Trans RINA, Vol 157, Part A3, Intl J Maritime Eng, Jul-Sep 2015 1000 /kg m 3 and , 8.0 10 / sec 4 kg m
respectively; an average of 2,000,000 cells were used in each computation example.
3.1 SHIP RESISTANCE VERIFICATION
To confirm the accuracy of the models, the resistance of an Azimut yacht was calculated and compared with the experimental data. The geometric parameters of the Azimut yacht comprise a length of 3.0 m, a width of 0.8 m, a draft of 0.2 m, and a displacement of 118.2 kg. Assuming the yacht travels at 5.04, 5.88, 6.72, and 7.98 m/s in the open waters in increments of 8 s, the average resistances (from 4–8 s) of
the yacht moving in the
x-direction were obtained. Table 2 shows the results and comparisons with experimental values (Wei [14]) of this model. One of the aims of the present numerical scheme is to show that the use of two grids to achieve good solutions with the comparisons of
the experimental
results. This has great advantage while dealing with three-dimensional flow problems. In order to validate the computer program developed to solve the governing equations for a yacht travels at various speeds in the open waters, a total of 300,000 (Grid1) and 600,000 (Grid2) computational cells were used for testing a grid independence study in this model. The direct numerical simulation method was used in the numerical model, which also considered the viscosity effect. The 6-axis coupled motions of the yacht were solved using general moving objects.
3.2 INVESTIGATING THE BANK EFFECTS OF TWO DIFFERENT SHIP TYPES
This study used two types of ships: a container ship (Ship 1) and a multipurpose ship (Ship 2). Ship 1 was a 3,600 TEU container ship, with a ship length of 230 m, a width of 32.2 m, and a draft of 10.8 m. Detailed ship parameters were
obtained ship from Lo et al.[2]. The
multipurpose ship had a length of 171 m, a width of 27.6 m, and a draft of 8.4 m. The ship models used in this study were constructed using the 3D coordinates in a table of
types. Based on these
parameters were input to the 3D CAD software to draft the ships. The node of the intersecting line of the still water level
coordinates, and the midship cross-section and the
vertical line of the stern were designated as the point of origin. A total of 60 body plans were created from the ship stern to the bow. Figure 1 shows the 3D diagrams of the two ship types completed using the CAD software. Figure 2 illustrates the schematics of each parameter of the ship and the embankment; B is the ship width, and is the angle of embankment, which varied from 0° (vertical), 30°, to 60° (sloped) in this study. The BS is 0.5 and 1.0 times the width of the ship. The ship navigated in the direction parallel to the bank at a low navigation speed set at 3 kn (1.54 m/s).
This section presents the conditions of the position of the ships’ centers of mass, sway force, yaw angular
velocity, yaw moment, velocity field, and pressure field with the variation of time. Figure 3 (a) and (b) present the deviation of the ships’ centers of mass when Ship 1 and Ship 2 navigated along embankments of various angles. Generally, a vertical bank caused a larger off-course deviation than sloped banks did, and Ship 1 caused a larger deviation than Ship 2 did because Ship 1 was larger and heavier than Ship 2. The x-axis in Figure 3 represents navigation time and the y-axis represents the distance of the ship’s center of mass away from the embankment, which was quantified using the ship width (B). Figure 3 shows that despite a ship speed of 3 kn, a d2b of 0.5 B resulted in obvious bank suction with substantial ship deviation. By simulating the navigation of two ship types along the embankment with fixed d2b, the bank effects were obvious even at low navigation speeds. Figure 4 (a) and (b) present the conditions of Ship 1 and Ship 2 under sway force with respect to time. The figure shows that sway forces were smaller in vertical banks than in sloped banks and smaller in Ship 2 than in Ship 1. Identical trends in the change of sway force for the two sloped banks were observed, oscillating in decline. Under continuous bank suction on the ships’ bows and sterns, the ships gradually drifted away from the bank; thus, when the ships evidently drifted from the bank, the bank effects on the ships gradually reduced.
The ships’ motion consisted of 6 DOFs; when moving along the x-axis, the bow direction simultaneously swayed against the z-axis. According to the right-hand rule, a positive angular velocity
represents a
counterclockwise rotation around the z-axis, the bow points to the left and the course angle is reduced, and vice versa. As shown in Figure 5, the off-course deviation angular velocities were collectively positive, indicating counterclockwise rotations around the z-axis, therefore the bow points to the left and the course angle reduces. By comparing the conditions of the three bank angles, the angular velocities of Ships 1 and 2 increased in oscillation with time, revealing that the ships were continually influenced by the bank effects during navigation and that the off-course angular velocity gradually increased, which raises the difficulty of ship operation;
thus, ship operators
navigating along embankments must preemptively respond to bank effects and always maintain course stability and high alertness. Figure 6 illustrates the variation of yaw moment with respect to time. Ship 1 had a larger tonnage than Ship 2 did, thus it had more obvious yaw moments than those of Ship 2, regardless of vertical or sloped banks. Yaw moments oscillated between positive and negative values with time. Generally, ships experience larger yaw moments when navigating through vertical banks than through sloped banks; however, special attention must be paid to submerged portions of sloped banks because they are difficult to detect, which increases the risk of collisions. Taking the ship 2 along the embankment
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©2015: The Royal Institution of Naval Architects
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