Showing papers on "Added mass published in 1974"

Motions of Large Structures in Waves at Zero Froude Number

Abstract: In this paper a three-dimensional source technique has been applied to a floating object in regular waves and the added mass and damping coefficients, the wave exciting force and moment and the motions in six degrees of freedom and the pressure distribution have been calculated. The horizontal drift force and moment have also been evaluated. Numerical calculations and experimental results for the case of a floating box of dimensions 90 m x 90 m and a draft of 20 m and 40 m show very good agreement in the measured quantities. On the basis of this comparison one may conclude that the method does indeed provide a solution to the dynamic analysis of large three-dimensional structures in waves, subject, of course, to the usual limitations of the linearized theory.

Hydrodynamics of Large Objects in the Sea Part I-Hydrodynamic Analysis

Abstract: This paper deals with the hydrodynamic forces exerted on a rigid object describing harmonic oscillations under or on a free surface as well as the forces resulting from the interaction of the object held fixed in a train of regular surface waves. The problem is formulated for a body of arbitrary shape in water of finite depth and the development of a numerical scheme for carrying out the calculations is described. An energy balance as well as Haskind's relations are used as a check on the accuracy of the numerical results. Numerical results are presented for a floating sphere, a vertical circular cylinder, and a practical semi-immersed caisson configuration. Nomenclature a = characteristic dimension of body Ct = wave force (or moment) coefficient, Eqs. (36) and (40) Cij - ith component force (or moment) coefficient associated with the ;th component motion d_ = depth of submergence dS - differential surface area / = source strength function G = Green's function g - acceleration of Gravity gi = (i = 1,2,...7) See Eq. (19) h_ - water depth H = wave height i,l,k = cartesian unit vectors L = wavelength Mtj — added mass coef., (added mass /pa3) or (added mom. of I/pa5) Ntj = damping coef., (linear damping/po-a3) or (angular damp

Wave forces on cylinders near plane boundary

Abstract: Potential flow theory predicts: (1) the added mass of a cylinder near a plane boundary is independent of the direction of the flow and more than twice as large as that of the same cylinder far away from the boundary; (2) a possible failure mechanism of alternating positive and negative lift force, which is proportional to horizontal velocity squared, for pipelines resting on the bottom boundary and subjected to waves and currents; and (3) the force from convective acceleration of the ambient flow can be as large as 30% of total acceleration force for waves. Laminar boundary-layer theory suggests that a phase shift exists between maximum velocity and maximum drag force for cylinders subjected to waves. Laboratory wave force experimentation confirms the aforementioned theoretical findings 1 and 2.

Dynamic Response Of Floating Bodies

Abstract: A numerical scheme is developed utilizing a Green's function and digital computer calculations to determine the excitation forces and moments as well as added mass and damping coefficients for floating bodies. The analysis is carried out within the framework of linear theory for bodies of arbitary shape, either submerged or semi-submerged, in water of finite depth. The calculated hydrodynamic coefficients associated with both the wave excitation and response of the body are utilized in conjunction with the equations of motion in order to determine the response of a free-floating body to wave excitation. In addition, the steady-state drift force and uplift force are discussed for the case of free-floating and fixed bodies. Numerical results are presented for several configurations.

Dynamic Forces Exerted by Oscillating Cables

Abstract: The manner in which the forces applied by a mooring cable to a floating buoy affect (in a linearized sense) the response of that body to regular waves is examined first by considering the case of a vertical, elastic cable of negligible weight density. Seeming infinities are encountered in the cable-alone solutions in view of neglect to the cable-motion-induced hydrodynamic damping forces. Bounded solutions for the buoy-cable system are shown to exist nevertheless. An outline of the treatment of the case of a cable of quite arbitrary scope and combination of parameters is given to demonstrate that the terms contributed by the cable can be identified and studied separately to determine their relative importance. A single set of results for cable force operations is given to show their frequency behavior. Much more numerical experience is needed. Further determinations should be made of cable force operators by measurements of the forces required to oscillate harmonically full-scale cables in a deep river with a known (or measurable) current profile. It is conjectured that, in some cases, cable forces may be approximated by their static force derivations in the wave-length range of interest. Verification of the validity of this conjecture requires examination of computer results for practical ranges of cable parameters. A new derivation of the equations of motion is given in Appendix A. It is found that the fluid added mass effects as given in other developments are incorrect. In addition, it is shown the effect of buoyancy has not heretofore generally been properly included in both the static and dynamic equations. The influence of these corrections on the dynamic response and the static shape requires further numerical evaluations by employing a suitably modified computer program.

Finite element solution for added mass and damping

Abstract: Two-dimensional cylinders undergo small-amplitude forced sinusoidal motion at the free surface of an ideal fluid. Using a finite element representation of the fluid region, discretized equations of motion are presented. A radiation boundary condition allows simulation of regions of infinite width. For heaving motion, computer solutions are obtained for three different hull forms. Results are compared with published analytic solutions for regions of finite depth and infinite depth. It is shown that the finite element method can give satisfactory solutions for these problem. The method is especially useful for fluid regions of non-uniform depth.

Two-dimensional calculations of the motion of floating bodies

Abstract: Attention is focused on the feasibility, the advantages, and the limitations of using numerical simulation techniques in the study of floating-body motions. Two simulation techniques, the Generalized Arbitrary Lagrangian-Eulerian (GALE) method and the Transient Potential Flow (TPF) method, have been considered. To assess their accuracy and practicality, both methods have been used to calculate the heaving of a semi-submerged circular cylinder in the free surface. (Modified author abstract)

Transverse Vibrations of a Ship Hull in Ideal Fluid, Determined Through Variational Methods

Abstract: A direct formulation for analyzing the vibration characteristics of an arbitrary body and predicting the three-dimensional velocity potential is presented. It consists of forming the Lagrangian for a Timoshenko-beam fluid system and minimizing Hamilton's principal function by the Rayleigh-Ritz method. Due to the boundary condition, the minimizing sequence for vibration amplitude transforms the velocity potential into an associated sequence of Neumann boundary-value functions. These are expressed further in terms of another minimizing sequence and evaluated after minimizing the corresponding energy functionals according to the method of minimal surface integrals. The theory is applied to a simple body and the resulting vibration characteristics are found to be more accurate than previous theoretical estimates and closer to published experimental data. Finally, the feasibility of the procedure is outlined for a submerged ship hull, employing approximate methods of numerical quadrature.

Some Transverse Resonant Vibration Characteristics of Wire Rope with Application to Flow-Induced Cable Vibrations

Abstract: The transverse resonant vibration of stranded wire rope was studied as part of an overall investigation of flow-induced cable vibrations It was found that an equivalent homogeneous string model adequately predicts the cable resonant frequencies Furthermore, the in-water damping and added mass exhibit no dependence on amplitude, mode shape, or wavelength, and the frequency dependence is slight The measured damping and added mass were combined to form a stability parameter which can be used to correctly estimate the maximum amplitude of the transverse vibrations induced by the interaction between a steady current and an elastic or elastically-mounted structure

On the added mass of a two dimensional rectangular cylinder vibrating in parallel to the free surface and in a perpendicular direction in restricted waters

Abstract: In the article [4], the authors have clarified that the hypercircle method is useful for estimating the added mass of two-dimensional cylinders with a rectangular or triangular section when vibrating in the horizontal direction very slowly. In this paper, the added mass of a rectangular cylinder floating on the surface of water and vibrating in the horizontal or vertical direction with a very high frequency is calculated by the same method also for the purpose of investigating the effect of the restricted waterway on the added mass.As the result, it is concluded that the clearance between the side wall of the waterway and the nearer side wall of the cylinder has a considerable effect on the horizontal added mass, while the vertical added mass is affected remarkably by the clearance of water under the bottom of the body.

On the Three-dimensional Correction Factor for the Added Mass in the Vertical Vibration of the Ship.

Abstract: The three-dimensional correction factor of the added mass of finite-length elliptic cylinders in vertical vibration in a free surface was calculated. This problem has already been dealt by T. Kumai[5] to contribute to analytical prediction of the three-dimensional correction factor for the added mass in vertical vibration of ships. In Kumai's work, the body boundary condition involved in the appropriate boundary value problem was approximately treated in the course of obtaining the solution. In this work, obtaining the solution derived from mathematically exact treatment of the body boundary condition, the author recalculated the three-dimensional correction factor for length-beam ratio , beam-draught ratio and number of nodes from 2 to 7. And the numerical results were compared with both Kumai's results and the author's experimental data for two and three-noded vibrations of the cylinder of beam-draught ratio 2.40 The comparison of the numerical results shows that the author's are always higher than the Kumai's as expected. And the comparison of the numerical results with experimental data shows that the Kumai's numerical results have less deviation in case of two-noded vibration, and that, in case of three-noded vibration, the author's numerical results are in fairly good correspondence.

On the Three-dimensional Correction Factors for the Added Mass and the Added Mass Moment of Inertia Related to Manoeuvrability in Shallow Water

Abstract: When we want to estimate the added mass and the added mass moment of inertia of a ship, the strip method which has proved to be very useful for the calculation of ship motion in waves, seems to be suitable, because it is possible to take the hull form of a ship into consideration in detail. However, since the strip method is based on two-dimensional consideration of the flow field around a three-dimensional body, both the added mass and the added mass moment of inertia obtained by the strip method are always excessively large compared with their exact values.Therefore in this paper, the correction factors are introduced separately for the added mass as to the swaying motion, and for the added mass moment of inertia as to the yawing motion in an analogous way to the correction factor for the added mass of the vertical ship hull vibration.Especially, the effect of finite water depth on these correction factors is investigated from the viewpoint of ship's manoeuvrability in restricted waters. As a result, it is found that the correction factors decrease with decrement of the water depth, and that the correction factor for the added mass moment of inertia is smaller than that for the added mass.