Aerodynamics is the way air moves around things. The
rules of aerodynamics explain how an airplane is able to fly. Anything
that moves through air reacts to aerodynamics. A rocket blasting off the
launch pad and a kite in the sky react to aerodynamics. Aerodynamics
even acts on cars, since air flows around cars.
Aerodynamics is a common application of CFD. CFD
allows the steady-state and transient aerodynamics of HVAC systems,
vehicles, aircraft, buildings, structures, wings and rotors to be
computed with extremely high levels of accuracy. System properties such
as mass flow rates and pressure drops and fluid dynamic forces such as
lift, drag and pitching moment can be readily calculated in addition to
the wake effects. This data can be used directly for design purposes or
as in input to a detailed stress analysis.
CFD analysis offers the ability to conduct
comprehensive, automated, multi-point optimization of designs. This
process allows engineers to automatically optimize a design to a given
set of performance parameters and can be used to minimize drag, or
maximize mass flow or lift forces to given targets.
Subsonic aerodynamics
In a subsonic aerodynamic problem, all of the flow
speeds are less than the speed of sound. This class of problems
encompasses nearly all internal aerodynamic problems, as well as
external aerodynamics for general aviation aircraft, model aircraft, and
automobiles.
In solving a subsonic problem, one decision to be
made by the aerodynamicist is whether or not 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 problem. When
the density is allowed to vary, the problem is called a compressible
problem. In air, compressibility effects can be ignored when the Mach
number in the flow does not exceed 0.3. Above 0.3, the problem should be
solved using compressible aerodynamics.
Transonic aerodynamics
Transonic aerodynamic problems are defined as
problems in which both supersonic and subsonic flow exist. Normally the
term is reserved for problems in which the characteristic Mach number is
very close to one.
Transonic flows are characterized by shock waves and
expansion waves. A shock wave or expansion waves is a region of very
large changes in the flow properties. In fact, the properties change so
quickly they are nearly discontinuous across the waves. Flow ahead of a
shock wave is supersonic; flow behind a shock wave is subsonic.
Transonic problems are arguably the most difficult to
solve. Flows behave very differently at subsonic and supersonic speeds,
therefore a problem involving both types is more complex than one in
which the flow is either purely subsonic or purely supersonic.
Supersonic aerodynamics
Supersonic aerodynamic problems are those involving
flow speeds greater than the speed of sound. Calculating the lift on the
Concorde can be an example of a supersonic aerodynamic problem.
Supersonic flow behaves very differently from
subsonic flow. The speed of sound can be considered the fastest speed
that "information" can travel in the flow. Gas travelling at subsonic
speed diverts around a body before striking it, it can be said to "know"
that the body is there. Air cannot divert around a body when it is
travelling at supersonic speeds. It continues to travel in a straight
line until it reaches a shock wave and decelerates to subsonic speeds.
Mathematically, supersonic flow is described by a hyperbolic partial
differential equation while subsonic flow is described by an elliptic
partial differential equation.
Another example of the difference between supersonic
and subsonic flow is the behaviour in a convergent duct (known as a
nozzle in subsonic flow and a diffuser in supersonic flow). Subsonic
flow in a convergent duct accelerates and supersonic flow decelerates.
Hypersonic aerodynamics
Hypersonic aerodynamics are characterized by viscous
interaction phenomena, that is, the viscosity of the flow significantly
affects the external flow, including shock waves. The curved shock waves
chemically alter the surrounding air or gas, creating a partially
ionized plasma with their high temperatures (caused in part by
significant aerodynamic heating of the body). "Hypersonic" is typically
considered to refer to the Mach 5 and faster region of aircraft speed;
however, some hypersonic phenomena can exist at speeds as low as Mach 3
(depending on the aircraft and the environment).
Typical applications include:
- Building & Structure Wind Loading
- Vortex Shedding
- External Aerodynamics of Vehicles
- Fan, Wing and Rotor Design
- HVAC Applications
- Airborne Particle Transport
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