EXPERIMENTAL AND NUMERICAL INVESTIGATION OF TIP CLEARANCE NOISE OF AN AXIAL FAN USING A LATTICE BOLTZMANN METHOD

摘要

The secondary flow through the tip clearance is one of the well-known sources contributing to the overall noise of axial fans. Aerodynamic losses and sound radiation increase significantly as the tip clearance is increased. The objective of this study is to revisit the mechanisms for tip clearance noise from a rotating fan impeller. The unsteady and compressible numerical Lattice-Boltzmann-Method (LBM) is utilized which allows a direct and simultaneous prediction of both aerodynamic and acoustic field. Overall aerodynamic and acoustic fan performance data as predicted with the LBM were validated with experimental data. The agreement was quite satisfactory which justified looking at the LBM-predicted field data in detail. The flow and acoustic field in the vicinity of an axial fan impeller's tip gap revealed important details of the sound generating mechanism. A large tip clearance is responsible for a complex vortex system with a considerable degree of inherent unsteadiness. The consequences are fluctuations of static pressure in the flow field in the adjacent tip region and on the blade surfaces, more pronounced on the pressure than on the suction side. Those pressure fluctuations generate sound that is then radiated away from the complete impeller upstream into the free field with the typical hemispherical directivity pattern.