Numerical study of aerodynamic noise behaviors for a vertically-installed flat strut behind an asymmetrical airfoil.

The aerodynamic noise generated from a cooling fan and its support frame is one of the major noise sources of many engineering machines, such as automobile engines and communication base-stations. A generic model composed of a vertically-installed flat strut behind an asymmetrical airfoil is used to investigate numerically the noise characteristics and generation mechanism of a cooling fan by using the SST-SAS turbulence model combining with FW-H formula. The airfoil–strut configuration is varied either by changing the perforation rate of the strut or by adjusting the streamwise gap between the airfoil trailing edge and the strut. The motivation is to investigate the effects of the two parameters on the aerodynamic and noise behaviors of the airfoil–strut​ model and evaluate the influences of noise control methods. The analysis of noise generation mechanism and far-field directivity indicates that in the vertical direction both the unsteady lift forces acting on the airfoil and strut contribute equally to the far-field noise, whereas in the upstream/downstream direction the noise is produced mainly by the unsteady drag on the strut. In addition, the whole model shows the directivity characteristic of an omnidirectional noise source. With the increase of the strut perforation rate, both the unsteady lift and drag on the airfoil and strut are reduced due to the attenuation of the Karman vortex street in the flow field, resulting in a far-field noise reduction. There exists a frequency lock-in (i.e. a resonance) between the airfoil and the strut in which the unsteady characteristic of the airfoil is induced by the downstream strut. However, this lock-in loses its strength with the increase of the streamwise separation between the airfoil and strut, accompanying with a change of far-field noise. [ABSTRACT FROM AUTHOR]