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 dynamic mesh (DM) 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 the 6DOF solver to calculate motion of the moving body. • Use the 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 assumes that you are familiar with the menu structure in FLUENT and that you have a good understanding of the basic setup and solution procedures. Some basic steps in the setup and solution procedure will not be shown explicitly. 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. The walls of the box undergo a rigid body motion and will displace according to the cal- culation performed by the 6DOF solver. Whenever the box and its surrounding boundary layer mesh are displaced, the mesh outside of the boundary layer will be smoothed and/or remeshed. c Fluent Inc. October 12, 2006 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 dynamic mesh (DM) case along with the six degree of freedom (6DOF) solverand the volume of fluid (VOF) multiphase model. The 6DOF UDF is used to calculate themotion 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 thebody and the dynamic mesh.
This tutorial demonstrates how to do the following:
• Use the 6DOF solver to calculate motion of the moving body.
• Use the VOF multiphase model to model the buoyancy force experienced by themoving 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 assumes that you are familiar with the menu structure in FLUENT and thatyou have a good understanding of the basic setup and solution procedures. Some basicsteps in the setup and solution procedure will not be shown explicitly.
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 undergo a rigid body motion and will displace according to the cal-culation performed by the 6DOF solver. Whenever the box and its surrounding boundarylayer mesh are displaced, the mesh outside of the boundary layer will be smoothed and/orremeshed.
i. Enter 0.056 m for Minimum Length Scale and 0.13 m for Maximum LengthScale.
The Minimum Length Scale and Maximum Length Scale can be obtained fromthe Mesh Scale Info panel. Click the Mesh Scale Info... button to open theMesh Scale Info panel.
ii. Enter 0.5 for Maximum Cell Skewness.
Gravitational acceleration must be specified in the Six DOF Solver tab. The nec-essary parameters have already been specified in the Operating Conditions panel.Hence, you need not do so again.
(f) Click OK to close the Dynamic Mesh Parameters panel.
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.
4. Enter 800 (pixels) for Width and 600 (pixels) for Height in the Resolution group box.
5. Click Apply and close the Graphics Hardcopy panel.
Step 12: Define commands to create TIFF files for animation purposes
Solve −→Execute Commands...
1. Set Defined Commands to 4.
2. Define the commands as follows:
Every When CommandCommand-1 100 Time Step display set-window 2Command-2 100 Time Step display contour water vof 0 1Command-3 100 Time Step display hard-copy "tiff-files/box-%t.tiff"Command-4 100 Time Step display set-window 3
Be sure to create the subfolder tiff-files before clicking the OK button.
1. Enable On for the four commands.
2. Click OK to close the Execute Commands panel.
Step 13: Solution
1. Set the iteration parameters.
(a) Enter 0.0005 for Time Step Size.
(b) Enter 10000 for Number of Time Steps.
(c) Set Max Iterations per Time Step to 50.
(d) Click Iterate.
Step 14: Postprocessing
1. Convert the TIFF files in the subfolder tiff-files to form an animation sequence forpostprocessing purposes, using a third party software package like QuickTime or FastMovie Player.
Figures 5—24 show the contours of volume fraction of water at different time steps.
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.