Optimisation of an Inertial Mechanism within a Uni-Axial Vibration Isolator to Suppress Internal Resonance

ABSTRACT Internal resonances within vibration isolators have been shown to increase force transmissibility and radiated noise from supporting structures. This paper theoretically investigates the optimal use of an inertial mechanism within a uni-axial vibration isolator to reduce the influence of these internal resonances. The inertial mechanism under consideration is associated with a device which exerts an inertial force proportional to the relative acceleration of its connection points. Examples of such devices include dynamic antiresonant vibration isolators, resonance changers and inerters. It has been shown that these devices can be used to establish suppression bands in vibration transmission. Previous research has examined the use of such a device for attenuating low frequency vibration transmission. This work considers the inertia of the isolator and minimises the force transmissibility over a wider frequency range to include the effect of internal resonances. The optimisation is carried out using a combination of a particle swarm and gradient based optimisation algorithm. It is shown that this isolator configuration has the potential to reduce the force transmissibility to levels approaching an ideal vibration isolator over a wide frequency range

Effective properties of acoustic metamaterial chains with low-frequency bandgaps controlled by the geometry of lightweight mass-link attachments

Available online 17 May 2019The effective mass of an acoustical metamaterial chain, which consists of a modified monatomic chain with a lightweight attached mass–link system, is derived and used to analyse its low-frequency vibration-isolation properties. The effective mass is expressed in terms of the resonant and anti-resonant frequencies of a basic element of the chain, and it is shown that the geometry of the attached system can be used to lower the resonant and anti-resonant frequencies, in turn lowering the bandgap of the chain, and producing efficient low-frequency vibration isolation with lightweight attached masses. In certain parameter limits, the chain is shown to degenerate to two previously proposed chains with contrasting band structures, and this provides insights into controlling the underlying vibration-isolation mechanisms. Numerical simulations are presented that illustrate the efficiency of the proposed system in terms of minimising transmission of a low-frequency incident wave packet with only two units of the attached system.Luke G. Bennetts, Malte A. Peter, Paul Dylejko, Alex Skvortso

Optimum resonance changer for submerged vessel signature reduction.

In maritime vessels, it is desirable to minimise the structural and acoustic responses for several reasons, including passenger comfort, minimisation of crew fatigue, and in the case of military vessels, to avoid detection. The propeller-shafting system represents one of the most critical areas which must be addressed in order to reduce the low frequency acoustic signature. The propeller-shafting system is primarily excited by axial oscillations at the propeller. The force transmitted along the propeller-shafting system from these disturbances results in axial excitation of the hull and subsequent sound radiation. The aim of this thesis is to apply a combination of passive and active control techniques, in order to minimise the low frequency radiated noise signature of a pressure hull submerged in a fluid.Dynamic models of the propeller-shafting system, foundation and cylindrical hull including complicating factors such as fluid loading, bulkheads and onboard equipment are developed and described using the transmission matrix approach. This modular description enables greater flexibility for dynamic modelling of the propeller shafting system, and can be easily manipulated for future design modifications. The far-field radiated sound pressure from the submarine hull is evaluated and related to the force delivered to the hull by the propeller-shafting system. A passive optimisation scheme involving a genetic and general non-linear constrained algorithm is used to minimise fitness functions associated with the vibration of the propeller, vibration transmission to the hull and far-field radiated sound pressure over a low frequency range. This results in optimal resonance changer parameters for single and multiple resonance changers in a variety of configurations.A new quasi-adaptive resonance changer system is proposed and optimised to minimise the radiated sound pressure or propeller velocity. The optimal use of an adaptive resonance changer is investigated in both the frequency and time domains to reduce the hull velocity and subsequently the far-field radiated sound pressure. Fully active control is also evaluated by introducing a control force to the resonance changer with the aim of minimising either the propeller velocity or the radiated noise level. Finally, the concept of hybrid control is investigated by coupling passive, active and semi-active control techniques together to improve the overall performance