Numerical modeling of fan noise in electronics

Potential users in the electronic industry - engineers, researchers, manufacturers - have a need to use the methods of computational aeroacoustics (CFD - CAA) and software to control and optimize the fan noise level of electronic devices with an accuracy of + / - (3 .. 4) dB. Such a software implementation would provide a practical opportunity to achieve the acceptable level of noise and its compliance with internationally accepted standards that requires PC software noise in the office no more than 45 -50 dB (A) with the desired level of 25-30 dB (A) . Currently available approaches for modeling the fan noise are mainly based on application of the Lighthill's equation , methods of so-called aeroacoustic analogy using the transformed Lighthill's equation, like well-known FW-H equation or application of Kirchhoff's theorem. The drawback of such methods is a lack of decomposition problem - separation of the acoustic and vortex (pseudosound) mode within the noise source that leads to significant errors in the noise amplitude prediction and inability to optimize the fan geometry. The proposed approach makes the following assumptions: ? tonal components at the blade passing frequency (BPF) and its higher harmonics dominate in the pressure pulsations and noise spectra ? the vortex mode ( pseudo sound) zones adjacent to the fan rotor locally limited in space comparing the whole volume of the electronic device flow path and present the tonal noise source ? Mach number is low ( subsonic flow) Taking these assumptions, the tonal noise modeling method, based on the numerical solution of the Helmholtz equation of Fourier transform with respect to the pressure pulsation with boundary conditions in the form of the complex impedance and each fan noise source defined on the surface surrounding the rotor: The fan sound power is determined by acoustic-vortex method (AVM) that provides the correct decomposition of equations of compressible medium motion and boundary conditions near the fan rotor . The AVM approach ensures high-precision determination of the sound amplitude giving correct absolute values of pressure pulsations. The source sound power is stored in the fans noise database for use in optimization calculations. To ensure the accuracy and versatility of the method, the nonlinear integro- differential equations approximated by a Cartesian grid in the spatial region of arbitrarily complex shape adapted to the complex boundary and solution characteristics using a finite volume approach. Specifically designed procedure solves the resulting algebraic system of equations. The main feature of this approach is the high performance in noise modeling (for a given fan sound characteristic) and correct accounting the interaction of acoustic waves with impedance boundary conditions, interference and diffraction of sound in the presence of boundary conditions of various type. The proposed method is a unique and highly effective for modeling of tonal noise phenomena caused by cooling fans of electronic devices.The code created on the base of this method can be effectively integrated into CAD systems