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Technical and research (T&R) | Bulletins and reports
The present paper deals with the wave motion caused by a solid body of arbitrary shape, in steady, rectilinear, horizontal motion at a certain depth beneath the surface of a liquid. The forces acting on the solid body are also calculated, namely the general formulae for both the wave resistance and for the lift of the body. The usual assumptions in problems of this kind are taken as a basis: namely, the fluid's ideal, the only force acting on it is gravity, and that the fluid motion is established. Further, the waves caused by the motion of the body are assumed to be small. In this case the subsequent assumption is equivalent to the hypothesis that the body disturbing the flow is at a sufficient depth below the free surface.
In the present work, we consider the problem of the wave motion generated in a heavy incompressible fluid by oscillations of a body under the free surface of the fluid. We have already considered this problem for the two-dimensional case by applying a general method that we had developed. We now give the solution to this problem for the case of three-dimensional motion.
In problems of the seaworthiness of a ship the study of the oscillation of the ship has an important place; it is necessary for the explanation of the mechanism of the phenomenon as a whole in order in order that one may foresee and guide in a desirable direction the seagoing qualities of a ship being design.
The solution to the basic problems of ship oscillation is closely connected with no other less important problems of seaworthiness such as the wetness of a ship and its resistance when moving through waves.
The Resistance and Propulsion Compilation of Project II-2 embodies the preparation and printing of three sets of sheets containing summaries of model test data and related data useful in the design of ships and propellers. These three sets have the following titles:
1. Resistance Data Sheets, abbreviated RD sheets
2. Propeller Data Sheets, abbreviated PD sheets
3. Self-Propulsion Data Sheets, abbreviated SPD sheets Group 1, beginning with sheet number 101 is a continuation of the series of Model Resistance Data and Expanded Resistance Data Sheets previously prepared and published by the Society, carrying sheet numbers I through 100.
In selecting the models for which Model and Expanded Resistance Data Sheets Numbers 1 through 150 have been prepared, an effort was made to include as wide a variety of ship types as possible. Most of these models were tested prior to adopting model basin techniques in which there is artificial stimulation of turbulence. Recent experience in model testing indicates that the resistance data of models of about 0.68 block coefficient and fuller should be accepted with some caution if tested without turbulence stimulation. This qualification applies to all usual model sizes in preparing additional Model and Expanded Resistance Data Sheets, beginning with RD sheet 151, only those models believed to be free of laminar flow will be selected.
The compilation of resistance and propulsion data has been prepared by the panel for project H-2 of the Society’s Hydrodynamics Committee and the panel’s successor, the Resistance and Propulsion panel. The aim of this project has been first, to develop a standard form for the reporting of model test results, and second, co make available on these forms some of the actual test results which have been obtained at model resting establishments in this country and abroad. The panels have also developed standard forms for reporting open. water propeller tests and self-propulsion tests, and actual test data on thee forms will be published.
The first chapter of this work, after a short introduction on the current need for a theory of ships capable of guiding experimentation, has as its object a brief exposition of the theories currently in use, There is an emphasis on the equivalence between the method that Sir Thomas Havelock has deduced from the notion of source, and the so-called method of wedges that the author has derived from a general formula of potential theory due to Michell. In the second chapter, there is an analysis of experimental results from the viewpoint of their comparison with the results of calculations. Results are given which show that the movement of a liquid that can be recorded on the surface of a ship is unreliable for comparison between experiment and theory. A new definition of experimental wave resistance is proposed. The third chapter shows some comparisons between calculations and experiments for ships.
The results obtained are interesting, but one can hardly hope for improvements to the calculations applied to the whole of an actual hull. On the other hand, the given comparisons indicate an important interference between viscosity and wave resistance for very narrow models, contrary to what one might think a priority. Finally, in a short conclusion, the established results are summarized and the author gives his opinion as to the direction in which work should be guided in order to exploit these results.
A paper by Dr. Guilloton on his method of calculating the wave-making resistance of ships was presented at the Fall meeting of the Society of Naval Architects and Marine Engineers in 1951, in conjunction with the Sixth International Towing Tank Conference. This paper is published in Volume 59 of the Society's Transactions. It became apparent to the Panel on Analytical Ship Wave Relations of the Society's Hydrodynamics Committee that this was an important paper and that the methods proposed by Dr. Guilloton should be tested by applying them to practical ship forms. The Panel, therefore, recommended that work be carried out to this end. An investigation was accordingly started at the Experimental Towing Tank of the Stevens Institute of Technology under Society sponsorship and later continued under sponsorship of the Office of Naval Research. Publication of the results of the investigation has been authorized by the Technical and Research Committees of the Society with the permission of the Office of Naval Research.
The present paper may be regarded as a manual for the design of hydrofoils or airfoils from specifications on the pressure distributions, The design procedure employs the well-known theories of incompressible, inviscid, steady flow and conformal transformations. A profile design is begun by tentatively specifying the pressure distribution desired at selected angles of attack. These specifications which usually conflict with each other are reconciled so as to correspond to an existing but unknown profile. The coordinates of the section are then accurately and rapidly calculated. The purpose of this paper is to give a connected and detailed development of both theory and numerical procedure in the hope that this useful profile design method may find more general application.
The mathematical-hydrodynamical considerations on the wave resistance of ship-shaped bodies present, in this writer's opinion, the most interesting problems met in theoretical Naval Architecture. The first experimental towing tanks were designed primarily for research on this aspect of ship resistance. A tremendous amount of experimental work has been done, the data on which are available in the transactions of the many engineering institutions. The goal, to express the wave resistance of a ship as a function of its shape, has nevertheless not yet been attained by the experimental method. On the other hand it cannot be denied that systematic model testing is the most important method of approach when considering improvements in the resistance characteristics of a ship.
About a year ago the Sea Weather: Office issued a small publication on wave statistics as the first result of wave observations by weather ships in the North Atlantic. This was done to overcome the existing need for information on the frequency of the various sea states in the North Atlantic.
The present publication is to be considered as complementary to the above-mentioned research. The basic period of the observations has been increased from one to two years; to wit from 1 November 1950 to 31 October 1952. In addition to giving the frequency distributions of wave heights and wave periods (viz. lengths) subdivided as in the previous publication, we have tried to derive the frequency distribution for specific combinations of both variables. On the basis of this representation, it is possible to make certain statements as to the wave steepness.
The observed data have been taken from the weather radio messages of the North Atlantic weather ships. Because the original logbook observations have not been published as yet, it has been necessary to take into account the reduction inaccuracy of the data due to the coding as well as the possibility of errors in transmission which always exist with the radio messages. However, spot checking of the original data against the radio messages have a good agreement, and the confidence in the weather news service has so increased that the possibility of transmitting errors can be considered as negligible.
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