Fan noise is caused by:
-inlet boundary layer or inflow distortion interacting with the fan
-self noise from the fan
-fan wakes interarcting with stators or struts
Fan Noise is divided into two major categories: broadband and tone noise
Broadband Noise
Broadband noise tends to be caused by inflow turbulence interacting with the fan or fan turbulent wakes impinging on stators and struts. This turbulence is generated by a number of phenomena:
-the boundary layer in the outer wall of the inlet duct
-wakes shed by fan blades
-atmospheric inflow turbulence (not normally significant
in production of broadband noise)
Tone Noise
Tone noise is primarily generated by rotor/stator/strut interaction with flow distortions and thus is also known as rotor-stator interaction noise. This rotor-stator interaction noise occurs at multiple of the blade passing frequency and its harmonics.
Click here to
find out about a code that computes turbofan tone noise.