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2.
Chen Shaohua Sun Mingguang This study is financially supported by National Science Foundation of China Postgraduate Zhongshan University Guangzhou Professor Zhongshan University Guangzhou 《中国海洋工程》1993,(4)
- In this paper, a six-order nonlinear dynamic model with three degrees of freedom is presented for the study of the "fishtail" motion of a Single Point Mooring System. The effect of parameter variations on the equilibrium state of the system is analized. In order to study the stability of the equilibrium state, the mooring-line length / is chosen as a bifurcation parameter, so that all eigenvalues of the Jacobian matrix under different parameters can be worked out, and then the Hopf-bifurcation point can be found . Finally, the Hopf-bifurcation periodic solution of the system is computed. 相似文献
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O.A. Montasir 《Ocean Engineering》2011,38(13):1417-1429
Spar platform has been regarded as a competitive floating structure for deep and ultra deep water oil and gas production. In this paper, an efficient methodology has been developed to determine the slow motion responses of slender floating offshore structures due to wave forces. Based on this methodology, a MATLAB program named ‘TRSPAR’ was developed to predict the dynamic responses in time domain and it was used in this study to obtain the numerical results of a typical truss spar platform connected to sea bed using nine taut mooring lines. The difference frequency forces were calculated using the principles of the extension of Morison equation for an inclined cylinder and the wave kinematics were predicted using hyperbolic extrapolation. Mooring lines were modelled as nonlinear springs and their stiffness was obtained by conducting the static offset simulation. Because of the lack of detailed calculations in literature, most of the equations used were derived and presented in this paper. The effects of the different sources of the second order difference frequency forces were compared for inertia and drag forces in terms of response spectra. To validate the TRSPAR code, its results were compared to results of a typical truss spar model test. 相似文献
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Open boundary conditions for nonlinear channel flow 总被引：1，自引：0，他引：1
Open boundary conditions are derived for the one-dimensional nonrotating two-layer shallow-water equations. The conditions are based on characteristics of the external and internal modes. It is possible to find exact nonlinear characteristic conditions for the external mode, as well as approximate nonlinear conditions for the internal mode. These conditions can also be linearised by Taylor expansion; the approximate linear conditions are similar to those used in several previous studies. Both of the nonlinear and linearised conditions perform well, indicating that either the nonlinear or linearised conditions may potentially be extended to the more general case of multi-layer flows. 相似文献
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A theory and numerical tool are developed for the coupled-dynamic analysis of a deepwater floating platform with polyester mooring lines. The formulas allow relatively large elongation and nonlinear stress–strain relationships, as typically observed in polyester fibers. The mooring-line dynamics are based on a rod theory and the finite element method (FEM), with the governing equations described in a generalized coordinate system. The original rod theory [Nordgren, R.P., 1974. On Computation of the Motion of Elastic Rods. Journal of Applied Mechanics, 41, 777–780] is generalized to allow larger elongation and nonlinear stress–strain relationship. The dynamic modulus of polyester is modeled following an empirical regression formula suggested by [Bosman, R.L.M., Hooker, J., 1999. The Elastic Modulus Characteristic of Polyester Mooring Ropes. In: Proceedings of the Offshore Technology Conference, OTC 10779. Houston, Texas], in which the axial stiffness is not constant, but depends on loading conditions. Two case studies, the static and dynamic behavior of a tensioned buoy and a classic spar with polyester mooring lines, are conducted. The time-domain simulation results are systematically compared with those from the original rod theory. The effects of large elongation and nonlinear stress–strain relations are separately assessed. It is seen that the mean offset, motions, and tension with polyester lines can be different from those by original rod theory with linear elastic lines. 相似文献
7.
对称式布置锚链系统的线性化处理 总被引：6，自引：2，他引：6
用悬链线方程求解了不同锚链状态的锚链力 ,并用多项式回归了锚链力与锚固点位移的关系 ,由此求得锚链恢复力刚度系数。分析了对称式布置锚链系统的非线性程度 ,阐述对称式布置锚链系统所提供的恢复力作线性化处理的可行性及方法 ,并给出了对称布置锚链系统的线性刚度矩阵。为各类受对称式布置锚链系统约束的浮动结构物的动力分析提供了理论依据 相似文献
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S. V. Shagalov V. P. Reutov G. V. Rybushkina 《Izvestiya Atmospheric and Oceanic Physics》2009,45(5):629-645
The mechanisms of the large-scale vortex structures formation in zonal jet flows (atmospheric blockings, cyclonic, and anticyclonic
vortices) is investigated. Nonlinear perturbations formed during the onset of barotropic instability of a long-wave mode in
weakly-dissipative and weakly supercritical jet flows with a symmetric velocity profile are considered in the β-plane approximation.
This analysis is performed within the framework of the asymptotic theory based on the concept of a nonlinear critical layer.
The equations describing the interaction of a wave with vorticity perturbations in a critical layer are derived. The regimes
of a quasi-stationary and nonstationary nonlinear critical layer are considered separately. Combined equations of evolution
covering the principle regimes of instability development are proposed. The existence of autowave-type structures characterized
by a balance between the energy receipt to the wave and its dissipation are obtained within the framework of a numerical simulation.
The dependence of the parameters of generated autowave structures on the shape of the zonal jet profile and the flow supercriticality
level is studied. 相似文献
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综合考虑多种变形因素的近岸规则波传播数学模型Ⅰ. 基本方程的导出 总被引：1，自引：0，他引：1
推广了Kirby的有环境水流影响的缓坡方程,得到了综合考虑环境水流(水流因子)、非线性弥散影响(非线性因子)、底摩擦波能损失(底摩擦因子)、非缓坡地形影响(地形因子)、折射、绕射、波浪破碎多种变形因素的波浪传播控制方程,并给出了非线性因子、地形因子、底摩擦因子、水流因子的确定方法。基于导出的方程做进一步推导,得到了波高和波向为变量的综合考虑多种变形因素的波浪传播基本方程,该方程有许多优点:1)其绕开了求解波势函数的困难,将椭圆型方程的边值问题化为初值问题;2)直接求解波高和波向;3)可采用有限差分法离散求解,对空间步长没有限制,适合大面积海区波场计算;4)综合考虑了多种波浪变形因素,方程更为合理,5)容易处理波浪破碎问题。 相似文献
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This study experimentally investigates the nonlinear response stability and transition behaviour of a submerged, moored ocean structural system which consists of a spherical buoy and attached multi-point mooring lines. The system is excited by a periodic wave field in a closed channel. System nonlinearities include complex geometric restoring (stiffness) force and coupled fluid-system interaction exciting forces. Experimental set-up, operating procedures and analysis of the measured results are presented. Characteristic motions observed include harmonic, subharmonic and ultraharmonic responses, which demonstrate a signature of the intricate pattern of the nonlinear global behaviour. Good agreements between the measured and most predicted responses are demonstrated in both time and frequency domains. These results confirm the validity of the analytical model presented and calibrate the accuracy of the existing numerical predictions. Primary and secondary resonances in the response are identified via frequency response curves. Response bifurcation cascades are observed in the experimental results and the possible existence of higher-order nonlinear responses is inferred. 相似文献
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主动式声纳列阵拖曳系统姿态数值计算 总被引：2，自引：1，他引：2
主动式声纳列阵拖曳系统是用于探测潜艇的新型声纳系统，为了准确探测潜艇的位置，必须首先预报声纳列阵的瓷态，本文通过对其三维力学模型的分析，得到该系统的运动微分方程，其中缆索的力学方程是基于Ablow和Milinazzo的模型，而对于拖体则运用六自由度空间运动方程模拟，结合边界条件，用有限差分法求解，通过对拖船的不同运动状态如匀速，变速和回转的计算，证明本文的方法对于预报声纳列阵的姿态是有效的。 相似文献
14.
Nonlinear Dynamics and Bifurcations of a Supercavitating Vehicle 总被引：1，自引：0，他引：1
In this effort, a numerical study of the bifurcation behavior of a supercavitating vehicle is conducted. The vehicle model is nonsmooth; this is a result of the planing force acting on the vehicle. With a focus on dive-plane dynamics, bifurcations with respect to a quasi-static variation of the cavitation number are studied. The system is found to exhibit rich and complex nonlinear dynamics including nonsmooth bifurcations such as the grazing bifurcation; smooth bifurcations such as Hopf bifurcations, cyclic fold bifurcations, and period-doubling bifurcations; and aperiodic behaviors such as transient chaotic motions and chaotic crises. The tailslap phenomenon of the supercavitating vehicle is identified as the consequence of a Hopf bifurcation followed by a grazing event. It is shown that the occurrences of these bifurcations can be delayed or triggered earlier by using dynamic linear feedback control laws employing washout filters. 相似文献
15.
Hideo Kawai 《Journal of Oceanography》2003,59(3):303-315
Distinctive features of the repulsive bifurcation of Kuroshio and Gulf Stream jets, induced from various observations, have
been examined by equations of motion of a 1 1/2 -layer model in natural coordinates, although causes of the bifurcation are
discussed little. From the conservation of volume flux through paths above a main thermocline, we have derived the equation
M
n
K
In + M
s
K
Is = 0 for northern (n) and southern (s), steady-state curved jets, just downstream of the bifurcation from a straight jet, where M is momentum flux through the path and K
I, positive for cyclonic deflection of the streamline, is the curvature of a certain intermediate streamline in the laminar
path. Assuming concentric streamlines in the curved path, which is narrower than 1–0.5 times the radius of curvature of the
center streamline of the path, and symmetric momentum flux through a path element of unit width with respect to the center
streamline, we found that K
I approximates to the curvature averaged across the path, relative errors being <0.25−(0.25)2. Bifurcation starts with a ‘Big Bang’ and the leading turbulent front of a split jet may be unsteady, but the succeeding
laminar part can be regarded as stationary or as translating at a constant velocity, as if frozen. Thus, such features are
described by the equation. A provisional classification is given of various warm extrusions along branches of both Streams,
all of which exemplify the repulsive bifurcation, except for the mainstreams with very narrow (< 30–40 km) branches, possibly
due to misapplication of the model.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
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A depth-integrated model for weakly dispersive, turbulent, and rotational fluid flows 总被引：2，自引：0，他引：2
A set of weakly dispersive Boussinesq-type equations, derived to include viscosity and vorticity terms in a physically consistent manner, is presented in conservative form. The model includes the approximate effects of bottom-induced turbulence, in a depth-integrated sense, as a second-order correction. Associated with this turbulence, vertical and horizontal rotational effects are captured. While the turbulence and horizontal vorticity models are simplified, a model with known physical limitations has been derived that includes the quadratic bottom friction term commonly added in an ad hoc manner to the inviscid equations. An interesting result of this derivation is that one should take care when adding such ad hoc models; it is clear from this exercise that (1) it is not necessary to do so – the terms can be included through a consistent derivation from the viscous primitive equations – and (2) one cannot properly add the quadratic bottom friction term without also adding a number of additional terms in the integrated governing equations. To solve these equations numerically, a highly accurate and stable model is developed. The numerical method uses a fourth-order MUSCL-TVD scheme to solve the leading order (shallow water) terms. For the dispersive terms, a cell averaged finite volume method is implemented. To verify the derived equations and the numerical model, four cases of verifications are given. First, solitary wave propagation is examined as a basic, yet fundamental, test of the models ability to predict dispersive and nonlinear wave propagation with minimal numerical error. Vertical velocity distributions of spatially uniform flows are compared with existing theory to investigate the effects of the newly included horizontal vorticity terms. Other test cases include comparisons with experiments that generate strong vorticity by the change of bottom bathymetry as well as by tidal jets through inlet structures. Very reasonable agreements are observed for the four cases, and the results provide some information as to the importance of dispersion and horizontal vorticity. 相似文献
17.
In this paper, motion response of a moored floating structure interacting with a large amplitude and steep incident wave field is studied using a coupled time domain solution scheme. Solution of the hydrodynamic boundary value problem is achieved using a three-dimensional numerical wave tank (3D NWT) approach based upon a form of Mixed-Eulerian–Lagrangian (MEL) scheme. In the developed method, nonlinearity arising due to incident wave as well as nonlinear hydrostatics is completely captured while the hydrodynamic interactions of radiation and diffraction are determined at every time step based on certain simplifying approximations. Mooring lines are modelled as linear as well as nonlinear springs. The horizontal tension for each individual mooring line is obtained from the nonlinear load-excursion plot of the lines computed using catenary theory, from which the linear and nonlinear line stiffness are determined. Motions of three realistic floating structures with different mooring systems are analyzed considering various combinations of linear and approximate nonlinear hydrodynamic load computations and linear/nonlinear mooring line stiffness. Results are discussed to bring out the influence and need for consideration of nonlinearities in the hydrodynamics and hydrostatics as well as the nonlinear modelling of the line stiffness. 相似文献
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Bound waves and triad interactions in shallow water 总被引：2，自引：0，他引：2
Boussinesq type equations with improved linear dispersion characteristics are derived and applied to study wave-wave interaction in shallow water. Weakly nonlinear solutions are formulated in terms of Fourier series with constant or spatially varying coefficients for two purposes: to derive higher order boundary conditions for regular and irregular wave trains and to derive evolution equations on constant or variable water depth. Wave transformation of monochromatic, bichromatic and irregular waves is studied and comparison with measurements and direct time domain solutions shows good agreement. The improvement relative to classical models from the literature is discussed. 相似文献
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《Coastal Engineering》1999,38(1):1-24
This paper presents a new and more accurate set of deterministic evolution equations for the propagation of fully dispersive, weakly nonlinear, irregular, multidirectional waves. The equations are derived directly from the Laplace equation with leading order nonlinearity in the surface boundary conditions. It is demonstrated that previous fully dispersive formulations from the literature have used an inconsistent linear relation between the velocity potential and the surface elevation. As a consequence these formulations are accurate only in shallow water, while nonlinear transfer of energy is significantly underestimated for larger wave numbers. In the present work we correct this inconsistency. In addition to the improved deterministic formulation, we present improved stochastic evolution equations in terms of the energy spectrum and the bispectrum for multidirectional waves. The deterministic and stochastic formulations are solved numerically for the case of cross shore motion of unidirectional waves and the results are verified against laboratory data for wave propagation over submerged bars and over a plane slope. Outside the surf zone the two model predictions are generally in good agreement with the measurements, and it is found that the accuracy of e.g., the energy spectrum and of the third-order statistics is considerably improved by the new formulations, particularly outside the shallow-water range. 相似文献
20.
We compare insolation results calculated from two well-known empirical formulas (Seckel and Beaudry’s SB73 formula and the
original Smithsonian (SMS) formula) and a radiative transfer model using input data predicted from meteorological weather-forecast
models, and review the accuracy of each method. Comparison of annual mean daily irradiance values for clear-sky conditions
between the two formulas shows that, relative to the SMS, the SB73 underestimates spring values by 9 W m-2 in the northern Adriatic Sea, although overall there is a good agreement between the annual results calculated with the two
formulas. We also elucidate the effect on SMS of changing the ‘Sun-Earth distance factor (f)’, a parameter which is commonly
assumed to be constant in the oceanographic context. Results show that the mean daily solar radiation for clear-sky conditions
in the northern Adriatic Sea can be reduced as much as 12 W m-2 during summer due to a decrease in thef value. Lastly, surface irradiance values calculated from a simple radiative transfer model (GM02) for clear-sky conditions
are compared to those from SB73 and SMS. Comparison within situ data in the northern Adriatic Sea shows that the GM02 estimate gives more realistic surface irradiance values than SMS, particularly
during summer. Additionally, irradiance values calculated by GM02 using the buoy meteorological fields and ECMWF (The European
Centre for Medium Range Weather Forecasts) meteorological data show the suitability of the ECMWF data usage. Through tests
of GM02 sensitivity to key regional meteorological factors, we explore the main factors contributing significantly to a reduction
in summertime solar irradiance in the Adriatic Sea. 相似文献