Aerodynamics

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Aerodynamics is a branch of dynamics concerned with studying the motion of air,
particularly when it interacts with a moving object. Aerodynamics is closely related
to 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. Most aerodynamicists use numbers between
5 and 8.

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.

Aerodynamic problems are solved using the conservation laws, or equations derived
from the conservation laws. In aerodynamics, three conservation laws are used:

Conservation of mass: Matter is not created or destroyed. If a certain mass
of fluid enters a volume, it must either exit the volume or increase the mass
inside the volume.
Conservation of momentum: Also called Newton’s second law of motion. The initial
sum of momentum (mass times velocity) must equal the ending sum of momentum.

Conservation of energy: Although it can be converted from one form to another,
the total energy in a given system remains constant.

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.