Shiphandling Simulation Application to Waterway Design (1992) / Chapter Skim
Currently Skimming:
5. Mathematical Models
5. Mathematical Models
Pages 43-58
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 43... ...
. The mathematical model, which is embedded In the simulation computer and invisible to the user, is difficult to generate and even more difficult to validate.
|
From page 44... ...
The framework typically consists of a data base that stores the waterway depth at specific locations and an interpolation scheme for these data that allows estimation of the water depth at an arbitrary point in the waterway. The structure of these data bases varies considerably.
|
From page 45... ...
Many available charts of waterways are not recent, and few of these include information on water depth that is dense enough for an adequate data base. Most field survey records provide discrete soundings at specific data points rather than a continuous bottom profile.
|
From page 47... ...
. The forces acting on the ship arise primarily from the combined effects of water surrounding the ship, wind, waterway geometry, and other external forces such as tug boat assistance and riding on anchor (Abkowitz, 1964; Bernitsas and Ke~dis, 1985; Eda and Crane, 1965; Norrbin, 19703.
|
From page 48... ...
Figure 5-1 depicts three threads of information (represented as thick horizontal lines) that affect several modules within the ship model.
|
From page 49... ...
Specific equations are not introduced in the following sections. The mathematical presentation of any of these models is algebraically intensive, as demonstrated by a mathematical model for the Esso Osaka in unrestricted shallow water (for further information on Esso Osaka, see Abkowitz, 1984; Ankudinov and Miller, 1977; Crane, 1979a,b; Dand and Hood, 1983; Eda, 1979b; Fujino, 1982; Gronarz, 1988; Miller, 1980; Report of the Maneuvering Committee, 1987~.
|
From page 50... ...
The coefficients that relate instantaneous motions to forces acting on the ship are most often determined experimentally by captive model tests using either an apparatus called the Planar Motion Mechanism (PMM) or a special facility called a rotating arm basin.
|
From page 51... ...
However, these programs may be as expensive to run as physical model tests, and their ability to reproduce physical model test results has not been demonstrated. The simulation of steering and maneuvering in deep water appears to be satisfactory for eng~neenng applications, as long as the coefficients of the mathematical model are identified by a properly conducted physical model test.
|
From page 52... ...
To perform either physical model tests or full-scale teals would require addressing significant modeling questions concerning the viscous flow in the gap between the ship and bottom and concerning the deformation of the mud bottom by the ship. The cost of performing the required tests is high because a new test parameter (water depth)
|
From page 53... ...
When the wind is parallel to the channel and in the same direction of travel as the ship, controlling the forward movement can be difficult, especially for diesel-powered ships where the minimum sustainable RPM corresponds to a significant speed and where the number of air starts may be limited. Significant wind forces usually arise when the wind velocity is much greater than the ship velocity, and as a result, a simple framework for these forces is usually adopted.
|
From page 54... ...
Reducing these dam to numerical formulas appears to have been accomplished by various facilities using proprietary techniques. The effect on the force system due to sudden changes in waterway depth, to a waterway bathymetry that is truly three-dimensional, or to currents that vary significantly along the length of the ship apparently have not been systemadcally studied.
|
From page 55... ...
Model tests to determine the force and moment history of two ships passing one another have been conducted in several contexts and particularly for He Panama Canal study (see Appendix C)
|
From page 56... ...
This situation is often referred to as blanketing the rudder and results in the rudder being almost ineffective. A characterization of these effects using elementary hydrodynamic analysis and empirical results is usually included in a semiempirical model for the propeller-rudder system.
|
From page 57... ...
lithe propeller-rudder interaction forces are often measured on a captive, towed model with the propeller spinning at a range of RPMs and at venous nodder angles. The results are scaled up to full scale using the information from separate propeller tests using larger models, performed in a facility called a propeller tunnel.
|
From page 58... ...
Models for complete steam turbine power plants are somewhat complex, but reliable models have been constructed by several different facilities (van Berlekom and Goddard, 19721. In today's fleet of merchant ships, diesel engines are much more popular choices for the main propulsion plant and are, unfortunately, much more difficult to characterize.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.