The equations of motion of a bubble, expanding adiabatically through an incompressible viscous fluid, are deduced when the centre of the bubble moves in a vertical plane in the presence of gravitational acceleration, ...The equations of motion of a bubble, expanding adiabatically through an incompressible viscous fluid, are deduced when the centre of the bubble moves in a vertical plane in the presence of gravitational acceleration, acting vertically downwards. The non-linear equations of motion obtained are solved numerically for different values of the various parameters of the problem. The path traced by the centre of the bubble and velocity of the centre, the change of radius R with time, and the influence of the buoyancy force, which is experienced by the expanding bubble for different values of the gravitational acceleration on these quantities, are investigated. The radius R（t） of the bubble is found to vary periodically with time when the acceleration due to gravity is small. But when the acceleration due to gravity increases, this periodicity in the value of R（t） with t is lost. The influence of viscosity in determining the periodicity of the bubble motion is also investigated.展开更多
Pollen analysis was carried out on the Core MD982194 of 29.78 m retrieved from the Middle Okinawa Trough which was dated as old as -200 ka BP. The results revealed that pollen assemblages mainly presented an alternati...Pollen analysis was carried out on the Core MD982194 of 29.78 m retrieved from the Middle Okinawa Trough which was dated as old as -200 ka BP. The results revealed that pollen assemblages mainly presented an alternation of coniferous and herb pollen. The coniferous saccate pollen, principally Pinus and Tsuga, predominated in most parts of the core, especially highlighted in the interstadial stages including MIS 1,3, 5 and MIS7, whereas the herb pollen significantly increased in the gla- cial periods. Thus the pollen flora and their percentages showed the sensitive changes under the influence of ice volume during the glacial and interglacial periods. Our record from this core has first documented that the percentage of Cyperaceae was ex- tremely high in the glacial stages with a notable increase in Artemisia, Gramineae, Asteraceae, Cbenopodiaceae, and freshwa- ter algae, which can be used as a proxy for sea-level change at the study site because of their close negative correlation of the orbital-scale changes in sea level. The distance between the continental coastline and the Okinawa Trough has deeply short- ened due to the sea-level drop in the LGM. As a result, the sediment materials from Yangtze River were extensively deposited on the flat, exposed continental shelf owing to the rapid decline of river flow speed, leading to that pollen grains from Okinawa Trough are derived mainly from the flat coastal vegetation of exposed continental shelf at glacial stages. Changes of pollen as- semblage were consistent with the variation of temperature and humidity, which showed that the percentage of arboreal pollen was highly augmented at MIS 7, 5 and MIS 1, corresponding to the strengthening of the East Asian summer monsoon and in- creasing of rainfall. Moreover, the peak of Pinus percentage in MIS 5.3, 5.1 and MIS 3.3 may be closely linked by orbital and sub-orbital cycles of solar radiation and monsoonal variability. The present study of core MD981294 implied lower tempera- ture and precipitation during the lowest sea-level stage （LGM）, and more visibly testified that the vegetation of the flat plain on the exposed continental shelf was dominated by intrazonal communities such as halophyte grasslands and freshwater wetlands instead of zonal steppe or semi-arid desert. All above evidence demonstrated that the fundamental changes of pollen assem- blage and their origins in Okinawa Trough since -200 ka BP were affected by combine factors including the coastline position and climate fluctuation. Moreover, the substantial shortening of distance between shoreline and the Okinawa Trough driven by orbital insolation cycles was clearly indicated by the pollen spectra, whereas the source-area climate signal of the pollen record was largely weakened.展开更多
Optical fiber sensors have attracted considerable attention in health monitoring of aerospace composite structures. This paper briefly reviews our recent advancement mainly in Brillouin-based distributed sensing. Dama...Optical fiber sensors have attracted considerable attention in health monitoring of aerospace composite structures. This paper briefly reviews our recent advancement mainly in Brillouin-based distributed sensing. Damage detection, life cycle monitoring and shape reconstruction systems applicable to large-scale composite structures are presented, and new technical concepts, ＂smart crack arrester＂ and ＂hierarchical sensing system＂, are described as well, highlighting the great potential of optical fiber sensors for the structural health monitoring （SHM） field.展开更多
文摘The equations of motion of a bubble, expanding adiabatically through an incompressible viscous fluid, are deduced when the centre of the bubble moves in a vertical plane in the presence of gravitational acceleration, acting vertically downwards. The non-linear equations of motion obtained are solved numerically for different values of the various parameters of the problem. The path traced by the centre of the bubble and velocity of the centre, the change of radius R with time, and the influence of the buoyancy force, which is experienced by the expanding bubble for different values of the gravitational acceleration on these quantities, are investigated. The radius R（t） of the bubble is found to vary periodically with time when the acceleration due to gravity is small. But when the acceleration due to gravity increases, this periodicity in the value of R（t） with t is lost. The influence of viscosity in determining the periodicity of the bubble motion is also investigated.
基金supported by the National Natural Science Foundation of China(Grant Nos.41072128 and 40772113)the Fundamental Research Funds for the Central Universities(Grant Nos.11LGJC13 and 10LGZD08)Young Scientist’s Fund of the State Oceanic Administration of China
文摘Pollen analysis was carried out on the Core MD982194 of 29.78 m retrieved from the Middle Okinawa Trough which was dated as old as -200 ka BP. The results revealed that pollen assemblages mainly presented an alternation of coniferous and herb pollen. The coniferous saccate pollen, principally Pinus and Tsuga, predominated in most parts of the core, especially highlighted in the interstadial stages including MIS 1,3, 5 and MIS7, whereas the herb pollen significantly increased in the gla- cial periods. Thus the pollen flora and their percentages showed the sensitive changes under the influence of ice volume during the glacial and interglacial periods. Our record from this core has first documented that the percentage of Cyperaceae was ex- tremely high in the glacial stages with a notable increase in Artemisia, Gramineae, Asteraceae, Cbenopodiaceae, and freshwa- ter algae, which can be used as a proxy for sea-level change at the study site because of their close negative correlation of the orbital-scale changes in sea level. The distance between the continental coastline and the Okinawa Trough has deeply short- ened due to the sea-level drop in the LGM. As a result, the sediment materials from Yangtze River were extensively deposited on the flat, exposed continental shelf owing to the rapid decline of river flow speed, leading to that pollen grains from Okinawa Trough are derived mainly from the flat coastal vegetation of exposed continental shelf at glacial stages. Changes of pollen as- semblage were consistent with the variation of temperature and humidity, which showed that the percentage of arboreal pollen was highly augmented at MIS 7, 5 and MIS 1, corresponding to the strengthening of the East Asian summer monsoon and in- creasing of rainfall. Moreover, the peak of Pinus percentage in MIS 5.3, 5.1 and MIS 3.3 may be closely linked by orbital and sub-orbital cycles of solar radiation and monsoonal variability. The present study of core MD981294 implied lower tempera- ture and precipitation during the lowest sea-level stage （LGM）, and more visibly testified that the vegetation of the flat plain on the exposed continental shelf was dominated by intrazonal communities such as halophyte grasslands and freshwater wetlands instead of zonal steppe or semi-arid desert. All above evidence demonstrated that the fundamental changes of pollen assem- blage and their origins in Okinawa Trough since -200 ka BP were affected by combine factors including the coastline position and climate fluctuation. Moreover, the substantial shortening of distance between shoreline and the Okinawa Trough driven by orbital insolation cycles was clearly indicated by the pollen spectra, whereas the source-area climate signal of the pollen record was largely weakened.
文摘Optical fiber sensors have attracted considerable attention in health monitoring of aerospace composite structures. This paper briefly reviews our recent advancement mainly in Brillouin-based distributed sensing. Damage detection, life cycle monitoring and shape reconstruction systems applicable to large-scale composite structures are presented, and new technical concepts, ＂smart crack arrester＂ and ＂hierarchical sensing system＂, are described as well, highlighting the great potential of optical fiber sensors for the structural health monitoring （SHM） field.
