Numerical Analysis of Airfoil Trailing-Edge Noise for Straight and Serrated Edges at Incidence

Abstract

The goal of this paper is to perform a detailed analysis of the hydrodynamic near field and acoustic far field of a NACA 64-618 airfoil with and without serrations. The impact of serrations is investigated across different airfoil angles of attack and serration flap angles. The natural boundary-layer transition cases 6 and 7 of the AIAA Benchmark Problems for Airframe Noise Computations V Workshop Category 1 are considered as benchmark for the straight edge cases. The numerical simulations are performed using the fully explicit, transient, and compressible lattice Boltzmann equation implemented in the computational fluid dynamics/computational aero-acoustics (CFD/CAA) solver SIMULIA PowerFLOW®. The acoustic far field is obtained by using the Ffowcs-Williams and Hawkings integral solution applied to the airfoil surface. A mesh resolution study is performed on the straight trailing-edge cases to demonstrate the grid independence of the numerical solution. The numerical results compare favorably against the experimental data. The impact of the serration flap angle on the effectiveness of sawtooth serrations in reducing noise is investigated by considering three different serration flap angles. It is found that the serration flap angle primarily affects the trailing-edge noise reduction through a modification of 1) the effective angle at which the turbulent structures are convected over the serrated edge; 2) the convection velocity and spanwise coherence length along the serration; and 3) the intensity of the hydrodynamic wall-pressure fluctuations that are scattered along the serrated edge. The first and last phenomena are expected to play the most important role on the far-field noise reduction.