Investigation of axial fan tip noise mechanisms by means of CAA and beamforming technique

Tip noise is known as an important sound generation mechanism in axial fans. It is caused by the flow through the tip gap between the rotating fan blades and the stationary housing, which is driven by the pressure difference between pressure and suction side. A possibility to reduce noise is the use of so called winglets in the blade tip region. With these geometrical modifications it is possible to influence the tip flow and the related vortices, so that the noise can significantly be reduced. To investigate fan noise generation mechanisms in detail one can principally use advanced measurement techniques like beamforming with microphone arrays or CAA simulation tools like LES. In this study both methods are used with the aim to complement and to benefit from each other. An axial fan of diameter 800 mm with two different blade tip shapes (build up by exchanging only the outer blade part) is investigated. The different geometries have nearly no effect on the aerodynamic fan performance but major impact on the aeroacoustics. The agreement between the beamforming and the CAA results concerning possible noise source locations on the fan blades is pretty good.