Tutorial: Heave and Pitch Simulation of Ship hull moving...

Tutorial: Heave and Pitch Simulation of Ship hull moving through head sea waves

Step 10: Dynamic Mesh Setup

1. Set the dynamic mesh parameters.

Dynamic Mesh

(a) Enable Dynamic Mesh.

(b) Enable Six DOF Solver.

Six DOF Solver Settings includes Gravitational Acceleration setting and the WriteMotion History option. You already have set the Gravitational Acceleration in theOperating Conditions dialog box. If you want the motion history, enable WriteMotion History and specify the File Name.

(c) Ensure that Smoothing is enabled.

(d) Enable Remeshing from the Mesh Methods group box ans click Settings....

i. Click Smoothing tab and set the Spring Constant Factor to 0.5.

ii. Click Remeshing tab and set the remeshing parameters.

14 c© ANSYS, Inc. February 25, 2011


A. Enter 0.011 m for Minimum Length Scale and 0.65 m for MaximumLength Scale.

The Minimum Length Scale and the Maximum Length Scale can be ob-tained from the Mesh Scale Info dialog box. Click on the Mesh ScaleInfo... button to open the Mesh Scale Info dialog box.

B. Enter 0.8 for Maximum Cell Skewness.

C. Enter 10 for Size Remesh Interval.

c© ANSYS, Inc. February 25, 2011 15


2. Set up the moving zones.

Dynamic Mesh (Dynamic Mesh Zones)−→ Create/Edit...

(a) Create the dynamic zone, hull.

i. Select hull from the Zone Names drop-down list.

ii. Ensure that Rigid Body is selected in the Type group box.

iii. Ensure that sdofproperty is selected from the Six DOF UDF drop-down list.

iv. Ensure that On is enabled in the Six DOF Options group box.

v. Click Create.

ANSYS FLUENTwill create the dynamic zone hull which will beavailable in the Dynamic Mesh Zones list.

vi. Click Close to close the Dynamic Mesh Zones dialog box.



Note: Open the six dof property.c file and check the properties of the hull body spec-ified. Mass and Moment of inertia are specified here. In addition to this followingcommands have been written,

prop[SDOF ZERO TRANS X] = TRUE;

prop[SDOF ZERO TRANS Y] = TRUE;

prop[SDOF ZERO ROT X] = TRUE;

prop[SDOF ZERO ROT Z] = TRUE;

These macros have been used to restrict the motion in the particular direction. Forexample first command restricts the translation motion in X direction. Like this wayfour motions have been restricted using four commands. Only two motions are allowedwhich is translation along Z axis and rotation about Y axis.

Step 11: Solution

1. Set the solution method parameters.

Solution Methods



(a) Ensure that SIMPLE is selected from the Scheme drop-down list.

(b) Select Green-Gauss Cell Based from the Gradient drop-down list.

(c) Select Second Order Upwind for all the equations except Volume Fraction.

(d) Select Compressive for Volume Fraction.

2. Set the solution control parameters.

Solution Controls

(a) Keep default values for the Solution Controls.

3. Set the solution monitors.

Monitors

(a) Residual Monitor.

Monitors −→ Residuals −→ Edit...

i. Ensure that Plot is enabled under Options.

ii. Retain 1000 for Iterations to Plot.

iii. Click OK to close the Residual Monitors dialog box.

(b) Force Monitor.

Monitors −→ Drag −→ Edit...



i. Enable Print to Console, Plot and Write under Options.

ii. Select hull under Wall Zones.

iii. Click OK to close the Drag Monitor dialog box.

4. Initialize the solution.

Solution Initialization

(a) Select up-inlet from the Compute From drop-down list.

(b) Select Flat from the Open Channel Initialization Method drop-down list.

(c) Click Initialize.

5. Set the auto save option.

Calculation Activities

(a) Enter 200 for Autosave Every (Time Steps) and click Edit....



(b) Enter hull 2dof.gz for File Name.

ANSYS FLUENTwill append the timestep number so that each file will have aunique filename.

(c) Click OK to close the Autosave dialog box.



6. Run the calculation.

Run Calculation

(a) Enter 0.005 s for Time Step Size.

Make sure that the time step size is in the range of 1/100 to 1/200 of the Period

(b) Enter 3000 for Number of Time Steps.

(c) Set Max Iterations/Time Step to 20.

Write the initial case and data files with the name hull-2dof-init.cas.gz beforeclicking Calculate

File −→ Write −→Case & Data

(d) Click Calculate.

Now you need to monitor the drag residual plot.Run the simulation till the dragvalue stabilize(Figure 3); after stabilization drag curve will become periodic.

Figure 3: Drag monitor plot



Step 12: Postprocessing

1. Read the case and data file for the 1600th time step (hull 2dof-1-01600.cas.gz).

File −→ Read −→Case & Data...

2. Create Iso-surface of water volume fraction 0.5.

Surface −→Iso-Surface...

(a) Select Phases... and Volume fraction from Surface of Constant drop-down list.

(b) Select water from Phase drop-down list.

(c) Enter 0.5 for Iso-Values

(d) Enter vof-0.5 for New Surface Name.

(e) Click Create and close the Iso-surface dialog box.



3. Display contours of Mesh (Z-coordinate) on the free surface (Figure 4).

Graphics and Animations −→ Contours −→ Set Up...

(a) Enable Filled and disable Global Range from the Options list.

(b) Select Mesh... and Z-Coordinate from the Contours of drop-down lists.

(c) Select vof-0.5 from the Surfaces list.

(d) Enter 25 for the Levels and click Display.

Figure 4: Wave elevation contour



4. Display contours of water volume fraction on the hull (Figure 5).

Graphics and Animations −→ Contours −→ Set Up...

(a) Select Phases... and Volume Fraction from the Contours of drop-down lists.

(b) Select water from the Phases drop-down lists.

(c) Select hull from the Surfaces list.

(d) Click Display.

Figure 5: Contours of water Volume Fraction on the hull (water line along hull hurface)



5. Create Heave and Pitch plots from the motion history data

In the dynamic mesh properties we have switch on the Write Motion History in thesix DOF options, so a motion history file (Figure 6) is written which consists of linearand angular positions. In the file, CG Z is the heave and THETA Y is the pitch.

Figure 6: Motion History File

(a) Create a text file heave.xy by copying the time and CG Z columns. Plot this fileto get the heave curve.

Plots −→ File −→ Set Up...

Figure 7: Heave motion curve



(b) Similarly create pitch.xy by copying the time and THETA Y columns. Plot thisfile to get the pitch curve.

Plots −→ File −→ Set Up...

Figure 8: Pitch motion curve

6. Wave elevation contours (contours of Z-Coordinate on the free surface) at differenttime steps are shown in Figures 9 to Figures 10.

Figure 9: Contours at t = 9 s Figure 9: Contours at t = 10 s



Figure 10: Contours at t = 11 s Figure 10: Contours at t = 12 s

Figure 10: Contours t = 13 s Figure 10: Contours t = 14 s

Summary

This tutorial demonstrated the setup and solution of ship hull heave and pitch simulation.


Tutorial: Heave and Pitch Simulation of Ship hull moving...

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