Acoustic Mode Coupling in a Circular-Section Duct Containing a Concentric Porous Windscreen

摘要

A coupled waveguide model which represents sound transmission in a flowing fluid which contains a windscreened microphone probe is developed. The flow is contained in an outer annular region of a circular section duct. In the central region the fluid is quiescent, and the two regimes are separated by a porous windscreen concentric with the duct. Acoustic coupling occurs across the windscreen and the central region represents the probe where the acoustic quantities are to be measured. The use of modal theory permits this situation to be formulated as an eigenvalue problem which results in an appropriate dispersion relation. Solving this equation analytically is difficult since the eigenvalues are complex. An alternative approach is to solve the governing equation for the acoustic pressure numerically. With these pressure profiles an iteration process is devised, which employs an optimization technique, to determine the wavenumbers and attenuation rates.