Showing papers in "Journal of Ship Research in 1974"

A Nonlinear Theory of Sloshing in Rectangular Tanks

Abstract: A two-dimensional rigid, rectangular, open tank without baffles is forced to oscillate harmonically with small amplitudes of sway or roll oscillation in the vicinity of the lowest natural frequency for the fluid inside the tank. The breadth of the tank is 0 (1) and the depth of the fluid is either (1) or infinite. The excitation is of the order epsilon and the response is of the order epsilon to the one-third. A nonlinear, inviscid boundary-value problem of potential flow is formulated and the steady-state solution is found as a power series in epsilon to the one-third correctly to the order epsilon. Comparison between theory and experiment shows reasonable agreement. The stability of the steady-state solution has been studied.

Minimum-trajectory pipe routing

Abstract: A dynamic programming method for routing pipe between two specified terminal points is developed and demonstrated. This method, modified to meet requirements of practical piping runs, could be an important contribution toward a totally computerized piping arrangement method.

Lobar buckling of thin-walled cylindrical and truncated conical shells under external pressures

Abstract: Numerical solutions have been produced for the asymmetric instability of thin-walled circular cylindrical and truncated conical shells under external pressure. The solutions for the circular cylinder have shown that the assumed buckling configurations of Nash and Kaminsky were quite reasonable for fixed ends. Comparison was also made of the finite-element solution of conical shells with other analyses. From these calculations, it was shown that the numerical solutions were superior to the analytical ones, as the former could be readily applied to vessels of varying thickness or those subjected to unsymmetrical loading or with complex boundary conditions.

Approximate probabilistic method of calculating ship longitudinal strength

Abstract: Recently, classification societies have taken an interest in the statistical approach to ship longitudinal strength and the probabilistic methods for defining new design criteria. This interest is partially a result of the development and the growing demand for large tankers; it is reflected in the increasing research done by the regulatory bodies in these areas. This paper presents a framework for an approximate probabilistic method to calculate ship longitudinal strength with possible implementation in classification societies rules in mind. The most important features of this method are that it is simple, realistic, consistent, and distribution-free. No assumptions are made with regard to the forms of the distribution functions of the random variables involved in the procedure. Nevertheless, the uncertainties associated with these variables are taken into consideration and are quantified by their coefficients of variation. Analyses of eighteen ships of different types are made in order to serve as a preliminary investigation of the appropriate level of safety as measured by a proposed safety index.

On the Linear Theory of an Actuator Disk in a Viscous Fluid

Abstract: The linearized theory of an actuator disk working in an incompressible viscous fluid is given. The optimum radial thrust distribution is derived. This gives information about the optimum radial thrust distribution of a propeller in a turbulent wake. It is shown that the efficiency of an actuator disk is greater in a viscous medium than in an inviscid fluid.

Linear Theory on Water Entry and Exit Problems of a Ventilating Thin Wedge

Abstract: A simplified mathematical model is formed for the flow field caused by the ventilating blade of a partially submerged propeller. The model is formed to investigate the difference between the phases of entry and exit; that is, a two-dimensional flat-sided wedge and a two-dimensional cambered wedge entering a free surface and exiting at another free surface, located at a distance equivalent to the length of circular path of blade section in the water. Pressure and drag are calculated for a flat plate and a parabolic cambered wedge, each at the instantaneous position, by a conformal mapping technique. A great difference in pressure distributions between the phases of entry and exit is obtained.

Stresses in Marine Propellers

Abstract: A rational approach to the general solution of the propeller strength problem is established. A finite-element displacement model is utilized to predict the behavior of an elastic body having an arbitrary shape and subjected to prescribed loading. Solid elements in their general form are adopted. This formulation bypasses the constraints of simplified assumptions commonly employed by many classical plate and shell theories, and allows a more realistic approximation to the true structural configuration than is possible by most other conventional approaches. The application of the present procedure is simple and straight-forward. Good agreements with measured displacements and experimental stress data were obtained for a highly skewed blade as well as a supercavitating blade.

The Effect of a Surface Layer of Viscous Fluid on the Wave Resistance of a Thin Ship

Abstract: A modification to the gravity-wave free-surface condition is suggested, to account for dissipative properties of thin surface layers of material such as broken-up ice, slush, or oil spills. A corresponding correction to Michell's wave-resistance integral is established, and computations are carried out for a parabolic strut.

A simple design theory and method for bulbous bows of ships

Abstract: A simple bulb design theory and method to minimize wave resistance are presented for bulbous bow ships, which attempt to determine how large a bulb should be for a given parent ship and design speed, where the bulb should be located, and how it should be faired into the ship

A numerical method for steady-state cavity flows

Abstract: : The objective of the work was to develop a numerical method for steady-state cavity flows to provide systematic correction of an initial, assumed free streamline position. The method is based on an inviscid, irrotational and incompressible flowfield, uses a numerical finite-difference representation with the fluid velocities as dependent variables, and obtains a solution by successive-overrelaxation. It has been implemented for two-dimensional fully cavitating flow past a vertical plate in a channel. Typical results and the computer program listing and output are given. (Author)

A time domain solution to the motions of a steered ship in waves

Abstract: The problem of ship motions in waves is formulated in the time domain by means of a convolution integral which relates the ship motion response to arbitrary exciting forces, under assumption that the response is linear The convolution integral is evaluated numerically to obtain the ship motions at discrete intervals of time Frequency independent nonlinearities of arbitrary form are incorporated into the model by considering them as part of the arbitrary exciting forces Nonlinearities with time lag, such as those arising from rudder motions, are particularly amenable to this treatment Nonlinearities that are functions of the instantaneous motions of the ship are approximated by continuously extrapolating the ship motions Thus one is able to include frequency-dependent linear force terms in what amounts to a stepwise solution of the nonlinear equation of motion, a capability not available in the conventional direct numerical integration techniques exemplified by, eg, Runge-Kutta (Author)

A Modified Froude Method for Determining Full-Scale Resistance of Surface Ships from Towed Models

Abstract: The Froude method for extrapolating the resistance of towed ship models to full-scale equivalent flat-plate resistance. A form factor for extrapolation is to be determined from a boundary-layer calculation of a uniquely defined equivalent body of revolution. Examples of form factors calculated for the LUCY ASHTON and a hull with a block coefficient of 0.8 are shown to be quite satisfactory.

A Linear Analysis of Planar Motion Mechanism Data

Abstract: A linear analysis of ship motions (for example, of directional stability and control) does not require that slow-motion derivatives be employed to represent fluid forces and moments. Indeed the notion of such a representation implies the use of a Taylor series in circumstances where it is not strictly tenable. For linearity, it is only necessary that the principle of superposition be employed. Experimental data show that, when used in this sense, the assumption of linearity is by no means unreasonable.

Finite element analysis and optimization of a web frame of a tanker with isolated ballast system

Abstract: Finite-element analysis was used in the design of an unconventional tanker web frame which satisfies certain requirements stipulated by an isolated ballast system The weights of the web frame was then minimized using an optimality criterion based on a fully stressed design A double iteration procedure was developed which allowed for the efficient use of the optimization program in conjunction with the finite-element analysis The reduction in the weight of the web frame is dependent on the minimum allowed plate thickness as shown in the results of the paper

Transverse Vibrations of a Ship Hull in Ideal Fluid, Determined Through Variational Methods

Abstract: A direct formulation for analyzing the vibration characteristics of an arbitrary body and predicting the three-dimensional velocity potential is presented. It consists of forming the Lagrangian for a Timoshenko-beam fluid system and minimizing Hamilton's principal function by the Rayleigh-Ritz method. Due to the boundary condition, the minimizing sequence for vibration amplitude transforms the velocity potential into an associated sequence of Neumann boundary-value functions. These are expressed further in terms of another minimizing sequence and evaluated after minimizing the corresponding energy functionals according to the method of minimal surface integrals. The theory is applied to a simple body and the resulting vibration characteristics are found to be more accurate than previous theoretical estimates and closer to published experimental data. Finally, the feasibility of the procedure is outlined for a submerged ship hull, employing approximate methods of numerical quadrature.

Observations of a Surge in a Forced Vortex Flow

Abstract: IN HIS PAPER "The Structure of Concentrated Vortex Cores," Hall (1966) ~ edited contributions made at the 1964 IUTAM symposium on the "vortex breakdown" phenomenon. The phenomenon deals with stagnation of the axial velocity component of a vortex flow and the resultant change in the vortex structure, principally in the region of the viscous core. Vortex breakdown has been observed in numerous flow situations such as in shear flows past corners, an example of which is flow over the deck near the sail region of a submarine. Due to the highly complex structure of vortex breakdown, few of its flow properties are established, much less understood. Those who have studied the phenomenon, such as Harvey (.1962), Lambourne and Bryer (1962), Gore and Ranz (1964), Chanaud (1965), Granger (1968), and Sarpkaya (1971) to name a few, agree that the change in the vortex flow field is largely due to a sudden rise in pressure. At some axial station in the viscous core the pressure and viscous forces are equal, thereby producing a stagnation region. In some cases the stagnation region may be stationary [see Lambourne and Bryer (1962)] and in others it may be moving [see Granger (1968)]. In the latter case, a few flow properties of vortex breakdown were presented. The purpose of this paper is to present an empirical formula for the speed of a surge that propagates along the eenterline of the vortex flow. A few questions arose from Granger's (1968) paper that presented an experimental investigation of a surge propagating in a vortex sink of constant fluid depth. Did the surge travel within the viscous core and, if so, what was the effect of sink flow rate both on the radial size of the viscous core and on the speed of the surge? How is the speed of the surge predicteda~ A very simplified mathematical model is presented which demonstrates the speed of the surge. The surge is proportional to the three-halves power of the circulation.

A Note on the Resistance of Bodies of Revolution and Ship Forms

Abstract: A simple so-called 'equivalent' body of revolution is proposed for reflex ship forms in an attempt to simplify calculation of the boundary layer over a ship's hull when there is no wavemaking. However, exhaustive testing of one body of revolution did not produce a favorable comparison with results for the corresponding reflex model. Gadd's recently proposed theory was used to calculate the boundary-layer development over the body of revolution. Reasonable agreement was obtained between the calculated and experimental results.

A theoretical and experimental study on the planform of superventilated wings

Abstract: This study explores the influence of the aspect ratio, the taper ratio, and the sweepback on the flow over trapezoidal superventilated wings with a flat wetted lower surface. The flow is first calculated by a numerical method in the scope of the linearized supercavitating lifting-surface theory. The calculated wings are then made and tested in a water tunnel at zero cavitation number. The measured force and moment coefficients are compared with the prediction.

Buckling of a circular cylindrical web-stiffened sandwich shell under axial compression

Abstract: Presented herein is a stability analysis of an infinitely long web-stiffened, circular cylindrical sandwich shell under uniform axial compression. The formulation begins with the establishment of a set of suitable large-deflection shell equations that form the basis for the subsequent development of the buckling equations. The mathematical model corresponds to two face layers which are considered as thin shells and a thick core which is capable of resisting both transverse shear and circumferential extension. The associated eigenvalue problem is solved. Results show that the lowest buckling load is associated with the axisymmetric mode rather than the asymmetric modes.

Unstable squat from lateral motion of ships in shallow water

Abstract: The theoretical possibility of instability exists in two-dimensional or nearly two-dimensional flow involving ship sections in shallow water, whenever the clearance is very low. This squat instability, which causes the ship to hit the bottom, occurs only for speeds greater than a certain critical value which varies as the three-halves power of the clearance/water depth ratio. Some applications to lateral ship maneuvers, as in tug-assisted docking, are discussed.