Airfoil self-noise is a common phenomenon for many engineering applications. Aiming to study the underlying mechanism of airfoil self-noise at low Mach number and moderate Reynolds number flow, a numerical investigation is presented on noise generation by flow past NACA0018 airfoil. Based on a high-order accurate numerical method, both the near-field hydrodynamics and the far-field acoustics are computed simultaneously by performing direct numerical simulation. The mean flow properties agree well with the experimental measurements. The characteristics of aerodynamic noise are investigated at various angles of attack. The obtained results show that inclining the airfoil could enlarge turbulent intensity and produce larger scale of vortices. The sound radiation is mainly towards the upper and lower directions of the airfoil surface. At higher angle of attack, the tonal noise tends to disappear and the noise spectrum displays broad-band features.
The purpose of this paper is to construct a general broadband impedance model, which is suited for predicting acoustic propagation problems in time domain. A multi-freedom broadband impedance model for sound propagation over impedance surfaces is proposed and the corresponding time domain impedance boundary condition is presented. Basing on the extended Helmholtz resonator, the multi-freedom impedance model is constructed through combing with a sum of rational functions in the form of general complex-conjugate pole-residue pairs and it is proved that the impedance model is well posed. The impedance boundary condition can be implemented into a computational aeroacoustics solver by a rectlrsive convolution technique, which results in a fast and computationally efficient algorithm. The two dimensional and three dimensional benchmark problems are selected to validate the accuracy of the proposed impedance model and time domain simulations. The numerical results are in good agreement with the reference solutions. It is demonstrated that the proposed impedance model can be used to describe the broadband characteristics of acoustic liners, and the corresponding time domain impedance boundary condition is viable and accurate for the prediction of sound propagation over broadband impedance surfaces.
