Computational Aeroacoustics of Inlet Geometry on Tip Noise for Low Pressure Axial Fans

Paper in proceeding, 2025

Ducted low-pressure axial fans are widely used for cooling and ventilation. These fans are often required to be installed in tight spaces. Therefore, the inlet nozzle, which conditions the flow entering the rotor, is often modified to meet the space requirements. The geometry of the inlet nozzle affects the tip-leakage flow and the tip-vortex. Consequently, the inlet geometry also affects the broadband tip-noise and the sub-harmonic tip noise, which are significant contributors to the overall noise produced by a rotor blade. This work computationally investigates the effect of the inlet-nozzle radius on both the sub-harmonic and broadband tip-noise. Three-dimensional, full-annulus, unsteady, turbulent, scale-resolving simulations are done using the Delayed Detached Eddy Simulations (DDES) turbulence model. The far-field noise is calculated using the permeable Ffowcs-Williams and Hawkings (FWH) analogy with the Farassat-1A formulation. Three different inlet-nozzle radii and an additional case without an inlet nozzle is investigated. The aeroacoustic effect of the inlet-nozzle radius is quantified through the change in the overall Sound poWer Level (SWL) and the acoustic spectra. The SWL increases by 3.4 dB between the shortest and longest investigated inlet nozzle radius. Decreasing the inlet nozzle radius leads to increased low-frequency noise (f < 1 kHz) and reduced dominance of the narrow band hump associated with the sub-harmonic tip noise. Additionally, increased broadband tip noise was observed for a frequency range between 1-2.5 kHz for larger inlet nozzle diameters due to increased tip leakage flow.