The prediction of tonal and broadband slat noise

by 1976- Agarwal, Anurag

Noise from high-lift devices such as slats and flaps can contribute significantly to the overall aircraft sound pressure levels, particularly during approach. The acoustic spectrum of the noise radiated from slats exhibits two distinct features. There is a high-frequency tonal noise component, and a high-energy broadband component ranging from low to mid-frequencies. The objective of this thesis is to predict both the tonal and the broadband slat noise. An aeroacoustic whistling mechanism is proposed to predict the tonal noise generation. When the vortex shedding frequency at the blunt trailing edge of the slat comes close to one of the normal modes of the gap between the slat and the main element, an intense tonal noise is produced. The normal modes are calculated based on the geometry of the wing. The vortex shedding frequency is predicted based on a linear stability analysis of the slat’s wake region. An efficient and robust scheme is developed by which the stability calculation can be performed by a modular algorithm in a relatively quick time. The broadband noise is predicted using a two-step process. First the noise sources are modeled based on the local turbulence information. Then, the sound from these sources is propagated by assuming that the flow past the wing is uniform. A Boundary Element Method is developed to find the Green’s function for wave propagation in a moving medium in the presence of the wing. The noise in the far field is then predicted by forming a convolution of the Green’s function with the modeled sources. Finally, a technique is presented to account for nonuniform flow around the wing. This requires a solution of the linearized Euler Equations. However, these equations support acoustic as well as instability waves. The instability waves can completely overwhelm the acousticwave solution. Thus it is imperative for an accurate noise-prediction scheme to suppress the unwanted instability waves. A detailed mathematical analysis is presented that demonstrates that the instability wave solution is suppressed if the governing equations are solved in the frequency domain. The main focus of this thesis is in the development of numerical schemes and models, and then their use to explore the physics of noise generation, and the prediction of noise iii radiation, from slats. iv