A Study on Flow Structure and Noise Generation in a Separated-and-Reattaching Flow in Multiblade Fan

Abstract

In order to investigate the sound generation from the flow field, the acoustic perturbation equation and the no-reflection boundary condition of the sound wave which is called Sommerfeld radiation condition are implemented in OpenFOAM, a set of computational fluid dynamics codes. Separated-reattaching flows between the blades of multi-blade fan model and the generated sound were calculated. Two types of blade models are used. One has a constant blade thickness, and is named original model. The other has a variable blade thickness, and is named filled model. The structure of the flow field contributing to the sound generation was extracted using the conditional average which refers to the calculated sound signal. As for the original model, it is difficult to extract the flow field contributing to sound generation because the space between the wings are filled with vortices. As for the filled model, it is suggested that sound is emitted at the moment when the coherent pressure fluctuation caused by the vortex exists near the reattachment point of the flow on the blade surface.