Showing papers on "Added mass published in 1986"

The force on an axisymmetric body in linearized, time-dependent motion: a new memory term

Abstract: In contrast to the steady Stokes equations for creeping motion, the time-dependent linearized Navier–Stokes equations have only been solved for very restricted geometries, the solution for the sphere being the sole solution for an isolated finite body. In the present paper, the linearized Navier–Stokes equations are further explored and a simple expression is derived which relates the force on an arbitrary axisymmetric body in oscillatory motion to the solution for the stream function in the far field. This result is applied to the case of a slightly eccentric spheroid and it is shown that the total hydrodynamic force contains four terms, three of which correspond to the classical solutions for the Stokes drag, added mass and Basset force on the perturbed sphere; the fourth term is only present when the body is non-spherical. In contrast to the three classical forces, the new term is not a simple power of the dimensionless frequency parameter iL2ω/ν, in which L is a length-scale, ω is the frequency of oscillation and ν is the kinematic viscosity of the fluid. A Laplace superposition is then used to find the force on the spheroid in an arbitrary axisymmetric motion with velocity U(t). The new memory term decays faster than the Basset force at large times and is bounded at short times.

Effects of Fluid Inertia and Turbulence on the Force Coefficients for Squeeze Film Dampers

Abstract: The effects of fluid inertia and turbulence on the force coefficients of squeeze film dampers are investigated analytically. Both the convective and the temporal terms are included in the analysis of inertia effects. The analysis of turbulence is based on friction coefficients currently found in the literature for Poiseuille flow. The effect of fluid inertia on the magnitude of the radial direct inertia coefficient (i.e., to produce an apparent added mass at small eccentricity ratios, due to the temporal terms) is found to be completely reversed at large eccentricity ratios. The reversal is due entirely to the inclusion of the convective inertia terms in the analysis. Turbulence is found to produce a large effect on the direct damping coefficient at high eccentricity ratios. For the long or sealed squeeze film damper at high eccentricity ratios, the damping prediction with turbulence included is an order of magnitude higher than the laminar solution.

Note on added mass and drift

Abstract: Several points of interpretation are reviewed bearing on the celebrated discovery by Darwin (1953) that the added mass for a body translating uniformly in an infinite expanse of perfect fluid equals the drift-volume times the density of the fluid The discussion focuses on the delicate qualifications needed to secure this equality as a mathematical proposition In § 2 a different approach to the matter is presented, leading to a new fact about added mass In § 3 a model of infinity in the fluid is proposed which clarifies an aspect of Darwin's original analysis

Modelling of radiation damping in fluids by finite elements

Abstract: A very efficient technique is presented to model the effects of radiation damping in the computation of added mass for the dynamic analysis of submerged structures. The structure is assumed to be surrounded by an infinite, incompressible and inviscid fluid field and the effect of the free surface is neglected. The technique is implemented in the finite element analysis of two-dimensional problems, assuming pressure to be the nodal unknown. The implementation procedure is quite simple and the symmetrical and banded form of the matrix of coefficients remains unchanged. With the use of the proposed radiation condition, the fluid field may be truncated at a relatively very short distance from the solid—fluid interface. This results in great computational advantages. Furthermore, a guideline is suggested for the selection of the geometry and the location of the truncation boundary to enhance the computational efficiency. The effectiveness and efficiency of the technique is demonstrated by analysing several cases for different geometries of the solid—fluid interface and the truncation boundary.

Time-periodic wave loading on a submerged circular cylinder in a current

Abstract: The time-periodic pressure loading, added mass, damping, and exciting forces on a horizontal submerged circular cylinder in a current are examined. The fluid layer is infinitely deep and the motion is two-dimensional. The boundary-value problem is solved by applying a source distribution along the contour of the body.

Two-Dimensional Analysis on the Lateral Drifting Force Between Two Floating Structures

Abstract: An analytical procedure for evaluating the lateral drifting forces between two structures by the near-field method is presented. The velocity potentials, including the hydrodynamic interactions, are evaluated by a two-dimensional sink-source technique. This method yields the mean drifting force consisting of four components of which the relative wave elevation term is dominant, whereas the Bernoulli quadratic term is secondary. The phenomema of negative drifting force have been found in the study, which are consistent with the standing wave or negative added mass. The near-field method clearly reveals the effects of the behavior of the response motions such as heave and roll resonances, although it seems to be complicated at first and difficult to use. It is also found that the hydrodynamic interactions between two bodies cannot be neglected.

Aerodynamic forces acting on parachutes in unste.4dy motion and thf consequential dynamic stability characteristics

Abstract: As parachutes descend they oscillate in pitch. In order that the effects of the most significant parameters influencing these oscillations are clearly identified it is important to be able to predict satisfactorily both their frequency and their damping. To do so it is first necessary to establish a simple and effective dynamic model, then to establish by experiment both themagnitudes and the variations with canopy shape of any as yet undetermined terms in that model. In this paper the ways are considered in which the unsteady aerodynamic forces and moments developed at an instant in time on bluff bodies like parachute canopies depend on both the instantaneous velocity and instantaneous acceleration. The concept of added mass is briefly re-examined. New experimental values of added mass coefficients, obtained for a range of canopy shapes during a recent test programme, have been tabulated and their significance in predicting parachute dynamic stability explained.

Concentrated Mass Effects on the Flutter of a Composite Advanced Turboprop Model

Abstract: The effects on bending-torsion flutter due to the addition of a concentrated mass to an advanced turboprop model blade with rigid hub are studied. Specifically the effects of the magnitude and location of added mass on the natural frequencies, mode shapes, critical interblade phase angle, and flutter Mach number are analytically investigated. The flutter of a propfan model is shown to be sensitive to the change in mass distribution. Static unbalance effects, like those for fixed wings, were shown to occur as the concentrated mass was moved from the leading edge to the trailing edge with the exception of one mass location. Mass balancing is also inferred to be a feasible method for increasing the flutter speed.

An optimal design problem for submerged bodies

Abstract: : When a body, floating on the surface of an infinite, ideal, inviscid, irrotational fluid is subjected to a periodic vertical displacement, a wave pattern is created in the fluid and the problem of determining this pattern from a knowledge of the body geometry and applied forces is well known in fluid mechanics. In problems with both partially and fully submerged objects, quantities of physical interest are not only the wave patterns which may be derived from the velocity potential but also functionals of the potential such as added mass and damping factors which measure the distribution of energy in the fluid. These factors are, of course, dependent on the body geometry. The present paper is devoted to showing how these quantities may be optimized over restricted classes of body geometry. Specifically we study the problem of the optimal design of a floating body, totally submerged in a fluid of finite depth. In the terminology of optimal control, this is a problem of optimization of geometrical elements.

Wave Forces Acting on Rough Circular Cylinders at High Reynolds Numbers

Abstract: Hydrodynamic forces acting on rough circular cylinders in a harmonically oscillating flow at high Reynolds numbers were experimentally investigated.Especially effects of the roughness height and roughness density were investigated. Two kinds of roughness height-5mm, 10mm and three kinds of roughness density-20, 60, 80% were used in experiments.Experiments were carried out using a large circular cylinder with 1.2m diameter and with 1.5m length at NKK's Tsu Ship Model Basin. The cylinder was fixed to the main carriage. The cylinder was forced to oscillate harmonically with the main carriage. In-line and lift forces acting on cylinders were measured at Keulegan-Carpenter numbers over 6 up to 50 and at two Reynolds numbers-0.5, 1.0×106. Drag coefficients, added mass coefficients, maximum lift coefficients and periods of lift force are obtained.Experiments show that the roughness has a great effect on the hydrodynamic coefficients even at high Reynolds numbers. In the range of the value of roughness parameters used in these experiments, a roughness height has a more considerable effect on the hydrodynamic coefficients than a roughness density.

Experimental Determination of Fluid-Coupling Coefficients in an Array of Hexagonal Prisms

Abstract: The problem is related to the seismic analysis of fast breeder reactor cores, where the fluid trapped between the subassemblies is known to have a considerable influence on the core response. The paper describes an experiment on a simple geometry which is representative of an actual LMFBR core. Experimental added mass coefficients are determined for various values of the gap, and compared with Finite Element results. The damping contribution is also analysed. The results indicate that experimental added mass coefficients are consistent with numerical estimations obtained with a plane model and a perfect fluid. The plane model, however, overestimates the coefficients by about 20%, which can be attributed to tridimensional effects in the tests. Experimental results indicate that the fluid damping cross terms are very small and that the damping ratio increases sharply as the gap decreases.

Analysis of radiating flexural shell sonar transducers using the finite element method

Abstract: New flexural shell transducers for low‐frequency applications are currently developed which are characterized by a large volume velocity and a drastic reduction of their resonance frequencies as soon as they are flooded, due to added mass effect. To design these transducers, a finite element modeling is very useful, because it can accurately handle the assembling of three‐dimensional and shell parts in the same structure, the piezoelectric driving force, the fluid‐structure interaction as well as the radiation damping. This paper describes the analysis of a test axisymmetric transducer with the ATILA code [J. N. Decarpigny et al., J. Acoust. Soc. Am. 78, 1499 (1985)] using dipolar damping elements and a new extrapolation method to obtain the transducer farfield characteristics [R. Bossut et al., J. Acoust. Soc. Am. Suppl. 1 79, S51 (1986)]. In‐air resonance modes, transmitting voltage response and directivity patterns are computed and compared to measurements, displaying a satisfactory agreement. Finally,...

Steady-state ice-structure interaction analysis

Abstract: Two-dimensional elastic wave theory is utilized to investigate the steady-state dynamic response of a cylindrical structure of elliptic cross section surrounded by an ice sheet of sinfinite extent. Fixed and frictionless boundary conditions at the ice-structure interface are considered under both plane stress and plane strain conditions. Numerical results are presented which show the effect of structural geometry, direction and frequency of vibration, and choice of boundary conditions on the dynamic characteristics of the ice-structure system. The analysis shows that the behavior of the ice-structure system is primarily controlled by the associated damping effects rather than the added mass. These results provide a measure of ice sheet resistance to the vibrational motion of structures surrounded by floating ice sheets and are applicable to the dynamic and seismic analysis of arctic offshore platforms.

On the Nonlinear Hydrodynamic Forces due to Large Amplitude Forced Oscillations

Abstract: The nonlinear hydrodynamic forces acting on a two-dimensional circular cylinder, oscillating with large amplitude in the free surface, are calculated by using the Semi-Lagrangian Time-Step-ping Method used by O.M. Faltinsen. In present calculation the position and the potential value of free surface are calculated using the exact kinematic and dynamic free surface boundary condition. At each time step an integral equation is solved to obtain the value of potential and normal velocity along the boundaries, consisting of both the body surface and the free surface. Some effort was devoted to the elimination of instability arising in the range of high frequency. Numerical simulations were performed up to the 3rd or 4th period which seems to be enough for the transient effect to die out. Each harmonic component and time-mean force are obtained by the Fourier transform of forces in time domain. The results are compared with others' experimental and theoretical results. Particularly, the calculation shows the tendency that the acceleration-phase 1st-harmonic component(added mass) increases as the motion amplitude increases and a reverse tendency in the velocity-phase 1st-harmonic component(damping coefficient). The Yamashita's experimental result also shows the same tendency. In general, the present result show relatively good agreement with the Yamashita's experimental result except for the time-mean force.

Fundamental Investigation on an Oil Damper : 5th Report, Effects of Eccentricity Between the Centers of Cylinder and Piston

Abstract: In a series of the work, we study on a simply constructed oil damper ; it has an annular cross-sectional clearance between the cylinder and the piston as a flow channel of oil. This report describes an analytical and experimental study of eccentric effects on the drag of the piston placed eccentrically within the cylinder. Assuming a laminar and unsteady flow, we analyze the oil stream through the eccentric annular channel, and derive an added mass and a damping coefficient of the oil damper. These values are compared with experimental results, and the following results are obtained : The eccentricity increases the added mass and decreases the damping coefficient of the oil damper. An increase in frequency of reciprocating motion of the piston reduces the eccentric effects.

Algorithm for energy-derived potential flow hydrodynamic coefficients

Abstract: An algorithm is described to compute steady and unsteady hydrodynamic coefficients from added mass tensors without the need to develop highly intense and system dependent algebraic expressions and computer codes, especially when complex motions are admitted. Lagrange's equation is applied in fixed coordinates. Coordinate transformations are accomplished to obtain an efficient algorithm for six-degrees-of-freedom coefficients for bodies hydrodynamically defined with time varying added mass tensors. In addition to infinite fluid, the algorithm is applicable to restricted classes of emerging body and free surface conditions.