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The relationship between flow structure and generation of aerodynamic sound from a multi-blade fan is experimentally investigated with enlarged two-dimensional blade models. Two blade models with different shapes are used. Sound pressure level and blade surface pressure are measured simultaneously where the sound transmittable board is used to find the position of sound source. In addition, flow velocity and blade surface pressure are also measured simultaneously. The results show that the sound pressure level takes highest values near the reattachment point of the flow on blade suction surface, where the RMS value of the surface pressure fluctuation takes highest value, and where coherence between the sound pressure and surface pressure takes high values. These values vary with the shape of blades. The size of separation bubble and the intensity of velocity fluctuation also differ by the shape of the blades. The large scale vortical flow structures are extracted by conditional averaging for blade surface pressure fluctuations. Sizes of the large scale vortices correspond to the peak frequencies of the blade surface pressure fluctuation and frequency characteristic of surface pressure fluctuation is explained by the convections of the vortices. In addition, the sizes of large scale vortices correspond to the size of separation bubble. This fact suggests that making separation bubble smaller is an efficient way for reduction of aerodynamic noise of a multi-blade fan.

Experimental study on the flow structure and sound generation between the blades of multi-blade fan