Showing papers on "Added mass published in 1978"

Added mass computation by the boundary integral method

Abstract: Computational techniques for the treatment of fluid-structure interaction effects by discrete boundary integral methods are examinde. Attention is focused on the computation of the added mass matrix by finite element methods for a structure submerged in an infinite, inviscid, incompressible fluid. A general computational procedure is presented that is based upon a variational approach involving the assumption of constant source strength over each surface element. This is followed by an analysis of the discretization error for a spherical body that is then used to develop a hierarchy of computational schemes. These schemes are than evaluated numerically in terms of ‘fluid boundary modes’ for a submerged spherical surface. One scheme has been found to be surprisingly accurate in relation to its computational demands.

The added-mass coefficients of a torus

Abstract: The generalised added-mass coefficients of a torus in translatory and rotational motion in an inviscid incompressible fluid are obtained via an exact solution of Laplace's equation in toroidal coordinates. Of the six possible independent coefficients three are found to have nonzero, finite and separate values, due to symmetry. These are translation in, and perpendicular to the ring plane and rotation around a diameter. For translation normal to the ring plane, the added mass is somewhat larger than the mass of the torus of equal density. This coefficient tends to the torus mass for slender tori (large ratio of ring to core diameters). For translation in the ring plane the added mass tends to one half the torus mass, and for rotation the added inertia is approximately the torus moment of inertia for such slender tori. Simple relations for the added-mass coefficients as a function of the diameter ratio for general tori are also presented.

Hydrodynamic confficients of some heaving cylinders of arbitrary shape

Abstract: The fluid forces acting on a uniform cylinder with an infinitely long axis, heaving in the free surface or an infinite ideal fluid, are described in terms of its ‘added mass’ and ‘damping coefficient’. The techniques of multipole expansion and multiparameter conformal transformation are adopted for such calculations and applied to shapes which cannot be adequately represented by the conventional, and more rudimentary, ‘Lewis form fit’. The shapes referred to are both relevant to ship bows, one being a ‘fine section’ and the other a ‘bulbous section’. The parameters which influence the accuracy of the solution are examined. Results for these two sections are computed and compared with results based on (a) the ‘Lewis form approximation’ and (b) the ‘Frank's close fit method’ which employs a singularity representation.

Noise transmission into a light aircraft

Abstract: An analytical study on noise transmission into a cabin of a twin engine G/A aircraft is presented. The solution of the governing acoustic-structural equations of motion is developed utilizing modal expansions and a Galerkin type procedure. The exterior noise pressure inputs are taken from available experimental data. A direct comparison between theory and experiments on cabin noise levels is given. Interior noise reduction by stiffening, mass addition, and damping treatments is investigated. It is shown that a combination of added mass and damping could significantly reduce interior noise levels for this aircraft.

A numerical determination of the entrained water in ship vibrations

Abstract: The hydrodynamic mass matrix for a vibrating ship is evaluated using a three-dimensional finite element discretization of the surrounding water. For the important case of vertical vibrations of the ship, values of the added masses obtained in this way are compared with those given by the conventional semi-empirical formulas currently in use in the shipbuilding industry. A numerical example is presented. It is concluded that conventional two-dimensional methods, the usefulness of which is confirmed when the first few modes of the hull girder only are sought, do not apply to higher modes, for which significant deformation of the cross-section may occur. On the other hand, a finite element approach has the advantage of its generality as well as flexibility. Suggestions for future research are given.

The Effect of the Boundary Condition on the Added Mass of a Rectangular Plate

Abstract: Using the elliptical cylindrical function, the added masses of thin rectangular plates vibrating elastically in an infinite ideal fluid are calculated. For the boundary conditions of the plates, two models are adopted. The plate which is simply-supported on two opposite edges while the other edges are clamped is one and the other is the plate which is simply-supported on two opposite edges while the other edges are free. Same examples are calculated numerically for the fundamental mode in each cases. And the effect of the boundary condition on the added mass are investigated by comparing these data with those of Kim's[4] which were calculated for the simply-supported plates by the same method. It is concluded that it is possible to predict the added mass of a rectangular plate, whose boundary condition is not treated in this report, by using the result of this investigation.

Response of open-bottom floating platforms to wind

Abstract: The motion response of a moored open-bottom floating platform is examined by a time-domain simulation technique, derived by the impulse response method that takes into consideration the frequency dependency of the fluid reactive force terms, i.e., added mass and damping force coefficients. The sway and roll responses of the moored platform to random wind loading are evaluated by solving the equations of motion using a simulated random wind loading history. The hydrodynamic coefficients of the open-bottom platform, with pressurized subdivided air chambers having a free air-water interface, are evaluated by modified Frank's close-fit method. Numerical examples of the motion response of the floating platform are presented. The results are used to study the effect of the variation of mooring system elasticity on platform response.

Dynamic analysis of the afterbody of a ship--towards a successful correlation between analytical and experimental results

Abstract: Calculations for predicting propeller-induced ship vibrations at the design stage involve the determination of four essential factors: rigidity, mass, damping and excitation, which the paper reviews separately. It is concluded that, although complex structural models yield a good approximation of the rigidity as well as the structural mass of the ship, there is on the other hand a lack of fundamental knowledge of added mass, damping and excitation forces, and that this deficiency is being filled by simplified methods which are often questionable. However, where it is possible to determine the added masses with reasonable approximation, a mass-rigidity model of the structure will permit the reliable determination of the ship's natural frequencies and modes. Also, difficulties inherent to any full-scale measurement are pointed out, and serve to partly explain poor correlations between calculations and measurements as have been recently reported in the literature. It is concluded that prediction of the actual vibration behaviour of a ship under service conditions is not possible at present. However, the response of the structure to a single vertical force of given intensity applied at the proper location, for arbitrarily-chosen viscous damping, can simulate, within the approximations introduced, the results of a full-scale exciter test, and may allow potentially dangerous frequencies to be recognised so as to permit a reliable choice of the number of propeller blades. The actual case of a 122,000 cubic meters LNG carrier is studied in detail, and serves to highlight some of the problems encountered at the computational as well as the experimental level.

On the influence of three-dimensional effects and restricted water depth on ship hull vibration

Abstract: In this note a finite element method for the calculation of the three-dimensional added mass of a vibrating ship hull is presented. The hull is approximated by a cylinder of the same overall dimensions as the ship and the mode of vibration is approximated by the vibration mode of a uniform free-free Bernoulli-Euler beam. The method has been compared with an analytical solution and with measured results. Finally, a parametric study of the effect of restricted water on the three-dimensional added mass is presented.

A Note on Blockage Correction.

Abstract: : It has recently been shown that a jump in velocity potential exists between infinite upstream and downstream directions when a body translates uniformly along a channel of finite cross section such as a towing tank or wind tunnel. In this report a new blockage correction formula for body speed is proposed. The speed correction formula due to blockage is obtained by dividing computed potential jump by body length, assuming that the body is slender or flat in the direction of motion. The potential jump is expressed explicitly in terms of the effective volume, i.e., the sum of the displaced volume and added mass/density of the submerged body, and the depth Froude number, if a free surface is present.

Approximate Evaluation of Added Mass and Damping Coefficients of Two-Dimensional SWATH Sections.

Abstract: : Derivation of approximate formulas for determining the added mass and damping coefficients of two-dimensional, small-waterplane-area, twin-hull (SWATH) sections is described. The added mass and damping coefficients of interest are those associated with a forced oscillation of SWATH sections in heave, sway, or roll mode in a free surface. The objective of deriving the approximate formulas for the hydrodynamic coefficients is to simplify the computation of motion of SWATH ships in waves without sacrificing its accuracy significantly. The approximate formulas are derived based on the potential-flow theory. The damping coefficients are obtained in terms of the outgoing wave amplitudes, and the added mass coefficients are obtained by using the damping coefficients through the so-called Kramers-Kronig relations. Within the frequency range of practical interest, the approximate formulas provide satisfactory results. (Author)

Effects of fluid inertia on particle collection

Abstract: The significance of fluid inertia in the computation of particle collection by impaction is explained by use of a numerical example.

The effect of fluid inertia on the diffusion of a plasma through magnetic fields

Abstract: For a magnetically confined cylindrical plasma, it is shown by using an appropriate boundary condition at the edge of the plasma, that fluid inertia reduces radial diffusion, albeit not drastically, and determines via the boundary conditions for the radial fluid velocity, the equilibrium distribution of magnetic fields and current densities. For a plasma confined within a torus, a formula is derived for the toroidal diffusion velocity, which is affected by fluid inertia.

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.