在公钥密码技术提出之初,人们比较苦恼的一件事情,就是在公钥密码实际应用时如何获取其他人的公钥并信任该公钥,又如何知道他人的私钥已失密、已更换等问题。为解决这些问题,先是有PGP系统提出的密钥环技术,后来又创造、发展和完善了PK...在公钥密码技术提出之初,人们比较苦恼的一件事情,就是在公钥密码实际应用时如何获取其他人的公钥并信任该公钥,又如何知道他人的私钥已失密、已更换等问题。为解决这些问题,先是有PGP系统提出的密钥环技术,后来又创造、发展和完善了PKI证书认证体系,形成了一整套密钥管理、证书管理、黑名单管理机制,给公钥密码技术进入实际应用创造了良好条件。组合公钥体制(Combined Public Key Cryptosystem,简称CPK)展开更多
安全性是通过设计赋予的一种产品特性,是航空航天飞行器设计必须满足的首要特性."系统安全"概念于20世纪60年代正式引用,它要求在系统的寿命周期中都应识别、分析和控制危险,强调在系统设计阶段应把可接受的安全性水平设计入...安全性是通过设计赋予的一种产品特性,是航空航天飞行器设计必须满足的首要特性."系统安全"概念于20世纪60年代正式引用,它要求在系统的寿命周期中都应识别、分析和控制危险,强调在系统设计阶段应把可接受的安全性水平设计入系统中,以保证系统在以后的试验、制造、使用、保障和退役处置中都是安全的.现行的美军规范"MIL-S-38130军用规格系统,次系统级装备的一般安全需求"提出:系统安全是在有限的时间、成本以及操作环境中,运用工程与管理的原理、原则以及技术使系统达到最佳的安全境界,最终足够安全投入使用(Release To Service)[1].展开更多
Flexible risers and steel catenary risers often provide unique riser solutions for today’s deepwater field development. Accurate analysis of these slender structures, in which there are high-speed HP/HT internal flow...Flexible risers and steel catenary risers often provide unique riser solutions for today’s deepwater field development. Accurate analysis of these slender structures, in which there are high-speed HP/HT internal flows, is critical to ensure personnel and asset safety. In this study, a special global coordinate-based FEM rod model was adopted to identify and quantify the effects of internal flow and hydrostatic pressure on both flexible and deepwater steel catenary risers, with emphasis on the latter. By incorporating internal flow induced forces into the model, it was found that the internal flow contributes a new term to the effective tension expression. For flexible risers in shallow water, internal flow and hydrostatic pressure made virtually no change to effective tension by merely altering the riser wall tension. In deep water the internal pressure wielded a dominant role in governing the riser effective tension and furthering the static configuration, while the effect of inflow velocity was negligible. With respect to the riser seabed interaction, both the seabed support and friction effect were considered, with the former modeled by a nonlinear quadratic spring, allowing for a consistent derivation of the tangent stiffness matrix. The presented application examples show that the nonlinear quadratic spring is, when using the catenary solution as an initial static profile, an efficient way to model the quasi-Winkler-type elastic seabed foundation in this finite element scheme.展开更多
In this paper we have investigated the reflection and the transmission of a system of two symmetric circular-arc-shaped thin porous plates submerged in deep water within the context of linear theory. The hypersingular...In this paper we have investigated the reflection and the transmission of a system of two symmetric circular-arc-shaped thin porous plates submerged in deep water within the context of linear theory. The hypersingular integral equation technique has been used to analyze the problem mathematically. The integral equations are formulated by applying Green＇s integral theorem to the fundamental potential function and the scattered potential function into a suitable fluid region, and then using the boundary condition on the porous plate surface. These are solved approximately using an expansion-cure-collocation method where the behaviour of the potential functions at the tips of the plates have been used. This method ultimately produces a very good numerical approximation for the reflection and the transmission coefficients and hydrodynamic force components. The numerical results are depicted graphically against the wave number for a variety of layouts of the arc. Some results are compared with known results for similar configurations of dual rigid plate systems available in the literature with good agreement.展开更多
This paper simulated the ultraviolet aging process of asphalt and used dynamic mechanic analysis （DMA） method to evaluate the effect of ultraviolet aging on the asphalt theological properties. After having experienc...This paper simulated the ultraviolet aging process of asphalt and used dynamic mechanic analysis （DMA） method to evaluate the effect of ultraviolet aging on the asphalt theological properties. After having experienced ultravio- let aging, the low temperature performance of asphalt binder decreased significantly, with its complex modulus increased and phase angle decreased along with changing rheological properties as compared to the performance of original asphalt binder. The ultraviolet aging process would make asphalt binder more sensitive to brittle and fatigue failure. On the basis of the time-temperature superposition principle （TTSP）, the viscoelastic transition frequency （coT） is proposed to evaluate the effect of ultraviolet aging. It is found that with the increase in ultraviolet aging time, the cox moves to the lower frequency range gradually. Since the viscoelastic transition frequency is sensitive to the effect of aging, it can be used as an indicator of ultraviolet aging.展开更多
文摘在公钥密码技术提出之初,人们比较苦恼的一件事情,就是在公钥密码实际应用时如何获取其他人的公钥并信任该公钥,又如何知道他人的私钥已失密、已更换等问题。为解决这些问题,先是有PGP系统提出的密钥环技术,后来又创造、发展和完善了PKI证书认证体系,形成了一整套密钥管理、证书管理、黑名单管理机制,给公钥密码技术进入实际应用创造了良好条件。组合公钥体制(Combined Public Key Cryptosystem,简称CPK)
文摘安全性是通过设计赋予的一种产品特性,是航空航天飞行器设计必须满足的首要特性."系统安全"概念于20世纪60年代正式引用,它要求在系统的寿命周期中都应识别、分析和控制危险,强调在系统设计阶段应把可接受的安全性水平设计入系统中,以保证系统在以后的试验、制造、使用、保障和退役处置中都是安全的.现行的美军规范"MIL-S-38130军用规格系统,次系统级装备的一般安全需求"提出:系统安全是在有限的时间、成本以及操作环境中,运用工程与管理的原理、原则以及技术使系统达到最佳的安全境界,最终足够安全投入使用(Release To Service)[1].
基金Supported by the National High-tech Research and Development Program of China (863 Program) under Grant No. 2010AA09Z303the Key Project of National Natural Science Foundation of China (Grant No. 50739004)the National Natural Science Foundation of China (Grant No. 11002135)
文摘Flexible risers and steel catenary risers often provide unique riser solutions for today’s deepwater field development. Accurate analysis of these slender structures, in which there are high-speed HP/HT internal flows, is critical to ensure personnel and asset safety. In this study, a special global coordinate-based FEM rod model was adopted to identify and quantify the effects of internal flow and hydrostatic pressure on both flexible and deepwater steel catenary risers, with emphasis on the latter. By incorporating internal flow induced forces into the model, it was found that the internal flow contributes a new term to the effective tension expression. For flexible risers in shallow water, internal flow and hydrostatic pressure made virtually no change to effective tension by merely altering the riser wall tension. In deep water the internal pressure wielded a dominant role in governing the riser effective tension and furthering the static configuration, while the effect of inflow velocity was negligible. With respect to the riser seabed interaction, both the seabed support and friction effect were considered, with the former modeled by a nonlinear quadratic spring, allowing for a consistent derivation of the tangent stiffness matrix. The presented application examples show that the nonlinear quadratic spring is, when using the catenary solution as an initial static profile, an efficient way to model the quasi-Winkler-type elastic seabed foundation in this finite element scheme.
基金Partially Supported by the Department of Science and Technology Through a Research Grant to RG(No.SR/FTP/MS-020/2010)
文摘In this paper we have investigated the reflection and the transmission of a system of two symmetric circular-arc-shaped thin porous plates submerged in deep water within the context of linear theory. The hypersingular integral equation technique has been used to analyze the problem mathematically. The integral equations are formulated by applying Green＇s integral theorem to the fundamental potential function and the scattered potential function into a suitable fluid region, and then using the boundary condition on the porous plate surface. These are solved approximately using an expansion-cure-collocation method where the behaviour of the potential functions at the tips of the plates have been used. This method ultimately produces a very good numerical approximation for the reflection and the transmission coefficients and hydrodynamic force components. The numerical results are depicted graphically against the wave number for a variety of layouts of the arc. Some results are compared with known results for similar configurations of dual rigid plate systems available in the literature with good agreement.
基金supported by the National Natural Science Foundation of China(NNSFC,Grant No.50808058)the New Century Excellent Talents in University,2007(NCETU)+1 种基金the National Science Foundation for Post-doctoral Scientists of China(NSFPSC,Grant No.20080430925)the Specialized Research Fund for the Doctoral Program of Higher Education of China(SRFDPHEC,Grant No.200902403)
文摘This paper simulated the ultraviolet aging process of asphalt and used dynamic mechanic analysis （DMA） method to evaluate the effect of ultraviolet aging on the asphalt theological properties. After having experienced ultravio- let aging, the low temperature performance of asphalt binder decreased significantly, with its complex modulus increased and phase angle decreased along with changing rheological properties as compared to the performance of original asphalt binder. The ultraviolet aging process would make asphalt binder more sensitive to brittle and fatigue failure. On the basis of the time-temperature superposition principle （TTSP）, the viscoelastic transition frequency （coT） is proposed to evaluate the effect of ultraviolet aging. It is found that with the increase in ultraviolet aging time, the cox moves to the lower frequency range gradually. Since the viscoelastic transition frequency is sensitive to the effect of aging, it can be used as an indicator of ultraviolet aging.
