Overset-LES for Airframe Noise Investigation

This work is dedicated to the establishment of a scale-resolving simulation tool for the prediction of airframe noise. The method is based on an evolution of the Computational Aeroacoustics code PIANO, in which the Non-Linear Perturbation Equations are extended with viscous terms leading to the full Navier-Stokes equations. The implementation of a subgrid-scale model enables a hybrid zonal RANS/LES method denoted as Overset-LES to emphasise its application on top of a time-averaged background flow. The sound sources related to airframe noise are often restricted to a region where a turbulent flow interacts with geometric edges. In a multi-step approach, these sound sources are first locally captured by means of an Overset-LES and subsequently propagated to the far-field with a 2D CAA propagation simulation. Stochastic volume forcing for turbulence reconstruction is applied at the inflow boundaries of the Overset-LES to expedite the development of realistic turbulence; which allows to keep the computational domain relatively small. The latter is validated for the flow over a flat plate. The entire process chain is applied to a well-defined NACA0012 simulation, which serves as a baseline case for subsequent studies related to noise reduction. Both the local turbulence statistics of the Overset-LES and the resulting far-field noise from the propagation simulation agree well with experimental reference data. The noise attenuation effect of a porous trailing-edge insert is numerically evaluated via a volume-averaged model considering a linear Darcy term and a non-linear Forchheimer term. A distinct noise reduction of up to 4 dB is observed, which is in good accordance with experimental findings that employed the same material. The noise reduction is explained by a breakdown of turbulent structures in the spanwise direction. The observation of high-frequency noise is attributed to increased surface roughness at the porous-fluid interface. Both phenomena are consistent with findings in literature. In a following study, the noise contribution from the slat of a three-element airfoil in high-lift configuration is evaluated by means of an Overset-LES and subsequent sound propagation. Two different geometries are compared under the same lift conditions, i. e., a well-defined baseline case and a long chord slat geometry for noise mitigation. The baseline case proved that the complex turbulent flow phenomena in the slat cove region are accurately captured. Furthermore, a noise reduction between 2 dB and 5 dB (depending on the radiation angle) is observed for the modified geometry compared to the reference, which is in agreement with experimental findings.