Zonal LES for Axial Fan Broadband Noise Prediction, Part 2: Computational Test Case

Broadband noise prediction is a major challenge in computational aeroacoustics for many industrial applications. A proper aeroacoustic simulation requires the use of accurate numerical approaches usually imposing a high computational burden. For this reason they are not suitable to be implemented in optimization loops, even if they constitute a valuable support to address the performances of various designs. In order to reduce the computational time for such kind of simulations, strong approximations have to be introduced in the numerical models. A validation and benchmark of the simulation procedure with experimental data sets is an essential step in order to rely on the numerical results. The objective of this paper is to provide to the scientific community a reference test-case for turbomachinery noise, assessing the potentialities of a novel breakthrough approach for Computational Fluid Dynamics (CFD): the zonal Large Eddy Simulation (LES). The test-case chosen is a 5-bladed axial fan tested in free field conditions with an outer diameter of 350 mm and an operating point at 1400 rpm. Aerodynamic and aeroacoustic performances have been investigated both from the experimental as well as from the numerical side, for different rotational speeds ranging from 1200 to 1600 rpm. Experimental results are given in the Part 1 [1] of this work, while Part 2 focuses on the computational results. The computational approach combines a fully resolved LES in the acoustic generation region coupled with a RANS solution in the outer region. The acoustic propagation is performed applying the Ffowcs Williams-Hawkings acoustic analogy. The numerical results are in good agreement with measurements for aerodynamics as well as for the sound level in a frequency range up to 10 kHz. A deep analysis of the computational issues as mesh optimization and simulation setup is provided. In addition the aerodynamic performance, noise sources location and noise pattern directivity are also given, providing a wide, reliable and complete testcase for further studies and benchmarks. The simulations are performed with ANSYS-Fluent, release 14.5.