The influence of realistic 3D viscous mean flow on shielding of engine-fan noise by a 3-element high-lift wing

The prediction of aeroacoustic shielding often rests on tools using constant mean flow thus neglecting mean flow inhomogeneities such as shear layers. This study analyses the influence of this simplification on shielding. As an example we considered engine-fan noise shielding at a 3-element high-lift wing. A Computational Aeroacoustic (CAA) approach was chosen. The simulations were carried out with the DLR CAA code PIANO. PIANO solves the Linearized Euler Equations (LEE) over steady viscous mean flow. To determine the mean flow influence three sets were computed. One rests on a realistic viscous Reynolds Averaged Navier-Stokes (RANS) solution, the second makes use of a simple constant mean flow and the third uses constant mean flow in conjunction with a flat plate with the same chord length to replace the wing. An axisymmetric solution of the finite element simulation code ACTRAN predicted the fan sound propagation from the engine intake through the non-uniform flow to a cylindrical interface. Subsequently, the data was coupled to the CAA computational domain via a Thompson boundary condition. It is shown that this condition meets the coupling requirements well. This study shows that the viscous mean flow has a significant influence on the predicted shielding potential of a high-lift wing. That is, the shielding benefit of a simplified prediction based on uniform flow propagation and a simplified geometrical significantly overestimate the potential shielding benefits considerably.