Task Report
Near-Field Tonal Fan Noise: Theoretical Modelling and Simulation
Introduction
The tonal noise produced by subsonic fans is a serious concern in heating, ventilating and air-conditioning (HVAC) systems and engine cooling systems.In all cases, the acoustic optimisation relies first on an adequate knowledge of the source mechanism, secondly on the proper accounting of the sound scattering within the duct system and towards the listener. In that respect, engine cooling systems are characterized by quite complex geometries surrounding the impeller. As a result, analytical methods based on tailored Green functions are irrelevant and the acoustic scattering must be computed numerically. The present work deals with the development and validation of accurate numerical methods for the prediction of subsonic tonal fan noise in complex enclosures.
In this Task, we focus on the role played by near-field terms in the acoustic scattering. Existing formulations, such as presented by Goldstein[1], discard explicitly the near-field terms to derive an elegant formulation for tonal fan noise,involving Bessel functions that represent the phase modulation related to Doppler effects. However,Roger[2] showed recently that the near-field terms can account for important phase-shifting effects, which can play a significant role, even in the amplitude of the acoustic far field, if for example the edge of a semi-infinite plane is present in the near field of the fan.
The work performed in this Task pursues this analysis for several cases of interest to the present project. Firstly, the importance of near-field terms is assessed in free-field, in order to quantify the distance from the fan beyond which the far-field approximation can be retrieved. Secondly, the implementation of the fan as a source in the context of Boundary Element and Finite Element Methods is validated by placing it within a cylindrical straight duct with anechoic boundary conditions at both ends. This permits validation of the approach by comparison with the analytical solution based on the ductmodes. The near-field solution is finally applied to the case of a VALEO fan, for which the BLHs have been obtained through CFD simulations.