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The continuing public concern with overfishing ignores the underlying problems that face fishery mangers. Attention needs to be given to (1) the transitory nature of the ocean environment, (2) the natural variation in fish stocks, (3) the role of the fishing industry and market forces in fishery management, and (4) the failure to focus on what it is we most need from the oceans, in what form we need it, and at what price. 相似文献
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Two diagnostic dynamic models for flow in hyperbolic and elliptic regions of a geophysical fluid are developed and compared. As the main interest here is in local dynamical processes, these models are used to study trajectories near stagnation points in the flow field. The simplest model presumes a balance between the Coriolis and geopotential accelerations. This model is equivalent to the classic approach that characterizes these regimes by the quadratic equation for the eigenvalues of the velocity gradient. However, since that model imposes geostrophic dynamics, the eigenvalues of the velocity gradient can be replaced by the local curvature or Hessian of the geopotential scaled by Coriolis. The general model adds both local and inertial accelerations to the dynamical balance. In contrast to the classic result the consequent frequency equation is a quartic that involves both the Hessian of the geopotential field, the components of the velocity gradient, and Coriolis. Roots of this equation give two distinct time scales, which are interpreted as Lagrangian time scales. Motion of the geopotential field produces a third Eulerian time scale. Critical trajectories are those whose initial positions and velocities are such that they are independent of the Lagrangian time scales. These simple models establish that within hyperbolic and elliptic regions of the geopotential field there may be trajectories whose time scales differ radically from even their nearest neighbors.A characteristic of critical trajectories in the ocean is that they often are found near stagnation points. These may be hard to identify even in model simulations, but a similar quantity, the null in the geopotential gradient, might be easier to obtain. To analyze the relation between the critical trajectories, stagnation points, and gradient null, evolution models for the later two objects are proposed. For a steady geopotential all three coincide. However with a time varying geopotential, they are distinct even though all have the same time scale. The analysis provides a metric for the separation of all three objects. 相似文献
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截至目前,国内外少有成功开展深水管道抗压承载力海上试验的案例,而海上试验对于验证室内压力舱模拟试验结果可靠性是非常必要的。对深水管道屈曲压溃及屈曲传播海上试验进行了设想和分析,并设计了试验流程。考虑到室内压力舱的加载特性,屈曲传播试验会受到若干因素影响,结合数值模拟,考虑屈曲传播海上试验中可能出现的各种加载条件对管道屈曲传播压力的影响进行了研究。数值模拟的结果与室内压力舱试验的结果之间存在一定差异,对该现象存在的原因进行了分析。还对试验中可能存在的问题及需要注意的关键难点提出了解决方案。 相似文献
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The effect of the shear and normal stress free surface boundary conditions on the laminar flow induced by wave propagation is discussed. The approximate form of the boundary conditions, as used by (Lin, P., Zhang, W., 2008. Numerical simulation of wave-induced laminar boundary layers. Coast. Eng. 55, 400–408), is valid only when the free surface slope is mild. 相似文献
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P. R. Payne 《Ocean Engineering》1994,21(3)
The results of several recent isolated investigations in planing theory are consolidated in this paper, together with new insights generated by a recent numerical solution of the vertically impacting wedge problem by Zhao and Faltinsen [(1992), Water entry of two-dimensional bodies. J. Fluid Mech. 246, 593–612]. As a result, in contrast to some earlier studies, it is found that the “wetted width” associated with the added mass is not that of the intersection of the wedge with the undisturbed water surface, but the wetted width of the splashed-up water, as originally proposed by Wagner [(1932), Uber Stoss-und Gleitvorgange an der Oberflache von Flussig-Keiten, Zeitschrift für Angewandte Mathematik und Mechanik, Band 12, Heft 4 (August)]. However, the splash-up ratio is not the value of (π/2–1) which he proposed, but a value which decreases with increasing deadrise, originally proposed in the late-1940s by Pierson (“Pierson's hypothesis” in the paper). For 30° deadrise, for example, Pierson's splash-up ratio is two-thirds that of Wagner's.The new equations are employed to determine the increase in the “added mass” of prismatic hull sections due to chine immersion, using experimental data. If mo′ is the added amss of the hull section whose chines are just wetted, Payne [(1988), Design of High-speed Boats. Volume 1: Planing. Fishergate, Inc., Annapolis, Maryland, U.S.A.] postulated that the increase in added mass due to a chine submergence (zc) would be where b is the chine beam and k is a constant which Payne [(1988), Design of High-speed Boats. Volume 1: Planing. Fishergate, Inc., Annapolis, Maryland, U.S.A.] gave as
.The present analysis includes the “one-sided flow” correction introduced in Payne [(1990), Planing and impacting forces at large trim angels. Ocean Engng 17, 201–234]. Partly for that reason and partly because of the more precise analysis of the experimental data, the present paper revises the value to k = 2 for wetted length to beam ratios normally employed. For deadrise angles in excess of 40° and wetted keel to beam ratios in excess of 2.0, there is some evidence that k < 2.0.The revised theoretical formulation is compared with eight different sets of experimental data for flat plate and prismatic hull forms and is found to be in excellent agreement when the speed is high enough for “dynamic suction” (a loss of buoyancy at low speeds and low wetted lenghts) to be unimportant. This is true for “chines-dry” operation with deadrise angles up to 50° and chines-wet operation at length to beam ratios far in excess of the most extreme conventional practice.The research involved in performing this analysis led to the realization that different towing tanks measure different wetted chine lengths for the same hulls and test conditions. Some consistently measure more splash-up than “theory” (based on Pierson's splash-up hypothesis) predicts and others measure somewhat less than the theory. Some examples are given in Appendix B. The reason for this is not understood. 相似文献
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W.J. Van Sciver 《Ocean Engineering》1973,2(5):223-229
The angular field observable in water by an observer in air depends on the configuration of the air-water interface. When the interface is a plane, the absolute limit to observable field is about
from the normal to the interface. A practical limit, because of lateral chromatic aberration is considerably less, approximately 30°. A conventionally used configuration in research submersibles, is a polymethyl methacrylate port with inner and outer surfaces parallel. This has the same optical limitations as the plane air-water interface. It is shown that if the inner and outer surfaces are not required to be parallel to each other, there are solutions which permit extending the observable field to nearly a full hemisphere with acceptably small distortion and lateral chromatic aberration. 相似文献
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Gloria was a fast-moving, intense hurricane that grazed the North Carolina coastline in September, 1985. The resulting storm surge was measured remotely using a telephone-linked wave data system as well as a local tide gage. The surge was hindcast using the model of Jelesnianski (1967) which was developed for shore-parallel storm tracks. The agreement with measurement was quite satisfactory. The data suggest extremely rapid rise times for the peak surge of hurricanes moving at high speed along the coast. 相似文献
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Morphological and chronological studies have been carried out on coralline algal buildups (“coralligène”) situated between 10 and 60 m depth near Marseilles, and in Corsica (France). Despite the presence of occasionally sizeable quantities of iron hydroxide, these constructions prove to be a reliable material for radiocarbon dating. Ages obtained using this method range from 640±120 yrs B.P. (Corsica, Scandola Natural Reserve, −15 m) to 7760±80 yrs B.P. (Marseilles, Grand Congloue, −52 m). Internal erosion surfaces within the buildups give evidence of discontinuous development. The accumulation rate of the coralligène constructions is very low (0.006–0.83 mm yr−1 according to the depth and time period). The higher values (0.53–0.83 mm yr−1) were recorded for the deeper constructions. They correspond to a period between 8000 and 6000 yrs B.P. After 6000 yrs B.P., the only appreciable accumulation rates (0.11–0.42 mm yr−1) were recorded for constructions situated between 10 and 35 m depth, whereas the accumulation rates of deep coralligène (> 50 m) appear to be low or zero. The age of the large constructions (overhang: 80 cm in width) is positively correlated with depth (r = 0.95; p < 0.005). Their development occurred during the Flandrian transgression. The oldest structures, today situated at 50 to 60 m depth, started to develop when water depth was probably no greater than 10 to 15 m. Apart from in strongly shaded fissures on rocky coasts and areas subjected to heavy sedimentation, the main framework building algal species was initially Mesophyllum lichenoides (Ellis) Lemoine, a high tolerant species to light, hydrodynamic energy and temperature. With the rise in sea level, the coralligène structure gradually became available to other less tolerant algal species (Lithophyllum, Lithothamnion), and the crustforming population diversified. Because of the good preservation of coralligène structures, the reliability of radiocarbon dating and the correlation between the age and bathymetric position of the large coralligène structures (except in areas of heavy sedimentation and fissures in shallow rocky coasts), these buildups are considered to be of use as biological indicators of variations in sea level. 相似文献
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R Pelet 《Marine Chemistry》1977,5(4-6)
- 1. (1) The nature of sediment—sea interactions depends on the time scale considered. At a time scale commensurate with human life, one can define a water—sediment interface, and the main exchanges are solutes exchanges through this interface by concentration diffusion. This condition will be termed as “Short time-scale equilibrium interaction”.
- On the other hand, at a geological time scale, there is a continuous accretion to the “sediment” of suspended particulate matter, bottom-current borne materials and sometimes precipitates of previously dissolved salts; to this sediment build-up corresponds a flux of water of reverse sense, from the sediment to the water column, due to the compaction of muds and oozes which reduce their porosity (their water content) under their own load. The concept of interface is then of limited utility, since physically it is constantly changing, and since the material balance of the exchanges does not depend on its characteristics at a first order of approximation. This condition will be termed “long time-scale geological interaction”.
- These situations are extreme ones. In areas of present important detrital sedimentation, even for short time spans it is doubtful if the definition of an interface has some utility: we are in a situation close to “geological time scale”. On the contrary, in abyssal zones remote from continents, the rate of sedimentation is so low that even for eons an interface separating two environments in physico-chemical equilibrium exists.
- 2. (2) If there were no internal sources of dissolved species in the sediment, the only concentration changes to occur would be due to the decrease in porosity (in water content) following gravitational compaction of sediments. But this phenomenon is the same as sedimentation, thus transfer of matter would be unappreciable within short time spans. The fact that this transfer can be measured at human time scale shows therefore that dissolved species are actually produced in the sediment. Some of these can originate from possible inorganic chemical reactions, but all the organic molecules, and an important part of the inorganic (such as phosphates, nitrates, NH4+, S2-) require processing of organic matter for their production. Whether or not this reworking is of biological origin remains controversial. On the whole, the quantities of matter thus transferred are very minute compared to the quantities present in oceanic waters. They cannot be considered in general as a significant input. But they may be important locally (nearshore restricted water bodies, or manganese nodules formation).
- 3. (3) At geological time scale, sedimentation, which adds solid material to the preexisting sediment, results also in the compaction of this sediment. At every depth in the sediment there exists an equilibrium value of the porosity, i.e. the fluids content, of the sediment; it tends to this value by expelling the corresponding quantity of fluids, with a rate determined by its permeability. This input may be important, but it is mainly water, and water formerly oceanic: therefore it is not a true input, but simply a delayed return. The transfer of other fluids (mainly oil and gas) is unsignificant generally speaking. Once more, it may be locally important (submarine seepages).
- 4. (4) On the whole, the processes of water—sediment interaction appear not to add any new matter into the oceanic pool, but rather to regulate the restitution by the sediment to the water of substances which were already present in ocean, in particulate or dissolved form, either free or combined. One can trace out two main processes, which differ in their rates and yields:
- 4.1. (a) the short time scale diffusion—high rate low yield restitution of organics and inorganics in dissolved state:
- 4.2. (b) the long time scale compaction: low rate high yield restitution of entrapped fluids, essentially water (devoid of dissolved species).
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精确获取剖面风数据对海上风电场规划选址、风能资源评估等至关重要。针对测风塔建设成本高、拆卸移动困难,而传统浮标电源供给较小、摇晃角度大等缺点,设计一种新型激光雷达浮标,实现了对目标海域的风资源剖面数据的观测、解算与自存储。在南海海域进行海试应用,并对观测的剖面风数据和传统大型海洋资料浮标风数据进行同步比测分析。结果表明,激光雷达测风浮标的测量结果与大型浮标测风数据吻合较好,风速R~2相关系数达到0.975,风向R~2相关系数达到0.932。该研究成果可为激光雷达测风浮标在海上风电开发和开展风场变化的科学研究等提供参考。 相似文献
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A hybrid thermal protection method using waste heat from a surface-mounted outboard motor is shown to create a warm “micro-climate” environment for divers. The effects of surface heater capacities, water flow rates, shelter volume and shelter insulation on micro-climate temperatures are characterized. During long, cold-water decompression stops this method offers a reliable, low-cost alternative to surface-supplied hot water suits or diver-carried heating systems. An added bonus for divers using closed-circuit breathing apparatus is prolonged durations of their carbon dioxide scrubbers when surrounded by the warm water “micro-climate”. Closed-circuit and open-circuit options of this diver decompression shelter concept are evaluated. 相似文献
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Coastline sand waves have been observed at “El Puntal” spit, located on the north coast of Spain. The spit has been monitored by an Argus video system since 2003 and the formation and destruction of sand waves has been observed. Coastline data from the video images are analyzed by means of principal components analysis, obtaining a mean sand wave length of 125–150 m and a maximum amplitude of ≈ 15 m. It is also observed that sand waves reach their maximum amplitude at about 15 days. No propagation of these sand waves is noticed during the approximately two-month-long events analyzed. Sand wave formation and evolution are examined in relation with the prevailing local wave conditions during that period. Incident waves at the west end of the spit approach from the east–northeast, with a very high angle with respect to the shoreline. Field observations suggest that sand waves may result from an instability in alongshore sediment transport caused by moderate-energy waves with a high-angle incidence. 相似文献