Showing papers on "Added mass published in 1987"

Effects of Fluid Inertia on Finite-Length Squeeze-Film Dampers

Abstract: The influence of fluid inertia on the SFD force response to circular-centered motions of arbitrary amplitude is analyzed in detail, For finite length, locally sealed SFDs, integro-differential equations are derived in terms of the mean flow components. Numerical predictions, using the finite-element method, show that the damping and added mass coefficients remain invariant as the Reynolds number increases from small values to a moderate Reynolds number equal to 10. An approximate, finite-length, solution for the fluid-film forces has been analytically obtained which accounts for the fluid-inertia effect as well as local end seal effects in symmetric SFD configurations. The approximate solution, strictly valid for small Reynold numbers (Re < 1), agrees well with the results from the numerical solution for most SFD configurations and orbit radii considered. Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Tribology Conference in Pittsburgh, Pennsylvania, October 20–22, 1986

Vibrations of Circular Cylindrical Structures Subjected to Two-Phase Cross Flows

Abstract: This paper reviews a vibration problem relating to industrial energy equipment such as steam generators, heat exchangers. and condensers. It focuses on unstable vibrations in circular cylindrical structures immersed in two-phase cross flows, which are caused by unsteady fluid forces induced by the interaction between structural vibration and the fluid flow. Added mass and fluid damping induced by a vibrating circular cylinder in a two-phase bubble fluid are introduced, and then the effects of air bubbles on vortex-induced vibrations such as in-line and cross-flow oscillations of a single circular cylinder are reviewed experimentally. Finally, fluidelastic vibrations in a single row of cylinders as well as in cylinder arrays are discussed in relatlon to air bubble effects on instability criteria with respect to reduced velocity and mass-damping parameters.

Calculated Dynamic Response of an Articulated Tower in Waves—Comparison With Model Tests

Abstract: Calculated dynamic response of an articulated tower in waves is compared with model tests. The theory used is based on Morison’s equation and linear wave theory and requires specified hydrodynamic force coefficients. Calculations are done with three different sets of coefficients. Firstly, coefficients are assumed not to vary with wave period. Secondly, they are selected from experimental data of oscillating flow past stationary cylinders. Thirdly, they are based on calculations using diffraction theory. Added mass and inertia coefficients have a predominant effect on calculated response, drag coefficients have almost no effect. Calculated tower top motion and horizontal force at the universal joint correlate well for all three sets of coefficients, indicating that hydrodynamic coefficients for normal flow are reasonably well selected and need not be specified with undue precision. In contrast, hydrodynamic coefficients for axial flow need to be chosen carefully. Calculated vertical force at the joint, using initially specified axial flow coefficients, correlates poorly with measurements. Correlation is greatly improved using reduced coefficients for axial flow. Calculated response is reasonably linear with wave height. Spectral analysis techniques are used to determine statistical measures for three irregular seastates. Agreement with corresponding model test results is satisfactory.

Prime mover frame structure for vertical shaft pump

Abstract: PURPOSE:To simply shift the directional property and the characteristic value of vibration and suppress the vibration, by providing an added mass connected through a spring to a prime mover frame, and adjusting the spring rigidity and the added mass. CONSTITUTION:A disk 2 for supporting added masses 4 is fixed to the inside of a prime mover frame cylinder 1. A plurality of less frictional ball bearings 3 are arranged and fixed on the disk 2. The added masses 4 are mounted on the ball bearings 3, and are connected through springs 5 to the frame body 1. The spring rigidity and the added masses are adjusted to thereby suppress the vibration of the prime mover frame body 1.

Hydrodynamic forces on subsea modules during lifting operations

Abstract: The paper presents a survey of results from model tests with some idealised, main types of subsea structures or modules. Most of the testing is devoted to decay tests for the evaluation of added mass and damping. It is shown that, even for a fully submerged body, the damping is best described by both a linear and a quadratic damping term. Wave forces acting on these structures in the splash zone are considered, and a comparison is made between calculated and measured forces.

Measurements of aerodynamic forces on unsteadily moving bluff parachute canopies

Abstract: : Equations which describe the unsteady motion of bluff bodies through fluids contain certain components, termed added mass coefficients, which can be determined only by experiment. From the solutions to such equations the ways in which the shapes of parachute canopies influence the frequency of their oscillatory motion in pitch and their corresponding damping rates are required. Although a full-scale parachute canopy descends through air, oscillating in pitch as it does so, experiments necessary to determine these added mass coefficients have been performed underwater, using for this purpose a large ship tank from the towing carriage of which the model parachute canopies were suspended. These experiments showed that the added mass coefficients for bluff parachute canopies differed appreciably from their corresponding potential flow values. The latter were obtained from the analysis of inviscid fluid flow round regular shapes which were representative of those parachute canopies. The significance for the prediction of the parachute's dynamic behavior in pitch of these differences is outlined.

Restraint effect of fluid in the annular region formed by coaxial circular cylinders.

Abstract: The fundamental characteristics of fluid restraint of the annular region in coaxial circular cylinders were clarified. The effects of gap width, axial length, leakage from the end, etc., on the added mass coefficients and added damping coefficient of two types of circular cylinders were evaluated by both an experiment utilizing one-degree of freedom cylindrical models and analysis. The method for evaluating the vibration characteristics of a beam which has fluid restraint was investigated, and a convenient formulation for estimating the damping ratio was derived.

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.

Sensitivity derivatives and optimization of nodal point locations for vibration reduction

Abstract: A method is developed for sensitivity analysis and optimization of nodal point locations in connection with vibration reduction. A straightforward derivation of the expression for the derivative of nodal locations is given, and the role of the derivative in assessing design trends is demonstrated. An optimization process is developed which uses added lumped masses on the structure as design variables to move the node to a preselected location; for example, where low response amplitude is required or to a point which makes the mode shape nearly orthogonal to the force distribution, thereby minimizing the generalized force. The optimization formulation leads to values for added masses that adjust a nodal location while minimizing the total amount of added mass required to do so. As an example, the node of the second mode of a cantilever box beam is relocated to coincide with the centroid of a prescribed force distribution, thereby reducing the generalized force substantially without adding excessive mass. A comparison with an optimization formulation that directly minimizes the generalized force indicates that nodal placement gives essentially a minimum generalized force when the node is appropriately placed.

Frequency-Dependent Damping Forces in an Oil Damper

Abstract: It is usually assumed that damping forces in a torsional oil damper are proportional to the relative velocity between the inner and outer rings. This is valid only when the frequencies of the relative velocity are sufficiently low. To express the damping forces correctly at high frequencies, an additional term proportional to the relative acceleration or the relative displacement is necessary, because the phrase lag occurs in the responses of velocity distribution in the narrow passage of the damper. In this paper the frequencydependent viscous forces between the two parallel plates are analyzed, assuming that the one of them oscillates harmonically with a small amplitude in the parallel direction, keeping a small, constant gap, and the added mass and pseudo-stiffness terms are determined.

On the Effect of Facet Discretisation on the Calculated Heave Added Masses for a Submerged Pontoon

Abstract: The effects of facet discretisation on the calculated heave added masses for a submerged pontoon are investigated by using the three dimensional calculation method as well as the two dimensional ones. The results show that : When the number of facets approximating the model shape is small, it may cause about 10% difference in the calculated added masses when it is compared with the results obtained by using a large number of facets. Particulary the approximation at the corner part of the pontoon is effective.Therefore, in the estimation of the heave added masses for a semisubmersible offshore structure by use of the singularity distribution method, the number of facets for the lowerhull part should be carefully chosen.

Dynamic AnalyaiB of Liquid.Pilled Tank. including Pluticity aDd Pluid Interaetion . Earthquake EfFect

Abstract: 1. IntroduetionProblem Tanks enclosing highly explosive or dangerous fluids are needed for purposes of energy supply as well for the chemical industry. In Germany containers with a capacity of more than 100 000 m3 are being planned. With regard to public safety such tanks also have to be designed against catastrophic external loads of dynamic character such as earthquakes, external explosions, impact and air plane crashes. While with small tanks very rough investigations may be tolerated from safety aspects as well as from economical consideration these structures have to be studied very thoroughly. The following contribution which reports on the fluid-structure interaction in a tank under earth­ quake excitation may help to solve this problems. In this article emphasis lies however more on modelling of the fluid-structure interaction in an elastoplatic steel structure than on the type of dynamic excitation. The latter can easily be exchanged in favour of other actions as e.g. explosion loads or impact. 2. Available Knowledge Finite elements have previously been developed for the numerical analysis of fluid-structure inter­ action. Along the lines of derivation the used methods can be categorised into three approaches: - "added mass" approximation - "Eularian" formulation - "Lagrangian" formulation In the "added mass" approach some fraction of the fluid mass is added to the structural model. So that spatial discretisation of the fluid is avoided. In the mid 1950's, HOUBner(1,2) formulated a mechanical model for estimating liquid response in seismically excited rigid, rectangular and cylindrical tanks. He divided the hydrodynamic pressure of the contained liquid into two com­ ponents; the impulsive pressure caused by the accelerated portion of the liquid within the tank and the convective pressure resulting from that part of the fluid, which sloshes in the tank. This