Experimental assessment of fan acoustic performance is limited by the difficulties of physical testing, and is typically performed in stand-alone test bench configurations. The results might not correlate well to performance when integrated into a real system because this method tends to substantially alter the flow and acoustic environment of the fan. With physical testing, it is very difficult or impossible to identify the source of a noise. Simulation has the potential to overcome these difficulties, but must meet the challenges of accurately capturing the key physical mechanisms related to flow-induced noise from cooling fans:
- By definition, noise generation is a transient phenomenon and traditional CFD codes typically have difficulty accurately predicting transient effects in reasonable time frames.
- The complex interaction of rotating blades with nearby stationary geometry is a primary source of noise, and the typical moving reference frame (MRF) technique fails to capture this effect.
- Tonal noise, especially associated with the blade-passing frequency, can have significant dependence on the quality of the incoming flow, rotor casing interactions, existence of rotating stall conditions, and unsteadiness of the flow field.
- Broadband noise, which is usually related to vortex shedding, flow detachments, turbulent boundary layer noise, and tip vortex noise.
- Installation effects, which tend to influence the inlet flow conditions and the acoustic response of the system.
- Radiation of small amplitude pressure fluctuations (acoustics) outside the convective near field.