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This paper focuses on the development of a three-dimensional panel method in time and frequency domains combined with the finite element method for analyzing the hydroelastic responses of rotating marine propellers in the wake of ships. A fully non-penetration boundary condition imposed on the deformed blade surface is conducted, in which the corrections of both the incoming flow velocities and the normal vectors imposed on the deformed and undeformed blade surface are taken into account. The added-mass and -damping matrices due to strongly coupled fluid-structure interaction are considered. Results of the present method are compared with experimental data available in the literature. It is observed that the fully non-penetration boundary condition applied on the deformed blade surface should be imposed to predict the unsteady performance of elastic propellers, which is due to the change of the added damping predicted by considering different non-penetration boundary conditions.

The coupled longitudinal and transverse vibration characteristics of a submarine propeller-shaft-hull system at low frequency are studied in this paper. Two substructures, which are the propeller-shaft subsystem and the submarine hull, are first modelled using the finite element method. The two substructures are then connected through bearings, which are simplified as longitudinal and lateral springs and dampers. The modes, the natural frequencies and the coupled vibration characteristics of the two substructures and their synthesized system are simulated. An experiment studying the dynamic characteristics of a large-scale submarine test-rig is proceeded and compared with the numerical results, showing great consistency. Finally, transfer path analysis of the low frequency excitation forces using power flow method is discussed.

An experimental study aiming at investigating the vibrational and acoustic characteristics of a submarine-like system is implemented in this article. For comparison purpose, a finite element model is used to calculate the dynamic behaviors and sound radiation of the system. With consideration of the interaction between the shaft and hull structure, the dynamic response and the radiated sound pressure of the system are presented. Associated with the finite element model, the modes that correspond to the main peaks in the radiated sound pressure are identified. The results indicate that the forces transmitted to the hull could be amplified at the natural frequencies of the shaft, and a relative high radiated sound pressure level can be observed if the hull is also resonating simultaneously. Due to the non-uniformity of the structure and external excitation, the bending modes (n= 1) and the longitudinal modes (n= 0) are coupled with each other and result in a more complex dynamic response of the system. Moreover, considering the foundation is critical to supporting the axial force transmitted from the propeller, and a lower foundation model is constructed to investigate its influence on the acoustic response of the system. The comparison results show that lower foundation would lead to a rise of the natural frequency of the mode that relates to the axial impedance of the foundation. And, a slight decrease in the radiated sound pressure can be achieved in the frequency range below these natural frequencies.

The vitality and fighting capacity of warships will be influenced by the underwater explosion–induced shock loadings. The foam materials have excellent energy dissipation capabilities and are widely used in mitigating impact/shock. In this article, a one-dimensional model for the shock mitigation of cellular claddings exhibiting small/significant strain hardening is proposed for both water blast loading and hydrostatic pressure. Special emphases are concentrated on the cellular material compression with actual stress–strain relationship and the effects of hydrostatic pressure on the shock mitigation performance of the cladding. Finite element models are developed to verify the analytical model. When considering the actual material relationship of the cellular claddings, the compaction mechanism is complex due to multiple reflections. A reflected secondary compression wave generates and propagates from the clamped end to the impinged end for a foam block with finite thickness. Later, there are even third or more compaction waves. It is worth to note that the part inside the secondary compacted zone continue to deform in quasi-static “Homogeneous” mode. Moreover, the effects of the hydrostatic pressure on the foam compression are revealed. The results can guide the design of such cellular claddings to water blast for naval ships.

The existence of mass eccentricity will lead to the energy transfer between axial and flexural vibrations of a beam. To study the coupling properties of a double-Timoshenko beam system, a non-uniform coupled double-beam system is modeled in which the upper beam is typical and the lower beam is mass eccentric simulated by a non-uniform two-layer Timoshenko beam. By incorporating Hamilton’s principle and spectral element method, the axial–bending coupled governing equations of the system are derived and the approach can also be easily used to analyze the influences of the parameters and other coupled beam systems. Both the free and forced vibration results of a double-beam system by this method are consistent with the corresponding finite element model’s and thus this method is validated. The coupled properties and their mechanism are revealed. The influences of axial and transverse flexible connection on the coupling properties including free and forced vibration are investigated. A systematic matching principle of reducing the vibration of the coupled system is proposed.

This article presents the experimental and numerical studies on the flow-induced vibration of propeller blades under periodic inflows. A total of two 7-bladed highly skewed model propellers of identical geometries but different elastic characteristics were operated in four-cycle and six-cycle inflows to study the blade vibratory strain response. A total of two kinds of wire mesh wake screens located 400 mm upstream of the propeller plane were used to generate four-cycle and six-cycle inflows. A laser Doppler velocimetry system located 100 mm downstream of the wake screen plane was used to measure the axial velocity distributions produced by the wake screens. Strain gauges were bonded onto the propeller blades in different positions. Data from strain gauges quantified vibratory strain amplitudes and excitation frequencies induced by the wake screens. The propellers were accelerated through the flexible propeller’s fundamental frequency to investigate the effect of resonance on vibratory strain response. The numerical work was conducted using large eddy simulation and moving mesh technique to predict the unsteady forces acting on the propeller blade when operating in a nonuniform inflow.

Owing to their locally resonant mechanism, internal resonators are usually used to provide band gaps in low-frequency region for many types of periodic structures. In this study, internal resonators are used to improve the vibration attenuation ability of finite periodic tetra-chiral coating, enabling high reduction of the radiated sound power by a vibrating stiffened plate. Based on the Bloch theorem and finite element method, the band gap characteristics of tetra-chiral unit cells filled with and without internal resonators are analysed and compared to reveal the relationship between band gaps and vibration modes of such tetra-chiral unit cells. The rotational vibration of internal resonators can effectively strengthen the vibration attenuation ability of tetra-chiral lattice and extend the effective frequency range of vibration attenuation. Two tetra-chiral lattices with and without internal resonators are respectively designed and their vibration transmissibilities are measured using the hammering method. The experimental results confirm the vibration isolation effect of the internal resonators on the finite periodic tetra-chiral lattice. The tetra-chiral lattice as an acoustic coating is applied to a stiffened plate, and analysis results indicate that the internal resonators can obviously enhance the vibration attenuation ability of tetra-chiral lattice coating in the frequency range of the band gap corresponding to the rotating vibration mode of internal resonators. When the soft rubber with the internal resonators in tetra-chiral layers has gradient elastic modulus, the vibration attenuation ability and noise reduction of the tetra-chiral lattice coating are basically enhanced in the frequency range of the corresponding band gaps of tetra-chiral unit cells.

In this paper, periodic and hybrid trichiral structure is developed as the raft in a floating raft system to enhance isolation performance in the medium and high-frequency range. Firstly, wave propagation in the periodic trichiral structure is analyzed through Bloch theorem. Further study of the hybrid trichiral structure, consisting of multilayer of cells with different geometry, demonstrates that the stop band of this assembly is the superposition of the band gap generated by individual cells. This feature can be used to extend the stop band without affecting the performance in other frequencies. Vibration transmission of the floating raft system is studied through frequency response function based substructuring method. Numerical examples indicate that the sound power radiated from the foundation can be greatly attenuated in the stop band of the periodic trichiral raft. Based on the feature of hybrid structure, an optimization scheme with respect to the trichiral raft is developed with the aim of minimizing the radiated sound power of the system. The proposed modeling and optimization method can be extended to design other regular raft structures with great flexibility.

This paper presents experimental and numerical investigation into the underwater sound radiation characteristics of a free-floating stiffened metal box covered with three different kinds of covering layers and subjected to mechanical excitation. One box is bare while the other three are, respectively, covered with solid covering layers, chiral covering layers, and chiral covering layers filled with expanded polystyrene (EPS) foams. The equivalent elastic modulus of chiral covering layer is obtained by the homogenization theory. The finite element method and boundary element method are used to calculate the underwater sound pressure. The measured and numerical results are illustrated and the sound insulation mechanisms of three covering layers are discussed. The measured results agree with the numerical results well. The covering layers can obviously reduce the underwater sound radiation of floating structures. Compared with the solid covering layer, the chiral covering layer is less effective in suppressing the sound radiation in the low-frequency range but more effective in the medium- and high-frequency range. The chiral covering layer filled with EPS foams shows the best performance, which is more effective in suppressing the sound radiation both in the low-frequency range and in the medium-frequency range. The EPS foams have a high contribution to the added damping of the chiral covering layer.

This paper is concerned with friction-induced vibration of a continuous propeller–shaft system excited by nonlinear friction due to contact between a water-lubricated bearing and a shaft. The dynamic equation is derived using Hamilton’s principle in conjunction with the finite element method. The drooping characteristics of the nonlinear friction and the torsional–lateral coupling via the bearing–shaft interaction laws are considered. Both stability analysis and responses analysis are then investigated through various system parameters. Stability is analyzed by determining the eigenvalues of the Jacobian matrix of the linearized system at the equilibrium point. Dynamic responses of the system are calculated on the basis of a reduced order modal model using the numerical integration method in order to validate the stability analysis. Numerical simulation proved to be consistent with the linear stability analysis. Analytical and numerical investigations reveal that friction-induced vibration of the proposed system is due to the combined action of nonlinear friction and coupled dynamics of the system, rather than the velocity-dependent friction alone as is commonly assumed.

A novel model-free control method for actively suppressing tonal structural vibration is investigated through numerical simulation and experiment. The model-free control method is established on the least-mean-squares-based adaptive feedback control principle, and an instrumental mechanism for adjusting the controller parameters is proposed to guarantee convergence of the algorithm. In the numerical verification of the model-free control method, a shaft–plate system is used to derive a lightly damped dynamic model, which is obtained by frequency response function synthesis, while in the experiment a shaft–cylinder system with time-varying dynamics is applied to evaluate the control system with respect to its potential in vibration control. In the control system, an electromagnetic actuator mounted on the shaft is enforced with the model-free control method to cancel vibration induced by the disturbance forces. Simulation results have demonstrated that the model-free control method is able to guarantee stability in the presence of changes in the disturbances and plant dynamics and that it is effective in attenuating vibration of the system. Experimental results have also demonstrated that the control system can tranquilize vibration of the shaft and suppress tonal vibrations in the shell effectively, but variation in the control channel should be sufficiently slow that the algorithm can complete its adaptation in time.

It is impractical to install sensors on every floor of a tall building to measure the full state vector because of the large number of degrees of freedom. This makes it necessary to introduce reduced-order control. A kind of system reduction scheme (dynamic condensation method) is proposed in this paper. This method is iterative and Guyan condensation is looked upon as an initial approximation of the iteration. Since the reduced-order system is updated repeatedly until a desired one is obtained, the accuracy of the reduced-order system resulting from the proposed method is much higher than that obtained from the Guyan condensation method. Another advantage of the method is that the reduced-order system is defined in the subspace of the original physical space, which makes the state vectors have physical meaning. An eigenvalue shifting technique is applied to accelerate the convergence of iteration and to make the reduced system retain all the dynamic characteristics of the full system within a given frequency range. Two schemes to establish the reduced-order system by using the proposed method are also presented and discussed in this paper. The results for a tall building with active tuned mass damper show that the proposed method is efficient for the reduced-order modelling and the accuracy is very close to exact only after two iterations. Copyright © 2001 John Wiley & Sons, Ltd.

A reliable technique is introduced to detect aerodynamic instability of compressors based on wavelet features and hidden Markov model (HMM). A single sensor is sufficient for stall warning if the position of the sensor is carefully selected. The method involves obtaining high-response pressure signal near the rotor tip close to the leading edge. Rotating instabilities band wavelet features are then extracted and trained for the HMM; using data under normal operating conditions, the performance index (PI) is calculated. Unsteady behaviors in prestall processes are discussed and casing wall pressure maps are implemented to explore the mechanism of tip leakage vortex (TLV), which are helpful in explaining the various PI results from different feature selections and probe locations. Experimental results show that the trend indices of PI suitably characterize the compressor aerodynamic instability in subsonic operation.