Computational Aeroacoustic Analysis of an Industrial Fan Application using a Hybrid Approach

B1 - Computational Aeroacoustics

Fans are used in many domains: ventilation, air conditioning, cooling systems for electric and thermal motors, electrical domestic, power generation in industrial compressors and turbines…With the growth of the acoustic comfort demand coupled to a more restrictive legislation, the new fan design are constrained to challenging noise control and reduction. Flow noise generated by a fan is often one of the major sources of noise.

Among the different techniques currently available to address this type of problem, a hybrid approach is recommended for industrial applications. In this approach, the unsteady flow field is first determined by a CFD solver like STAR-CCM+, and in a second stage the noise sources are propagated by a finite element acoustic software like ACTRAN. This allows clear identification of noise sources including those highly amplified during the acoustic propagation process. For many reasons, fan noise prediction using such approach is challenging and requires some advanced investigations. On the CFD side, acoustical non-reflective treatments and the manipulation of rotating-static domains to predict the unsteady compressible flow is complex and requires the use of advanced interfaces between static and rotating parts. On the acoustic side, the computation of moving CAA sources into the frequency domain requires the use of specific non-reflecting boundary conditions and a correct handling of the noise source and the acoustic propagation in a non-uniform medium.

In the first part of the present paper, the CFD model is described, highlighting the special features required for an efficient and accurate flow modeling inside a fan configuration. In a second step, the acoustic modeling technique is presented. This technique post-process the unsteady CFD results and computes the different excitations provided to the acoustic model. The acoustic model is described, focusing on the boundary conditions, propagation model in a non-uniform medium. In particular, the effect of a non-uniform flow field in the diffuser and in the outlet duct will be addressed by comparing the results of Lighthill and Möhring analogy. In both parts, best practices will be formulated and recommended. Some extensions involving aero-vibro acoustic concepts (aero acoustic sources exciting the fan casing and inducing noise in the external domain are presented.

The paper illustrates the application of a hybrid approach for fan noise modeling. The computational cost of the simulation is compared to the theoretical cost of a full direct simulation involving flow and acoustic simulation.