Competing Broadband Noise Mechanisms of a Generic Low-Speed Axial Fan Including Acoustic Scattering

D1 - Prediction of Axial Fan Noise by Hybrid Methods (i)

This paper focuses on the semi-analytical modeling of broadband noise generated by a low-speed axial fan and its scattered field by an obstacle. Two different broadband noise mechanisms will be investigated, turbulence-interaction noise -so called leading-edge noise- and trailing-edge noise. The former is due to impingement of the turbulent flow field to the leading edge of the fan blade. The later is due to the development and possible separation of the boundary layer and scattering by the trailing edge of the blade. This paper addresses both broadband noise mechanisms for a generic axial fan operating in different flow conditions.
The theory of Amiet on turbulence-interaction noise will be employed including a geometrical-near-field extension. The same geometrical-near-field extension will also be applied to the trailing-edge noise Amiet’s theory. The required inputs for both broadband Amiet’s theories will be provided from RANS CFD computations over a flat plate using OpenFOAM. The necessary incoming turbulence spectrum for the leading-edge theory will be computed from the von Karman spectrum model using flow data extracted upstream the leading-edge. The wall-pressure spectrum upstream the trailing-edge required in trailing-edge noise theory will be obtained from a wall-pressure reconstruction model using boundary layer informations. Such a model has been already published by the authors.
The second part of the paper focuses on the scattering of the broadband noise by an obstacle. The scattering of the broadband noise of the axial fan will be taken into account by means of Acoustic Transfer Vector (ATV) approach in the Boundary Element Method (BEM) framework. A flat screen will be introduced as a scattering obstacle in vicinity of the fan. The results will then be compared to an analytical solution which accounts for an infinite screen and an image source. The scattering of the turbulence-interaction noise has been already published by the authors. The final paper will also include the scattering of the trailing-edge noise generated by a low-speed axial fan.