Aerodynamics is a branch of
dynamics concerned with studying the
motion of air, particularly when it interacts with a moving object.
Aerodynamics is a subfield of
fluid
dynamics and
gas dynamics, with
much theory shared between them. Aerodynamics is often used
synonymously with gas dynamics, with the difference being that gas
dynamics applies to all gases. Understanding the motion of air
(often called a flow field) around an object enables the
calculation of forces and moments acting on the object. Typical
properties calculated for a flow field include
velocity,
pressure,
density and
temperature as a function of position and time.
By defining a
control volume around
the flow field, equations for the conservation of mass, momentum,
and energy can be defined and used to solve for the properties. The
use of aerodynamics through mathematical analysis, empirical
approximation and wind tunnel experimentation form the scientific
basis for
heavier-than-air
flight.
Aerodynamic problems can be identified in a number of ways. The
flow environment defines the first classification criterion.
External aerodynamics is the study of flow around solid
objects of various shapes. Evaluating the
lift and
drag on
an
airplane, the
shock waves that form in front of the nose of a
rocket or the flow of air over a hard drive
head are examples of external aerodynamics.
Internal
aerodynamics is the study of flow through passages in solid
objects. For instance, internal aerodynamics encompasses the study
of the airflow through a
jet engine or
through an
air conditioning
pipe.
The ratio of the problem's characteristic flow speed to the
speed of sound comprises a second
classification of aerodynamic problems. A problem is called
subsonic if all the speeds in the problem are less than the speed
of sound,
transonic if speeds both below
and above the speed of sound are present (normally when the
characteristic speed is approximately the speed of sound),
supersonic when the characteristic flow speed is
greater than the speed of sound, and
hypersonic when the flow speed is much greater
than the speed of sound. Aerodynamicists disagree over the precise
definition of hypersonic flow; minimum
Mach
numbers for hypersonic flow range from 3 to 12.
The influence of
viscosity in the flow
dictates a third classification. Some problems involve only
negligible viscous effects on the solution, in which case viscosity
can be considered to be nonexistent. The approximations to these
problems are called
inviscid flows.
Flows for which viscosity cannot be neglected are called viscous
flows.
History
and stories of flight have appeared throughout recorded history,
such as the legendary story of
Icarus and
Daedalus. Although observations of some
aerodynamic effects like wind resistance (a.k.a.
drag) were recorded by the likes of
Aristotle,
Avicenna,
Leonardo da Vinci and
Galileo Galilei, very little effort was made
to develop governing laws for understanding the nature of flight
prior to the 17th century.
In 1505,
Leonardo da Vinci wrote
the
Codex on the Flight
of Birds, one of the earliest treatises on aerodynamics.
He notes for the first time that the
center of gravity of a flying bird does
not coincide with its
center of
pressure, and he describes the construction of an
ornithopter, with flapping wings similar to a
bird's.
Sir Isaac Newton was the first person
to develop a theory of air resistance, making him one of the first
aerodynamicists. As part of that theory, Newton believed that drag
was due to the dimensions of a body, the density of the fluid, and
the velocity
raised to the second
power. These beliefs all turned out to be correct for low flow
speeds. Newton also developed a law for the drag force on a flat
plate inclined towards the direction of the fluid flow. Using F for
the drag force, ρ for the density, S for the area of the flat
plate, V for the flow velocity, and θ for the inclination angle,
his law is expressed below.F = \rho SV^2 \sin^2 (\theta)
Unfortunately, this equation is completely incorrect for the
calculation of drag (unless the flow speed is
hypersonic). Drag on a flat plate is closer to
being linear with the angle of inclination as opposed to acting
quadratically. This formula can lead one to believe that flight is
more difficult than it actually is, and it may have contributed to
a delay in human flight.
Sir George Cayley is credited as the first person to identify the
four aerodynamic forces of flight -
weight,
lift,
drag, and
thrust, and
the relationship between them. Cayley believed that the drag on a
flying machine must be counteracted by a means of propulsion in
order for level flight to occur. Cayley also looked to nature for
aerodynamic shapes with low drag. One of the shapes he investigated
were the cross-sections of
trout. This may
appear counterintuitive, however, the bodies of fish are shaped to
produce very low resistance as they travel through water. Their
cross-sections are sometimes very close to that of modern low drag
airfoils.
These empirical findings led to a variety of air resistance
experiments on various shapes throughout the 18th and 19th
centuries. Drag theories were developed by
Jean le Rond d'Alembert,
Gustav Kirchhoff, and
Lord Rayleigh. Equations for
fluid flow with
friction were developed by
Claude-Louis Navier and
George Gabriel Stokes. To simulate
fluid flow, many experiments involved immersing objects in streams
of water or simply dropping them off the top of a tall building.
Towards
the end of this time period Gustave
Eiffel used his Eiffel
Tower to assist in the drop testing of flat
plates.
Of course, a more precise way to measure resistance is to place an
object within an artificial, uniform stream of air where the
velocity is known. The first person to experiment in this fashion
was
Francis Herbert Wenham,
who in doing so constructed the first
wind
tunnel in 1871.
Wenham was also a member of the first
professional organization dedicated to aeronautics, the Royal Aeronautical Society of the
United
Kingdom. Objects placed in wind tunnel models are
almost always smaller than in practice, so a method was needed to
relate small scale models to their real-life counterparts. This was
achieved with the invention of the dimensionless
Reynolds number by
Osbourne Reynolds. Reynolds also
experimented with
laminar to
turbulent flow transition in 1883.
By the late 19th century, two problems were identified before
heavier-than-air flight could be realized. The first was the
creation of low-drag, high-lift aerodynamic wings. The second
problem was how to determine the power needed for sustained flight.
During this time, the groundwork was laid down for modern day
fluid dynamics and aerodynamics, with
other less scientifically inclined enthusiasts testing various
flying machines with little success.
1889,
Charles Renard, a French
aeronautical engineer, became the first person to reasonably
predict the power needed for sustained flight. Renard and German
physicist
Hermann von
Helmholtz explored the wing loading of birds, eventually
concluding that humans could not fly under their own power by
attaching wings onto their arms.
Otto
Lilienthal, following the work of Sir George Cayley, was the
first person to become highly successful with glider flights.
Lilienthal believed that thin, curved airfoils would produce high
lift and low drag.
Octave Chanute provided a great
service to those interested in aerodynamics and flying machines by
publishing a book outlining all of the research conducted around
the world up to 1893. With the information contained in that book
and the personal assistance of Chanute himself, the
Wright brothers had just enough knowledge of
aerodynamics to fly the first powered aircraft on December 17,
1903, just in time to beat the efforts of
Samuel Pierpont Langley. The Wright
brothers' flight confirmed or disproved a number of aerodynamics
theories. Newton's drag force theory was finally proved incorrect.
The first flight led to a more organized effort between aviators
and scientists, leading the way to modern aerodynamics.
During the time of the first flights,
Frederick W. Lanchester,
Martin Wilhelm Kutta, and
Nikolai Zhukovsky independently created
theories that connected
circulation of a fluid flow to
lift. Kutta and Zhukovsky went on to develop a two-dimensional wing
theory. Expanding upon the work of Lanchester,
Ludwig Prandtl is credited with developing
the mathematics behind thin-airfoil and lifting-line theories as
well as work with
boundary layers.
Prandtl, a
professor at Gottingen
University, instructed many students who would play important
roles in the development of aerodynamics like Theodore von Kármán and
Max Munk.
As aircraft began to travel faster, aerodynamicists realized that
the density of air began to change as it came into contact with an
object, leading to a division of fluid flow into the incompressible
and
compressible regimes. In
compressible aerodynamics, density and pressure both change, which
is the basis for calculating the
speed of
sound. Newton was the first to develop a mathematical model for
calculating the speed of sound, but it was not correct until
Pierre-Simon Laplace accounted
for the molecular behavior of gases and introduced the
heat capacity ratio. The ratio of the
flow speed to the speed of sound was named the
Mach number after
Ernst
Mach, who was one of the first to investigate the properties of
supersonic flow which included
Schlieren photography techniques to
visualize the changes in density.
William John Macquorn Rankine
and
Pierre Henri Hugoniot
independently developed the theory for flow properties before and
after a
shock wave.
Jakob Ackeret led the initial work on
calculating the lift and drag on a supersonic airfoil. Theodore von
Kármán and
Hugh Latimer Dryden
introduced the term
transonic to describe
flow speeds around Mach 1 where drag increases rapidly. Because of
the increase in drag approaching Mach 1, aerodynamicists and
aviators disagreed on whether supersonic flight was
achievable.
September 30, 1935 an exclusive conference was held in
Rome with the
topic of high velocity flight and the possibility of breaking the
sound barrier. Participants included
von Kármán, Prandtl, Ackeret,
Eastman
Jacobs,
Adolf Busemann,
Geoffrey Ingram Taylor,
Gaetano Arturo Crocco, and
Enrico Pistolesi. The new research
presented was impressive. Ackeret presented a design for a
supersonic wind tunnel. Busemann gave
perhaps the best presentation on the need for aircraft with
swept wings for high speed flight.
Eastman Jacobs, working for
NACA, presented his
optimized airfoils for high subsonic speeds which led to some of
the high performance American aircraft during
World War II. Supersonic propulsion was also
discussed. The sound barrier was broken using the
Bell X-1 aircraft twelve years later, thanks in
part to those individuals.
By the time the sound barrier was broken, much of the subsonic and
low supersonic aerodynamics knowledge had matured. The
Cold War fueled an ever evolving line of high
performance aircraft.
Computational fluid dynamics
was started as an effort to solve for flow properties around
complex objects and has rapidly grown to the point where entire
aircraft can be designed using a computer.
With some exceptions, the knowledge of
hypersonic aerodynamics has matured between the
1960s and the present decade. Therefore, the goals of an
aerodynamicist have shifted from understanding the behavior of
fluid flow to understanding how to engineer a vehicle to interact
appropriately with the fluid flow. For example, while the behavior
of hypersonic flow is understood, building a
scramjet aircraft to fly at hypersonic speeds has
seen very limited success. Along with building a successful
scramjet aircraft, the desire to improve the aerodynamic efficiency
of current aircraft and propulsion systems will continue to fuel
new research in aerodynamics.
Introductory terminology
Continuity assumption
Gases are composed of
molecules which
collide with one another and solid objects. If density and velocity
are taken to be well-defined at infinitely small points, and are
assumed to vary continuously from one point to another, the
discrete molecular nature of a gas is ignored.
The continuity assumption becomes less valid as a gas becomes more
rarefied. In these cases,
statistical mechanics is a more valid
method of solving the problem than continuous aerodynamics. The
Knudsen number can be used to guide
the choice between statistical mechanics and the continuous
formulation of aerodynamics.
Laws of Conservation
Aerodynamic problems are often solved using
conservation laws as applied to a
fluid continuum. In many basic problems,
three conservation principles are used:
Incompressible aerodynamics
An
incompressible flow is
characterized by a constant density despite flowing over surfaces
or inside ducts. A flow can be considered incompressible as long as
its speed is low. For higher speeds, the flow will begin to
compress as it comes into contact with surfaces. The
Mach number is used to distinguish between
incompressible and compressible flows.
Subsonic flow
Subsonic (or low-speed) aerodynamics is the study of
inviscid,
incompressible and
irrotational aerodynamics where the
differential equations used are a
simplified version of the governing equations of
fluid dynamics.. It is a special case of
Subsonic aerodynamics.
In solving a subsonic problem, one decision to be made by the
aerodynamicist is whether to incorporate the effects of
compressibility. Compressibility is a description of the amount of
change of
density in the problem. When the
effects of compressibility on the solution are small, the
aerodynamicist may choose to assume that density is constant. The
problem is then an incompressible low-speed aerodynamics problem.
When the density is allowed to vary, the problem is called a
compressible problem. In air, compressibility effects are usually
ignored when the
Mach number in the flow
does not exceed 0.3 (about 335 feet (102m) per second or 228 miles
(366km) per hour at 60
^{o}F). Above 0.3, the problem should
be solved by using compressible aerodynamics.
Compressible aerodynamics
According to the theory of aerodynamics, a flow is considered to be
compressible if its change in
density with
respect to
pressure is non-zero along a
streamline.
This means that - unlike incompressible flow - changes in density
must be considered. In general, this is the case where the
Mach number in part or all of the flow exceeds
0.3. The Mach .3 value is rather arbitrary, but it is used because
gas flows with a Mach number below that value demonstrate changes
in density with respect to the change in pressure of less than 5%.
Furthermore, that maximum 5% density change occurs at the
stagnation point of an object immersed in the gas flow and the
density changes around the rest of the object will be significantly
lower. Transonic, supersonic, and hypersonic flows are all
compressible.
Transonic flow
The term Transonic refers to a range of velocities just below and
above the local
speed of sound
(generally taken as
Mach 0.8–1.2). It is
defined as the range of speeds between the
critical Mach number, when some parts of the
airflow over an aircraft become
supersonic, and a higher speed, typically near
Mach 1.2, when all of the airflow is
supersonic. Between these speeds some of the airflow is supersonic,
and some is not.
Supersonic flow
Supersonic aerodynamic problems are those involving flow speeds
greater than the speed of sound. Calculating the lift on the
Concorde during cruise can be an example of
a supersonic aerodynamic problem.
Supersonic flow behaves very differently from subsonic flow. Fluids
react to differences in pressure; pressure changes are how a fluid
is "told" to respond to its environment. Therefore, since
sound is in fact an infinitesimal pressure difference
propagating through a fluid, the
speed of
sound in that fluid can be considered the fastest speed that
"information" can travel in the flow. This difference most
obviously manifests itself in the case of a fluid striking an
object. In front of that object, the fluid builds up a
stagnation pressure as impact with the
object brings the moving fluid to rest. In fluid traveling at
subsonic speed, this pressure disturbance can propagate upstream,
changing the flow pattern ahead of the object and giving the
impression that the fluid "knows" the object is there and is
avoiding it. However, in a supersonic flow, the pressure
disturbance cannot propagate upstream. Thus, when the fluid finally
does strike the object, it is forced to change its properties --
temperature,
density,
pressure, and
Mach number -- in an extremely violent
and
irreversible
fashion called a
shock wave. The presence
of shock waves, along with the compressibility effects of
high-velocity (see
Reynolds number)
fluids, is the central difference between supersonic and subsonic
aerodynamics problems.
Hypersonic flow
In aerodynamics, hypersonic speeds are speeds that are highly
supersonic. In the 1970s, the term generally came to refer to
speeds of Mach 5 (5 times the speed of sound) and above. The
hypersonic regime is a subset of the supersonic regime. Hypersonic
flow is characterized by high temperature flow behind a shock wave,
viscous interaction, and chemical dissociation of gas.
Associated terminology
The incompressible and compressible flow regimes produce many
associated phenomena, such as boundary layers and turbulence.
Boundary layers
The concept of a
boundary layer is
important in many aerodynamic problems. The viscosity and fluid
friction in the air is approximated as being significant only in
this thin layer. This principle makes aerodynamics much more
tractable mathematically.
Turbulence
In aerodynamics, turbulence is characterized by chaotic, stochastic
property changes in the flow. This includes low momentum diffusion,
high momentum convection, and rapid variation of pressure and
velocity in space and time. Flow that is not turbulent is called
laminar flow.
Aerodynamics in other fields
Aerodynamics is important in a number of applications other than
aerospace engineering. It is a significant factor in any type of
vehicle design, including
automobiles. It is important in the
prediction of forces and moments in
sailing.
It is used in the design of large components such as
hard drive heads.
Structural engineers also use
aerodynamics, and particularly
aeroelasticity, to calculate
wind loads in the design of large buildings and
bridges. Urban aerodynamics seeks to help
town planners and designers improve
comfort in outdoor spaces, create urban microclimates and reduce
the effects of urban pollution. The field of environmental
aerodynamics studies the ways
atmospheric circulation and flight
mechanics affect ecosystems. The aerodynamics of internal passages
is important in
heating/ventilation,
gas piping, and in
automotive engines where detailed
flow patterns strongly affect the performance of the engine.
See also
References
- Aydin
Sayili (1987), "Ibn Sīnā and Buridan on the Motion of the
Projectile", Annals of the New York Academy of Sciences
500 (1), pp. 477–82
Further reading
General Aerodynamics
Subsonic Aerodynamics
Transonic Aerodynamics
Supersonic Aerodynamics
Hypersonic Aerodynamics
History of Aerodynamics
Aerodynamics Related to Engineering
Ground Vehicles
Fixed-Wing Aircraft
Helicopters
Missiles
Model Aircraft
Related Branches of Aerodynamics
Aerothermodynamics
Aeroelasticity
Boundary Layers
Turbulence
External links
DARREN DEVLIN!!!!!!!!!!!!!!!!!!!!!!!!!!!!