Numerical Investigation of Mode Scattering in Bifurcated Annular Ducts

Kurzfassung

The scattering of sound waves by pylons (struts) in annular ducts is investigated numerically in quiescent conditions and with mean flows at Mach numbers ranging from 0.1 to 0.4. To this end, the propagation of acoustic modes in configurations with two or three pylons is computed over a wide frequency range using a frequency-domain finite-element solver. The transmitted and reflected duct modes are estimated using the single-plane pressure/velocity matching method. The sound pressure levels and modal amplitudes obtained are compared with experimental data, showing good overall agreement and thereby supporting the validity of the numerical approach. The amplitudes of the reflected and transmitted modes are found to depend strongly on the number of pylons and on frequency. The number of pylons governs the set of scattered azimuthal modes in agreement with the Tyler-Sofrin rule. At high frequencies, the scattering becomes broader as additional azimuthal and radial modes become cut on. For incident plane waves, the strongest reflected components are associated with azimuthal orders close to cut-off. For incident spinning modes, the scattered field is shown to follow the same selection rule, while the dominant transmitted component may, in some cases, have an azimuthal order of opposite sign to that of the incident mode, indicating a reversal of the spinning direction through the pylon row. The influence of mean flow on the scattered levels is significant but non-monotonic, with variations that depend on frequency, mode order, and number of pylons.