Vortex sound radiation in a flow duct with a dipole source and a flexible wall of finite length

Analytical coupled vibro-acoustic modeling of a cavity-backed duct-membrane system with uniform mean flow

Sound propagation in a flow duct is a complex and technically challenging problem. The presence of flexible vibrating walls inside the duct creates additional difficulties to the problem due to the complex vibro-acoustic and aero-acoustic couplings involved in the system. An accurate prediction of the coupled system response is of great importance for a good understanding of the underlying physics as well as the optimal design of relevant noise suppression devices. In the present work, a unified energy formulation is proposed for the fully coupled structural-acoustic modelling of a duct-mounted membrane backed by an acoustic cavity with a grazing flow. Sufficiently smoothed admissible functions, taking the form of a combination of Fourier series and supplementary polynomials, are constructed to overcome the differential discontinuities for various boundary and/or coupling conditions. The formulation allows the obtention of all relevant vibro-acoustic field information in conjunction with the generalized Lighthill equation and Rayleigh-Ritz procedure. The validation and convergence studies show the accuracy and the efficiency of the proposed model. Results show the strong structural-acoustic interaction in such a duct-membrane-cavity system, and the flow affects resonant amplitude of membrane-dominant modes significantly. Some cross-zones can be observed for the membrane kinetic energy frequency response with low Mach number cases, especially when a higher tension is applied to the membrane. Analyses on the structural-acoustic coupling strength indicate that the coupling between the odd-even structural modes becomes more significant at a higher Mach number compared with odd-odd and even-even mode pairs. It is also shown that adjusting the boundary constraint of the membrane or imposing a higher tensile force allows impairing the adverse influence of the flow in the duct on sound attenuation.

Transmission of Low-Frequency Acoustic Waves in Seawater Piping Systems with Periodical and Adjustable Helmholtz Resonator

The characteristics of acoustic wave transmitting in a metamaterial-type seawater piping system are studied. The metamaterial pipe, which consists of a uniform pipe with air-water chamber Helmholtz resonators (HRs) mounted periodically along its axial direction, could generate a wide band gap in the low-frequency range, rendering the propagation of low-frequency acoustic waves in the piping system dampened spatially. Increasing the air volume in the Helmholtz chamber would result in a sharply decrease in the central frequency of the resonant gap and an extension in the bandwidth in the beginning, yet very slowly as the air volume is further augmented. Acoustic waves will experience a small amount of energy loss if the acoustic–structure interaction effect is considered. Also, the structure-borne sound will be induced because of the interaction effects. High pressure loadings on the system may bring in a shrink in the band gap; nevertheless, the features of broad band gaps of the system is still be maintained.