Wavy leading edge airfoils interacting with anisotropic turbulence

Abstract

Leading edge noise reductions caused by serrations have been shown to be sensitive to the length scales of vortical disturbances. In order to improve the understanding of wavy leading edge airfoils as a noise reduction technology, this paper examines the effects of anisotropy on turbulence-airfoil interaction noise by means of computational aeroacoutic simulations. A synthetic turbulence method is used to generate fully three-dimensional, divergence-free, homogeneous anisotropic turbulence, which is injected in a linearized Euler equation solver to model the noise generation. Moderate variations in turbulence length scales, which are representative of the anisotropy in aero-engine fan wakes, are tested for a NACA 0012 airfoil with a wavy leading edge. This work focuses on the noise sources in the near-field by examining the distortion of the turbulent structures and velocity spectra in the vicinity of the noise sources, the unsteady pressure and its spectral density on the airfoil surface, the magnitude-squared coherence between velocity and pressure fluctuations on the noise sources, and the correlation between noise sources along the span for various degrees of anisotropy. Numerical results show that small variations in the turbulence length scales can produce significant changes in the spectral content of the noise sources at the peak and root regions. The loudest noise source is always located in the root region for the cases examined and this source is mainly affected by the transverse velocity fluctuations. To reduce the correlation between noise sources in the peak and root regions, the ratio between the chordwise length scale and the amplitude of the serrations, and the ratio between the spanwise length scale and the wavelength of the leading edge should satisfy lx/(2h)<1 and lz/λ≤0.5, respectively.

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