To address the difficulty in self-starting for vertical-axis water turbines with fixed pitch angles and to improve the efficiency of marine energy utilization,a hybrid power generation device was proposed that incorporated a Savonius-type wind rotor to assist in starting the vertical-axis water turbine.Numerical simulations were conducted to investigate the dynamic output characteristics of this integrated device.Firstly,a three-dimensional two-phase flow numerical tank was constructed using Fluent software,and the mesh partitioning scheme was determined by comparing the values of the power output parameters of the device under different mesh densities,and at the same time,the validity of the numerical tank was confirmed.Secondly,the established numerical tank was used to simulate the variation law of torque,speed and power coefficient of power generation devices under different flow field conditions with different water wheel blade numbers,radius ratios and height ratios.The results show that the device performs better in terms of torque stability,speed characteristics,and energy utilization in the simulated working condition range when the water wheel blades are 3 to 4,the radius ratio is 0.25 to 0.375,and the height ratio is 1/6 to 1/3.The device is able to start quickly in the presence of wind,and the power coefficient reaches 0.3 to 0.4 or even exceeds 0.4,with good starting and power generation performance.The research results can provide a reference for the design and application of the device.
The performance of Autonomous Underwater Vehicles(AUVs)is significantly influenced by their shape design.This study presents a novel AUV shape optimization method that integrates data-driven approaches and machine learning technique to focus on the impact of the head and tail profiles,and the configuration of the fins and rudders on AUV performance.A parameterized hydrodynamic analysis workflow was developed to automatically generate the hydrodynamic parameters required during the optimization process.Based on this workflow,a data-driven framework was constructed for multi-objective optimization of AUV shapes,with the aims of minimizing drag and maximizing maneuverability.Surrogate models for the two optimization objectives above were built using Multilayer Perceptron(MLP)neural networks and ensemble learning methods respectively,and their performances were compared with traditional surrogate models.The optimization problem was solved using the Non-dominated Sorting Genetic Algorithm II.Comparative analysis of the initial and optimized AUV shapes demonstrates significant improvements in hydrodynamic performance,confirming the feasibility and effectiveness of the proposed method.
In this study,the influence of opening parameters on the ultimate strength of perforated plates subjected to extreme cyclic loading in the presence of material kinematic hardening and isotro-pic hardening was analyzed.It is found that the ultimate strength of the perforated plates decreases rapidly and stabilizes in the first four cycles.Plates with oblong openings have a greater ultimate strength compared to plates with rectangular openings,while the relative strengthening ratio decreases over the duration of the cycle.The location of the openings is also an important parameter that affects the strength of the structure,as the plates with openings close to the edges in the longitudinal direction have higher strengths,while in the transverse direction the strengths are higher when the openings are close to the center.Among the three opening-strengthening methods compared,the Carling stiffener method maintains a better strengthening effect under cyclic loads for many periods.
There are numerous wave energy resources in the ocean, and how to convert them efficiently is still a problem to be solved. Considering the waves in China sea area are characterized by short period and small height, one new model combining three split heave point absorbers with a tension leg platform (TLP) is proposed in this paper. Based on the physical model test, the hydrodynamic performance and wave energy capture efficiency of the split absorbers are investigated, and compared with those of the corresponding unsplit absorber. The experimental results reveal that the split absorber presents new hydrodynamic characteristics, and that the efficiency of the new wave energy capture device is greatly improved for small-period waves in the low sea state. However, the pitch motion of the platform is significantly increased in some cases with specific incident wave direction, which needs to be solved by further optimizing the TLP mooring system.
Liquid sloshing is a common phenomenon in ocean engineering, and one which not only affects the stability of ship navigation, but also poses a threat to both the marine environment and human life. Ascertaining how best to reduce the amplitude of liquid sloshing has always been a key problem in ocean engineering. In this study, based on an improved moving-particle semi-implicit method, the BM-MPS method, the damping effect of a vertical slotted screen under rotation excitation was simulated and studied, and the influence of baffle porosity and the rotation amplitude on the resonance period and impact pressure was discussed. The results showed that the porosity had an obvious effect on the resonance period. A significant resonance period transformation happened when the porosity was 0.1, but a porosity of 0.15 was the point at which the maximum impact pressure in the resonance was at its minimum. Meanwhile, the impact duration curve was related to porosity. With the increasing of porosity, the impact duration curve changed from having no peak to a single peak, and then to double peak. In addition, the amplitude of rotation excitation was also one of the factors that affected the resonance period.
A plate system, composed of flat array is proposed to be one type of breakwater. At present, little research has been done on the wave elimination of the plate under long-period waves. The Reynolds-averaged Navier-Stokes (RANS) equations are treated based on three-dimensional closed RNG k-ε turbulence model and the governing equations are solved by VOF method. A numerical model of the interaction between wave and flat array breakwater is established to study the wave elimination performance of different wave period ang steep. The flow and vortex fields are solved to research the wave dissipation mechanism of breakwaters. Results show the wave transmission coefficient tends to increase with the increase of wave period. Wave transmitted coefficient are all less than 0.4 in this test, indicating that the flat array plate breakwater maintains a certain wave dissipation capacity for long-period waves. At the same period, the wave steepness change mainly affects the amount of water passing through the upper part of the flat plate, which has some influence on the way the breakwater dissipates energy. The plate spacing is the main wave energy dissipation area, which provides space for water exchange and effectively eliminate waves while reducing the amount of board. The results preliminarily demonstrated the potential application of flat plate array as a wave energy breakwater, but the flat plate parameter design and the interaction of waves and flat plates needs further research.
The hydrodynamic forces on the semi-submerged cylinder under the combination of steady and oscillatory flow are experimentally investigated. The semi-submerged cylinder is forced to move with different periods and amplitudes as towing in the still water to simulate the equivalent combined flow. Compared with the steady flow and oscillatory flow, the feasibility of Morison equation, the hydrodynamic coefficients including drag and added mass coefficients are respectively studied. The results show that the hydrodynamic forces on semi-submerged cylinder in these three types of flows under non-overtopping and overtopping have very different features. The Morison equation is further verified that it can well describe the hydrodynamic force on the semi-submerged cylinder in steady flow and oscillatory flow, while its limitation under combined steady and oscillatory flow when the overtopping occurs is clearly witnessed. The modified Morison equation by changing the of flow velocity squared term to the higher order index term is then proposed. The flow velocity indexes in the modified equation under the non-overtopping and overtopping conditions are respectively determined. The magnification of flow velocity indexes can be clearly found when the overtopping starts to happen. The maximum index can reach 4.4. Moreover, the distributions of drag and added mass coefficients with Froude number and KC number are also observed. This work can provide a useful reference for the design of related marine structures and more valuable guidance for future research.
Fracture failures of ship plates subjected to in-plane biaxial low-cycle fatigue loading are generally the coupling result of accumulative plasticity and biaxial low-cycle fatigue damage. A biaxial low-cycle fatigue crack growth analysis of hull structure that accounts for the accumulative plasticity effect can be more suitable for the actual evaluation of the overall fracture performance of the hull structure in severe sea conditions. An analytical model of biaxial low-cycle fatigue crack propagation with a control parameter for ∆ CTOD is presented for hull inclined-crack plate. A test was conducted for cruciform specimens made of Q235 steel with an inclined crack to validate the presented analysis. The biaxial accumulative plasticity behavior and the effects of biaxiality and stress ratios were investigated. The results of this study reveal a strong dependence of biaxial low-cycle fatigue crack propagation on biaxial accumulated plasticity.
This paper presents the repeated impact tests of a metal foam sandwich beam (MFSB) by using INSTRON 9350 drop tower and analysis of the deformation and energy absorption performance of the MFSB.Based on the rigid-plastic assumption,the theoretical model is established to analyze the plastic mechanical behavior of an MFSB suffering from repeated low velocity impacts,in which the bounds of dynamic solution of permanent deflection are derived.The theoretical predictions are compared with the results of impact tests.Results show that the displacement of the sandwich beam is linearly distributed,and the deflection increases with the impact number,while the increment declines.With the increase of the impact number,the plastic deformation energy produced in each impact decreases while the rebound energy increases.The permanent deflections predicted by theoretical solutions agree well with those obtained from the impact tests,confirming that the theoretical model is capable of predicting the plastic responses of MFSBs subjected to repeated impacts.The proposed analytical model can provide theoretical references and technical supports for the design of the MFSBs under repeated impact loadings.
The distributed liquid tanks are widely used in LNG carriers,aquaculture ships and some other ships.Due to the ship motions in waves,especially the pitching and rolling coupled motions in oblique waves,the liquid sloshing in tanks will take place,which will have a great impact on the living environment and the tank structures in aquaculture ships.Based on the motion data from the aquaculture ship model test,the CFD software STAR-CCM+ was used to carry out the numerical simulation of the tank sloshing of an aquaculture ship under single degree of freedom and double degrees of freedom.The magnitude of the sloshing pressure at different frequencies and measuring points were obtained,and the changes of the wave surface and the flow velocity distribution in the tank under different conditions were studied.The measured point pressure values obtained by simulation were compared with the experimental data to verify the accuracy of the numerical simulation method.The results can serve as a reference for the structure strength analysis and optimization of the liquid tanks of aquaculture ships.
Sandwich composites are widely used in marine structures due to their excellent comprehensive properties.However,the load-bearing characteristics analysis of marine sandwich composite structures faces challenges due to the different elastic moduli in tension and compression.Based on the modified Reissner theory assumptions,the governing equations for the deflection of rectangular sandwich plates with different tensile and compressive moduli are deduced,and the theoretical approximate solutions of simply-supported rectangular sandwich panels under uniform load are presented.Furthermore,based on the proposed material model with unequal elastic moduli in tension and compression,a finite element analysis model considering bi-modulus suitable for complex marine structures is established.Numerical example results show that the theoretical model and numerical method proposed in this paper have a high computational accuracy.The relevant calculation methods can serve as a reference for the structural design and numerical simulation of complex sandwich composites with bi-modulus.
This paper presents a new algorithm for active vibration control with both the eigenvalues and the eigenvectors designed simultaneously by multi-input feedback control.The emphasis here is placed on shaping the assigned eigenvector to reduce the structural frequency responses: (1) the subspace of the achievable eigenvectors is found with singular value decomposition;(2) the optimal eigenvectors are chosen by minimizing the modal energy;(3) the optimal eigenvectors and the desired eigenvalues can be assigned for vibration control with the proposed algorithm;and (4) a few numerical examples are given to demonstrate the effectiveness and accuracy of the proposed algorithm.
Freak waves can bring serious damage to vessels and maritime structures as well as to other facilities in the ocean due to the features of focused energy,strong nonlinearity,broad-banded spectrum,etc..Moreover such features are likely to be accompanied by other abnormal large waves occurring before or after them,such as deep troughs and successive large waves (wave group),which are also disastrous.Thus,more attention should be paid to both freak waves and associated abnormal large waves.In a real sea state,waves are usually characterized by multi-direction or short-crest.There are obvious differences between the dynamic behaviors of the two-dimensional and those of three-dimensional freak waves.Therefore,in this study,the complete generation and evolution processes of three-dimensional freak waves are experimentally simulated through dispersive and directional focusing of component waves,in order to discuss the dynamic behaviors of the abnormal large waves occurring during the generation and evolution process as well as the relationships among them.The results suggest that the abnormal large wave evolves along the main wave direction in a symmetric way during the generation and evolution process of the three-dimensional freak wave,and that the process is divided into three or four typical stages based on characteristic parameters of abnormal large waves,implying that the variation trend of abnormal large waves can be predicted based on characteristic parameters.In the present study,the spatio-temporal range of the generation and evolution process is shorter for three-dimensional freak waves compared to two-dimensional events,the process for three-dimensional case may skip wave group stage and deep trough stage,implying that the likelihood of deep troughs and wave group associated with a freak wave in multidirectional random seas is likely to be small.
With the growth of ship hull sizes and the emergence of new floating structures,the demand for the design values of wave loads in the new codes is becoming increasingly intense.At present,this design is primarily determined by means of numerical prediction,modelling and full-scale ship measurements.In making a numerical prediction,many factors like structural modelling of springing (wave-induced vibration) and hydroelasticity (wave-structural interaction),numerical methods and nonlinear effects should be considered.Key factors were summarized and proposed,and some prospects for future development were proposed.Large differences between numerical calculation results and model tests for high forward speed still remain.Cross-comparisons between results of different numerical calculation programmes and testing data are important for improving the prediction accuracy of numerical programmes in terms of nonlinearity,high-order responses and high speed.
The one-degree-of-freedom mass-spring-belt model is established for describing friction-induced stick-slip self-excited vibration by considering the stribeck-effect friction model and the normal load.The approximately analytical expressions for stick-slip oscillation amplitude are derived by using Krylov-Bogoliubbov-Mitropolsky method.Both the stable and the unstable steady-state solutions can be obtained by the analytical expressions.In addition,results from the analytical expressions agree well with results from numerical integration of the nonlinear governing equations of the model.Subsequently,the influences of kinetic friction coefficient and normal force on self-excited vibration amplitudes are discussed.The novelty of this work is the derived approximately analytical expressions for stick-slip oscillation amplitude,by which the stable and the unstable steady-state solution can be obtained.
In order to overcome the shortcomings of the traditional Rosenblatt-IFORM (Inverse-First-Order Reliability Method) environmental contour method,the use of the PCA (Principal Component Analysis)-IFORM contour method is proposed for generating more accurate environmental contours.The environmental contours at a chosen offshore site obtained by using the PCA-IFORM contour method are compared with those obtained by using the Rosenblatt-IFORM environmental contour method,and the accuracy and effectiveness of the PCA-IFORM contour method are convincingly validated.The 50-year extreme structural dynamic responses of a monopile-supported 5 MW offshore wind turbine installed at this chosen offshore site are calculated based on the highest nonlinear sea state from the PCA-IFORM contour method (or the traditional Rosenblatt-IFORM environmental contour method).The calculation results are systematically analyzed and compared,and the necessity and importance of using more realistic environmental contours (such as those generated using the PCA-IFORM contour method) are finally highlighted.
In order to eliminate danger and to reduce the frequency of ship accidents caused by ice forming on ship decks and superstructures,it is important to have a physical understanding and the ability to predict icing.By studying the freezing stages in an icing process,the influence of different parameters on the icing process is analyzed,which provides a theoretical basis for the prediction of ship icing.In this paper,the proportion of the ice phase after the water droplet recalescence stage is calculated,the water droplet profiles at different stages are discussed,and the freezing time of water droplets at different supercooling degrees and contact angles is simulated by Fluent.The results show that both the supercooling degree and the contact angle have a great influence on the freezing time of water droplets.The decrease of supercooling degree and the increase of freezing time are approximately exponential functions,and the increase of contact angle is also exponential functions for the increase of freezing time.
The Next Generation Subsea Production System (NextGen SPS) is an innovative concept for petroleum development in ultra-deep water areas,mainly consisting of artificial seabed (AS),rigid risers,flexible jumpers and mooring lines.To improve the overall performance and design efficiency of NextGen SPS,an integrated design approach for the NextGen SPS based on multidisciplinary design optimization (MDO) method was investigated in this paper by combing the multidisciplinary feasible (MDF) architecture and particle swarm optimization (PSO) algorithm to establish the design framework.Two sub-disciplines of hydrodynamic analysis and global performance analysis were defined,and analysis method in each sub-discipline was introduced.Surrogate models of hydrodynamic analysis and global performance analysis were developed by using Latin hypercube sampling method and back propagation neural network (BPNN).Surrogate models were incorporated into the design framework,through which an integrated design for NextGen SPS at a depth of 3000 m was implemented.It is concluded that both the overall performance and the design efficiency of NextGen SPS are improved.
To investigate the effect of the expansion characteristics of the cell on the hydrodynamic ram (HRAM),several types of cell structures with different configurations are designed,numerical simulations of cell structures under high-speed projectile penetration are carried out,and the effect of the wall thickness is discussed.The concept of 'expansion impedance' is proposed to quantify the cell expansion characteristics.The attenuating effect of the cell expansion characteristics on the cavitation pressure caused by HRAM is also analyzed.The results show that the smaller the expansion impedance is,the easier the cell is to expand and deform.When the expansion impedance of the cell structure decreases,the bulging process of the cell will reduce the cavitation pressure load strength and attenuate the HRAM.Besides,the expansion impedance value shows a correlation with the thickness of the side walls when the cavitation pressure load is small and the expansion impedance value is related to the matching relationship of the thickness of each wall when the cavitation pressure load is large.
