A computational study of the interaction noise from a small axial-flow fan

TLDR: It is argued that the dominant interaction mechanism is potential flow in nature, which differs from the usual rotor-stator interaction acoustics in which the downstream part is responsible for most of the noise.

Abstract: Small axial-flow fans used for computer cooling and many other appliances feature a rotor driven by a downstream motor held by several cylindrical struts. This study focuses on the aerodynamic mechanism of rotor-strut interaction for an isolated fan. The three-dimensional, unsteady flow field is calculated using FLUENT®, and the sound radiation predicted by acoustic analogy is compared with measurement data. Striking differences are found between the pressure oscillations in various parts of the structural surfaces during an interaction event. The suction surface of the blade experiences a sudden increase in pressure when the blade trailing edge sweeps past a strut, while the process of pressure decrease on the pressure side of the blade is rather gradual during the interaction. The contribution of the latter towards the total thrust force on the structure is cancelled out significantly by that on the strut. In terms of the acoustic contributions from the rotor and strut, the upstream rotor dominates and this feature differs from the usual rotor-stator interaction acoustics in which the downstream part is responsible for most of the noise. It is therefore argued that the dominant interaction mechanism is potential flow in nature.

Figures

FIG. 9. Comparison of experimental measurement and numerical prediction of sound at 0.5 m upstream of the fan center on the rotational axis. a Pressure wave forms. b Spectra.

FIG. 1. The computational and experimental configuration. a Photo of the r model fan with circular bellmouth. c Back view of the model fan showing e Mesh on a cylindrical cross section at 70% of the radial span.

FIG. 4. Contours of the sound source term p / rms on a the rotor pressu unwrapped view.

FIG. 8. Comparison of the acoustic directivity of sound pressure rms between experiment and prediction for the a BPF and b all frequencies. Both are scaled by the values on the pressure on the rotational axis. The directivity is taken on the horizontal plane while the fan stands in its upright position with flow from left to right.

FIG. 7. Rotor-strut acoustic interference. a Sound pressure decomposition into rotor upper curve and strut lower curve contributions. b Vectorial representation of complex sound amplitudes for the first three blade passing frequencies. The broken lines are the total sound amplitude consisting of the rotor horizontal line and strut oblique line contributions.

TABLE I. Geometry of the rotor blades and the incidence angle distribution.