Assessment of the broadband noise from an unducted axial fan including the effect of the inflow turbulence

This paper presents an assessment of the broadband noise computed by using the zonal LES coupled with the Ffowcs Williams - Hawkings (FWH) acoustic analogy for an unducted low speed axial fan. The results of the aerodynamic and aeroacoustic simulations are benchmarked with Particle Image Velocimetry (PIV) and noise measurements from the authors. An in-focus look is dedicated to Constant Temperature Anemometry (CTA) measurements for providing the numerical solver with the correct level of inflow turbulence. Far field noise results are presented for both laminar and turbulent inlet in a frequency range from 0.1 to 10 kHz. A specific analysis is carried out to detect noise sources on the blade surface, showing that broadband noise from 0.4 to 2 kHz is mainly due to the boundary layer scattering at the trailing edge and to the interaction of the tip vortex with the blade trailing edge, resulting rather non-sensitive to the inflow turbulence. At frequencies above 2 kHz, the main noise sources are detected on the front portion of the blade suction side, and their intensity is found to depend significantly on the inflow turbulence.