Inclusion of Upstream Turbulent Inflow Statistics to Numerically Acquire Proper Fan Noise Characteristics

To obtain realistic noise characteristics from CAA studies of subsonic fans, it is important to prescribe properly constructed turbulent inflow statistics. This is frequently omitted; instead it is assumed that the stochastic characteristics of turbulence, absent at the initial stage, progressively develops as the rotor inflicts the flow field over time and hence that the sound generating mechanism governed by surface pressure fluctuations are asymptotically accounted for. That assumption violates the actual interplay taking place between an ingested flow field and the surface pressure fluctuations exerted by the blades producing noise. The aim of the present study is to examine the coupling effect between synthetically ingested turbulence to sound produced from a subsonic ducted fan. The steady state inflow parameters are mapped from a precursor RANS simulation onto the inflow boundaries of a reduced domain to limit the computational cost. The flow field is resolved utilizing IDDES for turbulence handling and the computational domains are configured for both a single blade and a circumferential complete five bladed fan. The results clearly reveal the limitations of restricting the computational analysis to a single blade. Additionally, a separate investigation of the upstream inlet section shows that the deterministic flow structures generated at the inlet plane are self-sustained within the inlet section. The outcome stresses the importance of incorporating correlated inflow statistics for turbomachinery noise studies. Moreover, the acoustic analogy formulated by Ffowcs -Williams and Hawkings is employed to study the low frequency spectral distribution. Previously conducted measurements are used for validation of both the flow field statistics and the far-field sound field.