共查询到20条相似文献，搜索用时 186 毫秒
1.
This paper studies the moonpool resonance of two heaving rectangular bodies in a two-layer fluid. A mathematical model is proposed based on an eigenfunction matching approach. The motion of the two-dimensional bodies is assumed to be vertical and harmonic. Heave added mass and damping coefficients are computed to examine the hydrodynamic behavior of the twin bodies. The free surface and internal wave elevations are obtained near the resonant frequencies. The presented results and analyses reveal that there exist both Helmholtz and higher-order resonances in the two-layer fluid system, which is similar to the single-layer fluid case. It is also found that the resonances are closely associated with the free surface elevation inside moonpool gap, not the wave elevation at the interfacial surface. In addition, parametric studies have been performed to identify the dependencies of hydrodynamic behavior on geometry and density stratification. 相似文献
2.
Radiation impedance matrix and optimum power absorption for interacting oscillators in surface waves
Johannes Falnes 《Applied Ocean Research》1980,2(2):75-80
The two hydrodynamic coefficients, the damping coefficient and the added-mass coefficient are incorporated into a single complex coefficient, the radiation impedance. These coefficients become matrices for a system of interacting wave-generating oscillators. A derivation and an optimization of the power absorbed by the system are obtained by using a phenomenological theory. Subsequently the phenomenological parameters are related with hydrodynamics. Finally the optimum absorption by two heaving point oscillators is considered. It is demonstrated that the optimum velocity for maximum power absorption is not in all cases in phase with the excitation force. 相似文献
3.
对海洋中起伏运动(heaving)信号的时空分布研究能够帮助我们更好地了解气候系统中的年际和年代际变率。文章通过再分析资料和模式对太平洋区域的heaving主要模态进行了研究。研究结果表明: 太平洋区域主要存在两种heaving模态: 第一模态主要表现为赤道东西两侧的温跃层异常信号反位相; 第二模态表现为赤道区域和副热带区域的温跃层异常信号呈现反位相变化的规律。本文对这两个主要heaving模态所涉及的物理过程进行详细讨论, 结果表明: 东西反位相模态主要是受赤道波动调节的结果; 而经向结构模态则主要是由赤道地区的波动和副热带区域的风应力旋度异常作用共同导致。此外, 我们还讨论了heaving模态可以通过海洋波动以及Ekman输送等过程对海盆尺度的热输送(振幅约为5×1014W)以及海洋热含量(振幅约为1.5×1020J)的再分配起到了关键的调制作用, 进一步表明heaving模态对全球气候变化有着重要的作用。 相似文献
4.
An experiment on the correction of underwater images distorted by waves at the air-water interface was conducted using a laboratory modeling installation intended for experimental examination of light and image transfer across a water surface covered with waves. A digital color camera was used for the simultaneous formation of the image of the underwater test object through the disturbed surface and of the superimposed glitter pattern. Both images are spectrally separated. Processing the glitter pattern makes it possible to obtain the values of the surface slopes at a limited number of points and to use these slopes for retrieval of image fragments. The total corrected image is formed by integration of about 300 partially corrected fragments. This image is close to that obtained through a wave-free water surface. 相似文献
5.
The existence of the heaving plates can improve the heaving motion performance of an offshore structure significantly by providing both extra added mass and damping. In the current research, numerical investigation is carried out on the hydrodynamic characteristics of both isolated square heaving plate and double square heaving plates with opening by an immersed boundary-lattice Boltzmann method. The effects on hydrodynamic performance of plates due to Keulegan-Carpenter(KC) number, frequency number, opening ratio, opening distribution and spacing of plates are examined. It is found that the heaving plates with optimized opening ratio can provide additional damping compared with the plates without opening. Better hydrodynamic characteristics of double plates can be obtained with the increase of plate spacing. 相似文献
6.
The present paper reports on a study of the interaction of a current-free monochromatic surface wave field with a wave-free uniform current field in a three-dimensional flow frame. The wave and the current fields are not necessarily collinear with each other. The formulation of the wave-current field is done under the assumption of irrotational and inviscid flow. We have developed the three dimensional expressions describing the characteristics of the combined flow in terms of mass, momentum and energy transport conservation. These equations are found efficient to describe the sought-for combined wave-current field. The parameters describing the wave-current field after the interaction are the surface disturbance amplitude and length, mean water depth, mean current-like parameter and direction of the combined flow, which would be calculated from a set of equations that satisfy conservation of mean mass, momentum and energy flux and a dispersion relation on the free surface before and after the interaction. The results are shown in terms of relative changes in wave heights and lengths, current-like parameters and final directions obtained for the combined wave-current field with respect to current-free wave and wave-free current pre-interaction parameters. 相似文献
7.
The interaction between current-free higher-order water waves with a wave-free uniform current normal to the wave crests is considered. The combined wave-current motion resulting from the interaction is assumed stable and irrotational. The velocity potential, dispersion relation, the particle kinematics and pressure distribution up to the third order in wave amplitude are developed. The conservation of mean mass, momentum and energy, together with the dispersion relation on the free surface are used to derive a set of four nonlinear equations, through which the relationship between wave-free current, current-free wave and the combined wave-current parameters is established. Numerical results for a range of current values are also presented. 相似文献
8.
The coupled finite element and boundary element analysis of nonlinear interactions between waves and bodies 总被引：1，自引：0，他引：1
A coupled finite element (FEM) and boundary element (BEM) method is developed to analyse the nonlinear interaction between bodies and water waves. The former is used away from the body while the latter is used in a region near body. The combination is based on consideration of the efficiency of FEM and BEM in computation and mesh generation, respectively. Results for wave/body interactions are obtained by using auxiliary functions to decouple the mutual dependence of the body acceleration and the wave force. 相似文献
9.
带有弹簧支撑的水平板结构,可在波浪作用下起伏运动进行消浪,作为防波堤具有广泛的应用前景,但如何设计支撑刚度仍是当前研究的重点课题。基于黏性流理论建立数值波浪水槽,开展规则波与起伏板防波堤相互作用的模拟,得到起伏板透反射系数随支撑刚度的变化曲线,并分析最小透射系数对应刚度下起伏板运动相位对消浪的影响。为快速得到最小透射系数对应的支撑刚度,根据弹簧—阻尼—质量模型,建立一种以运动相位为输入反推起伏板最优支撑刚度的计算方法,并将推导结果与数值结果进行比较。结果表明,该方法推导的最优支撑刚度值与数值最优值接近,且可省去遍历取最优值的步骤,可为起伏板防波堤的支撑刚度设计提供参考。 相似文献
10.
M. Vantorre 《Ocean Engineering》1986,13(4)
An investigation of the hydrodynamic behaviour of bodies in a large-amplitude oscillatory motion, for example in view of wave power absorption by floating bodies, not only requires the knowledge of added mass and damping coefficients, which can be calculated by means of a linear theory, but also of higher-order forces. Especially the third-order values will have to be calculated, because they contain a first-harmonic component.A computation procedure has been developed in order to calculate hydrodynamic forces up to the third order, acting on axisymmetric bodies in an oscillatory heaving motion. The method only requires the knowledge of first- and second-order potential functions, even for the calculation of third-order forces.Experiments have been carried out on floating conical and submerged cylindrical models, in order to evaluate the theoretical procedure. 相似文献
11.
This paper outlines a procedure for the derivation of the differential equations describing the free response of a heaving and pitching ship from its stationary response to random waves. The coupled heave–pitch motion of a ship in random seas is modelled as a multi-dimensional Markov process. The partial differential equation describing the transition probability density function, known as the Fokker-Planck equation, for this process is derived. The Fokker-Planck equation is used to derive the random decrement equations for the coupled heave–pitch motion. The parameters in these equations are then identified using a neural network approach. The method is validated using numerical simulations and experimental results. The experimental data was obtained using an icebreaker ship model heaving and pitching in random waves. It is shown that the method produces good results when the system is lightly damped. An extension for using this method to identify couple heave–pitch motion in realistic seas is suggested. 相似文献
12.
《Ocean Engineering》1999,26(5):381-400
This paper outlines a procedure for the derivation of the differential equations describing the free response of a heaving and pitching ship from its stationary response to random waves. The coupled heave–pitch motion of a ship in random seas is modelled as a multi-dimensional Markov process. The partial differential equation describing the transition probability density function, known as the Fokker-Planck equation, for this process is derived. The Fokker-Planck equation is used to derive the random decrement equations for the coupled heave–pitch motion. The parameters in these equations are then identified using a neural network approach. The method is validated using numerical simulations and experimental results. The experimental data was obtained using an icebreaker ship model heaving and pitching in random waves. It is shown that the method produces good results when the system is lightly damped. An extension for using this method to identify couple heave–pitch motion in realistic seas is suggested. 相似文献
13.
A tether management system of a winch module in a marine environment is proposed. For the purpose of this study the subsea body is considered as a mass attached at the free end of a long tether which is wrapped around a circular drum controlled by an external torque. The winch is considered mounted on a heaving platform. The effect of the heaving platform on the motions of the drum, tether and attached mass are studied with respect to the longitudinal vibrations of the tether in one dimension. The hydrodynamic effects are considered on the deployed portion of the tether and the tethered mass, both of which are assumed submerged in otherwise still water. The resulting nonlinear system of equations of motion is developed and solved numerically for an example of a marine tether system. The effect of the tether extensibility on the operation threshold of the system is identified. Comparison with an inextensible tether case is provided. The numerical results and their analysis for the retrieval/deployment of the system are presented. 相似文献
14.
Nonlinear hydrodynamics of a twin rectangular hull under heave oscillation is analyzed using numerical methods. Two-dimensional nonlinear time-domain solutions to both inviscid and viscous problems are obtained and the results are compared with linear, inviscid frequency-domain results obtained in [26] to quantify nonlinear and viscous effects. Finite-difference methods based on boundary-fitted coordinates are used for solving the governing equations in the time domain [2]. A primitive-variables based projection method [6] is used for the viscous analysis and a mixed Eulerian–Lagrangian formulation [11] for inviscid analysis. The algorithms are validated and the order of accuracy determined by comparing the results obtained from the present algorithm with the experimental results of Vugt [22] for a heaving rectangle in the free surface. The present study on the twin-hull hydrodynamics shows that at large and non-resonant regular frequencies, and small amplitude of body oscillation, the fluid viscosity does not significantly affect the wave motion and the radiation forces. At low frequencies however the viscosity effect is found to be significant even for small amplitude of body oscillation. In particular, the hydrodynamic force obtained from the nonlinear viscous analysis is found to be closer to the linear inviscid force than the nonlinear inviscid force to the linear inviscid force, the reason for which is attributed to the wave dampening effect of viscosity. Since the wave lengths generated at smaller frequencies of oscillation are longer and therefore the waves could have a more significant effect on the dynamic pressure on the bottom of the hulls which contribute to the heave force, the correlation between the heave force and the wave elevation is found to be larger at smaller frequencies. Because of nonlinearity, the wave radiation and wave damping force remained nonzero even at and around the resonant frequencies – with the resonant frequencies as determined in [26] using linear potential flow theory. As to be expected, the nonlinear effect on the wave force is found to be significant at all frequencies for large amplitude of oscillation compared to the hull draft. The effect of viscosity on the force, by flow separation, is also found to be significant for large amplitude of body oscillation. 相似文献
15.
16.
Shape optimization of two-dimensional cavitators in supercavitating flows, using NSGA II algorithm 总被引：1，自引：0，他引：1
The reduction of energy consumption of high speed submersible bodies is an important challenge in hydrodynamic researches. In this paper, shape optimization of two-dimensional cavitators in supercavitating flows is studied. A two dimensional supercavitation potential flow passes a symmetric two dimensional cavitator, which is placed perpendicular to the flow in a channel of infinite length and immediately a cavity is formed behind the cavitator. This is because of the generation of a gas or vapor cavity between the body and the surrounding liquid due to the change in a high speed flow direction passing the cavitator. Drag force acting on this supercavitating body dictates the thrust requirements for the propulsion system, to maintain a required cavity at the operating speed. Therefore, any reduction in the drag force, by modifying the shape of the cavitator, will lead to decrease this force. This study concentrates on the optimization of two dimensional cavitators in order to decrease drag coefficient for a specified after body length and velocity in a potential flow. To achieve this goal a multi-objective optimization problem is defined to optimize cavitator shapes in supercavitating flow. The so-called NSGA II (Non-dominated Sorting Genetic Algorithm) algorithm is used as an optimization method. Design parameters and constraints are obtained according to supercavitating flow characteristics and cavitator modeling and objective functions are generated using Linear Regression Method. The obtained results are compared with other classic optimization methods, like the weighted sum method, for validation. 相似文献
17.
《Ocean Engineering》2004,31(8-9):1063-1082
An analytical method is presented to analyze the radiation and diffraction of water waves by a rectangular buoy in an infinite fluid domain of finite water depth. Analytical expressions for the radiated potentials and the diffracted potentials are obtained by use of the method of separation of variables. The unknown coefficients in the expressions are determined by use of the eigenfunction expansion matching method. The added masses and damping coefficients for the buoy heaving, swaying and rolling in calm water are obtained by use of the corresponding radiated potentials. Wave excitation forces are calculated by two different approaches, one is by use of the radiated potentials through Haskind’s theorem and the other is by the diffracted potential. It can be seen that the latter approach for wave forces on a rectangular buoy is much simpler than the former. To verify the correctness of the method, two specific examples in the past references are recomputed and the obtained results are in good agreement with those by use of other methods, which shows that the present method is correct. 相似文献
18.
Fenfang Zhao Takeshi KinoshitaWeiguang Bao Rong WanZhenlin Liang Liuyi Huang 《Applied Ocean Research》2011,33(3):169-177
In the present study, hydrodynamic interactions between water waves and porous bodies are investigated. Various hydrodynamic identities, such as the Haskind relation, Bessho-Newman relation etc., are systematically re-examined. Some of these identities, such as the symmetry of added mass and damping and the Haskind-Hanaoka relation, are still valid for porous bodies even without modification to the identities. However, when energy dissipation due to porosity is involved, appropriate supplementation is required to properly consider porous effects. In addition, the calculation of wave drift forces acting on a porous body is formulated either by pressure integration or using momentum conservation as basis. We conclude that porosity dissipation makes a more substantial contribution to wave drift forces than does conventional dissipation created by scattered and radiated waves. 相似文献
19.
Nonlinear interactions between waves and floating bodies are investigated using the weakly compressible Smoothed Particle Hydrodynamic (WCSPH) method. An improved algorithm based on the dynamic boundary particles (DBPs) is proposed to treat the moving boundary of the floating body. The force exerted on the floating body boundary particle by the particles surrounding it is evaluated using the volume integration of the stress tensors obtained from the momentum equation in its compact support. The improved WCSPH model is validated by the experimental results. The numerical test cases of the vertical oscillation of a rectangular box, the damped rolling oscillation of a floating box and the wave forces on a fixed rectangular box are then carried out to demonstrate the performance of the proposed model. Finally the evolution in time of the dynamic response of the freely floating body under nonlinear waves are discussed and compared with experimental results. 相似文献
20.
本文用二阶理论在时域范围内计算二维二阶非线性水波,一、二阶问题分别满足各自的自由表面条件和物面条件,流场内的速度势通过求解有限元方程得到,计算采用八结点四边形等参单元,采用人工阻尼来吸收反射波,对楔形体在水面上的振荡问题进行了计算,计算结果与有关文献相比符合较好。 相似文献