Rotating Coherent Flow Structures as a Source for Narrowband Tip Clearance Noise from Axial Fan
The sound radiation of axial fans typically increases significantly as the tip clearance is increased. In addition to the broadband tip clearance noise at the design flow rate, narrowband humps associated with tip clearance noise are also observed in the acoustic spectra measured in the free field at lower flow rate. In this study, both experimental and numerical methods are used to shed more light on the noise generation mechanism of this narrowband tip clearance noise. The successful validation of the aeroacoustic predictions from unsteady simulations with the Lattice Boltzmann Method (LBM) ensures that employing LBM data to conduct a comprehensive modal analysis for detecting rotating coherent flow structures which might be considered as noise sources. It is found that the narrowband humps are related to the tip clearance noise that is generated by an interaction of coherent flow structures being present in the tip region with the leading edge of the impeller blades. The visualization of the coherent structures is shown that they are indeed parts of the unsteady tip clearance vortex structures. They are hidden in a complex spatially and temporally inhomogeneous flow field, but can be recovered by means of appropriate filtering techniques.