Numerical Optimization of the Tonal Noise of a Backward Centrifugal Fan using a Flow Obstruction - Part II: Flow Obstruction Optimization

F1 - Lattice Boltzmann Methods (i)

International regulations on safety and acoustic comfort are making fans and integrated systems suppliers consider more and more sound quality as one of their first design criterion and market differentiator for their products. Indeed noise emissions can be perceived as a strong annoyance for people staying in their vicinity and can affect working conditions of operators. Therefore broadband and even more tonal contributions of the noise of such systems have to be addressed as early as possible in their development process.
In that context, the noise radiated by a rotating fan, and particularly its contributions at the Blades Passing Frequency (BPF) and its harmonics, must be controlled. Previous studies have shown that using a passive noise control device so-called flow obstruction in front of a centrifugal fan-heat exchanger module significantly reduces the BPF noise and having little effect on the broadband content [1]. But because this type of device has to be calibrated in axial and angular position, finding its optimum shape and location would require an expensively high number of prototypes and extensive experimental tests campaign.
A recent study [2] showed how PowerFLOW, a compressible and unsteady Computational Fluid Dynamics (CFD) solver based on the Lattice Boltzmann Method can be used to accurately predict the 3-dimensional turbulent flow of a truly rotating centrifugal fan and its corresponding acoustic field in a realistic underhood environment including a heat exchanger and a simplified engine. The noise reduction brought by a particular flow obstruction at the fan BPF was accurately captured and simulation results were used to bring more understanding on the flow mechanisms at the origin of the phenomenon.
In this paper, the same numerical approach will be used and associated to an optimization method in order to automatically design and calibrate optimized flow obstruction in a short development time. Transient and spectral analysis of the simulation results will be performed to bring more understanding on the flow features at the origin of the tonal noise reduction.

[1] A. Gerard, A. Berry, P. Masson, Y. Gervais, "Passive adaptive control of tonal noise from subsonic axial fans using flow control obstructions", Fan Noise 2007 international conference, Paper 09, Lyon, France
[2] F. Pérot, M-S. Kim, V. Le Goff, X. Carniel, Y. Goth, C. Chassaignon, "Numerical optimization of the tonal noise of a backward centrifugal fan using a flow obstruction", Noise Control Eng. J., Vol. 61, No. 3, 2013, pp. 307-319.