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This article presents a methodology for the reduction of ship structure vibration through local structural stiffness modifications. To identify the locations in the ship structure where the modifications should be made, three design criteria are considered: structural intensity, von Mises stress, and displacement. Two simple cases of cantilever beam and simplified ship were first tested under a harmonic excitation force at a frequency near resonance. The methodology was subsequently applied to a containership model subjected to propeller excitation forces. It is found that the von Mises stress is the most effective criterion in identifying the critical regions for structural modification. By increasing the thickness of plates in small localised regions, the global vibration level in the ship structure can be reduced by 50% when the excitation is near the resonant frequencies. The improvement varies with the structural damping present, and also on the model of mass distribution in the ship.

This article presents a methodology for the reduction of ship structure vibration through local structural stiffness modifications. To identify the locations in the ship structure where the modifications should be made, three design criteria are considered: structural intensity, von Mises stress, and displacement. Two simple cases of cantilever beam and simplified ship were first tested under a harmonic excitation force at a frequency near resonance. The methodology was subsequently applied to a containership model subjected to propeller excitation forces. It is found that the von Mises stress is the most effective criterion in identifying the critical regions for structural modification. By increasing the thickness of plates in small localised regions, the global vibration level in the ship structure can be reduced by 50% when the excitation is near the resonant frequencies. The improvement varies with the structural damping present, and also on the model of mass distribution in the ship.

In this hydroelasticity study, the fluid added-mass effect on a hemispherical shell structure under flexural vibration is investigated. The vibration response of the hemisphere is solved by using a commercial finite element software (ABAQUS) coupled with an in-house boundary element code that models the fluid as potential flow. The fluid-structure interaction is solved as a fully-coupled system by modal superposition to reduce the number of degrees of freedom. The need for an iterative scheme to pass displacement/force information between the two solvers is avoided by direct coupling between the fluid and structure equations. The numerical results on the downward shift in natural frequencies due to added-mass effect compare well with vibration measurements conducted on a stainless-steel bowl with interior and exterior fluid. For water and soap-water solution used in the experiments, the fluid viscosity (varying over a wide range) did not have any significant effect on the wet natural frequencies. This is due to the small viscous boundary layer (milimetre scale) compared to the nominal size of the bowl in centimetres. For such cases, the fluid-added mass only depends on the density of the fluid and the use of potential flow in the numerical model is applicable.

In this hydroelasticity study, the fluid added-mass effect on a hemispherical shell structure under flexural vibration is investigated. The vibration response of the hemisphere is solved by using a commercial finite element software (ABAQUS) coupled with an in-house boundary element code that models the fluid as potential flow. The fluid-structure interaction is solved as a fully-coupled system by modal superposition to reduce the number of degrees of freedom. The need for an iterative scheme to pass displacement/force information between the two solvers is avoided by direct coupling between the fluid and structure equations. The numerical results on the downward shift in natural frequencies due to added-mass effect compare well with vibration measurements conducted on a stainless-steel bowl with interior and exterior fluid. For water and soap-water solution used in the experiments, the fluid viscosity (varying over a wide range) did not have any significant effect on the wet natural frequencies. This is due to the small viscous boundary layer (milimetre scale) compared to the nominal size of the bowl in centimetres. For such cases, the fluid-added mass only depends on the density of the fluid and the use of potential flow in the numerical model is applicable.

To investigate flexural vibration of structures in a fluid, a numerical algorithm was developed to relate the added mass and damping effects of the fluid to each mode of vibration. These are separate from the traditional added mass associated with rigid body motion, such as the translational motion along Cartesian axes. In this formulation, small-amplitude free surface waves were accounted for by using a nonsingular implementation of the free-surface Green’s function for a potential flow solver based on Boundary Element Method. The formulation was applied to the forced vibration of structures, namely, a hemispherical shell and a reinforced half cylinder with typical dimensions of ships and offshore structures, to obtain their dynamic response at various excitation frequencies. It is observed that resonance frequency of the structure, in contact with water, decreases due to the added mass effect. The influence of the free-surface wave on the fluid loading was investigated for large structures. It is simpler to relate the fluid added mass to mode shapes rather than distribution of fluid load over wetted surface of the structure in engineering simulations. Moreover, it is found that the vibration energy radiated away by the fluid surface wave has little influence on the vibration response of the shell structures.

To investigate flexural vibration of structures in a fluid, a numerical algorithm was developed to relate the added mass and damping effects of the fluid to each mode of vibration. These are separate from the traditional added mass associated with rigid body motion, such as the translational motion along Cartesian axes. In this formulation, small-amplitude free surface waves were accounted for by using a nonsingular implementation of the free-surface Green’s function for a potential flow solver based on Boundary Element Method. The formulation was applied to the forced vibration of structures, namely, a hemispherical shell and a reinforced half cylinder with typical dimensions of ships and offshore structures, to obtain their dynamic response at various excitation frequencies. It is observed that resonance frequency of the structure, in contact with water, decreases due to the added mass effect. The influence of the free-surface wave on the fluid loading was investigated for large structures. It is simpler to relate the fluid added mass to mode shapes rather than distribution of fluid load over wetted surface of the structure in engineering simulations. Moreover, it is found that the vibration energy radiated away by the fluid surface wave has little influence on the vibration response of the shell structures.

A fast computation scheme is presented for simulating fluid–structure interaction of partially submerged structures. Fluid added mass is simulated using the boundary element method, which only requires a mesh at the fluid–structure interface while satisfying the far field boundary conditions in a semi-infinite domain. The boundary element solver is coupled to a finite element solver for structural vibration analysis. To speed up the boundary element simulation, a low resolution and geometrically simplified surface mesh was used. A projection scheme is introduced to map fluid elements to structural elements, allowing proper transfer of displacement and fluid loading between the two solvers. A series of low-resolution meshes for a realistic container ship model were used to study the efficiency of the proposed scheme, comparing the errors incurred in calculating the wet natural frequencies against the speed-up in computation. With a 20 times decrease in the number of boundary elements, the relative error in wet natural frequency calculation was found to be less than 10%, while computation speed-up was found to be almost 500 folds.

A fast computation scheme is presented for simulating fluid–structure interaction of partially submerged structures. Fluid added mass is simulated using the boundary element method, which only requires a mesh at the fluid–structure interface while satisfying the far field boundary conditions in a semi-infinite domain. The boundary element solver is coupled to a finite element solver for structural vibration analysis. To speed up the boundary element simulation, a low resolution and geometrically simplified surface mesh was used. A projection scheme is introduced to map fluid elements to structural elements, allowing proper transfer of displacement and fluid loading between the two solvers. A series of low-resolution meshes for a realistic container ship model were used to study the efficiency of the proposed scheme, comparing the errors incurred in calculating the wet natural frequencies against the speed-up in computation. With a 20 times decrease in the number of boundary elements, the relative error in wet natural frequency calculation was found to be less than 10%, while computation speed-up was found to be almost 500 folds.

The current study aims to investigate the effectiveness of two different designs of helmet interior cushion, (Helmet 1: strap-netting; Helmet 2: Oregon Aero foam-padding), and the effect of the impact directions on the helmeted head during ballistic impact. Series of ballistic impact simulations (frontal, lateral, rear, and top) of a full-metal-jacketed bullet were performed on a validated finite element head model equipped with the two helmets, to assess the severity of head injuries sustained in ballistic impacts using both head kinematics and biomechanical metrics. Benchmarking with experimental ventricular and intracranial pressures showed that there is good agreement between the simulations and experiments. In terms of extracranial injuries, top impact had the highest skull stress, still without fracturing the skull. In regard to intracranial injuries, both the lateral and rear impacts generally gave the highest principal strains as well as highest shear strains, which exceed the injury thresholds. Off-cushion impacts were found to be at higher risk of intracranial injuries. The study also showed that the Oregon Aero foam pads helped to reduce impact forces. It also suggested that more padding inserts of smaller size may offer better protection. This provides some insights on future's helmet design against ballistic threats.

Blast-induced traumatic brain injury has been on the rise in recent years because of the increasing use of improvised explosive devices in conflict zones. Our study investigates the response of a helmeted human head subjected to a blast of 1 atm peak overpressure, for cases with and without a standard polycarbonate (PC) face shield and for face shields comprising of composite PC and aerogel materials and with lateral edge extension. The novel introduction of aerogel into the laminate face shield is explored and its wave-structure interaction mechanics and performance in blast mitigation is analysed. Our numerical results show that the face shield prevented direct exposure of the blast wave to the face and help delays the transmission of the blast to reduce the intracranial pressures (ICPs) at the parietal lobe. However, the blast wave can diffract and enter the midface region at the bottom and side edges of the face shield, resulting in traumatic brain injury. This suggests that the bottom and sides of the face shield are important regions to focus on to reduce wave ingress. The laminated PC/aerogel/PC face shield yielded higher peak positive and negative ICPs at the frontal lobe, than the original PC one. For the occipital and temporal brain regions, the laminated face shield performed better than the original. The composite face shield with extended edges reduced ICP at the temporal lobe but increases ICP significantly at the parietal lobe, which suggests that a greater coverage may not lead to better mitigating effects.

We analyze the transmission characteristics of a Sagnac loop interferometer containing a polarization-dependent loss element and lossless polarization-converting elements by use of Jones matrices. We show that polarization independence in the transmission mode can be achieved in such a configuration and that maximum transmittance occurs when a half-wave plate is used for the polarization-converting element. The result is verified experimentally for a fiber acousto-optic tunable filter and cascaded long-period fiber gratings with either intrinsic or process-induced polarization-dependent filtering characteristics.

We describe a novel scheme for complete suppression of polarization-dependent loss/gain (PDL, PDG) for transmission-type optical devices (LPG, SOA) via a l/2-shifted all-fiber Sagnac loop interferometer. The results are explained theoretically and demonstrated experimentally. © 2004 Optical Society of America.

We analyze the transmission characteristics of a Sagnac loop interferometer containing a polarization-dependent loss element and lossless polarization-converting elements by use of Jones matrices. We show that polarization independence in the transmission mode can be achieved in such a configuration and that maximum transmittance occurs when a half-wave plate is used for the polarization-converting element. The result is verified experimentally for a fiber acousto-optic tunable filter and cascaded long-period fiber gratings with either intrinsic or process-induced polarization-dependent filtering characteristics.

We describe a novel scheme for complete suppression of polarization-dependent loss/gain (PDL, PDG) for transmission-type optical devices (LPG, SOA) via a l/2-shifted all-fiber Sagnac loop interferometer. The results are explained theoretically and demonstrated experimentally. © 2004 Optical Society of America.