An accelerated creep test method for deep-sea equipment viewport window was proposed in this paper based on the sensitivity of PMMA (Polymethyl methacrylate) to temperature and stress. A modified creep constitutive equation based on aging theory was proposed by introducing the influence function of temperature. The uniaxial compressive creep tests at different temperature and stress levels were carried out, and the creep constitutive relation of PMMA was obtained by stepwise fitting method. The conversion relationship of the creep behavior of the viewport under different temperatures and pressures was obtained by the finite element creep analysis. The accelerated creep test method of the viewport model under the condition of raised temperature and increased pressure was proposed, and the comparison between the test results and the calculated results was completed. It is shown that the stress sensitivity of the uniaxial compressive creep behavior of PMMA is related to temperature. The finite element solution of the conversion coefficient of viewport’s creep deformation under different temperatures and pressures is different from the calculated value. The results of the viewport model are in good agreement with those of finite element creep analysis. In conclusion, in a certain temperature and stress range, the creep constitutive relation of PMMA obtained in this paper can well describe the law of uniaxial compressive creep behavior, and that of the viewport can be well simulated by the finite element creep analysis. The calculation results show that the conversion coefficient of viewport’s creep displacements between conditions of 25 °C & 26.6 MPa and 3 °C & 20 MPa is 12.2, and this coefficient can provide a basis for the accelerated creep test.

Suppressing incipient cavitation in an underwater body is of great importance to reducing the adverse effects caused by cavitation. For the method of suppressing incipient cavitation by changing the surface roughness, the research on the optimal design of the rough band parameters based on surrogate model methods was conducted. Firstly, numerical calculation methods were used to analyze the influence mechanism of the rough band parameters on the incipient cavitation characteristics at the head of the underwater body, and an initial design range for the rough band parameters was established. Then, the surrogate model method was used for parameter optimization analysis. The results show that setting a rough band on the head surface of the body can change the pressure distribution. The front and rear boundaries of the roughness may cause slight pressure fluctuations, which can change the minimum pressure value and thereby affect the incipient cavitation characteristics. Through sensitivity analysis of the surrogate model, it can be observed that compared with the position and width of the rough band, its height has a greater influence on incipient cavitation. The final optimization results obtained were verified by numerical calculation, which can obviously reduce the incipient cavitation and achieve a better effect in suppressing it.

An equivalent source method based on plane wave expansion and mirror imaging is proposed for predicting radiation from elastic structures in shallow water waveguide. This method uses the equivalent source framework and represents the Green’s function with plane wave expansion for virtual source intensities and far-field acoustic calculations. For seabed reflection coefficients dependent on incident angles, the Green’s function decomposes the spherical wave from a point source into plane waves, facilitating accurate reflection calculations on non-ideal boundaries. Comparisons with other Green’s function validate the computational accuracy. Numerical simulations of an elastic cylindrical shell under various excitation frequencies in shallow water waveguide confirm the method’s accuracy in sound field reconstruction.

Marine risers are critical facilities connecting offshore oil and gas platforms to subsea wellheads, and vortex-induced vibration is the main cause of fatigue damage to risers. Therefore, a vibration suppression-energy harvesting turbine device was designed for risers to suppress vortex-induced vibration and harvest energy. Then experiments were conducted with the length of the blade of the turbine device’s impeller as a variable. On one hand, the vibration suppression and energy harvesting performance of the turbine device were verified; on the other hand, the influence of the impeller’s length of the blade on the device’s vibration suppression-energy harvesting performance was investigated. The results show that the overall vibration suppression efficiency of the turbine device is ideal, capable of reaching over 50%, with the best vibration suppression efficiency being 96.08%. The vibration suppression performance of the device increases positively with the increase in the impeller’s length of the blade, while the energy harvesting performance shows a negative trend. When the length of the blade is 12d, the vibration suppression-energy harvesting performance of the device increases synchronously with the increase in the incoming flow velocity.

Stiffened plates are the basic structural units of a ship hull, and the safety and reliability of such structures are of paramount importance during service. Therefore, accurate acquisition of physical parameters such as stress and strain in stiffened plate structures through real-time monitoring techniques can provide data support for the safety assessment and prediction of ship hull structures. By employing the inverse finite element method (IFEM) based on the least squares variational principle, strain-field reconstruction of stiffened plate structures was conducted. Initially, numerical simulations were performed on axially-loaded stiffened plates, and the simulation results were then input into the inverse finite element algorithm for strain field reconstruction. By designing different measurement point layout schemes, the errors between the reconstructed results and the simulation results were analyzed. Moreover, the Xgboost algorithm was applied to provide guidance for the selection of discrete measurement point quantities and locations. The results indicate that IFEM is applicable to strain reconstruction in stiffened plate structures of ship hulls, and by optimizing the positions of measurement points, a significant reduction in quantity of measurement points in the inverse finite element model can be achieved while maintaining high-precision reconstruction results. The findings of this study can provide technical support for the health monitoring and safety assessment of ship hull structures.

In the formation process of a sonar array, position deviation between array elements is inevitable, and the sound-field coupling between array elements is strong under the close-packed condition, thus resulting in the amplitude and phase inconsistency between array elements. So it is necessary to carry out array online calibration after array formation. An anechoic pool in the lab can only be used to simulate free field in a certain frequency band, which is difficult to satisfy the need of low frequency underwater acoustic measurement. Therefore, the study in this paper focuses on the low frequency calibration problem of cylindrical array, and proposes a calibration method for array amplitude-phase consistency which combines holographic acoustic field reconstruction in finite space with matching search compensation. Based on the sound field separation technique of near-field acoustic holography (NAH) in finite space, the sound field in finite space was reconstructed firstly. Then, the amplitude-phase consistency search function was established with the sound field reconstruction results to realize the amplitude-phase consistency calibration for complex array elements by combining the optimization idea of matching search algorithm. In this paper, a small cylindrical array was taken as the analysis object, the theoretical modeling and simulation of the finite space sound field in the cylindrical cavity were carried out, and combined with intelligent optimization algorithm to calibrate the amplitude and phase consistency between array elements. The simulation results show that the proposed method based on NAH can be used for on-line calibration of low frequency amplitude-phase consistency after array formation in the laboratory.

Currently, when predicting the ship maneuvering motion in waves based on the mathematical model of ship maneuvering motion, hydrodynamic coefficients are mostly obtained through the model test or numerical prediction in still water, without considering the influence of waves on ship maneuvering hydrodynamics. Therefore, establishing the prediction method of ship maneuvering motion hydrodynamics in waves is essential for accurately predicting ship maneuvering motion. In this paper, the multi-degree-of-freedom motion of ship in waves was numerically modeled based on the overset grid method. The hydrodynamic modeling, free surface treatment and wave simulation methods were presented. The numerical simulation method of ship maneuvering hydrodynamic forces in waves was established. The hydrodynamic model test and numerical prediction of S175-ship in regular waves were carried out, and the influence of waves on ship hydrodynamics was evaluated. The numerical prediction results were compared with the model test results to verify the reliability and practicability of the numerical simulation method. The research in this paper can provide a guidance for the improvement of the ship maneuvering prediction in waves.

The outboard discharge process of an underwater vehicle navigating in water results in an interaction between cross-flow and jet. The fluid dynamic characteristics generated by jet in cross-flow are important topics in the field of fluids. Based on Reynolds average and large eddy simulation method, a numerical calculation model of jet in cross-flow was established in this paper. Then the flow field characteristics of jet in cross-flow were explored in detail. The accuracy of the current method was verified by comparing the results of averaged velocity and fluctuating pressures with those in related references. The velocity and vorticity characteristics of the near-wall surface upstream and downstream of the orifice were investigated based on the results of numerical simulation. Additionally, the characteristics of the sound field in the vicinity of the orifice were analyzed. The results indicate that the impact of jet in cross-flow on the upstream near wall was confined within the range of 6 times the aperture from the orifice. Flow separation occurs in the downstreams of the orifice within a range of 2-14 times the bore size. Under the interaction between the crossflow and the jet flow, the characteristic vortex structure of counter-rotating vortex pairs (CVP) are formed in the downstreams of the orifice. The CVP is formed near the lower edge of the orifice and persists downstream. The vortex core gradually moves away from the wall along the flow distance, and the influence area gradually expands. The primary source of acoustic energy resulting from the interaction between the cross-flow and the jet is situated in close proximity to the wall. The acoustic energy level near the orifice is higher and exhibits a detached characteristic from the wall. Furthermore, the sound pressure level in the downstreams of the orifice is considerably higher than that upstream. The radiation of the sound field is dependent on the direction of flow and exhibits obvious directional characteristics.

Large vessel-shaped fish cages are promising large aquaculture structures developed in recent years, with maximum structure length of nearly 400 meters. The effects of the hydrodynamics on the nets and the frames will be significant for the cage deformation response in waves, which will increase the complexity of the cage design. In this paper, a coupled dynamic model of a large vessel-shaped fish cage is used to calculate the motion and structural response in the time domain. Firstly, the floating body of the cage is discretized into multi-module units, connected by equivalent elastic beams. The nonlinear effects of the net and steel frames are considered. A floating cage model considering the deformation of the floating body is then established in time domain for dynamic analysis. The radiation force of the floating body is solved by applying the added mass and damping directly or the state space method, and the hydrodynamic loads on the net and steel frames considering the disturbing effect of floating body are calculated by Morison formula. The results show that the hydrodynamics of the net and frames have an obvious influence on the response of the fish cage and the cross-sectional elasticity produces a certain degree influence on the net twine tension, which provides reference for structural analysis and cross-sectional design of the large vessel-shaped fish cages.

This paper focuses on proposing a finite-time command filtered backstepping robust adaptive dynamic positioning control method to deal with the thruster dynamics, parameters uncertainty, input saturation and unknown external disturbance problems. The proposed method not only has the advantages of the command filtered backstepping, but also can guarantee the control system convergence in finite time. Firstly, adaptive neural network was used to estimate uncertain functions in the system. Secondly, the thruster input saturation issue was addressed with a finite-time auxiliary dynamic system. Finally, based on the uncertain estimation and the FTADS, a finite-time command filtered backstepping (FTCFB) control law was introduced, and the system tracking errors and parameter estimating errors were proved to be convergent in finite time by using Lyapunov stability theory. Additionally, the effectiveness of the proposed positioning control method was verified by numerical simulations.