Showing papers in "Journal of Ship Research in 1991"

Effects of emulsified fuel on combustion in a four-stroke diesel engine

Abstract: While the use of emulsified fuel in diesel engines has been an area of much research interest in recent years, the promising results reported in laboratories have not been easy to reproduce in commercial practice. Many of these studies have only measured external effects such as fuel consumption and exhaust emissions. A single-cylinder research engine was operated with water/diesel fuel oil and hydrous ethanol/diesel fuel oil emulsions of varying percentages. Crank angle, cylinder pressure and injector lift were recorded electronically over 50 engine cycles, permitting calculation of the mean and standard deviation of key combustion parameters. The results showed decreased fuel consumption and increased ignition delay, peak cylinder pressure and maximum cylinder pressure rise rate for emulsion operation. While the standard deviation data showed little change in cycle-to-cycle variation for water emulsion operation, increases of over 200 percent were measured for operation at ethanol amounts over 20 percent.

Sum-and Difference-Frequency Wave Loads on a Body in Unidirectional Gaussian Seas

Abstract: When nonlinear effects are included in the diffraction of waves by a large body, there are, at second order, interactions at the sums and differences of the component frequencies of the incident waves In this paper, the complete deterministic and stochastic solutions of second-order (sum- and difference-frequency) wave loads in undirectional Gaussian waves are considered The deterministic results, namely the wave force quadratic transfer function (QTF) in bichromatic incident waves, is complete in the context of second-order diffraction theory in that all the relevant components including those due to the exact second-order potentials are obtained Statistical properties of second-order wave excitations are then investigated using the QTF results and a two-term Voltera series model For illustration, the exact second-order force spectra and probability distributions for the simple geometry of a truncated vertical cylinder are obtained and compared with those based on a number of existing approximation methods It is found that second-order exciting force variances and probability of extreme values may be significantly underestimated by existing approximation methods

The Transient Capsize Diagram—A New Method of Quantifying Stability in Waves

Abstract: It is argued that a plot of wave steepness against wave period, showing the combinations which cause capsize, is a well-defined measure of the stability of a ship or ocean vehicle in waves, provided the conditions are transient, that is, the vessel is initially in relatively calm water, and is suddenly hit by a train of regular waves. Such Transient Capsize Diagrams can obviously be obtained by model testing; it is also argued that they could be obtained by computer simulation on contemporary desktop computers, taking advantage of recent developments in nonlinear strip theory

Leading-Edge Corrections to the Linear Theory of Partially Cavitating Hydrofoils

Abstract: In this work, first, the partially cavitating hydrofoil problem is formulated in linear theory in terms of vorticity and source distributions on the projection of the hydrofoil to the free-stream direction. The resulting system of integral equations is inverted and the solution is expressed in terms of integrals of the horizontal perturbation velocity in fully wetted flow, multiplied by weighting functions that are independent of the shape of the hydrofoil. Second, the linearized dynamic boundary condition on the cavity is modified so that the total velocity on the cavity as predicted by applying Lighthill's leading-edge corrections is a constant

First- and second-order wave effects on a submerged spheroid

Abstract: Computations are presented for the linearised force and moment acting on a submerged slender spheroid in regular waves, the resulting pitch and heave motions, and the second-order mean force and moment. These numerical results, which are based on the use of a three-dimensional panel code, are compared with the approximations based on slender-body theory. The accuracy of the slender-body approximation is relatively good for the first-order forces and body motions, but substantial errors are revealed for the second-order mean drift force and pitch moment. Unlike the approximate result, the more correct numerical prediction of the mean pitch moment is non-zero, and generally acts in the bow-down direction in head seas. To explain this result it is shown that the wave elevation directly above the spheroid increases in amplitude from bow to stern, thus causing a greater upward force on the afterbody relative to the forebody.

Interaction between two bodies translating in an inviscid fluid

Abstract: The equations of motion of two bodies in translation motion in an inviscid fluid at rest at infinity are expressed in Lagrangian form. For the case of one body stationary and the other approaching it in a uniform stream, an exact, closed-form solution in terms of added masses is obtained, yielding simple expressions for the velocity of the moving body as a function of its relative position and for the interaction forces. This solution is applied to the case of a rectangular cylinder approaching a cylindrical one, for which the added-mass coefficients had been previously obtained in a companion paper by an integral-equation procedure. In order to compare results with those in the literature, and to evaluate the accuracy of the present procedures, results were calculated for a pair of circular cylinders by these methods as well as by successive images. Very good agreement was found. Comparison with published results showed good agreement with the added mass but very poor agreement on the forces, including disagreement as to whether the forces were repulsive or attractive. The discrepancy is believed to be due to the omission of terms in the Bernoulli equation, which was used to obtain the pressure distribution and then the force on a body. The Lagrangian formulation is believed to be preferable to the pressure-integral approach because it yields the hydrodynamic force directly in terms of the added masses and their derivatives, thus requiring the calculation of many fewer coefficients.

A Review of the Kelvin Ship Wave Pattern

Abstract: This paper finds that the Kelvin ship wave induced by a moving pressure point consists of a single system of divergent waves but that there are infinitely many different Kelvin wave patterns for a submerged point source.

Modified strip model for analysing the line heating method - part 2: thermo-elastic-plastic plates

Abstract: A modified strip method is developed for analysing the process of plate forming by the line heating method. The idea has been presented in Part 1 for analysing elastic plates, and it is extended here to simulate approximately the transient thermo-elastic-plastic plate bending. Two cases are discussed. First, the case of uncoupled thermo-elastic-plastic plate is studied illustrating the use of a modified strip model. Second, generalised coupled governing equations for the thermo-elastic-plastic modified strip are derived and discussed. The results, with the use of a regular strip model, indicate a qualitative error in comparison with the use of a plate model, whereas with the use of a modified strip model a qualitative agreement is reached. Finally, suggestions for future research work are given. See also abstract number 91092048.

Pontoon torsional strength analysis related to ships with large deck openings

Abstract: The torsional problem of a pontoon, consisting of channel middle part and rectangular tube peaks, is considered within the higher-order beam theory. The cross section and the contour compatibility conditions for assembling of the pontoon parts are investigated. The acceptability of the introduced assumptions is checked by three-dimensional finite-element model analysis. Some deficiencies of the classical beam theory regarding the girder stiffness are noticed. The finite-element formulation to be used for the torsional analysis of the ship's hull with large hatch openings is given.

Modified strip model for analysing the line heating method - part 1: elastic plates

Abstract: The line heating method can be most useful in the automation of plate forming and for removal of undesired residual distortions during shipbuilding. The three-dimensional transient temperature and stress fields complicate the problem of simulating the line heating process. In past research efforts, a strip model taken perpendicular to the heated line was used. However, recent experimental and theoretical works have demonstrated the need for a different model. Recognising that the existing strip model cannot be used effectively in the analysis of the stress-strain field during the line heating process, a modified strip model is suggested. In this paper the concept of a modified strip is introduced by developing a theory for an elastic modified strip. The results of selected examples show that the modified strip model gives excellent results for elastic plates in comparison with a regular strip model.

Computation of viscous flow around propeller-body configurations: iowa axisymmetric body

Abstract: Further validation of a viscous flow method for predicting propeller-hull interaction is provided through detailed comparisons with recent experimental data for the practical configuration of the Iowa axisymmetric body. Modifications are made to the k-e turbulence model and wall functions for axisymmetric bodies. Close agreement is demonstrated between the calculations and the data, which supports the conclusion that the present procedures can accurately simulate the steady part of the combined propeller-hull flow field. However, the present extensive comparisons also point out the critical role of turbulence modelling and detailed numerical treatments. Also, comparisons are made with Huang's inviscid flow method. Although both methods show similar trends, there are some important differences; for example, Huang's method predicts reduced propeller loading and larger axial velocities in the propeller plane near the body surface and propeller tip. Near the propeller tip, the present method exhibits a velocity defect region, which is absent in Huang's method. In consideration of the greater rigour of the present method, such differences imply that viscous effects play an important role in propeller-hull interaction even for the relatively simple case of an axisymmetric body and should be accounted for in the design procedures of wake-adapted propellers. However, part of the differences may be due to some of the present detailed numerical treatments, which indicate the need for continued refinement of comprehensive methods such as the present one, and more detailed experimental information for validation purposes.

Investigation of the Cone Angle of a Novel Swedged-Stiffened Pressure Hull

Abstract: This paper presents a theoretical analysis of the elastic general instability of two swedged-stiffened cylinders subjected to hydrostatic pressure loading and the effect of varying the angle of the cone section on this general instability pressure. From the standpoint of buckling pressure, available cabin space, and weight reduction, a reasonable cone angle range is proposed

Modelling of flow around a marine propeller using a potential-based method

Abstract: A nonlinear method is presented for calculating potential flow around lifting bodies. The perturbation potential is taken to be unknown at the surface of the body while its value is fixed inside the body. This method is particularly well suited to cases of thin structures. It is applied to calculate the flow around a non-cavitating marine propeller for which several difficulties are examined and solutions proposed: problem of the Joukowski condition, problem of propeller mesh, wake description. The influence of the hub and the calculated wake on numerical results is analysed.

Nonlinear Local Analysis of Steady Flow About a Ship

Abstract: A nonlinear local analysis of the steady potential flow at a ship bow and stern, and more generally at any point along a ship waterline, is presented. The hull boundary condition and the nonlinear kinematic and dynamic free-surface boundary conditions are satisfied exactly, at the actual position of the free surface, in this analysis. The bow-flow analysis shows that the free surface at a ship bow is tangent to the stem. This theoretical results appears to agree with existing experimental measurements of steady bow waves of the Wigley hull. Simple analytical expressions defining the fluid velocity at the bow and the stern, and more generally at any point along the wave profile, in terms of the elevation of the free surface at the corresponding point are also given. These analytical expressions and the available experimental measurements of wave profiles along the Wigley hull show that the velocity of the flow disturbance due to this hull is fairly small compared to the hull speed everywhere along the wave profile except in very small regions around the bow and the stern, where the total fluid velocity is nearly equal to the hull speed in magnitude but directed vertically. Nonlinearities therefore appear to be quite important, although only in very small regions surrounding a ship bow and stern. A genuine nonlinear method of calculation must then be able to represent the very rapid variation in the direction of the fluid velocity occurring within small regions around a ship bow and stern. In particular, a sufficiently fine discretisation is required in these regions.

Statistical analysis of failure time distributions for Great Lakes marine diesels using censored data

Abstract: Field-censored failure data collected from six identical Colt-Pielstick PC2-400 Great Lakes marine diesel engine components are analysed. Appropriate censored lifetime data analysis techniques are presented and applied to obtain the probability density function (PDF) parameter estimates for the entire population when only a portion of that population has failed. Weibull and exponential PDF parameters are evaluated for seven components - namely cylinder pistons, heads, jackets, liners and O-rings, connecting rod bearings, fuel cams and turbochargers. Reliability and hazard functions for these items are presented and discussed. It is shown that the exponential model is inappropriate for these items.

Stern flows at full-scale Reynolds numbers

Abstract: A numerical method for the solution of the Reynolds-averaged Navier-Stokes equations for three-dimensional flow of an incompressible fluid, used previously for calculations at model-scale Reynolds numbers, has been applied to the flow over the stern of an axi-symmetric body and a representative ship over a range of Reynolds numbers up to five billion. High Reynolds number experiments on axi-symmetric bodies strongly support the use of wall functions in the turbulence model. Comparisons with traditional ship wake scaling laws suggest that such laws do not properly describe the scale effects on the flow in the propeller plane.

Oblique Impact of Two Cylinders in a Uniform Flow

Abstract: The oblique motion of a circular cylinder through an inviscid and incompressible fluid, conveyed by a uniform flow at infinity, in the vicinity of another cylinder fixed in space is considered. In a relative polar 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 the cylinder pair. The Lagrange equations of motion are then integrated for the trajectories of the moving cylinder. In order to evaluate the added masses and their derivatives with respect to the separation distance between the cylinders in terms of the hydrodynamic singularities, the method of successive images, initiated by Hicks, and the Taylor added-mass formula are applied, and analytic solutions in closed form are obtained thereafter. The dynamic behaviour of a drifting body in close proximity of a fixed one is investigated by considering the limiting values of the fluid kinetic energy and the interaction forces on each body. The reliability of the numerical approximation of added masses and their derivatives is also discussed in the present study. The integral equations, in terms of surface source distributions and also discussed in the present study. The integral equations, in terms of surface source distributions and their derivatives on both circles, are carefully modified for obtaining accurate numerical solutions.

Analysis of the flow around supercavitating hydrofoils with midchord and face cavity detachment

Abstract: In this work, first the linearised supercavitating hydrofoil problem with arbitrary cavity detachment points is formulated in terms of unknown source and vorticity distributions The corresponding integral equations are inverted analytically and the results are expressed in terms of integrals of quantities which depend only on the hydrofoil shape These integrals are computed numerically, in an accurate and efficient way, to produce cavity shapes and pressure distributions on the foil and cavity The effect of the cavity detachment points on the shape of the cavity and the foil pressure distribution is investigated An inviscid flow criterion for the cavity detachment point is derived for the case where the cavity detaches in front of the trailing edge on the pressure side of the hydrofoil Finally, the accuracy of the linearised cavity theory is assessed for different foils and flow conditions, by analysing the produced cavity shapes with a nonlinear panel method

Some numerical results in 3-d transient linear naval hydrodynamics

Abstract: Some numerical results obtained by the code MELINA are given This code has been designed for the solution of three-dimensional steady and transient flows past a floating or submerged body; the fundamental equations of dynamics are taken into account in the case where the body is freely floating Only the case of an ocean of infinite depth is addressed here It is shown that convergence to a steady state is achieved in the case of a periodic forced motion if the history of the motion is correctly taken into account Also, the case of a freely floating sphere with initial vertical displacement from equilibrium is treated and compared with experiments

Modeling of Welding Distortion in Stiffened Rings

Abstract: A simple model is developed to describe the deformation of a ring stiffener under welding, with applications to submarine hulls. The model is based on the realisation that only a small part of the structure near the welding arc (near field) undergoes high temperature changes, and thus behaves thermo-elastic-plastically, while the rest of the structure (far field) is elastic in nature. The model is used to predict the overall axisymmetric residual shrinkage of different rings and the results are shown to be in excellent agreement with the finite-element method (FEM). This concept of near and far field could also be used to simplify three-dimensional FE models when predicting local, asymmetric welding distortions of various geometry.

On an iterative solution for nonlinear wave calculations

Abstract: This paper presents a study of a numerical ship-wave-resistance formulation based on control volume analysis with an iterative procedure, which accommodates nonlinear free surface conditions, parallel to Gadd's (1976) formulation. Gadd reported ship resistance calculations, but the exact nature of the approximations and assumptions used in the calculations were not easily available at the time. The present formulation attempts to rationalise the procedure already in use and establishes the limits of the approximations and inherent assumptions. Numerical results based on this iterative method are presented for the flow about a two-dimensional foil beneath the free surface. The numerical results are compared with published experimental measurements and other computations.

Nonlinear prediction of vertical motions of a fishing vessel in head sea

Abstract: Several years ago the authors developed a practical method for calculating vertical motions and wave loads of a high-speed craft which travels in regular head sea, and verified its validity by comparing the computed motions and wave loads with the results of model tests. In order to clarify further its validity, the method is applied herein to compute the vertical modions of a fishing vessel, and the computed motions are compared with the results of experiments conducted by Bales and others at the David W. Taylor Naval Ship Research and Development Center (DTNSRDC). Also, the vertical motions and ship-to-wave relative motions predicted by the method are compared with the numerical results of conventional linear strip theory computations performed at DTNSRDC. As a result, it is found that the present method, which in principle is based on the conventional Ordinary Strip Method synthesis but modified to be able to evaluate nonlinear hydrodynamic impact forces as well as dynamic lift in waves, can be applied to estimate vertical motions and ship-to-wave relative motions of fishing vessels travelling in head sea with enough accuracy for practical use.

Three-dimensional nonlinear dynamics of nonintegral riser bundle

Abstract: The dynamic behaviour of a nonintegral riser bundle is studied parametrically. The dynamics of each component-riser is analysed by a three-dimensional, nonlinear, large deflection, small strain model with coupled bending and torsion. Component-risers are slender, thin-walled, extensible or inextensible tubular beam-columns, subject to response and deformation dependent hydrodynamic loads. The connector equations of equilibrium are used to derive the connector forces and moments. Substructuring can thus be achieved even though in three dimensions connectors do not impose linearly dependent deflections at substructure interfaces. The developed time incremental and iterative finite-element computer code is used to analyse the effects of water depth, distribution of connectors, distance between component risers and number of finite elements in the numerical model. The problem of total CPU (central processor unit) time and the advantages of substructuring are discussed by running cases of up to 1094 degrees of freedom.

Ship lines using convexity-preserving rational cubic interpolation

Abstract: An algorithm is given and applied to the generation of ship lines from design station offset data. The specific advantages of this interpolant are its smoothness, simplicity, speed of computation and convexity-preserving properties