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
byAllison 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.
