Showing papers on "Added mass published in 1984"

The Aerodynamics of Hovering Insect Flight. II. Morphological Parameters

Abstract: Morphological parameters are presented for a variety of insects that have been filmed in free flight. The nature of the parameters is such that they can be divided into two distinct groups: gross parameters and shape parameters. The gross parameters provide a very crude, first-order description of the morphology of a flying animal: its mass, body length, wing length, wing area and wing mass. Another gross parameter of the wings is their virtual mass, or added mass, which is the mass of air accelerated and decelerated together with the wing at either end of the wingbeat. The wing motion during these accelerations is almost perpendicular to the wing surface, and the virtual mass is approximately given by the mass of air contained in an imaginary cylinder around the wing with the chord as its diameter. The virtual mass ranges from 0.3 to 1.3 times the actual wing mass, indicating that the total mass accelerated by the flight muscles can be more than twice the wing mass itself. Over the limited size range of insects in this study, the interspecific variation of non-dimensional forms of the gross parameters is much greater than any systematic allometric variation, and no interspecific correlations can be found. The new shape parameters provide quite a surprise, however: intraspecific coefficients of variation are very low, often only 1%, and interspecific allometric relations are extremely strong. Mechanical aspects of flight depend not only on the magnitude of gross morphological quantities, but also on their distributions. Non-dimensional radii are derived from the non-dimensional moments of the distributions; for example, the first radius of wing mass about the wing base gives the position of the centre of mass, and the second radius corresponds to the radius of gyration. The radii are called \`shape parameters' since they are functions only of the normalized shape of the distributions, and they provide a second-order description of the animal morphology. The various radii of wing area are strongly correlated, as are those of wing mass and of virtual mass: the higher radii for each quantity can all be expressed by allometric functions of the first radius. The overall shape of the distribution of a quantity can therefore be characterized by a single parameter, the position of the centroid of that quantity. The strong relations between the radii of wing area, mass and virtual mass hold for a diverse collection of insects, birds and bats. Thus flying animals adhere to \`laws of shape' regardless of biological differences. Aerodynamic and mechanical considerations are most likely to provide an understanding of these laws of shape, but an explanation has proved elusive so far. The detailed shape of a distribution can be reconstructed from the shape parameters by matching the moments of the observed distribution to those of a suitable analytical function. A Beta distribution is compared with the distribution of wing area, i.e. the shape of the wing, and a very good fit is found. With use of the laws of shape relating the higher radii to the first radius, the Beta distribution can be reduced to a function of only one parameter, thus providing a powerful tool for drawing a close approximation to the entire shape of a wing given only its centroid of area. Quite unexpectedly, the continuous spectrum of wing shapes can then be described in detail by a single parameter of shape.

The dispersive effects of Basset history forces on particle motion in a turbulent flow

Abstract: A calculation of the asymptotic particle diffusion coefficient in an unbounded stationary homogeneous turbulent flow is performed for a particle subjected to both added mass and Basset history forces, as well as the normal Stokes drag. The original value obtained by Tchen is shown to be in error by an additional term which for a sphere of diameter d is given by (d2/2πνf)σ, νf being the kinematic viscosity, and σ the covariance of the incipient relative velocity between particle and local fluid. Noticeably this term does not depend upon the particle density. This extra term is consistent with values obtained for the particle diffusion coefficient from a numerical simulation.

The occurrence of negative added mass in free-surface problems involving submerged oscillating bodies

Abstract: The phenomenon of negative added mass is studied by considering the heave oscillations of a submerged vertical cylinder. Free-surface effects are shown to be important for the occurrence of negative added mass. Rapid changes in the added mass and damping, as functions of frequency of oscillation, often associated with this phenomenon are explained in terms of near-resonant standing waves above the body.

The Motions of Adjacent Floating Structures in Oblique Waves

Abstract: This paper describes the theory on the effects of hydrodynamic interaction between two parallel slender structures in oblique waves. The method is based on the twodimensional diffraction theory including the interaction effect. According to Ohkusu's theory, the sectional interaction effects on the added mass, damping coefficient and wave exciting force are evaluated by analyzing incoming waves generated by the oscillatory motion of corresponding sections. Numerical results of the wave exciting force and moment and motions for the case of a combination of a ship and a rectangular barge are presented and compared with the results from model experiments. The comparison shows good agreement. Finally, some attention is given to the relationship between the arrangement of the two structures and responses in irregular waves.

A transfer function method of modeling systems with frequency-dependent coefficients

Abstract: The fluid forces that act on a floating body conventionally are represented in the frequency domain by frequency-dependent coefficients, the added mass, and damping. This paper describes a method of characterizing these forces in the time domain by a rational transfer function, an equivalent dynamic system that can be appended to the "dry" body dynamics. The technique is illustrated by an application to the roll motion of a flatbottomed barge; a low-order transfer function represents the infinite-order fluid dynamic system with acceptable accuracy. The influence of the frequency-dependent coefficients oh the overall system dynamics is discussed, and it is shown that multiple roll resonance peaks are possible.

Numerical simulations of compliant material response to turbulent flow

Abstract: Homogeneous and internally structured material coatings are studied numerically as candidates for drag and turbulence generated noise reduction on submersible hulls. Dynamic motions of compliant surfaces may conceptually interrupt random turbulent motions, reducing the turbulent interface stresses and absorbing turbulent energy, and consequently reducing the turbulence at the flow/coating interface. Time dependent, noninteractive, two and three dimensional Monte-Carlo turbulent pressure field models, interactive potential flow models, and interactive Navier-Stokes pseudo-spectral methods are used to represent the unsteady flow, while time-explicit finite element methods are applied to represent a variety of homogeneous layered, and internally structured material coatings. Attention is given to development and testing of an efficient but dynamically interactive fluid/solid interface coupling method, for two- or three-dimensional response simulations. The influence of added mass and of depth overburden on the material response in ocean water is discussed. Promising compliant material coatings include sandwiches of soft, homogeneous layers between thin, stiffer elastic materials and internally structured coatings combining streamwise ribs, spanwise voids and elastically deforming intervening stiffeners.

Hydrodynamic characteristics of large offshore units

Abstract: In this paper, several theoretical methods for the calculation of the wave forces, added masses, damping coefficients and mean drift forces for one-body and two-body configurations are presented.

Real time prediction of ship motions

Abstract: The theory of predicting ship motions in real time is outlined and the problems associated with its application are discussed. It is found that the most severe problems are related to the phase differences among the various motions and the frequency dependence of the added mass and damping hydrodynamic coefficients.

Fundamental Investigation on an Oil Damper : 3rd Report Comparison of Analyses Based on Cylindrical Coordinates and Cartesian Coordinates

Abstract: This paper deals with a theoretical research of an oil damper, whose piston is vibrating in a viscous fluid filling a fixed circular cylinder. In the previous report, transforming the annular clearance between the cylinder and the piston into a straight channel, the authors induced a drag force acting on the piston from an equation of motion of the fluid expressed in Cartesian coordinates. Therefore the result of this analysis contained an error due to the transformation. So in this report, the authors try to derive a more exact value using an equation represented in cylindrical coordinates. Comparing this solution with the former one, they proved that these are very close to each other, if there is not so much difference between the diameter of piston and the inner diameter of cylinder.

chapter 16 – FLUID MECHANICS

Abstract: Publisher Summary A fluid is a substance that cannot support a shearing stress. This characteristic gives fluids the ability to flow or change shapes. Both gases and liquids are fluids. Fluid mechanics is concerned with the behavior of fluids at the macroscopic level—the level at which measurements are made with pressure gauges, thermometers, and flow meters. Fluid mechanics is the analysis of the behavior of fluids. The description of fluid motion relies on Newton's laws of motion that is formulated in terms of measurable properties of the fluid, such as density, pressure, and flow velocity. A fluid has weight and, therefore, exerts a pressure on submerged objects. A fluid in motion transports momentum and energy. This chapter describes fluid statics and also explains the buoyant force exerted by fluids called Archimedes' principle. Fluid statics is concerned with fluids in which the center of mass of each fluid particle has zero velocity and zero acceleration. The chapter describes viscosity also known as unstable fluid motion.

Computing added mass coefficients

Abstract: Publisher Summary The objective of packages of elliptic problem solvers, such as ELLPACK and the library of the Club Modulef, is to obtain the accurate solution of general elliptic problems. The goal has only come close to being reached in reasonably general domains for linear, second-order differential equations (DE) in two space dimensions. Even for the simplest elliptic DE, the Laplace equation, Dirichlet-type boundary conditions are the only ones that are handled routinely, in theory and practice. This DE lies at the heart of the theory of potential flows, including especially that around a solid body moving through an ideal fluid. The theory of such flows, as developed by Kelvin and Kirchhoff, is by far the most satisfactory part of fluid dynamics from a mathematical standpoint. The resulting forces and moments on a body of general shape are all determined by a few added mass coefficients mjk. This chapter focuses on computing added mass coefficients. A package of subroutines for solving elliptic problems enable the user to compute all added mass coefficients for a solid or cylinder of general shape, by simply specifying its shape and size.