Numerical simulation on the NACA0018 airfoil self-noise generation

Abstract

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. c ⃝The noise radiated from an airfoil embedded in a uniform incoming flow is referred to as airfoil selfnoise. Airfoil self-noise, commonly heard from wind turbine, aircraft, helicopter rotors and various types of turbo-machineries, is produced by the interaction of the boundary layer and wake with the airfoil itself. Since such noise is an important noise source in many applications, the study of this phenomenon therefore has been an important research topic in the aeroacoustic community. Amiet's 1 classical trailing-edge noise theory is considered to be an attractive approach due to the fact that the far-field noise can be accurately predicted, provided the surface pressure difference. Based on a large experimental database, Brooks et al. 2 proposed five sound generation mechanisms responsible for the airfoil self-noise, among which trailing edge (TE) mechanism will dominate for most applications. Recent rapid advances in computational aeroacoustics, turbulence modeling and parallel computation, have made it possible for direct noise computation. Different approaches of solving unsteady Navier-Stokes equations, such as direct numerical simulation (DNS) and large eddy simulation (LES), have been resorted for directly computing airfoil self-noise. Sandberg et al. 3 performed DNS for direct noise computation of airfoil self-noise. Marsden et al. 4 conducted LES of the flow around a NACA0012 airfoil and associated sound. More recently, Tam and Ju 5,6 applied DNS technique to investigate the generation of airfoil tones at moderate Reynolds numbers.The main objective of the present paper is to simulate the noise generation by flow over airfoils and investigate the characteristics of the aerodynamic noise at various angles of attack. DNS is performed of flow over NACA0018 airfoil at Reynolds number of Re = a) Corresponding author. Email: lixd@buaa.edu.cn. α U Fig. 1. Schematic diagram of the computational model.

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