Showing papers on "Added mass published in 1976"

Added mass and damping of a vibrating rod in confined viscous fluids

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Effects of beam on the hydrodynamic characteristics of ship hulls

Abstract: Forced oscillation experiments were carried out with a systematic ship model family of which the length-beam ratio ranged from 4 to 20. The experiments also included a thin plate to simulate the case of an infinite length-beam ratio. Vertical and horizontal harmonic motions in calm water were considered and the corresponding hydrodynamic coefficients were determined. Moreover the vertical motions and added resistance in waves were measured. The results are presented in graphical form and are compared with some existing calculation methods.

Theoretical Prediction of Motion of Small-Waterplane-Area, Twin-Hull (SWATH) Ships in Waves.

Abstract: : Linear equations of motion with the hydrodynamic coefficients approximated by strip method are used to obtain the motion of small-waterplane-area, twin-hull (SWATH) ships in regular and irregular waves. The couplings among different modes of motion are separated into 3 independent groups: surge, heave-pitch and sway-roll-yaw; and the resulting equations of motions are treated in the frequency domain. The hydrodynamic coefficients of predominantly nonviscous nature, added mass and wavemaking damping, are obtained by strip theory. The effects of wave diffraction, viscous damping, and stationary stabilizing fins are included in the equations of motion within the framework of the linear frequency response of a body to waves. The assumptions involved in the evaluation of various hydrodynamic coefficients in the equation of motion are specified. The validity of the theoretical prediction of motion is checked by correlating with existing model experimental results, and the relevant discussions are made. In general, the theoretical prediction is satisfactory except the case of near zero-encounter frequency which can occur when a ship cruises with certain speed in stern-quartering waves.

Motion characteristics of floating bodies

Abstract: The motion characteristics of floating bodies, in and below a free surface, that are subject to surface waves are examined. The effects of various degrees of approximation of hydrodynamic coefficients on the results of the motion computations are investigated on the basis of linear theory. The behavior of hydrodynamic coefficients such as added mass, damping, and wave-exciting force with respect to frequency of oscillation (or wave length) and depth of submergence is also investigated. Numerical results are obtained for three cylindrical bodies of circular, rectangular, and rhombic cross sections. The results reveal that fairly crude approximations can be applied to compute the oscillatory motion of submerged bodies.

Added mass and damping forces on circular cylinders

Abstract: A series of experiments was performed to determine the fluid added mass and damping forces on harmonically oscillating cylinders in still water. The forces obtained in this case are simply related to the forces which would occur if the cylinder was stationary and the fluid oscillated with the same amplitude and frequency. Hence, the results have direct bearing on the calculation of wave forces on cylindrical structures. Primary attention is given to low amplitude motions where it is found that the fluid damping force is purely viscous (proportional to the velocity). Beyond an oscillation amplitude of 0.4 dia, the damping becomes proportional to velocity squared in agreement with previous investigations and common assumption. The added mass is found to be independent of amplitude and well represented by linear theory for the experimental conditions.

Wave Forces on Pipes Near the Ocean Bottom

Abstract: Where wake effects are negligible, potential flow calculations predict well the lift and added mass forces acting on pipelines near the ocean floor when subjected to time dependent flows. Wake effects have a significant influence on the flow conditions and measured frequencies of vortex shedding can predict the drag force acting on the cylinder. The Strouhal number is a function of the gap below the cylinder. The added mass coefficient is much larger when the cylinder is near the boundary than when it is a free stream.

Hydrodynamic damping and 'added mass' for flexible offshore platforms

Abstract: : The dynamic response of deepwater flexible platforms due to wind-generated ocean waves appears to be an important design consideration; therefore, a theoretical and experimental study was made of hydrodynamic damping and 'added mass.' Classical potential theory with linearized boundary conditions was used to study the hydrodynamic damping due to wavemaking and the coefficient of added mass on a vertical surface-piercing cylinder as a function of oscillation frequency, cylinder diameter, water depth, and mode shape. Experiments were conducted to verify the results of potential theory. Rigid vertical cylinders were oscillated with simple-harmonic motion in calm water. Total forces and radiated waves were measured. They compared very well with theoretical values. Other investigators' data also verified the theory. A small experimental study was made in an attempt to verify the hydrodynamic damping implied by the quasi-steady drag-force interaction term of the presently used modified Morison equation to represent the drag force on an oscillating cylinder in waves. Damping was measured for an elastically supported circular cylinder in a steady current. The measured values were up to 4 times lower than the theoretical values. The disagreement appears to be that the experiments were outside the range for which the quasi-steady assumption is valid. Coefficients of added mass were also measured and were found equal to the potential theory value irrespective of the velocity of the current.

High reynolds number oscillating flow by cylinders

Abstract: In order to determine the added mass, drag and lift coefficients of a smooth cylinder at various distances from a plane boundary, the forced cylinder oscillation tests at high Reynolds number 105 to 106 and the wave force tests at moderate Reynolds number 101* to 105 have been carried out at the Wave Research Facility at Oregon State University. It is found that the drag, lift and added mass coefficients are all strongly Reynolds number dependent. The effect of the near by plane boundary is to increase all of the force coefficients two to four times as compared to the free stream flow values. This is a most important factor to be aware of for design purposes.

Dynamical theory of potential flows with a free surface: a classical approach to strip theory of ship motions

Abstract: The application of the dynamical theory to problems involving a solid body moving in an unbounded medium of inviscid and incompressable fluid is well known in classical hydrodynamics. The distinctive feature of this method is that the fluid and the body together are treated as one dynamical system and the description of the problem requires only one single source of information, the kinetic energy of the fluid resulting from the motion of the solid body. In this paper, the application of this classical method to floating-body problems is presented. In particular, the method is applied to derive the dynamic coefficients in the equations of ship motion following the strip-theory approximation. Formulas for the added-mass and dampling coefficients are obtained, and the differences as compared with those used in the latest version of strip theory are discussed.

Diffraction theory and statistical methods to predict wave induced motions and loads for large structures

Abstract: This paper presents results from computations of wave loads on one fixed and one floating large volume structure using three-dimensional sink-source technique (diffraction theory). The motions and mean drift forces are also included for the latter. Theoretical calculations are compared with the results from model experiments. Generally, good agreement was found between computer programme results and experiments. Three-dimensional diffraction theory was further used to predict the effect a large volume caisson may have on the incident wave and to estimate the added mass pressure in vibration. The design wave concept and the use of statistical methods in connection with diffraction theory are discussed on the basis of different wave spectra forms. The application of two-dimensional sink-source method (strip theory) compared to the three-dimensional method is discussed with reference to sample calculations.

Experimental study on accelerating and decelerating ship motions

Abstract: In the previous report, the longitudinal forces acting on a ship under accelerating or decelerating motion were treated and an estimating method of the transient motion was proposed from a practical point of view.In this report, the lateral forces acting on a ship in the transient motion, and accordingly manoeuvrability are investigated.The model ship tested is a Series 60 parent type (L/B=7.0, CB=0.7) and experiments were carried out in a towing tank using a PMM.As a propeller working effects on a hull and on a rudder are different from each other, it is reasonable to divide the lateral force of a ship into two parts, the rudder force including rudder induced force on a hull and the remainder.Propeller working effects are unexpectedly strong on a hull and can be represented by a simple function of the apparent advance ratio Js, as seen in Fig. 6.The damping force acting on a rudder, in its dimensionless form, is proportional to the parameter k=√1+k8/π·KT/J2, while the turning force is proportional to k2.As a consequence, both longitudinal and lateral hydrodynamic forces on a ship are possible to be represented using Js.Hence, course stability of a ship and rudder effectiveness depend strongly on Js in the transient motion.The results are also confirmed by the free running tests of the model

A finite element-analytical method for modeling a structure in an infinite fluid

Abstract: A method is described from which the interaction of an elastic structure with an infinite acoustic fluid is determined. The displacements of the structure and the pressure field of the immediate surrounding fluid are modeled by finite elements, and the remaining pressure field of the infinite fluid region is given by an analytical expression. This method yields a frequency dependent boundary condition for the outer fluid boundary when applied to the frequency response of an elastic beam in contact with an acoustic fluid. The frequency response of the beam is determined using NASTRAN, and compares favorably to the exact solution which is also presented. The effect of the fluid on the response of the structure at low and high frequencies is due to added mass and damping characteristics, respectively.

Added Masses of Large Tankers Berthing to Dolphins

Abstract: The added masses of large tankers berthing to dolphins are studied both theoretically and experimentally. The movements of large vessels in shallow water in the directions normal to their planes of symmetry cause counterflows of appreciable velocities under the hulls. The inertia of these counter-flows is shown to have an important effect on the added masses of the vessels. A theoretical formula is derived to determine the mass factor of an ocean vessel in shallow water as a function of the ratio Draught/Water- depth, the Froude number of the vessel and the coefficient of head loss of the counter-flow under the hull. Experiment is made to determine the mass factor. Comparison:, between the theory and the experiment shows a good agreement.

An Axisymmetrical Dock in Waves

Abstract: Linearized motions of as axisymmetrical dock freely floating in a regular plane wave are discussed. An extension of the Bessho variational principle(Bessho[3]) is derived to obtain a numerical procedure for a solution of the boundary value problem associated with the fluid motion. The added mass and the damping coefficients for a circular dock in vertical(heave) and horizontal(surge) oscillations are evaluated numerically, and the resulte seems to be satisfactory.

Wave forces induced on very large crude carriers

Abstract: Describes a theoretical method for predicting the forces induced by irregular waves on very large crude carriers, in which the hull section is modeled as a prismatic elliptic cylinder. Diffraction of the wave around the elliptic cylinder is taken into account and results are presented for surge and sway force and yaw moment for incident wave angles from 0 degree (head-on) to 90 degrees (broadside-on) at several increments and water depth to draft ratios. Thus, the effect of underkeel clearance is taken into account. In order to verify the method, an experimental study utilizing the captive model technique was carried out at the Netherlands Ship Model Test Basin (NSMB). In this study, forces (and moments) in the surge, sway and yaw directions were measured as the real tanker shape models were subjected to irregular waves incident from 0 degree to 90 degrees at several values. The comparisons, which are presented in terms of the non-dimensional force (or moment) ratios, are considered to be excellent. The procedure described herein can be employed to synthesize external loading histories due to irregular waves acting on very large crude carriers of specified dimensions for a variety of incident wave angles and water depth/draft ratios. The technique can also be employed to derive added mass and damping coefficient functions for the specified tanker. The procedure is an intermediate step in the process of determining the behaviorial response of the moored vessel. The vessel can be considered as moored to conventional piers in relatively protected areas or to sea islands or sea berths in exposed locations. The combination of low computational cost and reasonable accuracy makes it an ideal procedure to employ for a number of engineering applications.

The Drag of an Accelerating Submarine. Part I. Skin Friction

Abstract: : Potential flow theory predicts that a body accelerated or decelerated in motion in a straight line in a fluid will experience an inertia force greater than that due to the mass of the body - a so called added mass effect. Submarine trials confirm the phenomenon but indicate that for motion in the fore and aft direction the added mass is much greater than predicted. This could be due to: (1) transient loss of thrust from the propeller, (2) changes in skin friction due to the acceleration, (3) changes in form drag due to the acceleration. Towing testa have been conducted on a body without propeller. These eliminate (1) as the primary cause. A theoretical and experimental study of (2) has been made for a flat plate. Theory predicts an increase in skin friction for both laminar and turbulent boundary layers, but experiment on the turbulent layer only partially supports the theory. Although further analysis of the data is desirable, the results suggest that acceleration may restore a measure of laminar flow in the boundary layer. Further work should concentrate on this aspect and on the effect of acceleration on form drag.

The Vibration of an Elastic Rectangular Plate in a Fluid

Abstract: It is a well-known phenomenon that, in the case of vibrations of an elastic body in a fluid such as water, the presence of the surrounding fluid has the effect of lowering the natural frequencies of the vibration as compared with those in air or vacuum on account of the increased inertia, i.e. added mass. In this report, defining the mass increase factor as the ratio of added mass to vibration mass of the body in air, the author investigated the mass increased factor of an elastic plate vibrating in the fluid. It is assumed that the edges of the plate are simply supported, and that the surrounding fluid is an infinite ideal one. For the problem formulation the elliptical cylindrical coordinate system is adopted, so that a rectangular plate may be represented by a sheet degenerated from an elliptical cylinder. By virtue of the coordinate system adopted, plates which are chordwisely finite and lengthwisely contineous could directly be treated, but plates which are chordwisely finite in both directions could not be treated directly. For the latter, hence, plates which are chordwisely finite and lengthwisely semi-finite are investigated as an appropriate approximation. Some examples of the mass increase factor are numerically calculated for the fundamental mode and modes of zero or one nodal line in each direction with the range of the aspect ratio from 1 to 10 or more.

Sway and Roll of an axi-symmetrical body

Abstract: Radiation problems of sway and roll of an axi-symmetrical body has been solved by means of Bessho's variational approach. Calculation results have been in good agreement with Ohkusu's calculations of a vertical cylinder and Kudos experiments of a sphere.Hydrodynamic coefficients and wave exciting forces of bulbous bodies have been calculated systematically for use of actual design.Motions of a semisubmersible offshore platform of footing type have been calculated directly using above hydrodynamic coefficients and wave exciting forces.Added mass coefficients calculated by variational method can be used for inertia coefficients applied in Morison's wave force equation.

Fluid-plasticity of thin cylindrical shells

Abstract: Dynamic plastic response of a thin cylindrical shell, immersed in a potential fluid initially at rest and subjected to internal pressure pulse of arbitrary shape and duration, is examined. The shell is assumed to respond as a rigid-perfectly plastic material while the fluid is taken as inviscid and incompressible. The fluid back pressure is incorporated into the equation of motion of the shell as an added mass term. Since arbitrary pulses can be reduced to equivalent rectangular pulses, the equation of motion is solved only for a rectangular pulse. The influence of the fluid in reducing the final plastic deformation is demonstrated by a numerical example.