The cylindrical shell is one of the main structural parts in ocean engineering structures.These cylinders are mostly of medium length,which means that the radius of the cross section is significantly smaller than the ...The cylindrical shell is one of the main structural parts in ocean engineering structures.These cylinders are mostly of medium length,which means that the radius of the cross section is significantly smaller than the length of the cylindrical shell.From the viewpoint of the shell theory,they belong to the mid-long cylindrical shell category.To solve mechanical problems on this kind of structure,especially a cracked cylindrical shell,analysis based on shell theory is necessary.At present the generally used solving system for the mid-long cylindrical shell is too complicated,difficult to solve,and inapplicable to engineering.This paper introduced the Sanders' mid-long cylindrical shell theory which reduces the difficulty of the solution process,and will be suitable for solving problems with complicated boundary conditions.On this basis,the engineering applications of this theory were discussed in conjunction with the problem of a mid-long cylindrical shell having a circumferential crack.The solution process is simple,and the closed form solution can usually be found.In practical engineering applications,it gives satisfactory precision.展开更多
The present study establishes a simple numerical model for the coupled response of a steel catenary riser(SCR) subjected to coplanar vessel motion and vortex-induced vibration(VIV). Owing to the large deflection of th...The present study establishes a simple numerical model for the coupled response of a steel catenary riser(SCR) subjected to coplanar vessel motion and vortex-induced vibration(VIV). Owing to the large deflection of the SCR, the geometric nonlinearity is considered in this model. The hydrodynamic force comprises the excitation force and hydrodynamic damping, where the excitation force that only exists when the non-dimensional frequency is located in the lock-in range, is associated with the VIV. The hydrodynamic force model is validated based on the published VIV test data.As for the seabed resistance at the touchdown zone(TDZ), integrated with an initial seabed trench, the hysteretic feature is modeled. Based on the model, the study emphasizes on the coupled response characteristics near the touchdown point(TDP) induced by coplanar vessel heave and VIV, and analyzes the sensitivity of the coupled response to the heaving amplitude and frequency. It is found that with the increase of the heave amplitude and frequency, the VIV can be obviously mitigated, but the heave-related response in the coupled analysis seems to be close to that in the heave-only simulation. Finally, the fatigue damage near TDP is parametrically investigated based on the separate analysis and the coupled analysis. The results demonstrate that the coupled effect plays a significant role in the fatigue assessment near TDP. Besides, the proportion of the coupled effect accounting for the total fatigue damage decreases with the increasing seabed stiffness, while increases with the increasing seabed trench depth.展开更多
In this work,a novel fluid-structure coupling method called the common node discrete element-smoothed particle hydrodynamics(DS-SPH)method is introduced.This framework combines the principles of the common node discre...In this work,a novel fluid-structure coupling method called the common node discrete element-smoothed particle hydrodynamics(DS-SPH)method is introduced.This framework combines the principles of the common node discrete element method(DEM)and smoothed particle hydrodynamics(SPH)to construct DEM-SPH particles situated on the same node.By doing so,the DEM particles can interact with the SPH particles within their support domain,enabling fluid-structure interaction(FSI).To determine the DEM microscopic parameters required for this method,uniaxial compression and three-point bending tests are conducted on sea ice.To verify the proposed model,we select the interaction between sea ice and structures as a case study.Through simulation,the model's capability of accurately depicting sea ice deformation and fracture has been demonstrated.The results indicate that the inclusion of SPH particles with fluid properties in the DEM model has minimal impact on the main mechanical parameters of sea ice.Additionally,it helps prevent the occurrence of particle splashing during cement failure.However,it is observed that the size of DEM particles and the friction between DEM particles and the structure significantly influence the macroscopic mechanical behavior of the common-node DEM-SPH model.Finally,we compare the fracture behavior of sea ice and the ice forces acting on structures obtained from the current model with on-site measured results.The agreement between the two sets of data is excellent,further validating the effectiveness of the proposed model in practical applications.展开更多
In this paper,we focus on the two-dimensional pulsating nanofluid flow through a parallel-plate channel in the presence of a magnetic field.The pulsating flow is produced by an applied pressure gradient that fluctuate...In this paper,we focus on the two-dimensional pulsating nanofluid flow through a parallel-plate channel in the presence of a magnetic field.The pulsating flow is produced by an applied pressure gradient that fluctuates with a small amplitude.A kind of proper transformation is used so that the governing equations describing the momentum and thermal energy are reduced to a set of non-dimensional equations.The analytical expressions of the pulsating velocity,temperature,and Nusselt number of nanofluids are obtained by the perturbation technique.In the present study,the effects of the Cu-H2O and Al_(2)O_(3)-H2O nanofluids on the flow and heat transfer in pulsating flow are compared and analyzed.The results show that the convective heat transfer effect of Cu-H2O nanofluids is better than that of Al_(2)O_(3)-H2O nanofluids.Also,the effects of the Hartmann number and pulsation amplitude on the velocity,temperature,and Nusselt number are examined and discussed in detail.The present work indicates that increasing the Hartmann number and pulsation amplitude can enhance the heat transfer of the pulsating flow.In addition,selecting an optimal pulsation frequency can maximize the convective heat transfer of the pulsating flow.Therefore,improved understanding of these fundamental mechanisms is conducive to the optimal design of thermal systems.展开更多
To realize the low-resistance shape optimization design of amphibious robots,an efficient optimization design framework is proposed to improve the geometric deformation flexibility and optimization efficiency.In the p...To realize the low-resistance shape optimization design of amphibious robots,an efficient optimization design framework is proposed to improve the geometric deformation flexibility and optimization efficiency.In the proposed framework,the free-form deformation parametric model of the flat slender body is established and an analytical calculation method for the height constraints is derived.CFD method is introduced to carry out the high-precision resistance calculation and a constrained Kriging-based optimization method is built to improve the optimization efficiency by circularly infilling the new sample points which satisfying the constraints.Finally,the shape of an amphibious robot example is optimized to get the low-resistance shape and the results demonstrate that the presented optimization design framework has the advantages of simplicity,flexibility and high efficiency.展开更多
Deepsea mining has been proposed since the 1960s to alleviate the lack of resources on land.Vertical hydraulic transport of collected ores from the seabed to the sea surface is considered the most promising method for...Deepsea mining has been proposed since the 1960s to alleviate the lack of resources on land.Vertical hydraulic transport of collected ores from the seabed to the sea surface is considered the most promising method for industrial applications.In the present study,an indoor model test of the vertical hydraulic transport of particles was conducted.A noncontact optical method has been proposed to measure the local characteristics of the particles inside a vertical pipe,including the local concentration and particle velocity.The hydraulic gradient of ore transport was evaluated with various particle size distributions,particle densities,feeding concentrations and mixture flow velocities.During transport,the local concentration is larger than the feeding concentration,whereas the particle velocity is less than the mixture velocity.The qualitative effects of the local concentration and local fluid velocity on the particle velocity and slip velocity were investigated.The local fluid velocity contributes significantly to particle velocity and slip velocity,whereas the effect of the local concentration is marginal.A higher feeding concentration and mixture flow velocity result in an increased hydraulic gradient.The effect of the particle size gradation is slight,whereas the particle density plays a crucial role in the transport.展开更多
This paper investigates the hydrodynamic performance of a cylindrical-dual or rectangular-single pontoon floating breakwater using the numerical method and experimental study. The numerical simulation work is based on...This paper investigates the hydrodynamic performance of a cylindrical-dual or rectangular-single pontoon floating breakwater using the numerical method and experimental study. The numerical simulation work is based on the multi-physics computational fluid dynamics(CFD) code and an innovative full-structured dynamic grid method applied to update the three-degree-of-freedom(3-DOF) rigid structure motions. As a time-marching scheme, the trapezoid analogue integral method is used to update the time integration combined with remeshing at each time step.The application of full-structured mesh elements can prevent grids distortion or deformation caused by large-scale movement and improve the stability of calculation. In movable regions, each moving zone is specified with particular motion modes(sway, heave and roll). A series of experimental studies are carried out to validate the performance of the floating body and verify the accuracy of the proposed numerical model. The results are systematically assessed in terms of wave coefficients, mooring line forces, velocity streamlines and the 3-DOF motions of the floating breakwater. When compared with the wave coefficient solutions, excellent agreements are achieved between the computed and experimental data, except in the vicinity of resonant frequency. The velocity streamlines and wave profile movement in the fluid field can also be reproduced using this numerical model.展开更多
Corrosion and fatigue cracks are major threats to the structural integrity of aging offshore platforms. For the rational estimation of the safety levels of aging platforms, a global reliability assessment approach for...Corrosion and fatigue cracks are major threats to the structural integrity of aging offshore platforms. For the rational estimation of the safety levels of aging platforms, a global reliability assessment approach for aging offshore platforms with corrosion and fatigue cracks is presented in this paper. The base shear capacity is taken as the global ultimate strength of the offshore platforms. It is modeled as a random process that decreases with time in the presence of corrosion and fatigue crack propagation. And the corrosion and fatigue crack growth rates in the main members and key joints are modeled as random variables. A simulation method of the extreme wave loads which are applied to the structures of offshore platforms is proposed too. Furthermore, the statistics of global base shear capacity and extreme wave loads are obtained by Monte Carlo simulation method. On the basis of the limit state equation of global failure mode, the instantaneous reliability and time dependent reliability assessment methods are both presented in this paper. Finally the instantaneous reliability index and time dependent failure probability of a jacket platform are estimated with different ages in the demonstration example.展开更多
The study focuses on the flexible jumper issue of Subsurface Tension Leg Production(STLP)system concept,which is considered as a competing alternative system to support well completion devices and rigid risers in ultr...The study focuses on the flexible jumper issue of Subsurface Tension Leg Production(STLP)system concept,which is considered as a competing alternative system to support well completion devices and rigid risers in ultra-deep water for offshore petroleum production.The paper presents analytical and numerical approaches for the optimum design and global analysis of the flexible jumper.Criteria using catenary concept are developed to define the critical length for optimum design.Based on the criteria,detailed hydrodynamic analyses including quasi-static analysis,modal analysis,and dynamic analysis are performed.Modal analysis with respect to the quasi-static analysis shows that the existence of resonant modes requires special consideration.The results of dynamic analysis confirm the effectiveness of the de-coupled effect from the jumper on STLP system.The approaches developed in the study also have wide application prospect in reference to the optimum design and analysis of any Hybrid Riser(HR)concept.展开更多
The evolution of energy, energy flux and modal structure of the internal tides (ITs) in the northeastern South China Sea is examined using the measurements at two moorings along a cross-slope section from the deep con...The evolution of energy, energy flux and modal structure of the internal tides (ITs) in the northeastern South China Sea is examined using the measurements at two moorings along a cross-slope section from the deep continental slope to the shallow continental shelf. The energy of both diurnal and semidiurnal ITs clearly shows a ~14-day spring-neap cycle, but their phases lag that of barotropic tides, indicating that ITs are not generated on the continental slope. Observations of internal tidal energy flux suggest that they may be generated at the Luzon Strait and propagate west-northwest to the continental slope in the northwestern SCS. Because the continental slope is critical-supercritical with respect to diurnal ITs, about 4.6 kJ/m^2 of the incident energy and 8.7 kW/m of energy flux of diurnal ITs are reduced from the continental slope to the continental shelf. In contrast, the semidiurnal internal tides enter the shelf because of the sub-critical topography with respect to semidiurnal ITs. From the continental slope to the shelf, the vertical structure of diurnal ITs shows significant variation, with dominant Mode 1 on the deep slope and dominant higher modes on the shelf. On the contrary, the vertical structure of the semidiurnal ITs is stable, with dominant Mode 1.展开更多
Point absorber wave energy device with multiple degrees of freedom(DOF) is assumed to have a better absorption ability of mechanical energy from ocean waves. In this paper, a coaxial symmetric articulated point absorb...Point absorber wave energy device with multiple degrees of freedom(DOF) is assumed to have a better absorption ability of mechanical energy from ocean waves. In this paper, a coaxial symmetric articulated point absorber wave energy converter with two degrees of freedom is presented. The mechanical equations of the oscillation buoy with power take-off mechanism(PTO) in regular waves are established. The three-dimensional numerical wave tank is built in consideration of the buoy motion based upon the CFD method. The appropriate simulation elements are selected for the buoy and wave parameters. The feasibility of the CFD method is verified through the contrast between the numerical simulation results of typical wave conditions and test results. In such case, the buoy with single DOF of heave, pitch and their coupling motion considering free(no PTO damping) and damped oscillations in regular waves are simulated by using the verified CFD method respectively. The hydrodynamic and wave energy conversion characteristics with typical wave conditions are analyzed. The numerical results show that the heave and pitch can affect each other in the buoy coupling motion, hydrodynamic loads, wave energy absorption and flow field.The total capture width ratio with two coupled DOF motion is higher than that with a single DOF motion. The wave energy conversion of a certain DOF motion may be higher than that of the single certain DOF motion even though the wave is at the resonance period. When the wave periods are high enough, the interaction between the coupled DOF motions can be neglected.展开更多
Surface charges can modify the elastic modulus of nanostructure,leading to the change of the phonon and thermal properties in semiconductor nanostructure.In this work,the influence of surface charges on the phonon pro...Surface charges can modify the elastic modulus of nanostructure,leading to the change of the phonon and thermal properties in semiconductor nanostructure.In this work,the influence of surface charges on the phonon properties and phonon thermal conductivity of GaN nanofilm are quantitatively investigated.In the framework of continuum mechanics,the modified elastic modulus can be derived for the nanofilm with surface charges.The elastic model is presented to analyze the phonon properties such as the phonon dispersion relation,phonon group velocity,density of states of phonons in nanofilm with the surface charges.The phonon thermal conductivity of nanofilm can be obtained by considering surface charges.The simulation results demonstrate that surface charges can significantly change the phonon properties and thermal conductivity in a GaN nanofilm.Positive surface charges reduce the phonon energy and phonon group velocity but increase the density of states of phonons.The surface charges can change the size and temperature dependence of phonon thermal conductivity of GaN nanofilm.Based on these theoretical results,one can adjust the phonon properties and temperature/size dependent thermal conductivity in GaN nanofilm by changing the surface charges.展开更多
The Multiple Column Platform(MCP) semi-submersible is a newly proposed concept, which differs from the conventional semi-submersibles, featuring centre column and middle pontoon. It is paramount to ensure its structur...The Multiple Column Platform(MCP) semi-submersible is a newly proposed concept, which differs from the conventional semi-submersibles, featuring centre column and middle pontoon. It is paramount to ensure its structural reliability and safe operation at sea, and a rigorous investigation is conducted to examine the hydrodynamic and structural performance for the novel structure concept. In this paper, the numerical and experimental studies on the hydrodynamic performance of MCP are performed. Numerical simulations are conducted in both the frequency and time domains based on 3D potential theory. The numerical models are validated by experimental measurements obtained from extensive sets of model tests under both regular wave and irregular wave conditions. Moreover, a comparative study on MCP and two conventional semi-submersibles are carried out using numerical simulation.Specifically, the hydrodynamic characteristics, including hydrodynamic coefficients, natural periods and motion response amplitude operators(RAOs), mooring line tension are fully examined. The present study proves the feasibility of the novel MCP and demonstrates the potential possibility of optimization in the future study.展开更多
With the rapid development of ocean technology, the deep-sea manned submersible is regarded as a high-tech equipment for the exploration and exploitation of ocean resources. The safety of manned cabin has a decisive e...With the rapid development of ocean technology, the deep-sea manned submersible is regarded as a high-tech equipment for the exploration and exploitation of ocean resources. The safety of manned cabin has a decisive effect on the whole system. Ti-6 Al-4 V with the superior strength-to-weight ratio and corrosion resistance has been used for the manned cabin. The manned cabin experiences loading spectrum with different maximum stresses and different dwell time during their service life. The load sequence effects on dwell fatigue crack growth behavior of Ti-6 Al-4 V under different dwell time are investigated experimentally in this paper. The experimental results show that the crack tip plastic zone is enlarged by the dwell time and the overload retardation zone increases with dwell time under the same overload rate. A dwell fatigue crack growth model is proposed by modifying the crack tip plastic zone under the loading history with combinations of the single overload and dwell time factors are included in the modified model. Based on the experimental data, the overload retardation zone and the crack growth rates of Ti-6 Al-4 V are predicted by the modified model. A reasonable model for the load sequence effect on the dwell fatigue crack growth rates of Ti-6 Al-4 V is verified.展开更多
The procedure of assessment of structural fatigue strength of an offshore floating production and storage and offloadingunit(FPSO) in this paper. The emphasis is placed on the long-term prediction of wave induced load...The procedure of assessment of structural fatigue strength of an offshore floating production and storage and offloadingunit(FPSO) in this paper. The emphasis is placed on the long-term prediction of wave induced loading, the refined finite el-ement model for hot spot stress calculation, the combination of stress components, and fatigue damage assessment based onS-N curve.展开更多
In this study,the effects of geometrical and physical factors on light particles dispersion in stirred tank were investigated by agitation characteristic curve.The experiments and CFD simulations with discrete phase m...In this study,the effects of geometrical and physical factors on light particles dispersion in stirred tank were investigated by agitation characteristic curve.The experiments and CFD simulations with discrete phase model(DPM)and volume of fluid model(VOF)were conducted in this paper.Five factors,which include four geometrical factors(submergence,impeller-to-tank ratio,number of impeller blades and baffling mode)and a physical factor(liquid viscosity)were considered.For each factor,the power consumption curve and agitation characteristic curve were drawn to compare the power consumption and mixing results in the stirred tank.Characteristics of the agitation characteristic curves were compared with the previous published literatures and theories.It is found that the agitation characteristic curves reflect the tendency of power consumption and particles distribution well in stirred tank.The good agreement indicates the applicability of the agitation characteristic curves for the study of light particles distribution in stirred tank.展开更多
Fluid resonance in a moonpool formed by two identical rectangular hulls during in-phase heaving motion is investigated by employing a two-dimensional numerical wave flume based on OpenFOAM package with Re-Normalizatio...Fluid resonance in a moonpool formed by two identical rectangular hulls during in-phase heaving motion is investigated by employing a two-dimensional numerical wave flume based on OpenFOAM package with Re-Normalization Group(RNG) turbulent model. The focus of the study is to examine the influence of heaving frequency and amplitude with various moonpool configurations on fluid resonant behavior. It is found that the resonant frequency of wave response in moonpool tends to decrease with the increase of moonpool breadth and hulls draft. The decrease of resonant amplitude can be observed for large moonpool breadth. The influence of hulls draft on resonant amplitude is not remarkable, especially for large heaving amplitude. The increase in heaving amplitude results in the decrease of relative resonant amplitude in an approximate power function, implying a complicated dependence of the resonant amplitude on heaving amplitude. Flow patterns in the vicinity of the moonpool are also analyzed, mainly regarding the dependence on the heaving frequency. The negligible influence of vortices on the wave response in moonpool is expected for low-frequency excitation because it is hard to observe the vortex structures. Intensive vortical flow and vortex structure can be identified under resonant condition, which gives rise to significant dissipation and accounts for the smaller relative resonant amplitude in moonpool. As for high-frequency excitation, the vortex motion is rather weak and dissipates rapidly, leading to insignificant effect on wave response amplitude.展开更多
Two marine structures arranged side by side with a narrow gap may suffer from violent free-surface resonance,which would cause green water on deck,dramatically raise hydrodynamic loads on structures and seriously thre...Two marine structures arranged side by side with a narrow gap may suffer from violent free-surface resonance,which would cause green water on deck,dramatically raise hydrodynamic loads on structures and seriously threaten the operation safety.The CFD-based open-sourced software,OpenFOAM?,is employed to simulate the twodimensional fluid resonance inside a narrow gap between a fixed box and a vertical wall induced by regular waves with different wave heights.The topographies with various plane slopes are placed in front of the wall.The focus of this article is on the influences of the incident wave height and the topographic slope on the nonlinear characteristics of various hydrodynamic parameters(including the wave height in the gap,the vertical wave force,and the horizontal wave force on the box)during gap resonance.The ratios of their high-order to the corresponding 1 st-order components under different sets of the incident wave height and the topographic slope are analyzed.It is found that the relative importance of all the high-order components increases gradually with the incident wave height for all the three parameters.The topographic influence on them closely depends on the type of the parameters and the incident wave height.In addition,the occurrence of the 2 nd-order gap resonance phenomenon can cause the 2 nd-order wave height and horizontal force to be significantly larger than the corresponding 1 st-order components.展开更多
基金Supported by the National Natural Science Foundation of China under Grant No.（50579023）.
文摘The cylindrical shell is one of the main structural parts in ocean engineering structures.These cylinders are mostly of medium length,which means that the radius of the cross section is significantly smaller than the length of the cylindrical shell.From the viewpoint of the shell theory,they belong to the mid-long cylindrical shell category.To solve mechanical problems on this kind of structure,especially a cracked cylindrical shell,analysis based on shell theory is necessary.At present the generally used solving system for the mid-long cylindrical shell is too complicated,difficult to solve,and inapplicable to engineering.This paper introduced the Sanders' mid-long cylindrical shell theory which reduces the difficulty of the solution process,and will be suitable for solving problems with complicated boundary conditions.On this basis,the engineering applications of this theory were discussed in conjunction with the problem of a mid-long cylindrical shell having a circumferential crack.The solution process is simple,and the closed form solution can usually be found.In practical engineering applications,it gives satisfactory precision.
基金financially supported by the National Natural Science Foundation of China (Grant No. 51979129)。
文摘The present study establishes a simple numerical model for the coupled response of a steel catenary riser(SCR) subjected to coplanar vessel motion and vortex-induced vibration(VIV). Owing to the large deflection of the SCR, the geometric nonlinearity is considered in this model. The hydrodynamic force comprises the excitation force and hydrodynamic damping, where the excitation force that only exists when the non-dimensional frequency is located in the lock-in range, is associated with the VIV. The hydrodynamic force model is validated based on the published VIV test data.As for the seabed resistance at the touchdown zone(TDZ), integrated with an initial seabed trench, the hysteretic feature is modeled. Based on the model, the study emphasizes on the coupled response characteristics near the touchdown point(TDP) induced by coplanar vessel heave and VIV, and analyzes the sensitivity of the coupled response to the heaving amplitude and frequency. It is found that with the increase of the heave amplitude and frequency, the VIV can be obviously mitigated, but the heave-related response in the coupled analysis seems to be close to that in the heave-only simulation. Finally, the fatigue damage near TDP is parametrically investigated based on the separate analysis and the coupled analysis. The results demonstrate that the coupled effect plays a significant role in the fatigue assessment near TDP. Besides, the proportion of the coupled effect accounting for the total fatigue damage decreases with the increasing seabed stiffness, while increases with the increasing seabed trench depth.
基金financially supported by the National Natural Science Foundation of China (Grant No.52201323)。
文摘In this work,a novel fluid-structure coupling method called the common node discrete element-smoothed particle hydrodynamics(DS-SPH)method is introduced.This framework combines the principles of the common node discrete element method(DEM)and smoothed particle hydrodynamics(SPH)to construct DEM-SPH particles situated on the same node.By doing so,the DEM particles can interact with the SPH particles within their support domain,enabling fluid-structure interaction(FSI).To determine the DEM microscopic parameters required for this method,uniaxial compression and three-point bending tests are conducted on sea ice.To verify the proposed model,we select the interaction between sea ice and structures as a case study.Through simulation,the model's capability of accurately depicting sea ice deformation and fracture has been demonstrated.The results indicate that the inclusion of SPH particles with fluid properties in the DEM model has minimal impact on the main mechanical parameters of sea ice.Additionally,it helps prevent the occurrence of particle splashing during cement failure.However,it is observed that the size of DEM particles and the friction between DEM particles and the structure significantly influence the macroscopic mechanical behavior of the common-node DEM-SPH model.Finally,we compare the fracture behavior of sea ice and the ice forces acting on structures obtained from the current model with on-site measured results.The agreement between the two sets of data is excellent,further validating the effectiveness of the proposed model in practical applications.
基金Project supported by the China Postdoctoral Science Foundation(No.2018M631909)the Doctor of Entrepreneurship and Innovation Project of Jiangsu Province(No.JSSCBS20221300)。
文摘In this paper,we focus on the two-dimensional pulsating nanofluid flow through a parallel-plate channel in the presence of a magnetic field.The pulsating flow is produced by an applied pressure gradient that fluctuates with a small amplitude.A kind of proper transformation is used so that the governing equations describing the momentum and thermal energy are reduced to a set of non-dimensional equations.The analytical expressions of the pulsating velocity,temperature,and Nusselt number of nanofluids are obtained by the perturbation technique.In the present study,the effects of the Cu-H2O and Al_(2)O_(3)-H2O nanofluids on the flow and heat transfer in pulsating flow are compared and analyzed.The results show that the convective heat transfer effect of Cu-H2O nanofluids is better than that of Al_(2)O_(3)-H2O nanofluids.Also,the effects of the Hartmann number and pulsation amplitude on the velocity,temperature,and Nusselt number are examined and discussed in detail.The present work indicates that increasing the Hartmann number and pulsation amplitude can enhance the heat transfer of the pulsating flow.In addition,selecting an optimal pulsation frequency can maximize the convective heat transfer of the pulsating flow.Therefore,improved understanding of these fundamental mechanisms is conducive to the optimal design of thermal systems.
基金financially supported by the National Natural Science Foundation of China(Grant No.52372356).
文摘To realize the low-resistance shape optimization design of amphibious robots,an efficient optimization design framework is proposed to improve the geometric deformation flexibility and optimization efficiency.In the proposed framework,the free-form deformation parametric model of the flat slender body is established and an analytical calculation method for the height constraints is derived.CFD method is introduced to carry out the high-precision resistance calculation and a constrained Kriging-based optimization method is built to improve the optimization efficiency by circularly infilling the new sample points which satisfying the constraints.Finally,the shape of an amphibious robot example is optimized to get the low-resistance shape and the results demonstrate that the presented optimization design framework has the advantages of simplicity,flexibility and high efficiency.
基金financially supported by the Hainan Provincial Joint Project of Sanya Yazhou Bay Science and Technology City(Grant No.520LH052)the National Natural Science Foundation of China(Grant No.51909164).
文摘Deepsea mining has been proposed since the 1960s to alleviate the lack of resources on land.Vertical hydraulic transport of collected ores from the seabed to the sea surface is considered the most promising method for industrial applications.In the present study,an indoor model test of the vertical hydraulic transport of particles was conducted.A noncontact optical method has been proposed to measure the local characteristics of the particles inside a vertical pipe,including the local concentration and particle velocity.The hydraulic gradient of ore transport was evaluated with various particle size distributions,particle densities,feeding concentrations and mixture flow velocities.During transport,the local concentration is larger than the feeding concentration,whereas the particle velocity is less than the mixture velocity.The qualitative effects of the local concentration and local fluid velocity on the particle velocity and slip velocity were investigated.The local fluid velocity contributes significantly to particle velocity and slip velocity,whereas the effect of the local concentration is marginal.A higher feeding concentration and mixture flow velocity result in an increased hydraulic gradient.The effect of the particle size gradation is slight,whereas the particle density plays a crucial role in the transport.
基金financially supported by the National Natural Science Foundation of China(Grant Nos.51579122,51609109,and 51622902)the Natural Science Found of Jiangsu Province(Grant No.BK20160556)+1 种基金the University Natural Science Research Project of Jiangsu Province(Grant No.16kjb70003)the Key Lab Foundation for Advanced Manufacturing Technology of Jiangsu Province(Grant No.CJ1506)
文摘This paper investigates the hydrodynamic performance of a cylindrical-dual or rectangular-single pontoon floating breakwater using the numerical method and experimental study. The numerical simulation work is based on the multi-physics computational fluid dynamics(CFD) code and an innovative full-structured dynamic grid method applied to update the three-degree-of-freedom(3-DOF) rigid structure motions. As a time-marching scheme, the trapezoid analogue integral method is used to update the time integration combined with remeshing at each time step.The application of full-structured mesh elements can prevent grids distortion or deformation caused by large-scale movement and improve the stability of calculation. In movable regions, each moving zone is specified with particular motion modes(sway, heave and roll). A series of experimental studies are carried out to validate the performance of the floating body and verify the accuracy of the proposed numerical model. The results are systematically assessed in terms of wave coefficients, mooring line forces, velocity streamlines and the 3-DOF motions of the floating breakwater. When compared with the wave coefficient solutions, excellent agreements are achieved between the computed and experimental data, except in the vicinity of resonant frequency. The velocity streamlines and wave profile movement in the fluid field can also be reproduced using this numerical model.
基金supported by the National Natural Science Foundation of China(Grant No.50609009)
文摘Corrosion and fatigue cracks are major threats to the structural integrity of aging offshore platforms. For the rational estimation of the safety levels of aging platforms, a global reliability assessment approach for aging offshore platforms with corrosion and fatigue cracks is presented in this paper. The base shear capacity is taken as the global ultimate strength of the offshore platforms. It is modeled as a random process that decreases with time in the presence of corrosion and fatigue crack propagation. And the corrosion and fatigue crack growth rates in the main members and key joints are modeled as random variables. A simulation method of the extreme wave loads which are applied to the structures of offshore platforms is proposed too. Furthermore, the statistics of global base shear capacity and extreme wave loads are obtained by Monte Carlo simulation method. On the basis of the limit state equation of global failure mode, the instantaneous reliability and time dependent reliability assessment methods are both presented in this paper. Finally the instantaneous reliability index and time dependent failure probability of a jacket platform are estimated with different ages in the demonstration example.
基金financially supported by the National Natural Science Foundation of China(Grant No.51221961)
文摘The study focuses on the flexible jumper issue of Subsurface Tension Leg Production(STLP)system concept,which is considered as a competing alternative system to support well completion devices and rigid risers in ultra-deep water for offshore petroleum production.The paper presents analytical and numerical approaches for the optimum design and global analysis of the flexible jumper.Criteria using catenary concept are developed to define the critical length for optimum design.Based on the criteria,detailed hydrodynamic analyses including quasi-static analysis,modal analysis,and dynamic analysis are performed.Modal analysis with respect to the quasi-static analysis shows that the existence of resonant modes requires special consideration.The results of dynamic analysis confirm the effectiveness of the de-coupled effect from the jumper on STLP system.The approaches developed in the study also have wide application prospect in reference to the optimum design and analysis of any Hybrid Riser(HR)concept.
基金The State Key Laboratory of Tropical Oceanography,South China Sea Institute of Oceanology,Chinese Academy of Sciences under contract No.LTO1915the National Natural Science Foundation of China under contract Nos 41630970,41876016,41676022 and 41521005the Instrument Developing Project of the CAS under contract No.YZ201432
文摘The evolution of energy, energy flux and modal structure of the internal tides (ITs) in the northeastern South China Sea is examined using the measurements at two moorings along a cross-slope section from the deep continental slope to the shallow continental shelf. The energy of both diurnal and semidiurnal ITs clearly shows a ~14-day spring-neap cycle, but their phases lag that of barotropic tides, indicating that ITs are not generated on the continental slope. Observations of internal tidal energy flux suggest that they may be generated at the Luzon Strait and propagate west-northwest to the continental slope in the northwestern SCS. Because the continental slope is critical-supercritical with respect to diurnal ITs, about 4.6 kJ/m^2 of the incident energy and 8.7 kW/m of energy flux of diurnal ITs are reduced from the continental slope to the continental shelf. In contrast, the semidiurnal internal tides enter the shelf because of the sub-critical topography with respect to semidiurnal ITs. From the continental slope to the shelf, the vertical structure of diurnal ITs shows significant variation, with dominant Mode 1 on the deep slope and dominant higher modes on the shelf. On the contrary, the vertical structure of the semidiurnal ITs is stable, with dominant Mode 1.
基金financially supported by the National Natural Science Foundation of China(Grant No.51579055)the Natural Science Foundation of Jiangsu Province(Grant No.BK20180980)
文摘Point absorber wave energy device with multiple degrees of freedom(DOF) is assumed to have a better absorption ability of mechanical energy from ocean waves. In this paper, a coaxial symmetric articulated point absorber wave energy converter with two degrees of freedom is presented. The mechanical equations of the oscillation buoy with power take-off mechanism(PTO) in regular waves are established. The three-dimensional numerical wave tank is built in consideration of the buoy motion based upon the CFD method. The appropriate simulation elements are selected for the buoy and wave parameters. The feasibility of the CFD method is verified through the contrast between the numerical simulation results of typical wave conditions and test results. In such case, the buoy with single DOF of heave, pitch and their coupling motion considering free(no PTO damping) and damped oscillations in regular waves are simulated by using the verified CFD method respectively. The hydrodynamic and wave energy conversion characteristics with typical wave conditions are analyzed. The numerical results show that the heave and pitch can affect each other in the buoy coupling motion, hydrodynamic loads, wave energy absorption and flow field.The total capture width ratio with two coupled DOF motion is higher than that with a single DOF motion. The wave energy conversion of a certain DOF motion may be higher than that of the single certain DOF motion even though the wave is at the resonance period. When the wave periods are high enough, the interaction between the coupled DOF motions can be neglected.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11772294,11621062,and 11302189)the Fundamental Research Funds for the Central Universities,China(Grant No.2017QNA4031)
文摘Surface charges can modify the elastic modulus of nanostructure,leading to the change of the phonon and thermal properties in semiconductor nanostructure.In this work,the influence of surface charges on the phonon properties and phonon thermal conductivity of GaN nanofilm are quantitatively investigated.In the framework of continuum mechanics,the modified elastic modulus can be derived for the nanofilm with surface charges.The elastic model is presented to analyze the phonon properties such as the phonon dispersion relation,phonon group velocity,density of states of phonons in nanofilm with the surface charges.The phonon thermal conductivity of nanofilm can be obtained by considering surface charges.The simulation results demonstrate that surface charges can significantly change the phonon properties and thermal conductivity in a GaN nanofilm.Positive surface charges reduce the phonon energy and phonon group velocity but increase the density of states of phonons.The surface charges can change the size and temperature dependence of phonon thermal conductivity of GaN nanofilm.Based on these theoretical results,one can adjust the phonon properties and temperature/size dependent thermal conductivity in GaN nanofilm by changing the surface charges.
基金financially supported by the 7th Generation Ultra Deep Water Drilling Unit Innovation Projectconducted at the State Key Laboratory of Ocean Engineering at Shanghai Jiao Tong University
文摘The Multiple Column Platform(MCP) semi-submersible is a newly proposed concept, which differs from the conventional semi-submersibles, featuring centre column and middle pontoon. It is paramount to ensure its structural reliability and safe operation at sea, and a rigorous investigation is conducted to examine the hydrodynamic and structural performance for the novel structure concept. In this paper, the numerical and experimental studies on the hydrodynamic performance of MCP are performed. Numerical simulations are conducted in both the frequency and time domains based on 3D potential theory. The numerical models are validated by experimental measurements obtained from extensive sets of model tests under both regular wave and irregular wave conditions. Moreover, a comparative study on MCP and two conventional semi-submersibles are carried out using numerical simulation.Specifically, the hydrodynamic characteristics, including hydrodynamic coefficients, natural periods and motion response amplitude operators(RAOs), mooring line tension are fully examined. The present study proves the feasibility of the novel MCP and demonstrates the potential possibility of optimization in the future study.
基金financially supported by the National Natural Science Foundation of China (Grant No. 51709134)the National Key R&D Program of China (Grant No. 2016YFC0300603-02)the Natural Science Foundation of Jiangsu Province (Grant Nos. BK20160559 and BK20170575)。
文摘With the rapid development of ocean technology, the deep-sea manned submersible is regarded as a high-tech equipment for the exploration and exploitation of ocean resources. The safety of manned cabin has a decisive effect on the whole system. Ti-6 Al-4 V with the superior strength-to-weight ratio and corrosion resistance has been used for the manned cabin. The manned cabin experiences loading spectrum with different maximum stresses and different dwell time during their service life. The load sequence effects on dwell fatigue crack growth behavior of Ti-6 Al-4 V under different dwell time are investigated experimentally in this paper. The experimental results show that the crack tip plastic zone is enlarged by the dwell time and the overload retardation zone increases with dwell time under the same overload rate. A dwell fatigue crack growth model is proposed by modifying the crack tip plastic zone under the loading history with combinations of the single overload and dwell time factors are included in the modified model. Based on the experimental data, the overload retardation zone and the crack growth rates of Ti-6 Al-4 V are predicted by the modified model. A reasonable model for the load sequence effect on the dwell fatigue crack growth rates of Ti-6 Al-4 V is verified.
文摘The procedure of assessment of structural fatigue strength of an offshore floating production and storage and offloadingunit(FPSO) in this paper. The emphasis is placed on the long-term prediction of wave induced loading, the refined finite el-ement model for hot spot stress calculation, the combination of stress components, and fatigue damage assessment based onS-N curve.
基金Supported by the National Natural Science Foundation of China(51474109,51609090,51679097).
文摘In this study,the effects of geometrical and physical factors on light particles dispersion in stirred tank were investigated by agitation characteristic curve.The experiments and CFD simulations with discrete phase model(DPM)and volume of fluid model(VOF)were conducted in this paper.Five factors,which include four geometrical factors(submergence,impeller-to-tank ratio,number of impeller blades and baffling mode)and a physical factor(liquid viscosity)were considered.For each factor,the power consumption curve and agitation characteristic curve were drawn to compare the power consumption and mixing results in the stirred tank.Characteristics of the agitation characteristic curves were compared with the previous published literatures and theories.It is found that the agitation characteristic curves reflect the tendency of power consumption and particles distribution well in stirred tank.The good agreement indicates the applicability of the agitation characteristic curves for the study of light particles distribution in stirred tank.
基金supported by the Fundamental Research Funds for the Central Universities with Grant No. of DUT 16RC(3)063
文摘Fluid resonance in a moonpool formed by two identical rectangular hulls during in-phase heaving motion is investigated by employing a two-dimensional numerical wave flume based on OpenFOAM package with Re-Normalization Group(RNG) turbulent model. The focus of the study is to examine the influence of heaving frequency and amplitude with various moonpool configurations on fluid resonant behavior. It is found that the resonant frequency of wave response in moonpool tends to decrease with the increase of moonpool breadth and hulls draft. The decrease of resonant amplitude can be observed for large moonpool breadth. The influence of hulls draft on resonant amplitude is not remarkable, especially for large heaving amplitude. The increase in heaving amplitude results in the decrease of relative resonant amplitude in an approximate power function, implying a complicated dependence of the resonant amplitude on heaving amplitude. Flow patterns in the vicinity of the moonpool are also analyzed, mainly regarding the dependence on the heaving frequency. The negligible influence of vortices on the wave response in moonpool is expected for low-frequency excitation because it is hard to observe the vortex structures. Intensive vortical flow and vortex structure can be identified under resonant condition, which gives rise to significant dissipation and accounts for the smaller relative resonant amplitude in moonpool. As for high-frequency excitation, the vortex motion is rather weak and dissipates rapidly, leading to insignificant effect on wave response amplitude.
基金financially supported by the National Key Research and Development Program of China(Grant No.2017YFC1404200)the National Natural Science Foundation of China(Grant Nos.51911530205 and 51809039)+5 种基金the Natural Science Foundation of Jiangsu Province(Grant Nos.BK20201455 and BK20210885)the Natural Science Foundation of the Jiangsu Higher Education Institutions(Grant No.20KJD170005)the Qing Lan Project of Jiangsu Universitiespartially supported by UK EPSRC(Grant No.EP/T026782/1)the Royal Academy of Engineering(Grant No.UK-CIAPP/73)the Royal Society(Grant No.IEC\NSFC\181321)。
文摘Two marine structures arranged side by side with a narrow gap may suffer from violent free-surface resonance,which would cause green water on deck,dramatically raise hydrodynamic loads on structures and seriously threaten the operation safety.The CFD-based open-sourced software,OpenFOAM?,is employed to simulate the twodimensional fluid resonance inside a narrow gap between a fixed box and a vertical wall induced by regular waves with different wave heights.The topographies with various plane slopes are placed in front of the wall.The focus of this article is on the influences of the incident wave height and the topographic slope on the nonlinear characteristics of various hydrodynamic parameters(including the wave height in the gap,the vertical wave force,and the horizontal wave force on the box)during gap resonance.The ratios of their high-order to the corresponding 1 st-order components under different sets of the incident wave height and the topographic slope are analyzed.It is found that the relative importance of all the high-order components increases gradually with the incident wave height for all the three parameters.The topographic influence on them closely depends on the type of the parameters and the incident wave height.In addition,the occurrence of the 2 nd-order gap resonance phenomenon can cause the 2 nd-order wave height and horizontal force to be significantly larger than the corresponding 1 st-order components.
