Large Eddy simulation of Trailing Edge Acoustic Emissions of an Airfoil

Abstract

The present investigation of trailing edge acoustic emission of an airfoil concerns the effects of the broadband noise generated by the interaction of turbulent boundary layer and airfoil trailing edge, and the tonal noise generated by the vortex shedding of trailing edge bluntness. Large eddy simulation (LES) is performed on an NACA0012 airfoil with blunt trailing edge at a Reynolds number Rec = 400; 000 based on the airfoil chord length for three different configurations with different angles of attack. In order to reproduce and compare with the result from experiment in the literature, numerical tripping is tested and chosen to control the boundary layer development to guarantee a similar boundary layer thickness near the airfoil trailing edge. The near wall region inside the boundary layer is directly resolved by LES simulation with Van Driest damping, in order to obtain the instantaneous data in that region. With these instantaneous data from aerodynamic simulation, the acoustic predication is conducted by the Curle's analogy, which is suitable for stationary surface in free ow. To validate the numerical solutions, both ow simulation and acoustic integration results are compared to experimental data and simulation results available in the literature, and good agreement is achieved. The aerodynamic results show that the similar boundary layer development of experimental result can be reproduced by simulation with a suitable choice of numerical tripping, and the similar instantaneous behavior of ow inside the boundary layer is therefore guaranteed, which is vital for the acoustic prediction. The aeroacoustic results show that the acoustic prediction changes with the lift and drag force provided by the airfoil. Basically speaking, it's a result that the unsteady force around the surface is closely related to the mean force provided by an airfoil, which means that the noise control of a given airfoil is coupled with the optimization of its aerodynamic performance. As for the approximation made in the implemetation of Curle's analogy, it is shown in the aeroacoustic results that the airfoil can be treated as a compact point only if low frequency acoustic emission is of interest, and such kind of approximation can cause obvious problem if very high frequency acoustic emission is concerned.