Tutorial: Solving a 2D Box Falling into Water Introduction The purpose of this tutorial is to provide guidelines and recommendations for setting up and solving a moving deforming mesh (MDM) case along with the six degree of freedom (6DOF) solver and the volume of fluid (VOF) multiphase model. The 6DOF UDF is used to calculate the motion of the moving body which also experiences a buoyancy force as it hits the water (modeled using the VOF model). Gravity and the bouyancy forces drive the motion of the body and the dynamic mesh. This tutorial demonstrates how to do the following: • Use 6DOF solver to calculate motion of the moving body. • Use VOF multiphase model to model the buoyancy force experienced by the moving body. • Set up and solve the dynamic mesh case. • Create TIFF files for graphic visualization of the solution. • Postprocess the resulting data. Prerequisites This tutorial is written with the assumption that you have completed Tutorial 1 from ANSYS FLUENT 13.0 Tutorial Guide, and that you are familiar with the ANSYS FLUENT navigation pane and menu structure. Some steps in the setup and solution procedure will not be shown explicitly. In this tutorial, you will use the dynamic mesh model and the 6DOF model. If you have not used these models before, see Sections 11.3 Using Dynamic Meshes and 11.3.7 Six DOF Solver Settings, respectively in the ANSYS FLUENT 13.0 User’s Guide. Problem Description The schematic of the problem is shown in Figure 1. The tank is partially filled with water. A box is dropped into the water at time t = 0. The box is subjected to a viscous drag force and a gravitational force. When the box is immersed in water, it is also subjected to a buoyancy force. c ANSYS, Inc. November 24, 2010 1
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Tutorial: Solving a 2D Box Falling into Water
Introduction
The purpose of this tutorial is to provide guidelines and recommendations for setting upand solving a moving deforming mesh (MDM) case along with the six degree of freedom(6DOF) solver and the volume of fluid (VOF) multiphase model. The 6DOF UDF is usedto calculate the motion of the moving body which also experiences a buoyancy force as ithits the water (modeled using the VOF model). Gravity and the bouyancy forces drive themotion of the body and the dynamic mesh.
This tutorial demonstrates how to do the following:
• Use 6DOF solver to calculate motion of the moving body.
• Use VOF multiphase model to model the buoyancy force experienced by the movingbody.
• Set up and solve the dynamic mesh case.
• Create TIFF files for graphic visualization of the solution.
• Postprocess the resulting data.
Prerequisites
This tutorial is written with the assumption that you have completed Tutorial 1 fromANSYS FLUENT 13.0 Tutorial Guide, and that you are familiar with the ANSYS FLUENTnavigation pane and menu structure. Some steps in the setup and solution procedure willnot be shown explicitly.
In this tutorial, you will use the dynamic mesh model and the 6DOF model. If you havenot used these models before, see Sections 11.3 Using Dynamic Meshes and 11.3.7 Six DOFSolver Settings, respectively in the ANSYS FLUENT 13.0 User’s Guide.
Problem Description
The schematic of the problem is shown in Figure 1. The tank is partially filled with water.A box is dropped into the water at time t = 0. The box is subjected to a viscous dragforce and a gravitational force. When the box is immersed in water, it is also subjected toa buoyancy force.
The walls of the box undergoes a rigid body motion and displaces according to the calcu-lation performed by 6DOF solver. Whenever the box and its surrounding boundary layermesh are displaced, the mesh outside the boundary layer is smoothed and/or remeshed.
Figure 1: Schematic of the Problem
Setup and Solution
Preparation
1. Copy the files (6dof-mesh.msh.gz and 6dof 2d.c) to your working folder.
2. Create a subfolder (tiff-files) to store the tiff files for postprocessing purpose.
3. Use FLUENT Launcher to start the 2D version of ANSYS FLUENT.
For more information about FLUENT Launcher see Section 1.1.2 StartingANSYS FLUENT Using FLUENT Launcher in ANSYS FLUENT 13.0 User’s Guide.
4. Enable Double-Precision in the Options list.
5. Click the UDF Compiler tab and ensure that the Setup Compilation Environment forUDF is enabled.
The path to the .bat file which is required to compile the UDF will be displayed as soonas you enable Setup Compilation Environment for UDF.
If the UDF Compiler tab does not appear in the FLUENT Launcher dialog box by default,click the Show Additional Options >> button to view the additional settings.
The Display Options are enabled by default. Therefore, after you read in the mesh, itwill be displayed in the embedded graphics window.
Step 1: Mesh
1. Read the mesh file (6dof-mesh.msh.gz).
File −→ Read −→Mesh...
As the mesh file is read, ANSYS FLUENT will report the progress in the console.
Step 2: General Settings
1. Define the solver settings.
General −→ Transient
(a) Select Transient from the Time list.
2. Check the mesh (see Figure 2).
General −→ Check
Figure 2: Mesh Display
ANSYS FLUENT will perform various checks on the mesh and will report the progressin the console. Make sure the minimum volume reported is a positive number.
2. Select 6dof 2d.c in the Select File dialog box.
ANSYS FLUENT displays a Warning dialog box warning you to ensure that the UDFsource files are in the same folder that contains the case and data files. Click OK.
ANSYS FLUENT sets up the folder structure and compiles the code. The compilationis displayed in the console.
3. Click Load to load the UDF library.
Step 5: Materials
Materials −→ Create/Edit...
1. Retain the properties of air.
2. Copy water-liquid (h2o<l>) from the FLUENT Database....
3. Modify the properties of water-liquid (h2o<l>).
(a) Select user-defined from the Density drop-down list.
i. Select water density::libudf from the User-Defined Functions dialog box.
(b) Select user-defined from the Speed of Sound drop-down list.
i. Select water speed of sound::libudf from the User-Defined Functions dialogbox.
(c) Click Change/Create and close the Create/Edit Materials dialog box.
A. Enter 0.056 m for Minimum Length Scale and 0.13 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 0.5 for Maximum Cell Skewness.
iii. Click OK to close Mesh Method Settings dialog box.
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.
i. Select moving box from the Zone Names drop-down list.
ii. Ensure that Rigid Body is selected in the Type group box.
iii. Ensure that test box::libudf is selected from the Six DOF UDF drop-down list.
iv. Ensure that On is enabled in the Six DOF Solver Options group box.
v. Click Create.
ANSYS FLUENTwill create the dynamic zone moving box which will beavailable in the Dynamic Mesh Zones list.
i. Similarly, create the dynamic zone, moving fluid by also enabling Passive fromthe Six DOF Solver Options group box.
Ensure that you enable Passive from the Six DOF Solver Options group box.When Passive for the rigid body is enabled, ANSYS FLUENT does not takeforces and moments on the zone into consideration.
ANSYS FLUENTwill create the dynamic zone moving fluid which will beavailable in the Dynamic Mesh Zones list.
The purpose of the preview is to verify the quality of the mesh yielded by the mesh motionparameters. Since the flow is not initialized, the motion of the box will be vertical due togravity.
(c) Enter 100 for Every and select Time Step from the When drop-down list.
(d) Enter the commands as shown in the Execute Commands dialog box.
Ensure to create the subfolder tiff-files before clicking the OK button.
(e) Click OK to close the Execute Commands dialog box.
12. Display filled contour plots, enter the following command in the ANSYS FLUENTconsole.
> display set contours filled-contours yes
13. Run the calculation.
Run Calculation
(a) Enter 0.0005 s for Time Step Size.
(b) Enter 10000 for Number of Time Steps.
(c) Set Max Iterations/Time Step to 50.
(d) Click Calculate.
Step 12: Postprocessing
1. Convert the TIFF files in the subfolder tiff-files to form an animation sequence forpostprocessing purpose, using the software package like QuickTime or Fast MoviePlayer.
The contours of volume fraction of water at different time steps are as shown in Figures 4through 23.
Figure 22: Contours at t = 4.75 s Figure 23: Contours at t = 5.0 s
Summary
This tutorial demonstrated the setup and solution of a dynamic mesh case along with the6DOF solver and the VOF multiphase model. The 6DOF UDF was used to calculate themotion of the box dropped into the water. The TIFF files created in the tutorial can beused to provide a graphic visualization of the solution.