The definition of the geometry is the first step to solve any problem. To create the hull, we are first going to create the NURBS lines defining the different waterlines of the model. The hull itself will be constructed as a NURBS surface based on those lines created in the previous step. In the following paragraph we will show the necessary steps to create the hull using the system pre-processor.
The surfaces defining the hull will be created using a batch file. This batch file is just a text file (ASCII format) containing the data points and the instructions listed in the appendix 1. The instructions included in this file can be also be inserted in the command line of the pre-processor module. Once the batch file have been created (and saved in a text ASCII file), it can be read by using the option Read Batch file in the menu Files (or pressing Cntr^B). By doing this, next actions are carried out:
The result of this process (half of the ship geometry, due to the symmetry of the problem) is shown in Figure 48.
Figure 48. Geometry definition, created with a batch file |
The analysis of the hydrodynamics flow is done within a so-called control volume, which represents the volume of water around the model, where the simulation is going to be carried out. The necessary steps to generate this control volume are described next.
The eight necessary points to define the control volume (see Figure 49) around the ship hull have been selected by the following procedure and listed below (please note that it is not necessary to reproduce all these steps as the points are listed below and can be introduced by the standard procedure):
1) Identify the two points extremes of the floating line (intersection between the ship and and the free surface of the water). These two points, which are placed in the bow and stern of the ship, will be hereafter named fore-point and aft-point respectively (the length of the ship is defined to be the distance from the fore to the aft point).
2) Copy the fore-point to point A, which will be placed at 70% the ship length ahead (upstream) along (-) X-axis.
3) Copy point A to point B placed at 80% the ship length along (+)Y-axis.
4) Copy the aft-point to point C, which will be placed at 140% the ship length downstream along (+) X-axis.
5) Copy point C to point D placed at 80% the ship length along (+) Y-axis.
6) Copy points A, B, C and D 70% the ship length along (-) Z-axis to create points E, F, G and H.
X | Y | Z | X | Y | Z | ||
Point A | -7.2 | 0.0 | 0.0 | Point E | -7.2 | 0.0 | -4.2 |
Point B | -7.2 | 4.8 | 0.0 | Point F | -7.2 | 4.8 | -4.2 |
Point C | 11.4 | 0.0 | 0.0 | Point G | 11.4 | 0.0 | -4.2 |
Point D | 11.4 | 4.8 | 0.0 | Point H | 11.4 | 4.8 | -4.2 |
The lines to be created have to join the following points A-B-C-D, E-F-G-H, A-E, B-F, C-G and D-H.
Finally the points A and D have to be joint to the aft and fore-points respectively to obtain the geometry shown in Figure 49.
Figure 49. Definition of the lines of the control volume |
The surfaces to be created are those defining the six faces of the box (control volume). These can be created following the one of the standard procedures available.
The previously created surfaces will define a single volume, which is the control volume. To create the volume, use the corresponding option in the Geometry menu. If the volume cannot be created for whatever reasons, check that all the surfaces were properly created, that there were duplicated lines and that all the surfaces belong to a single and eventually closed volume. The resulting geometry is shown in .
Figure 50. Control volume (final geometry) |
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