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The vibration properties of the submerged sandwich cylindrical shell with a viscoelastic core are investigated in this paper. Considering the acoustic-structure coupling, the analytical model of the submerged sandwich cylindrical shell that can handle three medium conditions including fluid-filled, fluid-loaded, and fluid-filled and -loaded is derived based on the wave propagation approach and the Flügge thin-shell theory. The vibration properties of the sandwich cylindrical shell under different medium and boundary conditions are analyzed, followed by a comparison of the damping effect of the constrained damping layer. Finally, an analysis is conducted on the influence of thicknesses of viscoelastic and constrained layers on vibration spectrum and natural frequency under fluid-filled and -loaded conditions. An experimental platform was established to conduct relevant experiments. Several important conclusions can be drawn. [ABSTRACT FROM AUTHOR]

[Objectives] In order to study the precise flow field of rudder cavitation and its related laws of unsteady hydrodynamic performance, [Methods] a propeller and rudder model of a certain ship is established, and a structure grid, SST k-ω turbulence model and VOF method are used to calculate the rudder cavitation. In order to understand the phenomenon of rudder cavitation, an observation experiment is carried out with a real ship. The calculated results are compared with the experimental results, proving the reliability of the numerical method. The periodic changes in rudder cavitation are then discussed and analyzed, and rudder cavitation is calculated on the basis of cavitation and non-cavitation under the two states of three kinds of rudder angles. [Results] The results show that when the cavitation range is small, it has little effect on average rudder force. As the cavitation range increases, its influence on average rudder force increases significantly while lateral rudder force decreases significantly. Once cavitation occurs, the amplitude of unsteady force increases greatly;the greater the cavitation range, the greater the amplitude of unsteady force. [Conclusions] The results of this study can provide technical support for assessing the hydrodynamic performance of rudders under cavitation conditions in the pursuit of optimal rudder design.

This paper aims to clarify the behaviour and mechanism of electromagnetic vibration in long sheet secondary tubular linear induction motors (LSS‐TLIMs). Firstly, the spatial and temporal characteristics of electromagnetic force acting in the stator are analysed. Secondly, based on the wave propagation method, an analytical model is established to calculate the vibration response of the stator, which is simplified as a finite cylindrical shell. The accuracy of the proposed model, as well as the rationality of the simplification, are verified by 3D FEM simulation. It is indicated that the electromagnetic vibration response of LSS‐TLIMs mainly depends on the "axial breathing mode" (ABM) and corresponding "modal participation factor". Moreover, the influences of the spatial distribution of electromagnetic force and the structural dimensions on the vibration are investigated. Finally, the "static vibration test" is performed to validate the above analysis. It is concluded that, compared with a rotary motor, the low‐frequency vibration of LSS‐TLIMs can be easily controlled since the natural frequency of ABMs tends to be much higher than that of the circumferential mode. [ABSTRACT FROM AUTHOR]

In order to study the bearing force caused by the machining errors of composite material propeller in non-uniform flow, the propeller was computed by changing the machining error of the propeller in six degrees of freedom, in which the main blade translates and rotates along the coordinate axis artificially by way of the statistical method. The bearing force of a DTMB P4119 propeller with a machining error was calculated according to the SST k-ω model and sliding meshing non-uniform flow to figure out the impact of various machining errors on the propeller, enabling a fundamental rule about the impact of various machining errors on a propeller's bearing force to be formed. The results show that vertical bearing force and horizontal bearing force increase linearly, and first-order blade frequency bearing force rarely changes with the increase of machining errors in each freedom; machining errors along the directions of diameter and pitch have a great impact on the propeller shaft's frequency bearing force. Therefore, we can put forward a more refined principle about the machining accuracy of propellers.

[Objectives] In order to improve the anti-cavitation performance of shaft strut on a surface ship,[Methods] we study the front strut, compare the pressure distribution and anti-cavitation performance of the improved hydrofoil with those of the original hydrofoil at different attack angles of incident flows, and analyze the hydrodynamic performance of the improved hydrofoil at different flow rates and attack angles. We also predict the initial cavitation speeds of three types of hydrofoil according to the cavitation conditions,and verify the predicted values according to the calculation results.[Results] The results show that the hydrodynamic performance and initial cavitation speed of the improved hydrofoil-1 have been significantly improved when compared with those of the original hydrofoil at the incident flow attack angles of 0°,5° and 10°;the improved hydrofoil-2 has better anti-cavitation performance at the attack angles of 0° and 5°,while worse anti-cavitation performance at 10°;the initial cavitation speed predicted according to the pressure distribution is relatively close to the results based on the cavitation, which shows that it is feasible to predict the initial cavitation speed by this method when the accuracy requirement is not high.[Conclusions] This provides a technical approach for vibration and noise reduction of a surface ship.

Based on the wave propagation method and improved Fourier series, the coupled vibration model of double-sided immersion (DI) cylindrical shell under arbitrary boundary conditions is established, and its coupled vibration characteristics are solved. The coupling vibration characteristic experiment of a cylindrical shell under the DI condition is carried out for the first time. The calculated natural frequency and vibration acceleration are compared with the model experimental results, which not only verifies the reliability of the theoretical method but also proves the effectiveness of the experimental results. Finally, the coupled vibration characteristics of DI cylindrical shells with arbitrary boundary conditions are analyzed. The results show that the natural frequency and acceleration of underwater immersed cylindrical shells are consistent with those of liquid filling cylindrical shells. The natural frequency and peak acceleration of the single-sided submerged cylindrical shell are greater than that of the DI cylindrical shell. The spring stiffness at both ends of the DI cylindrical shell can be divided into three sections: free boundary, elastic boundary, and rigid boundary. [ABSTRACT FROM AUTHOR]

In order to study the vibration isolation characteristics in water, based on the Flügge theory, a double-walled cylindrical shell model with spring connection is established using the wave propagation method. The fluid inside the inner shell is considered in the model. The reliability of the theoretical method is verified by comparing the calculation results of the mean square velocity with those of the finite element method. The effects of spring stiffness, spring number, fluid load, and inter-shell fluid thickness on vibration transmission of inner and outer shells are studied. The results show that the fluid loading effect of the water medium inside the inner shell can act on the inner shell and the outer shell simultaneously, while the fluid loading effect of the outboard fluid only acts on the outer shell. Under the condition of keeping the total spring stiffness unchanged, increasing the number of springs can reduce the vibration transmission from the inner shell to the outer shell, especially the vibration transmission from the inner shell to the outer shell at low frequencies. The "vibration short circuit" effect easily occurs in the inter-shell water, which is independent of the thickness of inter-shell water. When the elastic connection stiffness is small, the inter-shell water plays a leading role in vibration transmission in the whole frequency range. When the stiffness of elastic connection is large, the inter-shell water plays a leading role in vibration transmission at low frequencies, while the elastic connection plays a leading role in vibration transmission at high frequencies. [ABSTRACT FROM AUTHOR]

Based on the Flügge theory and orthotropic theory, the acoustic vibration coupling model of ring-stiffened cylindrical shell is established by using the wave propagation method and virtual source method. And the effects of water immersion on both sides, free surface, and hydrostatic pressure on the cylindrical shell are considered in the coupling model. Muller three-point iterative method is used to solve the coupling frequency. The calculation results of degradation theory are compared with COMSOL's calculation results and experimental results, respectively, which verifies the reliability of the theoretical method. Finally, the influence of fluid load, ring rib parameters, boundary conditions, hydrostatic pressure, and free surface on the coupled vibration of ring-stiffened cylindrical shell is analyzed by an example. [ABSTRACT FROM AUTHOR]