Axial fan noise: Towards sound prediction based on numerical unsteady flow data - a case study

Objective of this work is to evaluate modern numerical methods for predicting flow induced fan noise. A generic fan assembly is investigated consisting of a low pressure axial impeller (diameter 0.3 m, hub to tip ratio of 0.45) including an optional turbulence generator. The flow field is simulated with different state of the art unsteady computational fluid dynamic methods. All results are compared with each other and with hot wire flow velocity and surface pressure measurements. From the numerical data, the relevant dipole sound sources, i.e. the unsteady forces on the fan blades are derived. Eventually both, a free field formulation in the time domain (acoustical analogy by FFOWCS WILLIAMS and HAWKINGS), and a boundary element formulation in the frequency domain (SYSNOISE®) are employed to predict the radiated sound field based on the numerical source data. The acoustical results are compared and contrasted with measurements.