A CAA Study of Turbulence Distortion in Broadband Fan Interaction Noise
Résumé
The broadband noise generated by the interaction of the turbulent fan wakes on the outlet guides is a major noise source at approach conditions and its reduction is a challenge for turbofan engine manufacturers. In this paper, the linearized Euler equations are solved in the time-domain using a finite difference code to model the response of an isolated airfoil interacting with turbulence that is stochastically synthesized and injected in the computational domain through vorticity sources. The method of injection is firstly extended from one-component to two-component turbulence. The distortion of the turbulence by the mean flow near the leading edge is also investigated using this numerical method. Numerical simulations allow to collect simultaneously data for any variable and at any location in the computational domain, which is out of reach of experimental studies.
So far, only a small number of numerical studies have been performed on this subject and were mainly focused on qualitative observations of turbulence. It has been found that, upstream the airfoil, the turbulence starts to decrease uniformly from a threshold distance independent of the wavenumber. At low frequency, this decay is however inverted after a second threshold distance, much closer to the leading edge, which is wavenumber dependent. A parametric study shows that the turbulence distortion is independent of the incoming turbulence, and that only the geometry forward the position of the maximum thickness has an effect on the distortion. It also shows that the nose radius, the position of the maximum thickness and maximum thickness of an airfoil affect the turbulence distortion. However, these geometric parameters alone are not sufficient to fully account for the distortion of the turbulence.
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