Current installation costs of offshore wind turbines(OWTs) are high and profit margins in the offshore wind energy sector are low, it is thus necessary to develop installation methods that are more efficient and pract...Current installation costs of offshore wind turbines(OWTs) are high and profit margins in the offshore wind energy sector are low, it is thus necessary to develop installation methods that are more efficient and practical. This paper presents a numerical study(based on a global response analysis of marine operations) of a novel procedure for installing the tower and Rotor Nacelle Assemblies(RNAs) on bottom-fixed foundations of OWTs. The installation procedure is based on the inverted pendulum principle. A cargo barge is used to transport the OWT assembly in a horizontal position to the site, and a medium-size Heavy Lift Vessel(HLV) is then employed to lift and up-end the OWT assembly using a special upending frame. The main advantage of this novel procedure is that the need for a huge HLV(in terms of lifting height and capacity) is eliminated. This novel method requires that the cargo barge is in the leeward side of the HLV(which can be positioned with the best heading) during the entire installation. This is to benefit from shielding effects of the HLV on the motions of the cargo barge, so the foundations need to be installed with a specific heading based on wave direction statistics of the site and a typical installation season. Following a systematic approach based on numerical simulations of actual operations, potential critical installation activities, corresponding critical events, and limiting(response) parameters are identified. In addition, operational limits for some of the limiting parameters are established in terms of allowable limits of sea states. Following a preliminary assessment of these operational limits, the duration of the entire operation, the equipment used, and weather-and water depth-sensitivity, this novel procedure is demonstrated to be viable.展开更多
The lethality of a semi-preformed fragment warhead is closely related to the expand velocity and spatial distribution of the fragments from ruptured metal casing. The topic of how to improve the utilization of charge ...The lethality of a semi-preformed fragment warhead is closely related to the expand velocity and spatial distribution of the fragments from ruptured metal casing. The topic of how to improve the utilization of charge of have been drawing great attention from researchers and designer in this filed. In present paper,in order to investigate the influence of charge initiation manners on the scattering characteristics of semi-preformed fragment warhead, the numerical simulations and experimental test are conducted.Firstly, the influence of grid density on numerical results is investigated, and a proper numerical model with relatively high accuracy and effectiveness is determined. Then. numerical simulations of three kinds of different initiation position of a semi-preformed fragment warhead are carried out. An experimental test of the explosion of a semi-preformed fragment warhead is carried out. By comparing and analyzing the numerical results and experimental data, it is found that the initiation manners have great influence on scattering characteristics of semi-preformed fragment warhead. The researcher work of this paper would provide an effective alternative method to optimize the design of warhead.展开更多
For the design and operation of a floating bridge, the understanding of its dynamic behavior under a moving load is of great importance. The purpose of this paper is to investigate the dynamic performances of a new ty...For the design and operation of a floating bridge, the understanding of its dynamic behavior under a moving load is of great importance. The purpose of this paper is to investigate the dynamic performances of a new type floating bridge, the pontoon-separated floating bridge, under the effect of a moving load. In the paper, a brief summary of the dynamic analysis of the floating bridge is first introduced. The motion equations for a pontoon-separated floating bridge, considering the nonlinear properties of connectors and vehicles＇ inertia effects, are proposed. The super-element method is applied to reduce the numerical analysis scale to solve the reduced equations. Based on the static analysis, the dynamic features of the new type floating bridge subjected to a moving load are investigated. It is found that the dynamie behavior of the pontoon-separated floating bridge is superior to that of the ribbon bridge by taking the nonlinearity of eonneetors into account.展开更多
In this paper, a precise transfer matrix method is presented to calculate the struc- tural and acoustic responses of the conical shell. The governing equations of conical shells are written as a coupled set of first o...In this paper, a precise transfer matrix method is presented to calculate the struc- tural and acoustic responses of the conical shell. The governing equations of conical shells are written as a coupled set of first order differential equations. The field transfer matrix of the shell and non-homogenous term resulting from the external excitation are obtained by precise integra- tion method. After assembling the field transfer matrixes, the whole matrix describing dynamic behavior of the stiffened conical shell is obtained. Then the structural and acoustic responses of the shell are solved by obtaining unknown sound pressure coefficients. The natural frequencies of the shell are compared with the FEM results to test the validity. Furthermore, the effects of the semi-vertex angle, driving force directions and boundary conditions on the structural and acoustic responses are studied.展开更多
Based on relevant in-service experience, this paper discusses how risks associated with station-keeping systems can be controlled through adequate design criteria, inspection, repair and maintenance practice, as well ...Based on relevant in-service experience, this paper discusses how risks associated with station-keeping systems can be controlled through adequate design criteria, inspection, repair and maintenance practice, as well as quality assurance and control of the engineering processes. Particular focus must be placed on quantitative design for system robustness. The application of structural reliability analysis to quantify safety is briefly reviewed. In particular it was emphasized that reliability predictions based on normal uncertainties and variability yielded lower failure rates than those experienced for predictions of hulls and catenary mooring systems; gross errors in design, fabrication and operation were responsible. For this reason the broad safety management approach mentioned above was proposed. Moreover, it was found that this approach needed to be supported by a quantitative risk assessment. Finally, the challenges in dealing with the effects of human factors in risk management are outlined, along with means to deal with them in a qualitative manner, by the so-called barrier method to limit risk.展开更多
Based on linear water-wave theory, this study investigated the scattering of oblique incident water waves by two unequal surface-piercing thin vertical rigid plates with stepped bottom topography. By using the matched...Based on linear water-wave theory, this study investigated the scattering of oblique incident water waves by two unequal surface-piercing thin vertical rigid plates with stepped bottom topography. By using the matched eigenfunction expansion method and a least square approach, the analytical solutions are sought for the established boundary value problem. The effects of the incidence angle, location of step, depth ratio of deep to shallow waters,and column width between two plates, on the reflection coefficients, the horizontal wave forces acting on the two plates, and the mean surface elevation between the two plates, are numerically examined under a variety of wave conditions. The results show that the existence of the stepped bottom between two plates considerably impacts the hydrodynamic performances of the present system. It is found that the effect of stepped bottom on the reflection coefficient of the present two-plate structure is evident only with waves of the low dimensionless frequency.Moreover, the influence of the step location on the hydrodynamic performance of the present two-plate structure is slight if the step is placed in between the two plates.展开更多
To evaluate the trapped wave energy and energy loss, the problem of wave scattering by twin fixed vertical surface- piercing plates over a stepped bottom is numerically simulated using the open source package OpenFOAM...To evaluate the trapped wave energy and energy loss, the problem of wave scattering by twin fixed vertical surface- piercing plates over a stepped bottom is numerically simulated using the open source package OpenFOAM and the associated toolbox waves2Foam. The volume of fluid (VOF) method was employed to capture the free surface in the time domain. The validation of the present numerical model was performed by comparing with both the analytical and experimental results. The effects of the spacing between two plates and the configuration of stepped bottom on the hydrodynamic characteristics, such as reflection and transmission coefficients, viscous dissipation ratio, and relative wave height between the plates (termed as trapped wave energy), were examined. Moreover, the nonlinear effects of the incident wave height on the hydrodynamic characteristics were addressed as well. The results show that the step configuration can be tuned for efficient-performance of wave damping, and the optimum configurations of the step length B, the step height h1 and the spacing b, separately equaling λ/4, 3h/4, and 0.05h (λ and h are the wavelength and the water depth, respectively), are recommended for the trapping of wave energy.展开更多
After borrowing the idea of precise integration method, a precise integration transfer matrix method （PITMM） is proposed by modifying traditional transfer matrix method. The submarine hull can be modeled as joined c...After borrowing the idea of precise integration method, a precise integration transfer matrix method （PITMM） is proposed by modifying traditional transfer matrix method. The submarine hull can be modeled as joined conical- cylindrical-spherical shells. By considering the effect of the ring-stiffeners, the field transfer matrixes of shells of revolution are obtained accurately by PITMM. After assembling the field transfer matrixes into an entire matrix, the dynamic model is established to solve the dynamic responses of the joined shell. By describing the sound pressure in fluid by modified wave superposition method （MWSM） and collocating points along the meridian line of the joined shell, finally the structural and acoustic responses of a finite stiffened submarine hull can be predicted by coupled PITMM and MWSM. The effectiveness of the present method has been verified by comparing the structural and acoustic responses of the spherical shell with existing results. Furthermore, the effects of the model truncation, stiffness and thickness on the structural and acoustic responses of the submarine hull are studied.展开更多
The rigid-plastic assumption has greatly simplified the theoretical analysis of dynamic plastic response of structures.Within this framework,a common tool is the modal technique using approximate independent yield cri...The rigid-plastic assumption has greatly simplified the theoretical analysis of dynamic plastic response of structures.Within this framework,a common tool is the modal technique using approximate independent yield criteria,which leads to upper-and lower-bound solutions,but usually with poor accuracy.In this paper,by utilizing the membrane factor method(MFM),the large-deflection dynamic plastic response of square plates subjected to exponentially decaying pulse loading is analyzed by taking both the transient response phase and the exact yield criterion into account.Based on the combination of saturation analysis(SA)and MFM,the complete solutions and regressive formulae of saturated deflection and saturated impulse are obtained.As the dynamic behavior of plates under rectangular pulse loading serves as a benchmark of pulse-equivalent techniques,the large plastic deformation of square plates under short-duration rectangular pulse is also analyzed in detail.Moreover,by comparing the SA results of pulse-loaded square plates with different boundary conditions,it is found that the saturated deflection and saturated impulse of the fully clamped and simply supported square plates both increase linearly with the pulse amplitude,and the slopes are approximately the same,so the conversion between the SA quantities of plates with different boundary conditions can be easily achieved.展开更多
This paper discusses numerical results from three-dimensional large eddy simulations of an oscillating cylinder under prescribed movements in uniform flow. Six cases, namely pure in-line, pure cross-flow and two group...This paper discusses numerical results from three-dimensional large eddy simulations of an oscillating cylinder under prescribed movements in uniform flow. Six cases, namely pure in-line, pure cross-flow and two groups of ＇Figure of Eight＇ oscillation patterns are under investigation at Reynolds number Re = 24000. The ＇ Figure of Eight＇ pattern in each group is with identical shape but oppusite orbital directions. The numerical results on hydrodynamic forces, higher order force components, and vortex shedding modes are extensively studied and compared with the measured experimental data. It is found that the fluid force in phase with the velocity, which represents the energy transfer between the fluid and the cylinder, has opposite sign and different magnitude due to the opposite orbital direction. Higher order force components in cross-flow direction are found to occur at odd nmnber times of the oscillating frequency, while even nmbers dominate the higher order force components in in-llne direction. The 2C and 2T vortex shedding modes are well reproduced due to the opposite orbital direction effect. Comparisons between numerical and experimental results indicate that the present numerical model could be a rational tool for the identification of hydrodynamic coefficients which are normally applied in empirical models to predict the vortex-induced vibrations of slender marine structures.展开更多
This paper is concerned with the hydroelastic responses of a mat-like, rectangular very large floating structure(VLFS) edged with a pair of horizontal/inclined perforated anti-motion plates in the context of the direc...This paper is concerned with the hydroelastic responses of a mat-like, rectangular very large floating structure(VLFS) edged with a pair of horizontal/inclined perforated anti-motion plates in the context of the direct coupling method. The updated Lagrangian formulae are applied to establish the equilibrium equations of the VLFS and the total potential formula is employed for fluids in the numerical model including the viscous effect of the perforated plates through the Darcy’s law. The hybrid finite element-boundary element(FE-BE) method is implemented to determine the response reduction of VLFS with attached perforated plates under various oblique incident waves.Also, the numerical solutions are validated against a series of experimental tests. The effectiveness of the attached perforated plates in reducing the deflections of the VLFS can be significantly improved by selecting the proper design parameters such as the porous parameter, submergence depth, plate width and inclination angle for the given sea conditions.展开更多
OWTs （offshore wind turbines） are currently considered as a reliable source of renewable energy. OWT support structures account for 20%-25% of the capital cost for offshore wind installations. Pre-feasibility studie...OWTs （offshore wind turbines） are currently considered as a reliable source of renewable energy. OWT support structures account for 20%-25% of the capital cost for offshore wind installations. Pre-feasibility studies involving estimation of preliminary dimensions of the wind turbine structure need to be performed for initial costing to arrive at the commercial viability of the project. The main objective of the paper is to obtain preliminary configuration for commercial viability and approximate sizing of the foundation pile. Design equations and nomograms are proposed for quick preliminary design of monopile founded wind turbines located offshore of Gujarat. Parametric studies are carried-out on various configurations of a hollow monopile by varying water depths and properties of sand. A nonlinear static analysis of substructure is performed considering aerodynamic forces and hydrodynamic forces for various structural and soil parameters. The sub-structure design of wind turbine is based on API （American petroleum institute） standards. A simplified design methodology for monopile support structure under extreme loading condition is presented based on multivariable linear regression analysis. The input variables for the regression analysis are hydrodynamic data, angle of internal friction of sand, and the output variables are length and outer diameter of monopile. This simplified methodology is applicable in pre-studies of wind power parks.展开更多
Welding residual stress in the engineering structure has a non-negligible influence on crack propagation,and crack closure is a significant factor affecting the crack propagation.Based on the elastoplastic finite elem...Welding residual stress in the engineering structure has a non-negligible influence on crack propagation,and crack closure is a significant factor affecting the crack propagation.Based on the elastoplastic finite element method and crack closure theory,we studied crack closure and residual compressive stress field of butt-welded plates under constant amplitude loading and overloading regarding the stress ratio,maximum load,overload ratio,and number of overloads.The results show that the welding residual tensile stress can decrease the crack closure because of a decrease in the residual compressive stress in the wake zone,but the effect is gradually reduced with increased stress ratio or maximum load.And the combined effect of welding residual tensile stress and overload can produce a stronger retardation effect on crack propagation.展开更多
With a successful and rapid development of offshore wind industry and increased research activities on wave energy conversion in recent years,there is an interest in investigating the technological and economic feasib...With a successful and rapid development of offshore wind industry and increased research activities on wave energy conversion in recent years,there is an interest in investigating the technological and economic feasibility of combining offshore wind turbines(WTs)with wave energy converters(WECs).In the EU FP7 MARINA Platform project,three floating combined concepts,namely the spar torus combination(STC),the semi-submersible flap combination(SFC)and the oscillating water column(OWC)array with a wind turbine,were selected and studied in detail by numerical and experimental methods.This paper summarizes the numerical modeling and analysis of the two concepts:STC and SFC,the model tests at a 1:50 scale under simultaneous wave and wind excitation,as well as the comparison between the numerical and experimental results.Both operational and survival wind and wave conditions were considered.The numerical analysis was based on a time-domain global model using potential flow theory for hydrodynamics and blade element momentum theory(for SFC)or simplified thrust force model(for STC)for aerodynamics.Different techniques for model testing of combined wind and wave concepts were discussed with focus on modeling of wind turbines by disk or redesigned small-scale rotor and modeling of power take-off(PTO)system for wave energy conversion by pneumatic damper or hydraulic rotary damper.In order to reduce the uncertainty due to scaling,the numerical analysis was performed at model scale and both the numerical and experimental results were then up-scaled to full scale for comparison.The comparison shows that the current numerical model can well predict the responses(motions,PTO forces,power production)of the combined concepts for most of the cases.However,the linear hydrodynamic model is not adequate for the STC concept in extreme wave conditions with the torus fixed to the spar at the mean water level for which the wave slamming on the torus occurs and this requires further investigation.Moreover,based on a preliminary comparison of the displacement,the PTO system as well as the wind and wave power production,the STC concept will have a lower cost of energy as compared to the SFC concept.However,the cost of energy of either the STC or the SFC concept is higher than that of a pure floating wind turbine with the same floater.展开更多
Understanding and control of wake vortices past a circular cylinder is a cardinal problem of interest to ocean engineering.The wake formation and vortex shedding behind a variety of ocean structures such as spars,are ...Understanding and control of wake vortices past a circular cylinder is a cardinal problem of interest to ocean engineering.The wake formation and vortex shedding behind a variety of ocean structures such as spars,are subjected to fatigue failure limiting their life span.The additional influences due to ocean waves and currents further exacerbate these effects.In the present study,flow past an isolated circu-lar cylindrical structure subjected to an oscillatory upstream are numerically investigated.These studies involve high resolution simulations over the low Reynolds number range(100-200).Although the prac-tical range of interest is in high Reynolds number range of 103-105,the flow physics and a number of qualitative and quantitative aspects are similar to the low Reynolds number flows.In the high Reynolds number range,statistical averaging tools in conjunction with suitable closure models would be neces-sary.The control of wake vortices is achieved with the aid of two small rotors located in the aft of the main cylinder.A control algorithm was coupled to determine the quantum of actuation to the rotating elements.Although control of wake vortices was observed for harmonic in-let forcing,residual vortical structures were found to persist at higher amplitudes of oscillation.To study the efficacy of this control,numerical simulations were further extended,when the circular cylinder was flexibly mounted.The con-trol of flow induced vibrations was observed to be reasonably effective in controlling the wake generated behind the main cylinder due to oscillatory upstream.展开更多
基金financially supported by the Research Council of Norway granted through the Department of Marine Technologythe Centre for Ships and Ocean Structures(CeSOS) and the the Centre for Autonomous Marine Operations and Systems(AMOS) from the Norwegian University of Science and Technology(NTNU)the financial support from Escuela Politécnica Nacional(EPN)through the project PIMI-15-03"Investigación y evaluación de sistemas innovadores de propulsión distribuida con ingestión de capa límite para mejorar la eficiencia propulsiva y térmica de vehículos aéreos no tripulados aplicados en los sectores:agrícola,medicina y vigilancia"
文摘Current installation costs of offshore wind turbines(OWTs) are high and profit margins in the offshore wind energy sector are low, it is thus necessary to develop installation methods that are more efficient and practical. This paper presents a numerical study(based on a global response analysis of marine operations) of a novel procedure for installing the tower and Rotor Nacelle Assemblies(RNAs) on bottom-fixed foundations of OWTs. The installation procedure is based on the inverted pendulum principle. A cargo barge is used to transport the OWT assembly in a horizontal position to the site, and a medium-size Heavy Lift Vessel(HLV) is then employed to lift and up-end the OWT assembly using a special upending frame. The main advantage of this novel procedure is that the need for a huge HLV(in terms of lifting height and capacity) is eliminated. This novel method requires that the cargo barge is in the leeward side of the HLV(which can be positioned with the best heading) during the entire installation. This is to benefit from shielding effects of the HLV on the motions of the cargo barge, so the foundations need to be installed with a specific heading based on wave direction statistics of the site and a typical installation season. Following a systematic approach based on numerical simulations of actual operations, potential critical installation activities, corresponding critical events, and limiting(response) parameters are identified. In addition, operational limits for some of the limiting parameters are established in terms of allowable limits of sea states. Following a preliminary assessment of these operational limits, the duration of the entire operation, the equipment used, and weather-and water depth-sensitivity, this novel procedure is demonstrated to be viable.
基金supported by the Joint Foundation project for Young Scientists of Ministry of Education(6141A02033108)National Natural Science Foundation of China(11502180)
文摘The lethality of a semi-preformed fragment warhead is closely related to the expand velocity and spatial distribution of the fragments from ruptured metal casing. The topic of how to improve the utilization of charge of have been drawing great attention from researchers and designer in this filed. In present paper,in order to investigate the influence of charge initiation manners on the scattering characteristics of semi-preformed fragment warhead, the numerical simulations and experimental test are conducted.Firstly, the influence of grid density on numerical results is investigated, and a proper numerical model with relatively high accuracy and effectiveness is determined. Then. numerical simulations of three kinds of different initiation position of a semi-preformed fragment warhead are carried out. An experimental test of the explosion of a semi-preformed fragment warhead is carried out. By comparing and analyzing the numerical results and experimental data, it is found that the initiation manners have great influence on scattering characteristics of semi-preformed fragment warhead. The researcher work of this paper would provide an effective alternative method to optimize the design of warhead.
基金This project was supported by the Commission of Science Technology and Industry for National Defense .
文摘For the design and operation of a floating bridge, the understanding of its dynamic behavior under a moving load is of great importance. The purpose of this paper is to investigate the dynamic performances of a new type floating bridge, the pontoon-separated floating bridge, under the effect of a moving load. In the paper, a brief summary of the dynamic analysis of the floating bridge is first introduced. The motion equations for a pontoon-separated floating bridge, considering the nonlinear properties of connectors and vehicles＇ inertia effects, are proposed. The super-element method is applied to reduce the numerical analysis scale to solve the reduced equations. Based on the static analysis, the dynamic features of the new type floating bridge subjected to a moving load are investigated. It is found that the dynamie behavior of the pontoon-separated floating bridge is superior to that of the ribbon bridge by taking the nonlinearity of eonneetors into account.
基金supported by the National Natural Science Foundation of China(No.51409200)the Research Fund for the Central University(WUT:2014-IV-022)
文摘In this paper, a precise transfer matrix method is presented to calculate the struc- tural and acoustic responses of the conical shell. The governing equations of conical shells are written as a coupled set of first order differential equations. The field transfer matrix of the shell and non-homogenous term resulting from the external excitation are obtained by precise integra- tion method. After assembling the field transfer matrixes, the whole matrix describing dynamic behavior of the stiffened conical shell is obtained. Then the structural and acoustic responses of the shell are solved by obtaining unknown sound pressure coefficients. The natural frequencies of the shell are compared with the FEM results to test the validity. Furthermore, the effects of the semi-vertex angle, driving force directions and boundary conditions on the structural and acoustic responses are studied.
文摘Based on relevant in-service experience, this paper discusses how risks associated with station-keeping systems can be controlled through adequate design criteria, inspection, repair and maintenance practice, as well as quality assurance and control of the engineering processes. Particular focus must be placed on quantitative design for system robustness. The application of structural reliability analysis to quantify safety is briefly reviewed. In particular it was emphasized that reliability predictions based on normal uncertainties and variability yielded lower failure rates than those experienced for predictions of hulls and catenary mooring systems; gross errors in design, fabrication and operation were responsible. For this reason the broad safety management approach mentioned above was proposed. Moreover, it was found that this approach needed to be supported by a quantitative risk assessment. Finally, the challenges in dealing with the effects of human factors in risk management are outlined, along with means to deal with them in a qualitative manner, by the so-called barrier method to limit risk.
基金financially supported by the National Natural Science Foundation of China(Grant No.11702244)the Project of the Cooperation of Zhoushan City and Zhejiang University(Grant No.2017C82223)+1 种基金the Open Foundation of Key Laboratory of Port,Waterway and Sedimentation Engineering of the Ministry of Transport(Grant No.Yn216006)the Fundamental Research Funds for the Central Universities(WUT:2017IVA009)
文摘Based on linear water-wave theory, this study investigated the scattering of oblique incident water waves by two unequal surface-piercing thin vertical rigid plates with stepped bottom topography. By using the matched eigenfunction expansion method and a least square approach, the analytical solutions are sought for the established boundary value problem. The effects of the incidence angle, location of step, depth ratio of deep to shallow waters,and column width between two plates, on the reflection coefficients, the horizontal wave forces acting on the two plates, and the mean surface elevation between the two plates, are numerically examined under a variety of wave conditions. The results show that the existence of the stepped bottom between two plates considerably impacts the hydrodynamic performances of the present system. It is found that the effect of stepped bottom on the reflection coefficient of the present two-plate structure is evident only with waves of the low dimensionless frequency.Moreover, the influence of the step location on the hydrodynamic performance of the present two-plate structure is slight if the step is placed in between the two plates.
基金partially supported by the National Natural Science Foundation of China(Grant Nos.11702244,51679014,51809209)the Joint Fund of Zhoushan City and Zhejiang University(Grant No.2017C82223)the Open Fund of Hunan Provincial Key Laboratory of Key Technology on Hydropower Development(Grant No.PKLHD201707)
文摘To evaluate the trapped wave energy and energy loss, the problem of wave scattering by twin fixed vertical surface- piercing plates over a stepped bottom is numerically simulated using the open source package OpenFOAM and the associated toolbox waves2Foam. The volume of fluid (VOF) method was employed to capture the free surface in the time domain. The validation of the present numerical model was performed by comparing with both the analytical and experimental results. The effects of the spacing between two plates and the configuration of stepped bottom on the hydrodynamic characteristics, such as reflection and transmission coefficients, viscous dissipation ratio, and relative wave height between the plates (termed as trapped wave energy), were examined. Moreover, the nonlinear effects of the incident wave height on the hydrodynamic characteristics were addressed as well. The results show that the step configuration can be tuned for efficient-performance of wave damping, and the optimum configurations of the step length B, the step height h1 and the spacing b, separately equaling λ/4, 3h/4, and 0.05h (λ and h are the wavelength and the water depth, respectively), are recommended for the trapping of wave energy.
基金supported by the National Natural Science Foundation of China(Grant No.51409200)China Postdoctoral Science Foundation(Grant No.2015M582290)
文摘After borrowing the idea of precise integration method, a precise integration transfer matrix method （PITMM） is proposed by modifying traditional transfer matrix method. The submarine hull can be modeled as joined conical- cylindrical-spherical shells. By considering the effect of the ring-stiffeners, the field transfer matrixes of shells of revolution are obtained accurately by PITMM. After assembling the field transfer matrixes into an entire matrix, the dynamic model is established to solve the dynamic responses of the joined shell. By describing the sound pressure in fluid by modified wave superposition method （MWSM） and collocating points along the meridian line of the joined shell, finally the structural and acoustic responses of a finite stiffened submarine hull can be predicted by coupled PITMM and MWSM. The effectiveness of the present method has been verified by comparing the structural and acoustic responses of the spherical shell with existing results. Furthermore, the effects of the model truncation, stiffness and thickness on the structural and acoustic responses of the submarine hull are studied.
基金the support of the Wuhan University of Technology start-up fund for Distinguished Professors(No.471-40120163)the China Scholarship Council(CSC).
文摘The rigid-plastic assumption has greatly simplified the theoretical analysis of dynamic plastic response of structures.Within this framework,a common tool is the modal technique using approximate independent yield criteria,which leads to upper-and lower-bound solutions,but usually with poor accuracy.In this paper,by utilizing the membrane factor method(MFM),the large-deflection dynamic plastic response of square plates subjected to exponentially decaying pulse loading is analyzed by taking both the transient response phase and the exact yield criterion into account.Based on the combination of saturation analysis(SA)and MFM,the complete solutions and regressive formulae of saturated deflection and saturated impulse are obtained.As the dynamic behavior of plates under rectangular pulse loading serves as a benchmark of pulse-equivalent techniques,the large plastic deformation of square plates under short-duration rectangular pulse is also analyzed in detail.Moreover,by comparing the SA results of pulse-loaded square plates with different boundary conditions,it is found that the saturated deflection and saturated impulse of the fully clamped and simply supported square plates both increase linearly with the pulse amplitude,and the slopes are approximately the same,so the conversion between the SA quantities of plates with different boundary conditions can be easily achieved.
文摘This paper discusses numerical results from three-dimensional large eddy simulations of an oscillating cylinder under prescribed movements in uniform flow. Six cases, namely pure in-line, pure cross-flow and two groups of ＇Figure of Eight＇ oscillation patterns are under investigation at Reynolds number Re = 24000. The ＇ Figure of Eight＇ pattern in each group is with identical shape but oppusite orbital directions. The numerical results on hydrodynamic forces, higher order force components, and vortex shedding modes are extensively studied and compared with the measured experimental data. It is found that the fluid force in phase with the velocity, which represents the energy transfer between the fluid and the cylinder, has opposite sign and different magnitude due to the opposite orbital direction. Higher order force components in cross-flow direction are found to occur at odd nmnber times of the oscillating frequency, while even nmbers dominate the higher order force components in in-llne direction. The 2C and 2T vortex shedding modes are well reproduced due to the opposite orbital direction effect. Comparisons between numerical and experimental results indicate that the present numerical model could be a rational tool for the identification of hydrodynamic coefficients which are normally applied in empirical models to predict the vortex-induced vibrations of slender marine structures.
基金financially supported by the National Natural Science Foundation of China(Grant Nos.51490672,51579122 and51609109)the Natural Science Foundation 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 is concerned with the hydroelastic responses of a mat-like, rectangular very large floating structure(VLFS) edged with a pair of horizontal/inclined perforated anti-motion plates in the context of the direct coupling method. The updated Lagrangian formulae are applied to establish the equilibrium equations of the VLFS and the total potential formula is employed for fluids in the numerical model including the viscous effect of the perforated plates through the Darcy’s law. The hybrid finite element-boundary element(FE-BE) method is implemented to determine the response reduction of VLFS with attached perforated plates under various oblique incident waves.Also, the numerical solutions are validated against a series of experimental tests. The effectiveness of the attached perforated plates in reducing the deflections of the VLFS can be significantly improved by selecting the proper design parameters such as the porous parameter, submergence depth, plate width and inclination angle for the given sea conditions.
文摘OWTs （offshore wind turbines） are currently considered as a reliable source of renewable energy. OWT support structures account for 20%-25% of the capital cost for offshore wind installations. Pre-feasibility studies involving estimation of preliminary dimensions of the wind turbine structure need to be performed for initial costing to arrive at the commercial viability of the project. The main objective of the paper is to obtain preliminary configuration for commercial viability and approximate sizing of the foundation pile. Design equations and nomograms are proposed for quick preliminary design of monopile founded wind turbines located offshore of Gujarat. Parametric studies are carried-out on various configurations of a hollow monopile by varying water depths and properties of sand. A nonlinear static analysis of substructure is performed considering aerodynamic forces and hydrodynamic forces for various structural and soil parameters. The sub-structure design of wind turbine is based on API （American petroleum institute） standards. A simplified design methodology for monopile support structure under extreme loading condition is presented based on multivariable linear regression analysis. The input variables for the regression analysis are hydrodynamic data, angle of internal friction of sand, and the output variables are length and outer diameter of monopile. This simplified methodology is applicable in pre-studies of wind power parks.
基金supported by the National Natural Science Foundation of China (Grant No. 51779198)
文摘Welding residual stress in the engineering structure has a non-negligible influence on crack propagation,and crack closure is a significant factor affecting the crack propagation.Based on the elastoplastic finite element method and crack closure theory,we studied crack closure and residual compressive stress field of butt-welded plates under constant amplitude loading and overloading regarding the stress ratio,maximum load,overload ratio,and number of overloads.The results show that the welding residual tensile stress can decrease the crack closure because of a decrease in the residual compressive stress in the wake zone,but the effect is gradually reduced with increased stress ratio or maximum load.And the combined effect of welding residual tensile stress and overload can produce a stronger retardation effect on crack propagation.
文摘With a successful and rapid development of offshore wind industry and increased research activities on wave energy conversion in recent years,there is an interest in investigating the technological and economic feasibility of combining offshore wind turbines(WTs)with wave energy converters(WECs).In the EU FP7 MARINA Platform project,three floating combined concepts,namely the spar torus combination(STC),the semi-submersible flap combination(SFC)and the oscillating water column(OWC)array with a wind turbine,were selected and studied in detail by numerical and experimental methods.This paper summarizes the numerical modeling and analysis of the two concepts:STC and SFC,the model tests at a 1:50 scale under simultaneous wave and wind excitation,as well as the comparison between the numerical and experimental results.Both operational and survival wind and wave conditions were considered.The numerical analysis was based on a time-domain global model using potential flow theory for hydrodynamics and blade element momentum theory(for SFC)or simplified thrust force model(for STC)for aerodynamics.Different techniques for model testing of combined wind and wave concepts were discussed with focus on modeling of wind turbines by disk or redesigned small-scale rotor and modeling of power take-off(PTO)system for wave energy conversion by pneumatic damper or hydraulic rotary damper.In order to reduce the uncertainty due to scaling,the numerical analysis was performed at model scale and both the numerical and experimental results were then up-scaled to full scale for comparison.The comparison shows that the current numerical model can well predict the responses(motions,PTO forces,power production)of the combined concepts for most of the cases.However,the linear hydrodynamic model is not adequate for the STC concept in extreme wave conditions with the torus fixed to the spar at the mean water level for which the wave slamming on the torus occurs and this requires further investigation.Moreover,based on a preliminary comparison of the displacement,the PTO system as well as the wind and wave power production,the STC concept will have a lower cost of energy as compared to the SFC concept.However,the cost of energy of either the STC or the SFC concept is higher than that of a pure floating wind turbine with the same floater.
文摘Understanding and control of wake vortices past a circular cylinder is a cardinal problem of interest to ocean engineering.The wake formation and vortex shedding behind a variety of ocean structures such as spars,are subjected to fatigue failure limiting their life span.The additional influences due to ocean waves and currents further exacerbate these effects.In the present study,flow past an isolated circu-lar cylindrical structure subjected to an oscillatory upstream are numerically investigated.These studies involve high resolution simulations over the low Reynolds number range(100-200).Although the prac-tical range of interest is in high Reynolds number range of 103-105,the flow physics and a number of qualitative and quantitative aspects are similar to the low Reynolds number flows.In the high Reynolds number range,statistical averaging tools in conjunction with suitable closure models would be neces-sary.The control of wake vortices is achieved with the aid of two small rotors located in the aft of the main cylinder.A control algorithm was coupled to determine the quantum of actuation to the rotating elements.Although control of wake vortices was observed for harmonic in-let forcing,residual vortical structures were found to persist at higher amplitudes of oscillation.To study the efficacy of this control,numerical simulations were further extended,when the circular cylinder was flexibly mounted.The con-trol of flow induced vibrations was observed to be reasonably effective in controlling the wake generated behind the main cylinder due to oscillatory upstream.
