Fan, propeller and rotor noise is not a single phenomenon; at least two mechanisms act together, and confusing them leads to intervening in the wrong place.
Tonal noise
Sharp peaks at the blade passing frequency and its harmonics. The source is the periodic exposure of the blade pressure distribution to an observer. Non-uniform inlet flow strengthens it: the blade is loaded differently on each revolution.
Ways to reduce it:
- Straighten the inlet flow. A bend or obstruction upstream of the fan is the most common cause of a tonal component. See the secondary flow note.
- Increase the rotor-stator spacing.
- Choose blade and guide vane counts with no common divisor — otherwise the interactions superpose in phase.
- Skew the blades to spread the passage in time.
Broadband noise
A component spread across the frequency axis with no peaks. Its source is turbulence: fluctuation in the incoming flow, the boundary layer on the blade, and vortices shed at the trailing edge.
Ways to reduce it are lowering the tip speed (most effective but it also lowers capacity), trailing edge serrations, and reducing the tip clearance. The tip clearance vortex is a shared source of both noise and efficiency loss.
Modelling
The tonal component can be computed from the transient pressure fluctuation on the blade, obtained from a sliding mesh solution. See the rotating machinery note.
The broadband component requires resolving the turbulence itself; RANS cannot produce it. See the RANS/DES/LES note.
In practice most industrial studies do not target an absolute sound level; they find the sources and improve comparatively. See the duct noise note.
References
- Spalart, P. R., Jou, W.-H., Strelets, M. & Allmaras, S. R. (1997). Comments on the feasibility of LES for wings, and on a hybrid RANS/LES approach. In Advances in DNS/LES, Greyden Press, Columbus, OH.
- International Towing Tank Conference. ITTC Recommended Procedures and Guidelines 7.5-03-02-03: Practical Guidelines for Ship CFD Applications. ITTC.