Numerical and Experimental Investigations of the Sound Generation and Possible Optimization Techniques of Wires for Fan Guard Grilles

For modern axial fans optimized for low self-noise, additional noise emission from guard grilles mounted downstream of the fan can become one of the dominant sources of sound. In the present case, the overall sound power level increases by up to 6 dB. Based on narrow-band acoustic measurements and numerical Lattice-Boltzmann simulations of wind tunnel setups using round wires, it is observed that periodic flow separations behind the wires (von Kármán vortex street) lead to a pronounced hump in the noise spectrum. This occurs in a frequency range that corresponds to the grille-induced noise increase observed with an axial fan under comparable flow conditions. By examining various wire geometries, it was found that disrupting the von Kármán vortex street along the longitudinal direction of the wire and reducing the homogeneity of flow separation can significantly decrease sound generation. As a result, a guard grille prototype incorporating the most promising structures was manufactured for a modern low-noise axial fan. Comparative experimental results for the fan are presented.