Abstract
The tonal noise control of an axial low-speed fan system with flow obstruction has been achieved both numerically and experimentally. Its primary noise source caused by rotor-wake interaction has been directly and accurately simulated with a Lattice Boltzmann Method. The latter has then allowed deciphering the noise control mechanism of an upstream sinusoidal obstruction: the vortex rings shed by the obstruction yield a second noise source at the rotor-blade leading edge. The obstruction itself does not create any significant noise and is acoustically transparent. An industrially-applicable numerical methodology has then been proposed to obtain the optimal obstruction design for a given fan geometry and operating condition, with a maximum of six simulations of the fan system without and with the obstruction being static and slowly rotating. Simulations with rotating obstructions provide the optimal lobe amplitude and an optimal obstruction angular position, which are found to be 20 mm and about 16° respectively for the present fan system both numerically and experimentally. The frequency selectivity of the obstruction and the linear variation of the secondary source level with the lobe amplitude have also been confirmed.
Highlights
Abstract e tonal noise control of an axial low-speed fan system with flow obstruction has been achieved both numerically and experimentally
Around the optimal lobe amplitude Ao pt, the secondary source level 20 log10(ps ) varies linearly with the lobe amplitude. ey found that the separation of the primary noise radiated by the fan system and the secondary noise induced by the flow obstruction could be separated by rotating the obstruction in the optimization process, which shi s the secondary tone in the frequency spectrum [ ]
E La ice Boltzmann Method (LBM) simulations have highlighted vortex rings formed at the obstruction lobes that are convected downstream and impinge on the fan blades. ese structures create an azimuthal variation of the velocity profile which generates a periodic fluctuation of the blade load
Summary
Abstract e tonal noise control of an axial low-speed fan system with flow obstruction has been achieved both numerically and experimentally. Simulations with rotating obstructions provide the optimal lobe amplitude and an optimal obstruction angular position, which are found to be mm and about ◦ respectively for the present fan system both numerically and experimentally. E present study focuses on the tonal noise produced by low-speed axial fans that are o en used in cooling and airconditioning systems Such a noise component is a major part of the overall noise produced by such ventilation systems (about half or more depending on the operating condition), and the major source of annoyance (subjective noise) when the tones emerge by more than dB over the broadband level. Conclusions on the accuracy of the numerical method and the noise control strategy are drawn
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