Aerodynamic noise results from the propagation of disturbances through a compressible fluid (usually air), caused by the movement of objects or the fluid at some point in time and space. Whether the disturbance is caused by an object or the fluid itself, this results in fluctuations in fluid pressure (or equivalently, density) that propagate outwards from the source at the speed of sound relative to the fluid. The size of the pressure fluctuation relative to the background pressure is called the sound pressure level. Due to the extremely large range over which the human ear is sensitive, the sound pressure level is given on a logarithmic scale as a function of the pressure fluctuation:

(15–1)

where is the pressure fluctuation relative to the background pressure and is a reference pressure level, commonly chosen as 10^-5 Pa. The units of sound pressure level are the Decibel [dB]. When evaluating aerodynamic designs, predicting the sound power per unit area, or sound intensity level, is also important. This is related to the sound pressure level by:

(15–2)

where is the density and is the speed of sound in the background fluid. The sound intensity is also usually given on a logarithmic scale and is calculated using an equation of the same form as Equation 15–1 using the intensity level and a reference intensity of 10^-12 W/m².

In the context of a CFD calculation, aerodynamically generated noise is of primary interest. Ideally, one would like to directly predict the pressure fluctuations at any distance from the device. However, practical considerations limit the ability to make direct predictions with a CFD solution.

Pressure and velocity fluctuations, as well as movement of the simulated components on the CFD mesh, generally result in near field noise, although this may depend on the exact definition of the "near field." Regardless of its definition, the near field will likely include the locations where the maximum noise levels occur. For most CFD applications, the extent of the computational domain is usually such that near field noise can be both analyzed and predicted.

In contrast, far field noise is the noise level that occurs at some distance from the device, usually outside of the CFD computational domain. The near field pressure and velocity fluctuations play a key role in determining far field noise levels by acting as a localized source of noise for far field noise prediction. Usually far field noise is what most aerodynamic designers are concerned about. For example, it might be desirable to create designs that minimize the sound pressure level at a certain distance from the near field source.