The effect of mean flow/source motion on sound pressure fields with absorption

Abstract

The sound field for a moving source can be described in either a source-fixed or space-fixed reference frame. Noise source modules for prediction codes are usually formulated in the source frame since this simplifies the boundary conditions associated with the acoustic field. Also, this allows consideration of a distributed source, which is important for Doppler effects. For community noise studies, noise metrics are computed in a ground based coordinate system. In principle, the problem can be formulated and a solution sought in either frame of reference. Once the problem is solved in a particular reference frame, coordinate transformations allow the pressure field to be expressed in different reference frames. In this study, the problem is formulated in a source-fixed frame. By using Fourier transforms and separation of variables, a solution is determined in the source frame. Directivity patterns are constructed for a cylindrical radiator, which show the influence of Mach number, frequency and displacement pattern on the radiated sound field. The far-field acoustic relations are transformed to emission coordinates that depict what is observed from a space-fixed (ground) reference frame. Effects of mean flow/forward flight on absorption due to viscosity are also presented.