CAA boundary conditions for airfoil noise due to high-frequency gusts

Abstract

This paper proposes inflow and outflow boundary conditions for direct computation of airfoil noise under the influence of impinging gusts with a particular interest in the high frequency response. The proposed boundary conditions are based on an existing zonal sponge technique that has been used mostly to attenuate outgoing disturbances and absorb reflections from computational boundaries. A modified form of the sponge technique is presented in this paper in order to specify an incoming disturbance. The proposed boundary conditions still maintain the genuine non-reflective features that lead to accurate calculations of far-field sound intensity and directivity. It is also shown that the proposed boundary conditions enable the use of a significantly smaller domain size, and hence fewer grid cells, than used in conventional airfoil calculations, which enables the calculation of high-frequency gust-airfoil noise at a much lower computational cost. The proposed boundary conditions are validated against CAA (computational aeroacoustics) benchmark solutions after a variety of parametric tests, through which an optimal combination of the domain size, sponge thickness and a sponge coefficient is obtained for the highest efficiency. The proposed boundary conditions yield more accurate and consistent solutions particularly at the far field than the conventional ones. Further applications to high-frequency gust responses are performed to observe and demonstrate significant changes in the sound intensity and directivity varying with different frequencies and gust incidence angles.