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1.
Influence of non-linearities on the limits of stability of ships rolling in head seas 总被引：3，自引：0，他引：3
The present paper describes an investigation on parametric resonance in head seas in which a new third-order coupled mathematical model is considered. The restored modes of heave, roll and pitch are contemplated. The discussion is illustrated for the case of a transom stern fishing vessel at different speeds. It is pointed out that numerical simulations employing the new model are successfully compared to experimental results previously obtained for the vessel.Considering that analyticity is an important tool when handling complex stability issues, some theoretical dynamic characteristics of the equations are discussed. By means of the analysis of the coupled linear variational equation derived from an extended third-order model, the appearance of super-harmonics and increased rigidity proportional to wave amplitude squared due to third-order terms is demonstrated.In the present paper, an important tool is explored, that is the analysis of the limits of stability obtained from the new model. Limits of stability are a well-known and practical way of looking into the problem of parametric resonance. New limits of stability are derived and compared to the more conventional Strut diagram. Dynamic characteristics associated with the new limits of stability are discussed. The influence of different parameters is investigated, including vessel speed, damping and tuning. Consistent and revealing results are obtained through the analysis of the new limits of stability for different speeds and damping. 相似文献
2.
Madden J.D.W. Schmid B. Hechinger M. Lafontaine S.R. Madden P.G.A. Hover F.S. Kimball R. Hunter I.W. 《Oceanic Engineering, IEEE Journal of》2004,29(3):738-749
A decade of research into electroactive polymer actuators is leading to the exploration of applications. These technologies are not ready to compete with the internal combustion engine and electric motors in high power propulsion systems but are suitable for intermittent or aperiodic applications with moderate cycle life requirements, providing an alternative to solenoids and direct drive electric motors. Polypyrrole, an emerging actuator material, is applied to drive hydrodynamic control surfaces and in particular to change the camber of a foil. The foil is intended for use in the propeller blade of an autonomous underwater vehicle. A scaled prototype is constructed which employs polypyrrole actuators imbedded within the blade itself to vary camber. The kinematics required to generate camber change are demonstrated, with >30/spl deg/ deflections of the trailing edge being observed from both bending bilayer and linear actuator designs. Forces developed in still conditions are five times lower than the 3.5 N estimated to be required to implement variable camber. The observed 70 kJ/m/sup 3/ polypyrrole work density however is more than sufficient to produce the desired actuation from within the limited blade volume, enabling an application that is not feasible using direct drive electric motors. A key challenge with the polypyrrole actuators is to increase force without sacrificing speed of actuation. 相似文献
3.
Second-order springing on an elastic body with forward speed is analyzed by numerical simulations. The boundary-value problem for the velocity potential is solved by means of the direct time-domain higher-order boundary element method (HOBEM). The free-surface boundary condition in the boundary-value problem is approximated on the mean surface up to second order by use of perturbation and Taylor-series expansion methods. The body boundary condition for an elastic body is derived with various quantities which are redefined in the generalized mode. These variables such as mode shape, normal vector, etc. are obtained by using directional derivative and continuum mechanics, and the same mathematical expressions are used to obtain several second-order generalized forces. To validate the numerical results, the second-order hydrodynamic force on the bottom-mounted rigid/elastic cylinders without forward speed is compared with other semi-analytic results. The property of second-order forces on an elastic ship is studied by changing the flexural rigidity and forward speed with elastic response. It is confirmed that the second-order velocity potential is important for a body with forward speed and investigation should be made more on numerical methods for accurate computation of the second-order velocity-potential force with forward speed. 相似文献
4.
Beach nourishment is an environmentally preferred method of shore protection, but the annual sand requirement may lead to substantial maintenance costs. The shoreline processes, involving the surf zone, beach and dune, are reviewed with the aim of reducing the annual sand requirement of eroding shorelines. It is shown that surf zones with equilibrium profiles, on which the wave energy conversion is evenly distributed across the surf zone, from experience for given conditions indicate least loss of sand. On steep, eroding shorelines it may be difficult to establish an equilibrium profile. For such cases, the use of perched surf zones is recommended, which are supported at the seaward limit by an underwater sill. For reduction of littoral transport, the use of pervious pile groynes is recommended. These are arguably more efficient than impervious groynes. The sand loss from a usually dry beach by raised water levels is shown to be a function of the beach slope and is least when the storm waves at raised water levels do not cut an erosion escarpment. The loss of sand from a dune by infrequent severe storm tides can be prevented with the aid of a built-in membrane. These sand losses are usually large and constitute an uneconomic use of this sand resource. The proposed concepts and measures are linked to existing knowledge, augmented by data from the large wave flume (LWF) in Germany and field data from the North and Baltic Sea coasts. 相似文献
5.
The finite element method(FEM) is employed to analyze the resonant oscillations of the liquid confined within multiple or an array of floating bodies with fully nonlinear boundary conditions on the free surface and the body surface in two dimensions.The velocity potentials at each time step are obtained through the FEM with 8-node quadratic shape functions.The finite element linear system is solved by the conjugate gradient(CG) method with a symmetric successive overelaxlation(SSOR) preconditioner.The waves at the open boundary are absorbed by the combination of the damping zone method and the Sommerfeld-Orlanski equation.Numerical examples are given by an array of floating wedgeshaped cylinders and rectangular cylinders.Results are provided for heave motions including wave elevations,profiles and hydrodynamic forces.Comparisons are made in several cases with the results obtained from the second order solution in the time domain.It is found that the wave amplitude in the middle region of the array is larger than those in other places,and the hydrodynamic force on a cylinder increases with the cylinder closing to the middle of the array. 相似文献
6.
The finite element method (FEM) is employed to analyze the resonant oscillations of the liquid confined within multiple or an array of floating bodies with fully nonlinear boundary conditions on the free surface and the body surface in two dimensions. The velocity potentials at each time step are obtained through the FEM with 8-node quadratic shape functions. The finite element linear system is solved by the conjugate gradient (CG) method with a symmetric successive overelaxlation (SSOR) preconditioner. The waves at the open boundary are absorbed by the combination of the damping zone method and the Sommerfeld-Orlanski equation. Numerical examples are given by an array of floating wedge- shaped cylinders and rectangular cylinders. Results are provided for heave motions including wave elevations, profiles and hydrodynamic forces. Comparisons are made in several cases with the results obtained from the second order solution in the time domain. It is found that the wave amplitude in the middle region of the array is larger than those in other places, and the hydrodynamic force on a cylinder increases with the cylinder closing to the middle of the array. 相似文献
7.
Isamu Aida 《Journal of Oceanography》1972,28(5):220-227
Estimation of the offshore form of tsunami waves based on an observed tidegage record at a bay head is attempted by means of an impulse response method and a characteristics method. For several tsunamis in Onagawa Bay, the results estimated by both methods are compared with the observed data at an offshore station. The correlation coefficient between the estimated and the observed values is 0.9 to 0.95, and the present methods are proved to be practical. The validity of the characteristics method is further confirmed in the case of Ofunato Bay. 相似文献
8.
通过系列模型试验,对透空水平板下波浪总上托力进行了研究,结果表明：平板下最大总上托力并不与最大冲击压强同步发生。为此,依据试验结果对波浪总上托力产生机理和影响因素进行详细分析,提出透空水平板波浪最大总上托力的计算公式,试验结果表明,该公式与实验值有着较好的一致性。 相似文献
9.
With the method of separation of variables and the eigenfunction expansion employed, an analytical solution is presented for the radiation and diffraction of a rectangular structure with an opening near a vertical wall in oblique seas, in which the unknown coefficients are determined by the boundary conditions and matching requirement on the interface. The effects of the width of the opening and the angle of incidence on the hydrodynamic characteristics of a rectangular structure with an opening near a vertical wall are mainly studied. The comparisons of the calculation results with wall-present and with wall-absent are also made. The results indicate that the variation trends of the heave added mass and excitation force with wall-present are almost the same as those with wall-absent, and that the peak values in the former case are obviously larger than those in the latter due to the reflection of the vertical wall. 相似文献
10.
基于实测传播损失数据对比分析Marsh-Schulkin和Rogers经验传播损失模型。通过讨论东海声学调查具体的海洋环境,较详细地标定了两模型对应的声学参数,并融入模型,结合实测数据进行对比验证,得出关于两模型在东海适应性的如下结论:两模型预报结果与实测数据吻合较好,且两者预报曲线大致相同;其中MarshSchulkin模型预报曲线斜率略大于Rogers模型,预报差异随距离增大有所增加;各频率预报曲线存在交点,交点左侧Rogers模型更接近实测数据,右侧Marsh-Schulkin更为准确。 相似文献
11.
G. D. Aburjania 《Izvestiya Atmospheric and Oceanic Physics》2011,47(4):491-502
The generation and further linear and nonlinear dynamics of planetary magnetized Rossby waves (MRWs) in the rotating dissipative ionosphere are studied in the presence of a zonal wind (shear flow). MRWs are caused by interaction with the spatially nonuniform geomagnetic field and are ionospheric manifestations of ordinary tropospheric Rossby waves. A simplified self-consistent set of model equations describing MRW-shear flow interaction is derived on the basis of complete equations of ionospheric magnetohydrodynamics. Based on an analysis of an exact analytical solution to the derived dynamic equations, an effective linear mechanism of MRW amplification in the interaction with nonuniform zonal wind is ascertained. It is shown that operators of linear problems are non-self-adjoint in the case of shear flows, and the corresponding eigenfunctions are nonorthogonal; therefore, the canonically modal approach is of little use when studying such flows; a so-called nonmodal mathematical analysis is required. It is ascertained that MRWs effectively get shear flow energy during the linear stage of evolution and significantly increase (by several orders of magnitude) their energy and amplitude. The necessary and sufficient condition of shear flow instability in an ionospheric medium is derived. Nonlinear self-localization begins with the development of shear instability and an increase in the amplitude, and the process ends with the self-organization of strongly localized isolated large-scale nonlinear vortex structures. Thus, a new degree of freedom and a way for perturbation evolution to occur appear in medium with shear flow. The nonlinear systems can be a pure monopole vortex, a vortex streets, or vortex chains depending of the shape of the sheared flow velocity profile. The accumulation of such vortices in the ionospheric medium can produce a strongly turbulent state. 相似文献
12.
在航道治理工程中,往往通过丁坝群来实现其稳定航槽等目的,而坝田作为缓流区,其与主槽的水沙交换主要取决于横向的紊动交换。基于长江口北槽丁坝群实测资料分析和物理模型水槽试验研究发现不同长宽比坝田内的流态、淤积形态、坝田与相邻河段水沙交换的机理均不同,在长江口北槽丁坝群坝田建成后的淤积初期阶段,长宽比为0.30～0.40的坝田内的平均淤积强度最大。水槽试验研究表明,长宽比为0.50的坝田内淤积的主要部位即为主环流所在位置(坝田外侧),而在副环流位置,则出现微淤或冲刷的趋势;而长宽比为0.33的坝田内的淤积分布相对比较均匀。长宽比为0.33的坝田内淤积速率明显大于长宽比为0.50的坝田,长宽比为0.33的坝田达到冲淤平衡的时间较长。坝田淤积强度与随坝田回流强度、坝田与主槽水沙交换系数的增加而增加。 相似文献
13.
In this work, we carried out an asymptotic analysis, up to the second order in a regular expansion, of the interaction of linear long waves with an impermeable, fixed, submerged breakwater composed of wavy surfaces. Below the floating breakwater, there is also a step with a wavy surface. The undulating surfaces are described by sinusoidal profiles. The effects of three different geometric parameters — the amplitude of the wavy surfaces and the submerged length and width of the structure — on the reflection and transmission coefficients are analyzed. The hydrodynamic forces are also determined. The governing equations are expressed in dimensionless form. Using the domain perturbation method, the small wavy surfaces of the breakwater are linearized. The wavy surfaces of the breakwater generate larger values of the reflection coefficient than those obtained for breakwaters with flat surfaces, and the largest values of this coefficient are obtained when the length of the breakwater is of the same order of magnitude as the wavelength. The asymptotic solution is compared with the theoretical solutions that have been reported in the specialized literature and with a numerical solution. The present mathematical model can be used as a practical reference for the selection of the geometric configuration of a submerged floating breakwater under shallow flow conditions. 相似文献
14.
胶州湾北部水域 叶绿素-a含量和初级生产力 总被引：6，自引：1，他引：5
Chai Xinyu 《中国海洋大学学报(自然科学版)》2000,30(Z1):45-52
据1995年3月至11月胶州湾北部水域内调查资料 ,分析叶绿素-a含量和初级生产力的分布,月变化和日变化以及同环境因子的关系。叶绿 素-a含量的月变幅为1．194～10．715mg/m3,平均值为3.386mg/m3。初级生产力月变 幅为68.85～1417.41mg.C/m2.d,平均值为353.01mg.C/m2.d。叶绿素-a含量和初 级生产力均以8月>9月gt;5月gt;3月gt;11月。 相似文献
15.
基于无机氮浓度响应系数的厦门湾沿岸污水处理厂排放口选址优化 总被引：2，自引：0，他引：2
基于美国普林斯顿大学的河口、陆架和海洋模式(ECOM),建立了厦门海域三维变边界的物质输运模型,模拟了厦门海域污水处理厂尾水排放口附近无机氮的响应系数场.结果表明,厦门市尾水排放口中,石渭头排放口响应系数明显小于其他排放口,其排放口中心点的响应系数仅为其他排放口的0.08～0.50;其次是澳头、海沧和筼筜排放口;而杏林、同安、集美以及翔安排放口的响应系数相对较大,说明该区域水动力条件较弱,污染物扩散缓慢,不是理想的排放口.建议将杏林排放口优化调整至海沧排放口,集美、同安、翔安排放口优化调整至澳头排放口进行排放.该研究可为厦门海域尾水排放口选址优化提供科学依据,为厦门海域入海污染物总量控制和海洋环境管理提供参考价值. 相似文献
16.
M. Barbaros Okan 《Ocean Engineering》1982,9(2):127-134
This paper is concerned with the vibrations of an orthotropic rectangular plate with one edge free while the rest are elastically restrained against rotation. The plate is assumed to be thin and its vibration amplitudes small. The plate is in contact with an ideal fluid of infinite extent and the plate-fluid interaction is considered to be quasi-coupled. The calculations are carried out by the Ritz method and the fundamental frequencies for the free and damped vibrations of the rectangular plate are presented. The fluid effect upon the fundamental frequency is briefly discussed. 相似文献
17.
通过对舟山群岛表层沉积物样品的粒度分析和黏土矿物含量测定,研究了表层沉积物的粒度分布特征及黏土矿物组成。利用Flemming三角图示法分析了研究区域的沉积动力环境,并探讨了沉积物的物质来源。结果表明,舟山群岛表层沉积物共有5种类型,其中粉砂含量最高,呈片状广泛分布于舟山群岛东部宽阔海域;研究区表层沉积物分选性较差,以正偏为主;沉积物样品在Flemming三角图上投影显示,粉砂沉积物集中在D-Ⅱ,E-Ⅱ,E-Ⅲ区,砂质粉砂等沉积物零星分布在D-Ⅱ,C-Ⅱ及S区,表明研究区沉积物整体粒径较细、岛屿间水动力较强,开阔水域水动力较弱。研究区黏土矿物中伊利石为优势矿物,质量分数均值为61.5%;其次为绿泥石,质量分数均值为15.0%;高岭石和蒙脱石含量较少,质量分数均值分别为12.7%和10.9%。黏土矿物组合类型以伊利石-绿泥石-高岭石-蒙脱石为主,伊利石-绿泥石-蒙脱石-高岭石次之,具有类长江型沉积物特征。舟山群岛表层沉积物以陆源成因为主,物质来源以长江为主、浙江河流入海输沙以及舟山群岛岩石风化产物为补充。 相似文献
18.
A diagnostic study is performed in the paper on the process of typhoon Norris (1980) transforming into an ex-tratropical cyclone after its landing over Southeast China. The main findings are as follows:The changes of kinetic energy are mainly attributed to the generation due to non-divergent wind. During the early stage of the typhoon landing, there exits only a small quantity of kinetic energy exchanging with the environment. And after it is transformed into an extratropical cyclone, a large amount of kinetic energy is exported from the system toward the environment.The horizontal and vertical flux-divergence terms of eddy available potenlial energy are the prominent sinks in the budgets of eddy kinetic energy. The generations of eddy kinetic energy due to both the barotropic and baroclinic processes are source terms. The former is remarkable during the initial stage. But after the depression is transformed into an extratropical cyclone, the roles of the generation by the barotropic and baroclinic proc 相似文献
19.
The present work is an analytical study of the influence of geometrical parameters, such as length, thickness and immersion of the plate, on the reflection coefficient of a regular wave for an immersed horizontal plate in the presence of a uniform current with the same direction as the propagation of the incident regular wave. This study was performed using the linearized potential theory with the evanescent modes while searching for complex roots to the dispersion equation that are neither pure real nor pure imaginary. The results show that the effects of the immersion and the relative length on the reflection coefficient of the plate are accentuated by the presence of the current, whereas the plate thickness practically does not have an effect if it is relatively small. 相似文献
20.
This paper presents a method to statistically predict the magnitude of impact pressure (including extreme values) produced by deep water waves breaking on a circular cylinder representing a column of an ocean structure. Breaking waves defined here are not those whose tops are blown off by the wind but those whose breaking is associated with steepness. The probability density function of wave period associated with breaking waves is derived for a specified wave spectrum, and then converted to the probability density function of impact pressure. Impacts caused by two different breaking conditions are considered; one is the impact associated with waves breaking in close proximity to the column, the other is an impact caused by waves approaching the column after they have broken. As an example of the application of the present method, numerical computations are carried out for a wave spectrum obtained from measured data in the North Atlantic. 相似文献