Showing papers on "Added mass published in 1983"

Added Fluid Mass and the Equations of Motion of a Parachute

Abstract: While it has long been known that added fluid mass may be important in the dynamics of parachutes, due to inadequate or incorrect derivation and/or implementation of the added mass tensor its full significance in the stability of parachutes has yet to be appreciated. The concept of added mass is outlined and some general conditions for its significance are presented. Its implementation in the parachute equations of motion is reviewed, and the equations used in previous treatments are shown to be erroneous. A general method for finding the equivalent external forces and moments due to added mass is given, and the correct, anisotropic forms of the added mass tensor are derived for the six degree-of-freedom motion in an ideal fluid of rigid body shapes with planar-, twofold- and axisymmetry, These derivations may also be useful in dynamic stability studies of other low relative density bodies such as airships, balloons, submarines and torpedoes. Full nonlinear solutions of the equations of motion for the axisymmetric parachute have been obtained, and results indicate that added mass effects are more significant than previously predicted. In particular, the component of added mass along the axis of symmetry has a strong influence on stability. Better data on unsteady forces and moments on parachutes are needed.

The Impact Loads of Ships Colliding With Fixed Structures

Abstract: When a ship moves forward in water and is decelerated by some obstacle or structure, it is not only the mass of the ship that is decelerated but also a certain amount of water moving along the ship. It is this so-called hydrodynamic added mass which is the crucial factor in the design calculations. The magnitude of this added mass is influenced by: dimensions and shape of the underwater hull, underkeel clearance, spring characteristics of the obstacle and collision mode. In order to identify more precisely the influence of all the different contributing parameters, a very extensive model test programme has been carried out at the Netherlands Ship Model Basin. A series of some 750 test runs was performed, during which the sensitivity of the hydrodynamic mass for fender stiffness, collision mode and collision speed was tested. The results suggest that the accepted and frequently used hydrodynamic added mass values are too low, a finding which is of importance in the design of fendering systems and flexible structures.

The Added Mass and Damping Coefficients of and the Excitation Forces on Four Axisymmetic Ocean Platforms

Abstract: This paper presents numerical results of the added mass and damping coefficients of vertical axisymmetric bodies on or under the free surface. Also computed are the excitation forces on these bodies due to an incident regular wave system. The numerical scheme employs a localized finite-element method, which is based on the theory of the calculus of variations. The excitation forces and moments on a submerged half-spheroid lying on the bottom are computed and compared with the results obtained by others. he agreement is good. Several specific types of floating vertical axisymmetric platforms are considered for ten different wave lengths, in connection with the design of an ocean-thermal-energy converter platform. The added mass and damping coefficient, as well as the excitations, are presented. It is shown that simple strip theory gives a good approximation of the sway(and pitch) added mass for a disc platform having a long circular cylinder.

Vibrations of a body in a bounded volume of viscous fluid

Abstract: The problem of the vibrations of a body in a bounded volume of viscous fluid has been studied on a number of occasions [1–4]. The main attention has been devoted to determining the hydrodynamic characteristics of elements in the form of rods. Analytic solution of the problem is possible only in the simplest cases [2]. In the present paper, in which large Reynolds numbers are considered, the asymptotic method of Vishik and Lyusternik [5] and Chernous' ko [6] is used to consider the general problem of translational vibrations of an axisymmetric body in an axisymmetric volume of fluid. Equations of motion of the body and expressions for the coefficients due to the viscosity of the fluid are obtained. It is shown that in the first approximation these coefficients differ only by a constant factor and are completely determined if the solution to the problem for an ideal fluid is known. Examples are given of the determination of the “viscous” added mass and the damping coefficient for some bodies and cavities. In the case of an ideal fluid, general estimates are obtained for the added mass and also for the influence of nonlinearity. Ritz's method is used to solve the problem of longitudinal vibrations of an ellipsoid of revolution in a circular cylinder. The hydrodynamic coefficients have been determined numerically on a computer. The theoretical results agree well with the results of experimental investigations.

Evaluation of fluid-modeling techniques in the seismic analysis of LMFBR-reactors

Abstract: Fluid modeling is of great importance in the seismic analysis of the LMFBR primary system. If the fluid model used in the analysis is too simplified, the results could be very uncertain. On the other hand, if the model is too detailed, considerable difficulty might be encountered in the analysis. The objectives of this study are to examine the validity of the two commonly used fluid modeling techniques. i.e. simplified added mass method and lumped mass method and to provide some useful information on the treatment of fluid in seismic analysis. The validity of these two methods of analysis is examined by comparing the calculated seismic responses of a fluid-structure system based on these two methods with that calculated from a coupled fluid-structure interaction analysis in which the fluid is treated by continuum fluid elements.

User's Manual for UCIN-CABLE III: A Two-Dimensional, Finite Segment Computer Code for Submerged and Partially Submerged Cable Systems

Abstract: : This report presents a User's Manual for the computer program UCIN- CABLE III. The program is designed to predict the two dimensional dynamics of submerged and partially submerged towing cable systems. A towing cable system includes the towing cables as well as a set of towed bodies. The system may have one or many branches, but no closed loops may be formed. The cable is modeled as a series of rigid cylinders connected end to end by hinge joints. The fluid forces on each segment include the effects of normal and tangential drag, added mass, and buoyancy. The fluid forces on the towed bodies are calculated from a set of 25 hydrodynamic coefficients. These account for fluid drag and added mass effects. Buoyancy forces are included separately. This manual provides instruction for using CABLE to study such cable systems. It also provides sample input and output data.

Added Mass and Damping for Ice Floes by Long Water Wave Theory

Abstract: Added mass and damping for ice floes are important parameters in determining floe collision forces with arctic structures. They are measures of the hydrodynamic resistance to the change of the floe motion. An analytical approach is presented based on long water wave theory to determine: (1) The added mass and damping for a circular ice floe for a wide frequency range and with general floe movements including two translations (surge and heave) and one rotation (pitch); and (2) the hydrodynamic impulse function on a circular ice floe resulting from its impact with a rigid structure. The governing equations are solved by: (1) The Fourier transformation, or (2) the Laplace transformation method, depending on the problem. Closed-form solutions and plots are provided for estimating the added mass, damping, and the hydrodynamic impulse function for a circular ice floe.

Seismic response of large suspended tanks

Abstract: The response of a large-diameter liquid-filled tank to seismic disturbances is studied in this paper with the emphasis on the effects of the bottom plate vibration on fluid dynamic pressure and free-surface sloshing. The fluid in the tank is treated as a continuum medium, which is quite different from the conventional method that treats the fluid as an added mass attached on the tank wall. As a result, important effects such as free-surface sloshing, fluid-structure interaction, and flexibility of the tank are included in the analysis.

Added-mass computation for impellers in nuclear power pumps. Final report. [PWR; BWR]

Abstract: In this investigation, a method has been developed for determining the hydrodynamic mass of the impeller stage of the rotor which accounts for the three-dimensional geometry of the impellers and the effects of the fluid velocity By using this method, more-accurate calculations of resonant frequencies of the rotor and hence the seal forces can be made In comparison with previous approaches, two main features have been included: 1) an accurate representation of the shape of the impeller and the pump geometry; and 2) the effects of the velocity of the fluid Results have been obtained for a prototypical centrifugal nuclear pump The hydrodynamic masses which are obtained are significantly larger than predicted by commonly-used formulas based on the mass of the entrained fluid or computed by the Fritz formulas Furthermore, the magnitude of the hydrodynamic mass has been found to depend strongly on the velocity of the working fluid, which indicates that its effects on critical frequencies will depend on the operational speed of the pump