Equatorial Windows and Barriers for Stationary Rossby Wave Propagation

Rossby waves can cross the equator and connect the Northern Hemisphere (NH) and Southern Hemisphere (SH), or be blocked in the vicinity of the equator. This work explores the windows and barriers for the cross-equatorial waves (CEWs) by the wave ray ensemble method. The eastern Pacific and Atlantic regions are identified as common windows in both boreal winter and summer, while the Africa–Indian Ocean section exists as a window only in boreal summer. The western–central Pacific is found to be a barrier section. These results are consistent with correlation analysis of reanalysis data. Moreover, the dependence on the wavenumber of CEWs is investigated, revealing that they are restricted to long waves with zonal wavenumbers less than 6 and that their wavenumber vectors exhibit a northwest–southeast (southwest–northeast) tilt when they cross the equator from the NH to SH (from the SH to NH). This long-wave dominance of CEWs results from the spectral-selective filtering mechanism, which suggests that long waves have narrower equatorial barriers than short waves. Finally, the main wave duct associated with each window is obtained by the global passing CEW density distribution. The results indicate that the main CEW ducts roughly follow a great circle–like pathway, except for the Africa–Indian Ocean window in boreal summer, which may be modulated by the cross-equatorial monsoonal flow.

Investigation on acoustic energy harvesting based on quarter-wavelength resonator phononic crystals

This article presents the development of an acoustic energy harvester using a quarter-wavelength resonator phononic crystals together with piezoelectric vibrators. The quarter-wavelength resonator phononic crystals consist of a main wave duct, several quarter-wavelength resonators, and equivalent piezoelectric vibrators. The acoustic energy is converted to mechanical energy when the sound incident in the quarter-wavelength resonator generates an oscillatory pressure as of localization efficiency, which in turns causes piezoelectric vibrators vibrating. Transfer matrix method is used to provide a physical insight into the structure band of the quarter-wavelength resonator phononic crystals, and finite element method is used to analyze the sound localization effect and evaluate the reclaimed energy of the quarter-wavelength resonator phononic crystals. Results indicate that the numerical analysis agrees well with experiments. When the frequencies of the incident sound are near both sides of the bandgap, the maximum output voltage always can be obtained.

A Standing Wave-Duct System for Acoustical Property Prediction of Multi-Layered Noise Control Materials

Based on a hybrid transfer-matrix method, a new standing wave-duct system with four microphones for acoustical property prediction of multi-layered noise control materials is developed in this paper. In this system, the two-load method (TLM) and two-cavity method (TCM) are used for computing the transfer matrix of each material sample. The transfer matrices saved in a database in the system hardisk may be selected for predicting both the absorption ratio and transmission loss of a multi-layered treatment of materials. Verification results suggest that the newly designed standing wave-duct system is effective for acoustical prediction of multilayer material configurations.