal of reaching 28,000 students by 2010. This
growth, which is positively impacting the campus and the
state's economy, is in keeping with UA's vision to be the
university of choice for the best and brightest students. UA,
the state's flagship university, is an academic community
united in its commitment to enhancing the quality of life for
all Alabamians.
Skin of the Teeth
Shark scales are tiny compared
with those of teleosts (bony fishes) and have a characteristic
tooth-like structure. Although they are often termed placoid
("plate-like") scales in older texts, most biologists today
prefer the more descriptive phrase, dermal denticles
(literally, "tiny skin teeth"). These denticles typically have
a broad basal plate, a narrow stalk, and a broad, ridged or
otherwise highly sculptured crown. In general, the crowns of
dermal denticles have cusps pointing tailward, which is why a
shark feels relatively smooth if stroked from head-to-tail but
sandpapery coarse if stroked the other way. (An interesting
exception is the Basking Shark [Cetorhinus maximus], in which
the crowns seem to point every which way; Norwegians, who have
commercially harvested this species for decades, have come up
with a clever use for this peculiarity: they paste a strip of
Basking Shark skin on the soles of their boots, preventing
slippage on wet, rolling decks.) The White Shark is furnished
with dermal denticles, too, and it is worth taking a moment to
consider briefly their many functions.
Dermal Denticles of a White
Shark (head to the left).
A. Dorsal(top) view of the
crowns of three denticles.
B. Lateral (side) view of a
single denticle.
Redrawn after Radcliffe
(1916)
Dermal denticles are built on the
same engineering principles as the most durable of man-made
compounds, such as fibreglass and reinforced concrete.
Embedding a hard material inside a softer one combines the
best properties of both, providing the rigidity of the former
without brittleness and the plasticity of the latter without
distortion. The dentine layer of dermal denticles is composed
of a hard, crystalline mineral called apatite, embedded in a
soft protein, our old friend collagen. Due to their
microstructure, dermal denticles are about as hard as granite
and as strong as steel. Not surprisingly, dermal denticles
afford sharks no small measure of physical protection. Yet
they do so without sacrificing mobility, like a built-in suit
of chainmail armor. The dermal denticles of the White Shark
have crowns shaped like miniature horseshoe crabs, so tiny as
to be barely visible to the naked eye. These crowns overlap
tightly, providing protection from both large potential
predators — including other Great Whites — and tiny skin
parasites.
The denticle crowns of the White Shark are highly sculptured,
each with three longitudinal ridges that terminate in a
rearward-pointing cusp. Although it may seem counter-intuitive
for an aquatic animal to be anything but smooth as possible,
there are actually sound hydrodynamic benefits to be gained
from such sandpaper roughness. How strategic roughness can
yield aero- and hydro-dynamic benefits has elicited a great
deal of research in recent years. Consider the humble golf
ball. Those characteristic dimples are not created equal: the
indentations around the equator of the ball are actually
slightly deeper than those at the poles. This deceptively
simple design feature grants a golf ball in flight and with
the proper backspin an additional two seconds of 'hang-time' —
increasing driving range by as much as 80 feet (24 metres) —
and reduces the incidence of hooks and slices by as much as
75%. Similarly, in fighter jets or fast ships, the secret to
their phenomenal speed lies in fine, V-shaped grooves. These
grooves must be very closely spaced — about as close together
as the grooves on an old-fashioned phonograph record (Anyone
remember those?). Such closely-spaced grooves appear to reduce
drag by preventing eddies from coming in contact with the
surface of a moving body. Nowadays, there is hardly an
American military aircraft or vessel that does not somehow
benefit from the fluid dynamic efficiency of incorporating
strategically-placed, V-shaped grooves along the fuselage,
hull, and foils. But, whenever there is a physical principle
that provides an elegant solution to a practical environmental
challenge, it seems that Nature always beats us to the punch.
Collectively, the tiny, three-ridged dermal denticles of the
White Shark create closely-spaced grooves similar to those on
high-speed air or water craft. These denticles very probably
impart similar drag-reducing properties to the shark. Thus,
without understanding the first thing about golf balls or
military craft, the White Shark has been employing many of the
same fluid dynamics principles for millions of years.
In a short but fascinating 1982 paper, Wolf-Ernst Reif and his
co-worker A. Dinkelacker reviewed the hydrodynamics of dermal
denticles in fast-swimming sharks. Reif and Dinkelacker found
that the crowns of dermal denticles in the Shortfin Mako and
other fast-swimming sharks are smooth and almost ridgeless on
the tip of the snout and leading (anterior) edges of the fins,
but elsewhere on the body the crown ridges are quite steep,
with depths one-half to two-thirds their width. They also
found that the alignment of these crown ridges varies over the
body, closely approximating path-of-least-resistance flow of
water over the surface of the shark. The smoothness of
denticles on the leading edges of the snout and fins offer the
least resistance to these areas of minimal boundary layer
thickness. In contrast, the alignment of crown ridges with the
'natural' flow-direction of water over the shark's body can be
expected to maximize drag reduction by reducing turbulence,
thereby preventing eddy formation. The arrangement of dermal
denticles in the White Shark is probably very similar to that
exhibited by the Shortfin Mako. Thus, like a dimpled golf
ball, a grooved Great White may glide farther on a given
amount of energy than would a smooth one.
In a 1986 paper, biologists William Raschi and Jennifer Elsom
reviewed the drag-reduction properties of shark dermal
denticles. Raschi and Elsom examined the denticles of 15
species of shark and found that those of fast-swimming pelagic
species — such as the Shortfin Mako — are consistently smaller
and lighter than those of sluggish or bottom-dwelling species.
Therefore, the relatively small, light-weight dermal denticles
of the White Shark are probably adapted for fast swimming more
than armor-like protection — yet another compromise between
form and function. In addition, they found that the Shortfin
Mako and other fast-swimming species consistently had ridge
characteristics nearer those values predicted as optimal for
burst speeds. Raschi and Elsom also found that, despite
growth-associated increases in the crown size of denticles,
the height and spacing of the scales' longitudinal ridges
remained nearly constant in all species examined. This
suggests that some important functional feature may be
maintained throughout a shark's life. That feature is very
probably drag reduction — in a 1984 report, Raschi and
ichthyologist Jack Musick discovered that the longitudinal
ridge system created by shark dermal denticles is responsible
for drag reductions as great as 8%. That percentage represents
a substantial energy savings, and it seems unlikely that the
White Shark would not take advantage of the benefits afforded
by this mechanism.
There is at least one further benefit of sharks'
hydrodynamically sculpted dermal denticles: stealth. Despite
pioneer undersea explorer Jacques-Yves Cousteau's poetic
description of the marine environment as a "silent world", the
ocean is full of noise. Mournful songs of lonely whales,
exuberant squeals and clicks of cavorting dolphins,
multitudinous croaks and yaps of reproductively-ripe fishes,
and the incessant, static-like chorus of snapping shrimps vie
with the mechanical rumble of ocean-going ships and the
frenetic buzz of speedboats. If you were to lower a hydrophone
(underwater microphone) near a school of teleost fishes, you
would quite easily hear the sloshing sounds of water
turbulence, created by their swimming movements. The large,
overlapping scales of teleosts are not nearly as
hydrodynamically 'clean' as the dermal denticles of sharks. If
you were to place the same hydrophone near a cruising shark,
no such swimming sounds would be heard. Sharks are, literally,
"silent hunters". For the Great White, this hydrodynamic
side-effect probably confers tremendous advantages when
stalking prey: the hapless fish or sea lion almost never hears
the shark that caught it.
WO2008121418
A PASSIVE DRAG MODIFICATION
SYSTEM
Abstract
-- A micro-array surface that provides for drag
reduction. In one aspect, an aerodynamic or hydrodynamic wall
surface that is configured to modify a fluid boundary layer on
the surface comprises at least one array of micro-cavities
formed therein the surface. In one example, the interaction of
the micro-cavities with the boundary layer of the fluid can
delay transition of the fluid over an identical smooth surface
without the micro-cavities.
A PASSIVE MICRO-ROUGHNESS ARRAY
FOR DRAG MODIFICATION
US2007194178 // WO2008103663
Abstract
--vThe present invention is directed to a micro-array surface
that provides for either drag reduction or enhancement, hi one
aspect, an aerodynamic or hydrodynamic wall surface that is
configured to modify a fluid boundary layer on the surface
comprises at least one array of roughness elements disposed on
and extending therefrom the surface. In one example, the
interaction of the roughness elements with a turbulent
boundary layer of the fluid reduces the skin friction drag
coefficient of the surface over an identical smooth surface
without the roughness elements.
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