No abstract

No abstract

The paper focuses on the evaluation of the impact associated with various geometrical and material properties on the overall acoustic performance of generic multi-layer thermo-acoustic sources. First, a generalized numerical framework is developed using a state-of-the-art thermo-acoustic emission model for multi-layered devices and is used to forecast the effects associated with different parameters (thickness, density, thermal conductivity, and specific heat capacity), based on a set of 65 536 simulated architectures. Then, the acoustic facility is designed, assembled, and instrumented, and the findings of the simulation campaign are validated against experimental measurements for 32 different samples, manufactured via various vacuum deposition techniques. The results of the experimental campaign corroborate the simulation's prediction and indicate that the variables that have the strongest impact on the thermo-acoustic performance are the thicknesses of the substrate and thermophone layers, as well as the backing's thermal conductivity. Finally, the experimental results are directly comparable with the simulation predictions and the deviation between the two values is within the limits of the experimental accuracy, with an average deviation of 12% (maximal divergence of 28%) and best absolute performance of [Formula: see text] when measured from a distance of [Formula: see text]. Overall, the findings provide an insight into the effect of analyzed properties and offer a set of tangible guidelines that can be applied in the future toward the design optimization process that can potentially result in higher-efficiency thermophone-on-substrate thermo-acoustic emitters.

Finite amplitude dynamics of ships in head seas due to parametric instabilities is a subject of renewed interest with an increasing demand of operation in severe and variable environmental conditions. In this current study we investigate the nonlinear dynamics and internal resonances of a ship with a rectangular cross-section in head seas. We employ an asymptotic averaging method to obtain the slowly varying system evolution dynamics for the weakly nonlinear response, complemented by numerical integration in the strongly nonlinear regime. A weakly nonlinear frequency response is obtained analytically for a principal parametric resonance and a 1:1 roll—pitch internal resonance. Comparison of results with three degrees of freedom numerical simulations reveals a good fit. A strongly nonlinear numerical analysis reveals that beyond the stability thresholds, the system’s responses included quasiperiodic dynamics. This combined approach resolves both parametric instabilities and internal resonances induced for both weak and finite nonlinear interactions, and culminates with criteria for orbital stability thresholds describing the onset of quasiperiodic response and magnification of energy transfer between coupled pitch-heave and ship roll.

No abstract