Showing papers on "Added mass published in 1992"

An arbitrary Lagrangian-Eulerian finite rigid element method for interaction of fluid and a rigid body

Abstract: A computational procedure is developed to solve the problems of coupled motion of a rigid body and a viscous incompressible fluid; the former is mounted on elastic springs, and the latter is surrounding the rigid body. The arbitrary Lagrangian-Eulerian method is employed to incorporate the interface conditions between the body and the fluid. The streamline upwind/Petrov-Galerkin finite element method is used for the spatial discretization of the fluid domain, and the predictor-corrector method is used for the time integration. The method is applied to evaluate the added mass and the added damping of a circular cylinder as well as to simulate the vibration of a circular cylinder induced by vortex sheddings.

On the influence of the Basset history force on the motion of a particle through a fluid

Abstract: A parameter study on the development of the Basset integral appearing in the equation of motion of a particle moving through a fluid is presented. The particle motion is investigated numerically for the flow across an aerodynamic shock. It is found that, for this type of flow, the Basset history force acting upon the particle described by the Basset integral can be many times larger than the viscous drag in the immediate shock region. It is demonstrated how this force affects the particle motion across the shock for different particles. Nevertheless, the results obtained show that it is justified to neglect the Basset integral for the theoretical description of the motion of the types of particles commonly used in flow measurement tracer techniques for the type of flow considered here.

Chaotic dynamics of particle dispersion in fluids

Abstract: An analysis of the Lagrangian motion for small particles denser than surrounding fluid in a two‐dimensional steady cellular flow is presented. The Stokes drag, fluid acceleration, and added mass effect are included in the particle equation of motion. Although the fluid motion is regular, the particle motion can be either chaotic or regular depending on the Stokes number and density ratio. The implications of chaotic motion to particle mixing and dispersion are discussed. Chaotic orbits lead to the dispersion of particle clouds which has many of the features of turbulent dispersion. The mixing process of particles is greatly enhanced since the chaotic advection has the property of ergodicity. However, a high dispersion rate was found to be correlated with low fractal dimension and low mixing efficiency. A similar correlation between dispersion and mixing was found for particles convected by a plane shear mixing layer.

Active/passive damping apparatus

Abstract: A damping apparatus for large structures operable both passively and actively is disclosed which comprises a passive type damping mechanism to be mounted on the structure and including an added mass, spring, dampener, etc., hydraulic cylinders mounted on the structure and connected to the added mass, an electro-hydraulic servo mechanism to switch the hydraulic cylinders between passive and active type operation, and a control unit for switching the electro-hydraulic servo mechanism between passive and active type operation in response to the velocity and displacement of the added mass as well as the velocity of the structure.

Closed-form vibration analysis of sagged cable/mass suspensions

Abstract: A theoritical model is presented which describes the three dimensional non linear motion of a sagged cable that supports an array of discrete masses. An asymptotic form of this general model is derived for the linear response of a cable/mass suspension having small equilibrium curvature and horizontal supports

Hydrodynamic Forces on Fixed Submerged Cylinders

Abstract: Wave loads on the cylindrical members of fixed offshore structures are generally calculated by using Morison’s Equation. The inertia force component of this equation is conventionally quoted in a form derived from theoretical calculations for a uniformly accelerating fluid. In this paper the correct form for the inertia force in a general fluid flow is derived from first principles by pressure integration and, independently, from earlier work, by energy arguments. It is shown that, for the thin cylinder limiting case, the transverse force on a circular cylinder is incorrectly given by the conventional approach, in that the product of transverse fluid velocity (in the direction of the required force) with the longitudinal velocity gradient should be added to the water particle acceleration, when computing the added-mass component of the force. Axial divergence, in other words, appears to play the role of a rate-of-change of added mass. It is shown that the mathematical origin of this extra term is the classical three-dimensional flow feature of a ‘zonal harmonic’, which produces a convective fluid acceleration but zero loading. A more elaborate formula is derived for non-circular cylinders, and the nature of point loads occurring at cylinder ends is also discussed.

The Magnitude of Basset Forces in Unsteady Multiphase Flow Computations

Abstract: This paper reports on the equation of motion of a small spherical particle moving in a fluid which is solved numerically with the radius of the sphere and the ratio of fluid to particle densities being parameters. The Basset force term is computed and compared to the total force on the particle for the case of turbulent flow in a duct. It is found that the Basset force may be neglected in the equation of motion of the particle only when the fluid to particle density ratio is very high and the particle diameter is greater than 1[mu]m. A dimensional analysis is also performed for the case when the particle size and the characteristic flow dimension are of the same order of magnitude. In the latter case, it is deduced that the Basset force is significant whenever the flow Reynolds number is greater than one.

Added masses and forces on two bodies approaching central impact in an inviscid fluid

Abstract: An integral-equation procedure has been developed to determine interaction forces on two bodies approaching central impact in an inviscid fluid. The accuracy of the results from that procedure is evaluated by applying it to a pair of circles and a pair of spheres for which exact solutions are available. A second purpose was to refine the procedure so that accurate solutions could be obtained at closer distances between the bodies. In the first part of this work, the classical theory is extended by deriving truncation corrections for the infinite series representing the exact solution and asymptotic formulas for computing interaction forces at small gaps. In the second part, two problems were resolved: one on the treatment of the sharp peaks of the integrands when the gap between the bodies was small, the other on reducing the errors in the numerical differentiation required to evaluate the forces. Results for various combinations of circle pairs, for equal spheres, and for an elliptical cylinder approaching a circular one are presented. A new relation between the interaction forces on a wall and on a body moving normal to it is presented.

Semi-analytical solutions for the hydrodynamics of submerged pontoons of finite length

Abstract: A semi-analytical solution procedure for the derivation of first-order wave forces, added mass and damping terms is presented for rectangular pontoons which may be floating, submerged or resting on the seabed. The development is based on a solution matching technique for the scattered and radiated wave potentials and gradients across common boundaries of the subdivided solution domain. The final expressions are true for waves of arbitrary angle of incidence and singularities due to the case of normal incidence are eliminated. The approach is also generalised for the case where the pontoon is a unit of a TLP offshore production system by referring all quantities to a global co-ordinate system. Numerical results are extracted for a number of benchmark problems and compared with independent values where available in the literature. Excellent agreement is obtained for wave forces, added mass and damping values for cases involving floating, submerged and bottom-founded pontoon configurations.

On the planar translation of two bodies in a uniform flow

Abstract: The general planar translation of two bodies of revolution through an inviscid and incompressible fluid is considered. The moving trajectories and the hydrodynamic interactions are computed based on the generalised Lagrange equations of motion, including the effects of solid constraints, external forces in the plane of motion, and a uniform stream in any direction parallel to the plane of motion. In a relative co-ordinate system moving with the stream, the kinetic energy of the fluid is expressed as a function of six added masses due to motions parallel and perpendicular to the line joining the centres of two bodies. Analytical solutions of added masses in series form are obtained for the motion of two spheres. A new iterative formula based on the analysis of velocity potentials around each body is developed for added masses and their derivatives with respect to the separation distance due to the transverse motion. The method of successive images and Taylor's added-mass formula are applied to determine the added masses and their derivatives due to the centroidal motion. These results are compared with the numerical solution of added masses computed by the boundary0integral method and the generalised Taylor added-mass formula. The integral equations, in terms of surface-source distributions on both surfaces, are modified for obtaining accurate numerical solutions. Numerical results are given for several practical engineering problems.

An Improved Method for Calculating the Contribution of Hydrodynamic Chain Damping on Low-Frequency Vessel Motions

Abstract: The computation of low frequency mooring forces on a tanker exposed to survival conditions requires knowledge of the hydrodynamic excitation and reaction forces. The low frequency excitation is caused by wave drift forces. The reaction forces on the tanker are a combination of added mass forces, wave drift damping forces, viscous forces and damping forces due to interaction with the mooring chains. In this paper a direct simulation method for a chain-turret moored tanker is applied for the correct instantaneous low and high frequency motions of the tanker, including the dynamic behaviour of the mooring chains. The latter is modelled by a lumped mass method. Comparison is made of the direct simulation method and an approximate method using the average chain damping. An evaluation of the chain damping contribution is made using model test results and computations.

Flow-Induced Vibration of Long-Span Gates due to Shed Vortices : Vibration Criteria, Level of Fluid Excitation and Added Mass

Abstract: Submerged long-span gates which dam wide rivers can undergo violent streamwise vibrations caused by vortex shedding beneath the gate. This study presents flow-induced vibration characteristics which were obtained in a model gate test. From the measured vibration frequencies and damping ratios in air and water, the level of fluid excitation and the added mass for small-amplitude gate vibrations are calculated and reduced to a dimensionless form. Thus, the vibration criteria are obtained. In addition, the average values of the maximum amplitudes of gate vibration were measured. The results of these experiments, taken as a whole, suggest that the flow-induced vibration characteristics of the long-span gates are well predicted by dimensionless parameters, such as the reduced gate opening and the reduced velocity.

A virtual mass and a drag coefficients for an oscillating particle

Abstract: We report on a fluid force acting on a sphere oscillating sinusoidally in a fluid at rest, and the fluid force is evaluated as a virtual mass coefficient and a drag coefficient. And we propose equations for the virtual mass and drag coefficients of an oscillating sphere

Optimizing tuning masses for helicopter rotor blade vibration reduction including computed airloads and comparison with test data

Abstract: The development and validation of an optimization procedure to systematically place tuning masses along a rotor blade span to minimize vibratory loads are described. The masses and their corresponding locations are the design variables that are manipulated to reduce the harmonics of hub shear for a four-bladed rotor system without adding a large mass penalty. The procedure incorporates a comprehensive helicopter analysis to calculate the airloads. Predicting changes in airloads due to changes in design variables is an important feature of this research. The procedure was applied to a one-sixth, Mach-scaled rotor blade model to place three masses and then again to place six masses. In both cases the added mass was able to achieve significant reductions in the hub shear. In addition, the procedure was applied to place a single mass of fixed value on a blade model to reduce the hub shear for three flight conditions. The analytical results were compared to experimental data from a wind tunnel test performed in the Langley Transonic Dynamics Tunnel. The correlation of the mass location was good and the trend of the mass location with respect to flight speed was predicted fairly well. However, it was noted that the analysis was not entirely successful at predicting the absolute magnitudes of the fixed system loads.

Low frequency added mass of a semi-submersible influenced by incident waves

Abstract: It has been reported that the low frequency added mass of a semi-submersible is increased by incident waves, similar to low frequency damping. Previous work has stated that low frequency added mass sometimes increases proportionally to the wave amplitude of incident waves depending on the wave frequency. It can be twice the value for still water and affects the resonant frequency of the slow drift motion as well as the amplitude. This is a very interesting phenomenon, and as yet there is no physical explanation for it. To investigate this phenomenon, forced oscillation tests were carried out, not only for a total semi-submersible model but also several partial models, those being the lower-hull, column, brace, column lowerhull, column brace, column brace of 2/3 shortened length and column brace of 2/3 narrowed breadth. The experiments show that the brace plays the most important role in the change of added mass of the semi-submersible model, and that the added mass increases proportionally to the square of the wave amplitude of incident waves rather than the wave amplitude itself.

Vibration control device of structure

Abstract: PURPOSE: To provide a positive vibration control device of the double acting vibration absorber, wherein the device is actuated by small control force and control mechanism thereof is simple and tuning of vibration period of added mass body with respect to the natural period of a structure can be made easier. CONSTITUTION: A first added mass body 1 sliding along a structure and a guide rail 7 is connected by a hollow laminated rubber 3. Auxiliary laminated rubbers 4 for adjusting period are provided at the front and rear in the direction of vibration to tune the vibration period of the body 1 to the natural period of the structure. Oil dampers 5 are provided on either side of the rubber 3 to give a predetermined damping coefficient to the body 1. A second added mass body 2 is provided on the body 1 and both are connected by a coiled spring 6. A pinion 12 is turned by an AC servomotor 10 provided on the body 2 based on the response of the structure due to earthquake so that the body 2 is driven back and forth by engagement of the pinion 12 with a rack 13 provided on the body 1. COPYRIGHT: (C)1993,JPO&Japio

Hybrid dynamic absorber

Abstract: PURPOSE:To obtain a vibration suppressing effect over a wide frequency range so as to greatly reduce the degree of energy which is externally applied, by producing a control force necessary of suppressing vibration in accordance with a quantity of state of a structure and an added mass, from a variable damper and an actuator. CONSTITUTION:Since an actuator 5 comprises a variable damper 4, if a vibration is effected in a structure A by an external force caused by earthquake, wind or the like, a control force necessary for damping the vibration is determined in accordance with a predetermined control rule so that the variable damper 4 and the actuator 5 are operated in accordance with a quantity of state of the structure A and an added mass 1. Further, whether the control force is produced or not is determined in accordance with the relationship between the relative speed of the structure A and the added mass 1 and the necessary control force. If it is possible, the variable damper produces the necessary control force, but if it is impossible, the actuator 5 gives a control force.

Method for analyzing coupled vibration of fluid and structure

Abstract: PURPOSE:To apply to a complex structure by noting only a necessary vibration mode of the structure and calculating an added mass of fluid corresponding to it CONSTITUTION:The number of natural vibrations and the intrinsic modes of fluid and a structure at the time of coupled vibration are obtained with an added mass of fluid taken into consideration At this time the number of natural vibrations and the intrinsic modes of the structure at the time of non-coupled vibration are used to transform the equation of the structure to a mode coordinate system, wherein the matrix is reduced and an added mass of the fluid is calculated corresponding to the respective modes This is added to each mode mass of the structure, eigenvalue analysis is performed in the mode coordinate system, and the number of natural vibrations and the intrinsic modes are calculated Using these as the new number of natural vibrations and intrinsic modes of the structure, a corresponding added mass is calculated This is repeated until for example their changes are at a specified value or lower and converged, to obtain the number of natural vibrations and the intrinsic modes at the time of coupled vibration Thus the matrix is reduced and the method can be applied to a complex structure

An Approximate Method for Computation of Added-Mass Coefficients of Two-Dimensional Bodies. Two-Dimensional Potential Theory.

Abstract: This solution is based on a discrete singularity method in two-dimensional potential theory. For computational examples, the following bodies were chosen : groups of circular cylinders and elliptic cylinders in an infinite flow field, and a group of circular cylinders in a cylindrical container. Their added-mass coefficients were then calculated. As a result, it was clarified that this approximate method of solution is effective for calculation of the added-mass coefficients of two-dimensional bodies of any form, and that it is better than the imaginary method in view of its generality.

On the coupled vibration of an ideal fluid with a linear elastic structure

Abstract: The purpose of this paper is to analyse theoretically and numerically the coupled vibration of an ideal fluid with a linear elastic structure. It is proved in the paper that the natural frequencies of the coupled vibration do exist and are all real positive. The paper presents an efficient method to transform a coupled fluid-structure system to the structure with added mass and the vibrational analysis of the former is replaced by the latter in vacuum only. Numerical solution is outlined for the transformed problem and a compact frequency equation is derived in which fluid variables do not appear. This simplifies the analysis significantly. A convergent proof has been given to guarantee the reliability of the solution. The paper also offers a general algorithm bombined with Ritz method, boundary element method, and finite element method to analyse the transformed problem. Based on this algorithm, one can apply a known structural analysing program, with a little modification, to solve many different kinds of fluid-structure coupling problems. Some numerical results are given to show the efficiency of the algorithm.

Low Frequency Surge-Added-Mass of a Moored Semi-Submersible Influenced by Incident Waves

Abstract: Low frequency added mass of a semi-submersible is increased by incident waves, similarly to low frequency damping. It can be 1.6 times value for still water and affects the resonant frequency of the slow drift motion as well as the amplitude of it. This is a very interesting phenomenon and there is no physical explanation to explain this yet. In order to investigate this phenomenon forced oscillation tests in regular waves were carried out not only for a total semi-submersible model but also several constituent models, those being the lowerhull, brace, column, column lowerhull, columm brace, column brace of 2/3 narrowed breadth, column brace of 2/3 shortened length. It is important to elucidate this phenomenon on the design of mooring of a floating body.The experiments show that the brace plays the most important role of the change of added mass of the semi-submersible model, and that the added mass increases proportionally to square of wave amplitude of incident waves rather than the wave amplitude itself.

Hydroelastic vibration analysis of structures in contact with fluid

Abstract: In the vibration analysis of submerged or floating bodies such as ships and offshore structures, the coupled system between fluid and structure should be considered using the compatibility conditions on the wetted surface. It is well known that the hydroelastic vibration analysis of structures in contact with fluid can be done by applying the finite element method to structures and the boundary element method to fluid domain. However, such and approach is impractical due to the characteristics of the fluid added mass matrix fully coupled on whole wetted surface. To overcome this difficulty, an efficient approach based on reanalysis scheme is proposed in this paper. The proposed method can be applied for the cases of higher local modes and beam-like modes for which three-dimensional reduction factors are not known. The three dimensional reduction factors are not needed and thus the restrictions can be removed in the analyses of non-beam like modes or local vibration modes by considering fluid-structure interaction. The validity and the calculation efficiency of the proposed method are proved through numerical examples.

Hydroelastic Analysis of Structural Vibration in Contact with Fluid

Abstract: In the vibration analysis or submerged of floating bodies such as ship and offshore structures, the coupled system between structure and fluid satisfying the compatibility conditions on the wetted surface should be considered. It is well known that the hydroelastic analysis of structural vibration in contact with fluid can be solved by applying the finite element method to structure and the boundary element method to fluid domain. However such an approach is impractical, because fluid added mass matrix is fully coupled on whole wetted surface. To overcome this shortcoming, an efficient approach based on reanalysis scheme is proposed in this paper. The proposed method can be applied for cases with higher modes lacking 3-D reduction factor J as well as beam-like modes of marine structures. It is well known the traditional method using 2-D added mass and J-factor is good only for beam-like modes with reliable J values. The validity and the calculation efficiency of the proposed method are proved with numerical examples.