MFC actuators, widely used in intelligent sensing/actuation, energy harvesting, underwater bionic robots and other fields, have increasingly attracted attention due to their good flexibility, large actuation force and excellent waterproof performance. In this paper, the vibration characteristics and dynamic response of underwater flexible structure driven by MFC actuators were studied. The driving force of the MFC actuators were calculated, and the additional inertia force and additional damping force of the fluid were derived according to Morrison's semi-empirical formula. Based on the Euler-Bernoulli beam theory, the assumed mode method and the second kind of Lagrange equation, a coupled nonlinear dynamic model of MFC-actuated underwater flexible structures was established. The harmonic balance method was used to convert the nonlinear damping into linear. The numerical simulation and experimental results show that the local stiffness of the flexible beam structure with the MFC actuator is increased, and the measured modal shape is basically consistent with the simulated one. Affected by the hydrodynamic force of the surrounding fluid, the first two orders resonance frequency and dynamic response of the MFC-actuated cantilever beam underwater decrease significantly. The amplitude-frequency response curve predicted by the model is in good agreement with the measured curve, which confirms the validity of the coupling dynamic model. This study provides a reference for underwater bionic propulsion devices based on smart materials.

The failure of ice sheets caused by various forces applied vertically is an important scenario in the polar exploration and utilization engineering. The characteristics of ice strength has a direct impact on the design and evaluation of the structural strength and safety manipulation of platform structures active in this area. In this paper, relying on the Small Ice Model Basin of China Ship Scientific Research Center (CSSRC SIMB), the columnar saline model ice was made to carry out circular plate center loading tests. During the test, the center of the ice specimen on the evenly-distributed circumferential support was loaded vertically, the force curve of the ice specimen from the initial loading to the flexural failure was recorded by a force measurement system, and the flexure strength was obtained according to the peak force. At the same time, the failure details of the ice specimen were documented by a high-speed video camera, and then the process and mode of ice destruction were analyzed. On this basis, a series of experimental tests were carried out to analyze the influence on the flexural strength of the model ice by varying the loading speed and the ice temperature. The work of this paper provides a feasible method for measuring and analyzing the strength characteristics of the model ice for CSSRC SIMB and establishes a preliminary foundation for the further research of the mechanism of ice loading under the vertical interaction between the structure and the ice sheet.

Active control can directly control the low-frequency vibration and noise of a structure by applying control force on the structural system, so it plays an important role in vibration and noise control. In this paper, a method of left eigenvector assignment based on structural receptances was proposed, and the active control of structural vibration was realized by using the relationship between left eigenvector and excitation of vibration system. Firstly, the assignment of eigenvalues and left eigenvectors was derived based on the structural receptances, so there is no need to establish the system model of the structure and know the M, C and K matrices. Secondly, using the redundant space of the left eigenvector assignment, the left eigenvector of the closed-loop system was assigned in the form of orthogonality with the excitation force vector, and the active control of structural vibration was realized. Finally, the numerical examples were given to verify the effectiveness of the method.

The current Level 2 vulnerability criteria assessment method for the surf-riding/broaching stability failure mode of the IMO Second Generation Intact Stability Criteria only applies to ships using conventional propellers. However, waterjet propelled ships are also prone to this stability failure mode. Therefore, it is necessary to establish a regulatory assessment method for such a ship type as well. This study established a mechanical model for waterjet propulsion systems through the analogy to the model for propellers. The Level 2 vulnerability assessment method applicable to waterjet propelled ships was proposed and validated based on the actual pump data and sample ship calculations. A specific assessment method for waterjet propelled ships was proposed to overcome the limitation on propulsion type, which not only makes the regulation more complete, but also provides technical support for the stability safety assessment of waterjet propelled ships.

The icing problem seriously threatens the safety of ship navigation in the polar environment. The droplet collection coefficient is to determine how effectively a body collects water droplets and is the key parameter for prediction of ice accretion. In this paper, the Euler method was used to numerically simulate the two-phase flow around a circular cylinder. Analysis of the influence on the trajectory of the droplet by studying the droplet Stokes number (St) and droplet Reynolds number (Re_w). Research shows that St has a significant effect on the trajectory of droplets. When St is large (St>1), the collection coefficient depends entirely on the St, Re_w influence is weaker, the total collection coefficient and local collection coefficient slowly decrease with the increase of Re_w. When St is small (0.26<St<1), as Re_w increases, the local and total collection coefficients significantly decrease. Because the fluid forms a vortex downstream of the cylinder, it has a greater flow-following feature at St<0.26, driving droplet coiling to be sucked into the near-wall area at the end of the cylinder, even colliding with the back wall of the cylinder, such that the local and total collection coefficients become larger with the increase of Re_w.

In this paper, for the demand of low-frequency hydrodynamic noise control in submarine sonar dome, based on the coupled vibration equations of the plate and acoustic cavity, the hydrodynamic self-noise calculation model of the multi-layer composite plate was established by using the acoustic vibration transfer matrix, modal expansion method and wave vector-frequency spectrum of turbulence boundary layer pressure. According to the analysis of dynamic vibration absorption characteristics, the vibration and noise equations of the plate under the control of distributed energy absorption unit were formed, and the hydrodynamic self-noise of the distributed power absorption composite sonar domes was evaluated. The hydrodynamic self-noise reduction effect was verified through the large cavitation channel tests, providing technical support for the design of advanced low-noise sonar domes.

Offshore wind energy resources is richer than land wind energy, and water depth of the continental shelf in China's waters increases slowly as the distance offshore increases. Based on the characteristics, how to optimize the design of mooring systems to adapt to the water depth conditions in China is one of the major problems encountered in the development of floating wind turbines. In this paper, a 5MW-OC4 semi-submersible floating wind turbine was used as the research object, the floating wind turbine was moored by suspended chain lines, and the frequency domain and time domain calculations of the floating wind turbine were performed under 40 m water depth in a sea area of Bohai Sea using SESAM software. Mooring accessories were used separately and in combination for parameter sensitivity analysis, and then the mooring system was optimized by combining buoys and clump weight blocks. The results of the study show that under shallow water conditions, the combination of mooring fittings has the same effect on the overall response of the floating wind turbine as changing the same mooring parameters when used alone, but the effect of changing the mooring parameters on the optimisation of the performance of the mooring system is more obvious when used in combination, the optimisation difference in the mean value of the counterweight block position parameters when used in combination can reach 15.7%, and the optimisation difference in the longitudinal oscillation, longitudinal rocking and the tension response are all within 10% of each other. The optimisation difference of longitudinal oscillation, longitudinal rocking and tension response is basically within 10%. Therefore, choosing a reasonable combination of accessories can significantly change the overall characteristics of the floating system and affect the safety and cost of the system.

For a high-speed planing craft, remarkable variation of the sailing state may cause abnormal distribution of air-water on the bottom for numerical calculation. In order to match the mesh layout and free-surface, a numerical wave tank based on Reynolds-averaged Navier-Stokes (RANS) method was established with dynamic mesh and manual six degrees of freedom (6-DOF) motion model. The high-resolution interface capturing with volume-of-fluid model (HRIC-VOF) scheme was applied to calculate the bottom’s water-air distribution on the ship model. The influences of angle factor, sharpening factor, Courant number’s upper bound, Courant number’s lower bound and time step on the calculation results of water-air distribution and total resistance were explored. The comparison of calculation and experimental results indicates that the current method is feasible for high-speed crafts’resistance forecast and for capture of free-surface. The relative error is less than 4.5% for ship model’s velocity at 2-13 m/s when FV=0.96-5.78.

A rigid polyurethane foam (RPUF) buffer was designed to reduce the load of a projectile during high-speed water entry. Based on the Hopkinson compression bar technique, the density and strain rate effects of RPUF under impact loading were obtained, and its macroscopic constitutive model was established. Based on the Arbitrary Lagrangian-Eulerian (ALE), the numerical simulation model of the projectile during high-speed water entry was established. The numerical simulation of the projectile during high-speed water entry with different densities of RPUF was carried out. The dynamic failure process and motion parameters of the buffer during the water entry were obtained, and the influence law of the density and thickness of RPUF on the load reduction characteristics was analyzed. It can be found that the strain rate effect of RPUF is not obvious, but the density effect is obvious, and that, as the density and thickness of RPUF increase, the load reduction performance of RPUF increases.

In this paper, the dynamic response characteristics of aluminium honeycomb sandwich panels under repeated rigid wedge impacts and ice wedge impacts were experimentally studied by using the horizontal impact test apparatus. The impact force-displacement curves and the structural deformation properties were obtained. Results show that with the increase of collision numbers, the peak value of collision force increases continuously, the contact time decreases significantly, the local indentation and global bending deformation of face sheet increase gradually, the compressive deformations of the honeycomb cores enlarge gradually and finally the densification phenomenon appears. Due to the ice fragmentation phenomenon in the collision process, the contact area under ice wedge impact increases. Compared with rigid wedge impact, the midpoint permanent deflection of top facesheet under ice wedge impact is smaller, but the local damage area is obviously larger. With the increase of collision numbers, the plastic accumulated deformation of honeycomb sandwich panel evolves from a plastic hinge line to an elliptic plastic zone.