Example 6: Wigley Hull. Using Sink & Trim Wizard.

Sink & Trim wizard automatically updates the free surface, and sink and trim of a geometry (ship hull), based on a previous calculation (it is necessary to perform an initial analysis before using this wizard).

In this example we will use the results obtained in the previous example to perform this analysis.

Layer

Entities

body_wet

Surface 1

symm_oy

Lines 10, 11, 12

Figure 72. Entities to be splitted in the Sink&Trim wizard layers

Sink & Trim wizard easily updates the geometry, generates the control volume around a body (hull), assigns the main data, and generates a mesh to finally perform the simulation. Moreover, Sink & Trim wizard will also help you in the post-processing and reporting of the results.

Sink & Trim Wizard can be accessed through the menu option Utilities>Tdyn Wizard>Sink&Trim Wizard.

Sink & Trim wizard can be used for the following types of problems:

Case 1. Semi-submerged floating body with free surface. Geometry and boundary conditions with symmetry in y=yo plane. Main flow in +x or -x direction.

Case 2. Semi-submerged floating body with free surface. Geometry and boundary conditions without symmetry. Main flow in +x or -x direction.

Figure 73. Sink and Trim Wizard cases

Geometry definition

The first step is creating the surfaces defining the hull. These will be created using the batch file used in the previous example. Once the hull geometry has been created, we can start the Sink & Trim wizard and follow its instructions.

Starting the Sink&Trim Wizard

As said before, Sink & Trim wizard can be started through the menu option Utilities>Tdyn Wizard>Sink&Trim Wizard.

The first step of the Sink & Trim wizard will automatically open the Layers window and indicate us to split the geometry in different layers, as:

1.      Layer body_wet with all the surfaces of the body.

2.      Layer symm_oy with the lines of the body (if any) belonging to the symmetry plane (this symmetry plane must be of type y = yo). This layer will only be used in case 1 (see above).

A summary of the contents of every layer is shown in Figure 72.

It is also possible to read the above data from the previous geometry definition, but since the free surface will be updated, the quality of the resulting geometry may be worse than reading the original geometry.

A control volume generation using the Sink & Trim wizard may be performed by using the Read previous data window.

Figure 74. Read previous data window

The following commands are available in the Read previous data window (see Figure 74):

File: GiD file containing the previous calculation. After selecting the file, sinkage and trim angle may be changed.

Read hull geometry: If this option is selected, hull geometry of the previous calculation will be read and used.

Read free surface: If this option is selected, free surface obtained in the previous calculation will be read and used.

Sinking: Sinking to be applied. It is read from the previous calculation, but may be modified at any time.

Trimming: Trim to be applied. It is read from the previous calculation, but may be modified at any time.

Apply: Next actions are done: read hull geometry from selected file (if selected), read updated free surface (if selected) obtained from previous calculation, apply sink and trim data to the hull geometry, calculate hull-free surface intersection and split the resulting geometry in different layers. This action may take some minutes.

OK: Control volume is generated. This action may take some minutes.

To use this tool, click the folder button and select the “*.gid” directory containing the results of the previous calculation. Then the Sinkage and Trim angle fields will update with the calculated values. Note that this values can be changed at any moment. Then press the Apply button to apply the sinkage and trim angle to the geometry and read the free surface. The result of this procces is shown in Figure 75.

Figure 75. Results of the application of the sink, trim and free surface results

Due to the huge variety of possible problems, the Apply button may not work correctly in all cases. In case of an error it is possible to manually calculate hull-free surface intersection and split the resulting geometry in different layers, following next indications:

  1. body_wet layer: Surfaces of the ship within the fluid have to be assigned to the body_wet layer.
  2. body_dry layer: Surfaces and lines of the ship out of the fluid (if any) have to be assigned to the body_dry layer.
  3. free_surf layer: Free surface and its lines and points.
  4. symm_oy layer: Lines of the body (if any) belonging to the symmetry plane (sym. plane must be of type y = yo) and being below the floating line have to be assigned to the symm_oy layer. Note that only wet entities (those below free surface have to be included in this layer).

Once everything is ready, press OK button to create the control volume.

Finally, press Next> button in the Wizard window to go to the next step.

The following steps are similar to those used in the Tdyn wizard. Follow the instructions included in the previous example in the Define Data and Generate Mesh paragraphs to perform this analisis.

Figure 76. Wave profiles on hull. Comparison with experimental data and previous results

As a concusion, in the a comparison of the results obtained in this case (labeled as Tdyn S&T), the results of the previous example (labeled as Tdyn) and the results of the experiments are shown. As can be seen, the accuracy of the results has been increased, due to the mesh updating, based on the previous results.

The resulting forces data can also be compared with the resistance values of the free to sink and trim model experiments.

The new result of the viscous coefficient of our calculation is very similar to the obtained in the previous example Cv  = 2.90·10-3, while the pressure force obtained in this simulation is Cw = 1.95·10-3. These values gives a total resistance coefficient of CT  = 4.85·10-3, very similar to the experimental data found.