## Aerodynamics | CFD Aerodynamic Analysis | Aerodynamics Concepts | Aerodynamics Introduction

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