Zonal large eddy simulation for the numerical prediction of the acoustic performance of an axial fan

Advanced aerodynamic and aeroacoustic simulation methodologies are becoming a major topic in the modern vehicle industry and even more in the design of Heating, Ventilation and Air-Conditioning (HVAC) systems. The computation of airborne noise requires highly accurate numerical approaches to deal with the complexity of phenomena involved, such as turbulence, transition to turbulence and laminar instabilities. Moreover the requirements in terms of time and space resolution for the aeroacoustics as well as the identification and calculation of aeroacoustic sources make the computation of aerodynamic generated noise often extremely time demanding and thus not suitable for implementation within design and optimization loops. The objective of the present paper is to perform the aeroacoustic simulation of an axial fan by using an innovative LES technique, the Zonal LES. The proposed approach consists of a fully resolved LES in the acoustic generation region (embedded into the LES sub-domain) merged with a RANS solution in the outer flow region. In this way, the impact of the LES computational burden is significantly reduced and complex geometries can be simulated within reasonable computational time. The acoustic propagation is performed by using the Ffowcs Williams-Hawkings (FWH) acoustic analogy. The test-case chosen for this study is based on the geometry of a 5-bladed axial fan with an outer diameter of 350 mm, blade Reynolds number of 0.06 M at the root and 0.16 M at the tip and tested in free field condition. The aeroacoustic and aerodynamic predictions of the numerical model are compared with the experimental data collected by the authors.