Computational Fluid Dynamics and Aeroacoustics

Numerical Aero-Acoustics of Fans

       
Rotor-stator interaction and fan downstream noise
(resp. elsA and sAbrinA computations)

The recent growth in codes solving the equations of fluid mechanics has brought significant progress in research on the theme of numerical aero-acoustics of fans. In particular, CFD computations from the Onera’s elsA platform (an aerodynamic simulation software package are now able to provide unsteady pressures on blade cascades directly (sources of load noise) and to partly simulate the acoustic propagation in the close vicinity of the wheels. The deterministic noise sources from which the noise lines originates are now accessible by Reynolds-Averaged Navier Stokes (RANS) computations, and the first Large Eddy Simulations (LES) are also being conducted to simulate the turbulent phenomena that create the broadband noise. The integral formulations of Ffowcs, Williams and Hawkings that are widely used for computing the radiation of these sources in free space (rotor and propeller noise) are unfortunately not directly applicable to fans because of the presence of the fairing, which leads to a veritable integral equation (as the Green function is precisely the unknown in the problem). The approach proposed by Goldstein at the end of the 1970’s consists in imposing an analytical solution of the Green function (in the frequency domain) by assuming the inlet duct to be cylindrical (or annular) and the mean flow to be uniform. Using these assumptions, direct numerical integration becomes possible and can be used to simulate the acoustic propagation in the duct and the power radiated. Computation codes based on these integral methods were thus developed at Onera for predicting the noise lines and the broadband noise of the fans.

At the same time, studies conducted in cooperation with other units of the department led to the development of the sAbrinA platform for studies of the noise of aerodynamic origin. This platform notably makes it possible to solve the (non-linearized) Euler equations with a high order (non-dissipative schemes), which is in practice indispensable for correctly simulating the propagation of the acoustic waves in industrial configurations. Among other things, this code is used for applications to rotating machines in order to simulate the propagation in the duct along with the upstream and downstream radiation. The elsA code was recently interfaced with sAbrinA’s Euler solver using a modal decomposition technique that follows the approach proposed by Rienstra (University of Eindhoven) in the framework of the European TurboNoise CFD project. This approach gave rise to a module for computing equivalent sources (being transferred to Safran), providing the source terms of the Euler equations solved with sAbrinA. We were thus able to establish an aero-acoustic chain for simulating the noise lines of the fans, from the generation of the sources out to the far field (the acoustic farfield is found using a Kirchhoff integral). The convection effects due to the flow heterogeneity, and the acoustic transmission by the air intake and the exhaust are thus within the scope of current simulations.

Last Update: 6 June 2006 - © ONERA 2009 - Terms of use