A study of sound sources and unsteady surface pressure in an axial fan

The noise emitted from an axial fan has become one of the primary concerns for many industrial applications. This paper presents the work to predict the noise generation and investigate sound sources in a low speed axial fan. Computational fluid dynamics modeling is conducted using Scale Adaptive Simulation for the unsteady flow field. The sound predictions by the acoustic analogy are in good agreement with the experimental data. The results from this study show that the aerodynamic interaction between the blades and outlet vanes has a major contribution to the radiated noise spectrum. Two types of sources of narrowband humps are identified in the axial fan. The first is found at the leading edge of the blade tip, which is related to the interaction of coherent flow structures in the blade tip region. The second is found in the vicinity of the blade hub, which can be attributed to the recirculating flow and hub vortex. The noise below the frequency of 1500 Hz is mainly due to the blade-outlet vane aerodynamic interaction, manifested as the tonal sound at BPF and its harmonics, whereas above 1500 Hz the broadband component of sound is mainly related to the turbulent boundary layers.