The goal of the present paper is to report verification and validation studies carried out by Exa Corporation in the framework of turbofan engine noise prediction through the hybrid Lattice-Boltzmann/Ffowcs-Williams & Hawkings approach (LB)-(FW-H). The underlying noise generation and propagation mechanisms related to the jet flow field and the fan are addressed separately by considering a series of elementary numerical experiments. As far as fan and jet noise generation is concerned, validation studies are performed by comparing the LB solutions with literature experimental data, whereas, for the fan noise transmission through and radiation from the engine intake and bypass ducts, LB solutions are compared with finite element solutions of convected wave equations. In particular, for the fan noise propagation, specific verification analyses are carried out by considering tonal spinning duct modes in the presence of a liner, which is modelled as an equivalent acoustic porous medium. Finally, a capability overview is presented for a comprehensive turbofan engine noise prediction, by performing LB simulation for a generic but realistic turbofan engine
The Lattice Boltzmann method was used to perform numerical simulations of the sound and turbulent flow inside a standing wave tube terminated by a circular orifice in presence of a forced mean flow. The computational domain comprised a standard virtual impedance tube apparatus in which sound waves were produced by periodic pressure oscillations imposed at one end. An orifice plate was located between the driver and the tube termination. All waves transmitted through the orifice were effectively dissipated by a passively non-reflecting (i.e. anechoic) boundary at the tube termination. A turbulent jet was formed at the discharge of the orifice by the forced mean flow inside the tube. The acoustic impedance and sound absorption coefficient of the orifice plate were calculated from a wave decomposition of the sound field upstream of the orifice. Simulations were carried out for different excitation frequencies, and orifice Mach numbers. Results and trends were in good quantitative agreement with available analytical solutions and experimental data. The Lattice Boltzmann method was found to be an efficient numerical scheme for prediction of sound absorption by realistic three dimensional orifice configurations.
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