Investigation of noise generation due to interference between leading edge separation bubbles and blade surface

Abstract

Sirocco fans are used in familiar applications such as car air-conditioners due to their compactness, and reducing the noise they generate is an important social issue. However, the mechanism of their noise generation has not been well studied. The objective of this study is to identify the physical quantities contributing to noise generation by examining the “interference between the leading-edge separation bubble and the blade surface” as an fundamental process. To achieve this, numerical simulations were conducted for the cascade models with three blade shapes of varying thicknesses. The length of the separation bubble, its thickness, and the size of the vortex all decreased with increasing blade thickness. Generated noise was predicted using Curle’s equation, and noise measurement experiments were performed with a similar system to validate the prediction accuracy. The results showed good agreement, and the generated noise was reduced at almost all frequencies with increasing blade thickness. To clarify the reasons for this, the cospectra of the time derivative of the fluctuating pressure on the suction surface were analyzed in detail. The following conclusions were drawn: To reduce the noise generated by a sirocco fan, it is effective to reduce the peak value and broaden the shape of the fluctuating pressure distribution on the blade surface and decrease the vortex size.