File Name: water waves and ship hydrodynamics .zip
Not a MyNAP member yet? Register for a free account to start saving and receiving special member only perks. An improved method to measure diffraction and radiation waves around a ship model running in waves at forward speed is proposed. Measured wave pattern especially with blunt bow and shallow draft ships is used to investigate validity of an analytical method to predict the diffraction and radiation waves and added resistance of a ship.
We are developing more and more sophisticated analytical method to understand and predict realistic nonlinear hydrodynamics of ships in waves. Validity of those methods is still tested, however, with traditional way of comparing the theoretically predicted and experimentally measured global hydrodynamic force on ships. The global force is an integrated effect and the implication of the test results is not always clear. The more sophisticated the analytical methods are, the more strict and direct experimental test such as the comparison of fluid pressure without any integration effect will be necessary to prove their advantage over traditional heuristic approaches.
This kind of test is also plausible from practical point of view because ship designers do need analytical tool accurate enough to predict local sea loads as well as global sea loads. The purpose is to prove by more direct manner validity or invalidity of our current analytical methods to predict the flow and the pressure regarding the ships in waves. Measurement of the flow and the pressure themselves naturally serves better practically as well as academically this purpose.
Reason of the wave observation instead is that it undergoes less integration effect than the global force. Moreover the measurement is easier in terms of experimental instrumentation and its accuracy is much more reliable than that of the flow or pressure measurement. Investigations based on this idea have already produced some successful results. A large discrepancy, for example, found between added resistance measured as a force and one directly derived from measured wave energy leads to an idea of breaking of the diffraction waves in front of the ship bow and new method has been developed to predict extra added resistance due to the breaking of the diffraction waves Naitoh et al.
Yet we found recently that the difference is unusually large with hull forms of shallow draft or at ballast condition corresponding to the shallow draft SR No theoretical explanation is available now for this large difference. We have some to be improved in the original technique proposed by the present author to measure diffraction and radiation waves.
It gave only wave distribution on a line parallel to the track of a ship model running in experimental tank. One can not construct two dimensional image of the wave pattern by those data alone. This was one reason to derive the amplitude function of the component wave composing the diffraction and radiation waves from the data and to reconstruct the global wave characteristics. This process is possible only when we can assume a mathematical expression of the waves, a linear superposition of component waves satisfying dispersive relation corresponding to its direction of propagation.
Therefore some part of wave energy will not possibly be captured with this process. So it is plausible if we could measure two dimensional wave pattern and compare them without any processing with theoretical prediction.
It is also desirable to separate the first and second order wave components to compare the former with the predicted by linear theories. In this report we return to the present author's old approach of measuring waves but we attempt to improve it in order that we can obtain directly the global image of the diffraction and radiation waves at forward speed and separate them into the first and sec-. We investigate various information on the diffraction and radiation waves observed by the improved technique of wave measurement particularly for seeking any explanation of the discrepancy of added resistance of shallow draft ships.
We then propose a linear analytical approach to predict the fluid pressure on ships particularly due to the diffraction wave. This approach accounts effectively for nonuniform steady flow which is prominent in the vicinity of the blunt bow and is supposed to have large influence on accurate prediction of the wave pressure on the bow part.
We test the accuracy of our analytical approach by comparing the diffraction wave field predicted with the measured with our improved technique. We presume that if the prediction of the diffraction wave elevation around the bow is accurate, then the prediction of wave pressure on the bow part will be accurate and the added resistance, on which the wave loads at the bow is the most influential, will be predicted accurately. Neither radiation wave nor diffraction wave field generated by a ship advancing in waves is visible at tank test because of the coexistence of other waves such as steady wave generated by the forward speed of the ship on otherwise a calm water and incident waves.
So we need a technique to separate each of those waves. It is relatively simple to exclude the effect of the incident waves. We measure them upstream where they are not yet disturbed by a ship model and extrapolate them to near it.
The extrapolated incident waves are subtracted from the measured wave. Another problem is that instantaneous distribution of wave elevation around the model is not a full information of the radiation and the diffraction waves. One can obtain a complete picture of the radiation and the diffraction waves only when the distribution of the amplitude and the phase of the wave motion is measured.
When the ship model advances in the tank, each wave probe comes to a location relative to the ship model at different time instant. In other words the wave probes record the wave elevation at every location on the line parallel to the ship model's track on several different time.
U is the speed of the ship model. Temporal and spatial variation of the diffraction or radiation waves generated by the ship model running at forward speed in the monochromatic incident waves or sinusoidally oscillating is given to the second order by.
The first term on the right of 1 is the steady wave elevation corresponding to the Kelvin wave pattern. Naturally it includes the second order steady component. The second term is the linear oscillatory part and the third the second order oscillatory component.
We used. With them we can compose the pattern of the radiation or diffraction waves as shown in Fig. The relative position of the ship is drawn by a slender rectangle.
Several features of the diffraction wave shown in Fig. Energy of other wave component than the component of the fundamental frequency is apparently very small. This means that added resistance due to the wave component other than the linear one may not be considerable.
The second harmonic component is significant at the fore front of the diffraction wave pattern where naturally the wave steepness is the largest. It might suggest the wave breaking at the front of the diffraction waves. No significant difference between the diffraction waves at full load and ballast conditions is observed. Practically an extrapolation of the measured wave record to the location of x larger than some value is necessary since the record far behind the ship model is affected by the tank wall reflection and not to be used as the data for the transform.
The expression 2 of the wave elevation is asymptotic one which is correct when x is sufficiently large but y is kept constant. This transverse transform is possible when the measured wave data as shown in Fig. The wave pattern in Fig.
It is, however, supposed to be less accurate for the components whose crest lines are parallel or almost parallel to the y axis. On the other hand x -wise transform is on the waves measured close to the bow. In Fig. They are of the diffraction waves due to a Series 60 model presented in the previous section. The agreement of regardless of the full load Fig. This agreement suggests that our linear model of the wave given by equation 2 is correct in describing diffraction waves around a ship.
The hull form has a big bulbous bow. Added resitance of this hull form particularly in head waves of short wave length and at ballast condition is unusually large as shown later. The disagreement suggests anomaly in the wave pattern of this hull form one can not describe by equation 2. However we can not find any considerable feature suggesting this anomaly of the wave in Fig. We may conjecture the the wave energy is disspated during its propagation from near to behind the ship. But we can not conclude definitely so and we can not either say that this anomaly of the wave is the cause of unusually large added resistance of this ship.
Full nonlinear treatment of radiation or diffraction waves generated by a running ship is not straightforward. Even linear or qusi-linear theory is not so easy to be numerically implemented Sclavounos , Iwashita et al. A panel method using 3D Green function satisfying a full linear free surface condition at forward speed, for example, needs formidable computer time and the results are some-.
We should develop a theoretical method robust as well as rational in predicting the diffraction waves particularly close to the ship bow. We give the first priority to accurate prediction of the diffraction wave, the divergent wave component in particular, close to the bow part.
We suppose the diffraction wave at the bow almost determines the fluid pressure that decides added resitance of the ship in the incident waves of short wave length. The divergent wave prevails at the bow part and therefore is to be accurately predicted.
We must consider interacting effect by nonuniform steady flow close to the bow of the ship. Consider a ship running at forward speed U in incident regular waves on deep water. Let x, y, z be a right-handed coordinate system fixed to the mean position of the ship; the z is vertically upward through the fore end of the ship, —x is in the direction of forward motion and the origin is in the plane of the undisturbed free surface. Here we restrict ourselves to diffraction waves for brevity of explanation.
The problem is formulated in terms of potential flow theory. The total velocity potential is expressed by. The first assumption is quite appropriate when we are concerned with the flow close to a relatively bluff bow, where the flow variation down the ship length is supposed to be not so small.
The governing equation of near the ship is two dimensional Laplace equation:. This free surface condition, though it is linear with respect to , apparently produces an intricate situation for the diffraction wave; the incident waves coming far upstream which is usually assumed linear will be diffracted on the displaced free surface and the ship surface. Numerical implementation of this formulation has not been successful for the diffraction waves. Their formulation is yet attractive because it can deal with the bow flow rather correctly but by simpler way.
It can account for the interaction effect of steady wave elevation considerable at the bow part on the diffraction wave. We introduce a rather inconsistent and simpler approach similar to it. Our simpler approach considers only weak interaction with the steady flow. While we ignore terms, we retain almost all other terms even if they are redundant. It yields the free surface condition when the ship is in head waves:.
They will represent partly the effect of the steady flow on the unsteady wave making. As a consequence the diffraction wave involves a part to correct the difference of the actual free surface condition from 13 and This part might be called a diffraction of 0 by the nonuniform steady flow close to the ship. The last two terms of the right side of equation 11 and the last of equation 12 represent this effect.
In other wave conditions than head seas some more terms appear to represent the effect. The boundary value problem posed by equation 10 , 11 , 12 and the body boundary condition is solved starting from an appropriate condition at the bow to downstream.
Not a MyNAP member yet? Register for a free account to start saving and receiving special member only perks. An improved method to measure diffraction and radiation waves around a ship model running in waves at forward speed is proposed. Measured wave pattern especially with blunt bow and shallow draft ships is used to investigate validity of an analytical method to predict the diffraction and radiation waves and added resistance of a ship. We are developing more and more sophisticated analytical method to understand and predict realistic nonlinear hydrodynamics of ships in waves. Validity of those methods is still tested, however, with traditional way of comparing the theoretically predicted and experimentally measured global hydrodynamic force on ships.
Request PDF | Water waves and ship hydrodynamics: An introduction | In this book an introduction is given to aspects of water waves that play.
Not a MyNAP member yet? Register for a free account to start saving and receiving special member only perks. The shallow-water wave equations of Boussinesq type are used to simulate the ship waves at subcritical, transcritical, and supercritical speeds.
Jetzt bewerten Jetzt bewerten. In this book an introduction is given to aspects of water waves that play a role in ship hydrodynamics and offshore engineering. At first the equations and linearized boundary conditions are derived describing the non-viscous free surface water waves, with special attention to the combination of steady and non-steady flow fields. Then some simple kinds of free wave solutions are derived, such as plane waves and cylindrical waves. For several situations, steady and unsteady, the source singularity function is derived.
With the enhanced ship hydrodynamic in Xicheng Canal, the shipgenerated waves are the main factor inducing turbidity in the water column and damages of the channel revetments. In this study, to clarify the characteristics of ship-generated waves in Xicheng Canal, field observations and numerical analysis were carried out. The field observations were conducted at a straight section of Xin xiagang River and the ship-generated waves model is established based on depth averaged non-linear shallow water NLSW equations. The water level fluctuations at different wave gauges calculated by Delft3D-FLOW model agree fairly well with the observed data.
Ltd, Xiaogan , China. In recent years, the development and construction of islands and reefs has been proposed by the government and commercial company. However, as a large cargo carrier cannot reach islands and reefs if the harbor is not available, such type of carrier which has well deck is designed to meet the requirements of delivering people and equipment. It is a possible way to connect the island and supply cargo ships. This paper firstly summarizes the domestic and foreign research progress of hydrodynamic analysis of ships with well deck.
Volume 7B: Ocean Engineering. Glasgow, Scotland, UK. June 9—14, Sinkage and trim, which often occur to ships moving in shallow water, do not only have an effect on the ship-ship hydrodynamic interaction forces, but also increase the risk of grounding. An algorithm based on the potential theory has been devised for real-time simulation of the hydrodynamic interaction between two ships in shallow water accounting for sinkage and trim. The shallow water condition is modeled using the mirror image method; while the sinkage and trim are solved iteratively based on the principle of hydrostatic balance, where a mesh trimming procedure is performed when the waterline is changed. Simulations are performed with and without accounting for the sinkage and trim, and comparison with experimental results shows a fair agreement.
It seems that you're in Germany. We have a dedicated site for Germany. In this book an introduction is given to aspects of water waves that play a role in ship hydrodynamics and offshore engineering. At first the equations and linearized boundary conditions are derived describing the non-viscous free surface water waves, with special attention to the combination of steady and non-steady flow fields. Then some simple kinds of free wave solutions are derived, such as plane waves and cylindrical waves. For several situations, steady and unsteady, the source singularity function is derived. These functions play a role in numerical codes used to describe the motion of ships and offshore structures.
The author has provided the reader with comprehensive coverage of ship hydrodynamics with a focus on numerical methods now in use. The book provides a global overview of experimental and numerical methods for ship resistance and propulsion, manoeuvring and seakeeping. As boundary element techniques are now in standard use, these are covered in sufficient detail for independent code development.
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of water waves that play a role in ship hydrodynamics and offshore engineering. Included format: EPUB, PDF; ebooks can be used on all reading devices.