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1.
Diffraction of a directionally spread wave group by a cylinder   总被引：2，自引：0，他引：2  
The problem of diffraction of a directionally spread focused wave group by a bottom-seated circular cylinder is considered from the viewpoint of second-order perturbation theory. After applying the time Fourier transform and separation of vertical variable the resulting two-dimensional non-homogeneous Helmholtz equations are solved numerically using finite differences. The detailed formulation of the second-order radiation condition is presented. Numerical solutions of the problem are obtained for JONSWAP amplitude spectra for the incoming wave group with various types of directional spreading. The results are compared with the corresponding results for a unidirectional wave group of the same amplitude spectrum. Finally we discuss the applicability of the averaged spreading angle concept for practical applications.  相似文献   

2.
The reconstruction process of two-dimensional and three-dimensional sound-soft obstacles from a set of scattered field measurements is formulated as a linear filtering problem. Under coherent and incoherent insonification, the impulse response of the equivalent filter completely characterizes the acoustical imaging process. An energy-based criterion is proposed to describe the impulse response features in both cases. Numerical simulations are also presented, showing the better performance of the incoherent insonification  相似文献   

3.
In this study, a new analytical solution for the wave-induced seabed response in a multi-layered poro-elastic seabed is developed. The seabed is treated as a multi-layered porous medium and characterized by Biot’s theory. The displacements of the solid skeleton and the pore pressure are expressed in terms of two scalar potentials and one vector. Then, the Biot’s dynamic equation can be solved using Fourier transformation and reducing to Helmholtz equations. To obtain the general solutions for the multi-layered poro-elastic seabed in the frequency-wave-number domain, the transmission and reflection matrices (TRM) method is used to form the equivalent stiffness. Using the boundary conditions and continuous conditions, the frequency-wave-number domain solutions are obtained. Finally, the time-space domain solutions for the multi-layered poro-elastic seabed are obtained by means of the inverse Fourier transformation with respect to the horizontal coordinate. Based on the new solution, a parametric study is carried out to examine the effects of soil characteristics (number of layers, permeability and shear modulus) and wave characteristics (water depth and wave steepness) on seabed responses. The results indicate that the seabed response is affected significantly by permeability, shear modulus and relative water depth.  相似文献   

4.
A standard inverse problem in underwater acoustics is the reconstruction of the ocean subbottom structure (e.g., the density and sound speed profiles) from an aperture- and bandlimited knowledge of the reflection coefficient. In this paper we describe an inverse solution method due to Candel et al. [12] which is based on the scattering of acoustic plane waves by a one-dimensional inhomogeneous medium. As a consequence of applying the forward scattering approximation to a local wave representation of the acoustic field, they obtain an expression for the reflection coefficient in the form of a nonlinear Fourier transform of the logarithmic derivative of the local admittance. Inversion of this integral transform enables the recovery of the admittance profile via the numerical integration of two first-order differential equations which require as reflection data a single impulse response of the medium. Separate recovery of both the density and sound speed profiles requires two impulse responses for two different grazing angles. In this case, four differential equations need to be integrated instead of two. To illustrate the capability of the method, we present numerical reconstructions which are based on synthetic reflection data for a geoacoustic model that represents the acoustic properties of the surficial sediments for a site in the Hatteras Abyssal Plain.  相似文献   

5.
In the paper two types of numerical models – a lumped-parameter model and a high-resolution two-dimensional hydrodynamic model – are used to analyse the response of a system of partially-connected tidal basins to inhomogeneous open sea forcing. The equations of the lumped-parameter model, suitable for an arbitrary number of basins with sloping walls, are formulated based on one-dimensional continuity and momentum equations. Numerical solutions to the equations are thoroughly examined, showing the influence of inhomogeneous open sea forcing and of geometrical parameters of the basins on the tidal range and the water transport through the system, with particular emphasis given to inter-basin water exchange and cumulative water transport through basins boundaries. The results of the lumped-parameter model simulations for the tidal basins of the German Wadden Sea are successfully compared with the results of calculations with the two-dimensional hydrodynamic model, which is used to investigate in more detail circulation patterns and the influence of specific local features of inlet bathymetry on the hydrodynamic processes in the study area. The influence of wind on the basins response is discussed as well.  相似文献   

6.
A time domain finite element method （FEM） for the analysis of transient elastic response of a very large floating structure （VLFS） subjected to arbitrary time-dependent external loads is presented. This method is developed directly in time domain and the hydrodynamic problem is formulated based on linear, inviscid and slightly compressible fluid theory and the structural response is analyzed on the thin plate assumption. The time domain finite element procedure herein is validated by comparing numerical results with available experimental data. Finally, the transient elastic response of a pontoon-type VLFS under the landing of an airplane is computed by the proposed time domain FEM. The time histories of the applied force and the position and velocity of an airplane during landing are modeled with data from a Boeing 747-400 jumbo jet.  相似文献   

7.
In Part I of these series of papers, the complete problem formulation in a linearized form was presented. In order to provide the engineer with an in-depth knowledge about the exact solution of the problem, it is natural and essential to start with a linear solution. This will be the objective of Part II, together with an exposition to the analysis of numerical technique utilized.A truncated infinite Fourier series-type solution is adopted for the linearized boundary value problem. It is shown that such a solution is mathematically consistent and represents the phenomenon properly by satisfying all of the field equations and the imposed boundary conditions. The dependence of the Fourier coefficients on the truncation limit has been investigated. The best lower and upper “cutoff limits” for the truncation of an infinite series are determined. An error analysis of the solution technique is performed.  相似文献   

8.
A complete analytical solution is presented for the linear diffraction of oblique waves by horizontal rectangular cylinders either fixed at the free surface or mounted on the sea bed in a finite-depth of water. Helmholtz equation is employed as the governing differential equation obtained by reducing the 3-D oblique wave scattering problem to a 2-D case. According to the method proposed, the fluid region is divided into three sub-regions in which the governing differential equation is solved by the separation of variables. The solutions for each region are then matched on the common boundaries of sub-regions to determine the unknowns of the eigen series expansions and Fourier series. Thus transmitted and reflected waves are obtained in the far-field, and forces and moments acting on the rectangular cylinder fixed at the free surface are also given. Comparisons are made in order to check the accuracy of the method.  相似文献   

9.
As an important part of lifeline engineering in the development and utilization of marine resources, the submarine fluid-filled pipeline is a complex coupling system which is subjected to both internal and external flow fields. By utilizing Kennard’s shell equations and combining with Helmholtz equations of flow field, the coupling equations of submarine fluid-filled pipeline for n=0 axisymmetrical wave motion are set up. Analytical expressions of wave speed are obtained for both s=1 and s=2 waves, which correspond to a fluid-dominated wave and an axial shell wave, respectively. The numerical results for wave speed and wave attenuation are obtained and discussed subsequently. It shows that the frequency depends on phase velocity, and the attenuation of this mode depends strongly on material parameters of the pipe and the internal and the external fluid fields. The characteristics of PVC pipe are studied for a comparison. The effects of shell thickness/radius ratio and density of the contained fluid on the model are also discussed. The study provides a theoretical basis and helps to accurately predict the situation of submarine pipelines, which also has practical application prospect in the field of pipeline leakage detection.  相似文献   

10.
赵明  滕斌 《中国海洋工程》2004,18(2):267-280
The incompressible viscous uniform and shear flow past a circular cylinder is studied. The two-dimensional Navier-Stokes equations are solved by a finite element method. The governing equations are discretized by a weighted residual method in space. The stable three-step scheme is applied to the momentum equations in the time integration. The numerical model is firstly applied to the computation of the lid-driven cavity flow for its validation. The computed results agree well with the measured data and other numerical results. Then, it is used to simulate the viscous uniform and shear flow over a circular cylinder for Reynolds numbers from lO0 to lO00. The transient time interval before the vortex shedding occurs is shortened considerably by introduction of artificial perturbation. The computed Strouhal number, drag and lift coefficients agree well with the experimental data. The computation shows that the finite element model can be successfully applied to the viscous flow problem.  相似文献   

11.
A differential model of the upper turbulent layer in the ocean is considered. A closed system of equations includes equations of motion, balance, and dissipation of kinetic turbulence energy. Boundary conditions at the surface are determined using a solution of the atmospheric problem taking into account the interaction between the two media. The formulated algorithm allows for a relationship between turbulent energy dissipation and flux and the parameters of wind disturbance. The vertical profiles of turbulence and drift current characteristics are presented as well as parameters of the ocean-atmosphere interaction for various values of impulse jump within the limits of the wave layer with waves collapsing and not collapsing.UDK 551.456.152  相似文献   

12.
The three-dimensional coupled behavior during the interaction of buoys with their mooring systems is numerically analyzed. A time-domain model was developed to predict the response of a tethered buoy subject to hydrodynamic loadings. External loadings include hydrodynamic forces, tethers tensions, wind loadings and weight. System nonlinearities include large rotational and translational motions, and non-conservative fluid loadings. The mooring problem is formulated as a combined nonlinear initial-value and two-point-boundary-value problem which is directly integrated both in time and space. Buoy equations of motion are derived using small Eulerian angles. Coupling between rotational and translational degrees of freedom is included and coupling between the buoy and cable is effected by adopting the buoy equations of motion as boundary conditions at one end for the mooring problem. Numerical examples are provided to validate the formulation and solution technique; predicted responses of three types of buoy (sphere, spar, and disc) are compared with experimental results.  相似文献   

13.
This paper is concerned with the very large, sudden pressures produced by water-wave impact on a vertical wall. Numerical and theoretical models are briefly described and we show that there are significant pressure gradients along an impermeable sea bed, away from the wall. This aspect of wave impact pressure has not previously been noted. The effects of such transient pressure gradients on bodies lying on the bed are examined in the light of pressure impulse theory. Our theoretical results indicate that bodies may be displaced by these pressure gradients when ordinary fluid drag is insufficient to move them. We discuss the impulse and the speed imparted to a rigid body lying within a region of pressure impulse which has constant horizontal gradient, and we show that the impulse is related to the body's volume, and shape. Analytic results are given for a hemi-ellipsoidal “boulder”, which is free to translate in a direction perpendicular to the wall. The analysis for the hemi-ellipsoid encompasses the special cases of a semi-circular cylinder and a semi-circular disc. The case of a circular cylinder which touches the bed is also discussed, and it is shown that there is a significant vertical impulse on the body in addition to the horizontal component of the impulse.  相似文献   

14.
We present a numerical model of Black Sea circulation based on primitive equations with improved spatial resolution in the coastal zone. The model equations are formulated in a two-pole orthogonal coordinate system with arbitrary locations of the poles and a vertical σ coordinate. Increased horizontal resolution is gained by displacing the pole into the vicinity of the separated subdomain. The problem is solved over a grid with a variable step. The northern coordinate pole is displaced to the vicinity of Gelendzhik; the grid step varies from 150 m in the coastal zone to 4.6 km in the main basin. We simulated the fields of currents, sea level, temperature, and salinity under the given atmospheric forcing in 2007. The model is capable of reproducing the large-scale Black Sea circulation and submesoscale variations in the coastal currents.  相似文献   

15.
The problem of numerical modeling and analysis of the large-scale World Ocean circulation variability under variations of the external forcing is considered. A numerical model was developed in the INM RAS and is based on the primitive equations of the ocean circulation written in a spherical generalized σ-coordinate system. The model’s equations are approximated on a grid with resolution of 2.5° × 2° × 33, and the North Pole is displaced to the continental point (60°E, 60.5°N). There are two stages for the numerical experiments. The quasi-equilibrium circulation of the World Ocean under the climatological atmospheric forcing is simulated at the first stage. The run is carried out over a period of 3000 years during which a quasi-equilibrium model regime is formed. At the second stage, the sensitivity of the model ocean circulation to the atmospheric forcing perturbations in the Southern Hemisphere is studied. According to the results, the strongest regional changes in the hydrography take place in the Arctic Ocean. Substantial changes of sea’s surface height and local anomalies of the temperature and salinity are formed there.  相似文献   

16.
现行的场预报方法,一般是根据预报量之间的统计关系,进行场分解,而后用经验正交函数建立预报方程,但却很少考虑到外界因子的作用。本文尝试把影响因于引入经验正交预报方法中。在这种优选因子场预报方法中,作者不仅考虑预报量之间内部的相互关系,而且也考虑了外部因子的影响,借助于从综合因子中优选因子,可以建立对于预报数量和等级都适用的最优预报方程。提出了权重相关系数的概念。与其他相关系数之间的明显差别是,它具有场的特征。本文还介绍了水湿的盐度预报的一些实例。  相似文献   

17.
The dynamic response of beam–pile–soil system under vertical transient excitation is investigated. Both piles and beam are assumed to be one-dimensional rods and subjected to vertical exciting forces. The uniformly distributed Voigt models are introduced to simulate the pile tip resistances, and the dynamic interactions between piles and beam are simplified as a set of concentrated point loads. Then, the plane strain model, the theory of longitudinal vibration of one-dimensional rod, and the Timoshenko beam theory are used to establish the mathematical models for the motion of soil, piles, and beam, respectively. On this basis, the matrix equation for solving the governing equations is constructed in the Laplace domain and the time-domain response is then obtained by the discrete inverse Fourier transform. Comparisons with numerical simulations and model tests are conducted to evaluate the rationality of the present solution. The results show that the dynamic responses calculated by the proposed solution are generally consistent with simulated curves and experimental data.  相似文献   

18.
Sinee the predietion equations for EOF(Entpirical Orthogonal Fune-494青岛海洋大学学报1 9 89年tion)methods《belonging to 6eld foreeasting)are generally eonstrueted onthe basis of statistieal relations among predietands without eongidering theinnuenee of external faetors,while the regression analysis methods(晚long-ing to single·station foreeasting)eonsidering sueh in口uenee faeto“eannot show the internal relations among the predietand.〔l一3几The Pr暇ntPaper attemPts to introduee the innue…  相似文献   

19.
A two-dimensional scour model based on coupled system of shallow water equations (SWEs) and sediment transport on unstructured mesh is developed. The coupled system of hydrodynamic and morphodynamic equations is solved by finite volume method using Godunov scheme. Roe's approximate Riemann solver is used to calculate the inviscid fluxes. The use of unstructured mesh makes the model applicable to complex domains. However, it is difficult to evaluate the eigenvalues and eigenvectors of the Jacobian matrix in the global coordinate. The method proposed herein to deal with this difficulty is to transform the system into the local coordinate with one of the axes in the same direction as the interface outward normal vector. In the local coordinate system, the Jacobian matrix is simplified and the eigenvalues are analyzed using asymptotic method. Regular expansion breaks down when the flow is near critical. Uniformity of the expansion is achieved by changing the scales. Rotational invariance theorem is used to relate the interfacial fluxes in the global and local coordinate systems. Special treatment of the source term on unstructured grid makes the scheme stable and physically balanced (both mass and momentum). The method proposed in this paper for the eigen-system is very efficient comparing to iterative numerical methods. Results from the test cases show good agreement with the experiments.  相似文献   

20.
The 3-D flow field and bed stress surrounding a short cylinder in response to combined wave and mean-flow forcing events is examined. Model simulations are performed with a 3-D nonhydrostatic computational fluid dynamics model, FLOW-3D. The model is forced with a range of characteristic tidal and wave velocities as observed in 12-15 m of water at the Martha's Vineyard Coastal Observatory (MVCO, Edgartown, MA). The 2.4-m-long and 0.5-m diameter cylinder is buried 10% of the diameter on a flat, fixed bed. Regions of incipient motion are identified through local estimates of the Shields parameter exceeding the critical value. Potential areas of sediment deposition are identified with local estimates of the Rouse parameter exceeding ten. The model predictions of sediment response are in general in agreement with field observations of seabed morphology obtained over a one-week period during the 2003-2004 MVCO mine burial experiment. Both observations and simulations show potential transport occurring at the ends of the mine in wave-dominated events. Mean flows greater than 10 cm/s lead to the formation of larger scour pits upstream of the cylinder. Deposition in both cases tends to occur along the sides, near the center of mass of the mine. However, the fixed-bed assumption prohibits the prediction of full perimeter scour as is observed in nature. Predicted scour and burial regimes for a range of wave and mean-flow combinations are established.  相似文献   

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