Fan broadband noise is a major aircraft noise challenge currently and will be even more important in the future. Novel low-noise engine architectures, such as ultra-high-bypass-ratio engines and lower-speed fans, can help address jet noise and fan tone noise, but they are unlikely to reduce fan broadband noise significantly. The accurate prediction and control of fan broadband noise is therefore essential if aircraft noise is to be reduced.

The most significant broadband noise sources are believed to be generated by the different interaction mechanisms between:

• the blade tip vortex of the rotor fan and the turbulent boundary layer on the inlet-duct (rotor boundary layer interaction noise)
• turbulent eddies convected in the rotor boundary layer past the rotor trailing edge (rotor self noise)
• the impingement of the rotor wake onto the downstream outlet guide vanes (OGV interaction noise), and
• turbulent eddies convected in the vane boundary layer and the vane trailing edge (OGV self noise)

These four mechanisms each generate a whole spectrum of frequencies, making it difficult to use conventional noise measurements to isolate the contribution of each mechanism. Furthermore, the broadband noise generation process is very complex to model, requiring representation of the fine length scales involved in turbulence generation and propagation.

Consequently, current methods for industrial broadband noise prediction are almost exclusively semi-analytic in nature. They rely largely on correlating measured noise levels to a few relevant aerodynamic and geometric parameters, but are unable to predict the effects of different blade geometries. The advances in purely numerical methods, which have revolutionised tone noise prediction, have yet to make an equivalent impact on broadband noise prediction.