# FLUENT - Tutorial - Dynamic mesh - Missile Silo Launch

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```					Tutorial: Missile Silo Launch

Introduction
The purpose of this tutorial is to provide guidelines to set up and run a dynamic mesh
(DM) case using the layering scheme. The tutorial models a missile being launched from a
silo. The motion of the missile is predicted using the six degree of freedom (6DOF) solver
in FLUENT.
In this tutorial, you will learn how to:
• Read a mesh ﬁle and perform a dynamic mesh calculation.

• Enable and conﬁgure the 6DOF solver.

• Compile a UDF that governs the missile motion.

• Set up moving zones.

• Set up a dynamic mesh event to control the mesh motion.

• Deﬁne custom commands to be executed at regular intervals during the simulation.

• Run an unsteady calculation for the problem.

• Write image ﬁles that can be played as an animation.

Prerequisites
This tutorial assumes that you are familiar with the FLUENT interface, and that you have
a good understanding of the basic setup and solution procedures. In this tutorial, you will
use the dynamic mesh model. If you have not used this model before, refer to Section 10.6:
Dynamic Meshes in the FLUENT 6.2 User’s Guide.

Problem Description
Consider the launch of a missile from its silo (Figure 1). The 6DOF solver is used to predict
the motion of the missile. The UDF (silo.c) is used to specify the properties of the missile
(mass and moments of inertia) as well as any body forces that are present. The 6DOF
solver applies Newton’s law to determine the acceleration of the missile.

c Fluent Inc. March 20, 2006                                                               1
Missile Silo Launch

Thrust, gravity, and pressure forces are considered in the calculation, whereas the drag on
the missile is neglected. The missile engine starts to ﬁre at t = 0 seconds but the missile is
held ﬁxed in the silo and is not allowed to move until t = 0.1 seconds. The ﬂow is assumed
to be inviscid and the domain is axisymmetric.

Figure 1: Problem Schematic of a Missile Inside the Silo

Preparation

1. Copy the ﬁles, silo.msh.gz and silo.c to the working directory.
2. Start the 2D, double-precision (2DDP) version of FLUENT.

Setup and Solution
Step 1: Grid

1. Read the mesh ﬁle, silo.msh.gz.
2. Scale the grid to inches.
Grid −→Scale...
(a) Under Units Conversion, select in from the Grid Was Created In drop-down list.
(b) Click Scale and close the panel.

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3. Set the unit of length to inches.
Deﬁne −→Units...

4. Check and display the grid (Figure 2).
Display −→Grid...

Grid                                                  Feb 10, 2006
FLUENT 6.2 (2d, dp, segregated, lam)

Figure 2: Grid Display

c Fluent Inc. March 20, 2006                                                                    3
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5. Set the view so that the missile points upward (Figure 3).
Display −→Views...
(a) Click Camera....

i. Select Up Vector in the Camera drop-down list.
ii. Set X to 1, Y to 0, and Z to 0.
iii. Click Apply and close the Camera Parameters panel.
The view updates to show the missile pointing upward.
(b) Under Mirror Planes, select axis-in and axis-out.
(c) Click Apply.
The view updates to show the display mirrored about the axis of symmetry. Do
not close the Views panel.
(d) Zoom and pan in the display to center the view (Figure 3).
(e) Click Save to save the view and close the Views panel.
The saved view, view-0 appears in the Views list.

6. Create zone surfaces for the two ﬂuid zones.
Surface −→Zone...
(a) Under Zone, select ﬂuid-inner and click Create.
(b) Similarly, create the ﬂuid-outer zone.

7. Display ﬂuid-inner zone surface (Figure 4).
The ﬂuid-inner zone contains the missile and is meshed with quadrilateral elements.
This zone is bounded on the top and bottom sides by the interior zones internal-top
and internal-bot, respectively.
The task is to move this ﬂuid zone in accordance with the dynamics of the missile.
During this process, the layering algorithm builds layers of quadrilateral elements at
internal-bot and collapses layers at internal-top.
A non-conformal interface has to be used between ﬂuid-inner and ﬂuid-outer zones,
because the ﬂuid-inner zone will move relative to the rest of the domain.

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Grid                                                         Feb 10, 2006
FLUENT 6.2 (2d, dp, segregated, lam)

Figure 3: Rotated and Mirrored View of the Grid

Grid                                                       Mar 17, 2006
FLUENT 6.2 (2d, segregated, lam)

Figure 4: The ﬂuid-inner Zone

c Fluent Inc. March 20, 2006                                                                           5
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Step 2: Compile the UDF
Deﬁne −→ User-Deﬁned −→ Functions −→Compiled...

1. Click Add... and select silo.c.

2. Click Build.
The Information dialog box is displayed. Read the instructions carefully and click OK.
FLUENT creates the appropriate directory structure, makeﬁles, and compiles the code
for you. The progress of the compilation is shown in the FLUENT console window.
You can monitor the progress of the compilation for any linking errors. Alternatively,
you can also view the log ﬁle with the compilation history that will be created in the
working directory.

Step 3: Models

1. Enable the Coupled solver with Explicit formulation, Unsteady time condition, and
Axisymmetric space.
Deﬁne −→ Models −→Solver...

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(a) Select Coupled under Solver, Explicit under Formulation.
(b) Select Axisymmetric under Space, and Unsteady under Time.
Dynamic mesh features are available only in the unsteady solver with ﬁrst-order
implicit time discretization.
(c) Retain the default settings for the remaining parameters and click OK.

2. Enable the Energy equation.
Deﬁne −→ Models −→Energy...

3. Enable the Spalart-Allmaras turbulence model.
Deﬁne −→ Models −→Viscous...
(a) Under Model, select Spalart-Allmaras (1 eqn).
(b) Retain the default settings for the other parameters and click OK.

4. Enable the Species Transport model.
It helps you in distinguishing between air and exhaust gases.
Deﬁne −→ Models −→ Species −→Transport & Reaction...

(a) Under Model, select Species Transport.
(b) Click OK.

c Fluent Inc. March 20, 2006                                                               7
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Step 4: Materials

1. Create a new ﬂuid material named exhaust to represent the exhaust gases of the
missile.
Deﬁne −→Materials...

(a) Select ﬂuid in the Material Type drop-down list.
(b) Change the Name from air to exhaust and deﬁne its properties as shown in fol-
lowing table:
Parameter          Value
Cp                 4000 j/kg-k
Molecular Weight   50 kg/kgmol
(c) Click Change/Create.
Click No when asked to overwrite air.

2. Edit the mixture material, mixture-template.
(a) Select Mixture in the Material Type drop-down list.
(b) Select mixture-template in the Fluent Mixture Materials drop-down list.
(c) Click Edit... for Mixture Species.

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i. Add exhaust and air (in the same order) to the Selected Species list.
ii. Remove h2o, o2, and n2 from the Selected Species list and click OK.
(d) Select ideal-gas for Density.
(e) Click Change/Create and close the Materials panel.

Step 5: Operating Conditions
Deﬁne −→Operating Conditions...

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1. Keep the default operating pressure of 101325 Pa.

2. Enable Gravity and set the Gravitational Acceleration to -9.81 m/s2 in the X direction.

3. Set the Operating Temperature to 300 K.

4. Set the solution limits for pressure and temperature.
Solve −→ Controls −→Limits...

(a) Set the Minimum Absolute Pressure to 10000 Pa and Maximum Absolute Pressure
to 2500000 Pa.
(b) Set the Minimum Static Temperature to 50 K and the Maximum Static Temperature
to 2800 K.
(c) Set the Maximum Turb. Viscosity Ratio to 1e06.
(d) Click OK.

Step 6: Boundary Conditions
Deﬁne −→Boundary Conditions...

1. Set the boundary conditions for the nozzle-exit zone.

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(a) Set the Mass Flow-Rate to 50 kg/s and Total Temperature to 2700 k.
(b) Set the Supersonic/Initial Gauge Pressure to 101325 Pa.
(c) Set Direction Speciﬁcation Method to Normal to Boundary.
(d) Set the Turbulence Speciﬁcation Method to Intensity and Hydraulic Diameter.
(e) Set Turbulence Intensity 10 % and Hydraulic Diameter to 8.4 in.
(f) Set Species Mass Fractions for exhaust to 1.
(g) Click OK.

c Fluent Inc. March 20, 2006                                                              11
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2. Set the boundary conditions for the far-ﬁeld zone.

(a) Retain the default value of 0 for Gauge Pressure.
(b) Retain the default values of the other parameters and click OK.

3. Ensure that the Type for zones, axis-in and axis-out is axis.

4. Ensure that the Type for zones, interface-in, interface-out-1, and interface-out-2 is in-
terface.

Step 7: Dynamic Mesh Setup

1. Merge interfaces into one zone.
Grid −→Merge...

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(a) Under Multiple Types, select interface.
(b) Under Zones of Type, select interface-out-1 and interface-out-2.
(c) Click Merge and close the panel.
The zones interface-out-1 and interface-out-2 are merged into one zone. The new zone
is given the name of the ﬁrst zone, namely interface-out-1.

2. Deﬁne the non-conformal interface between the moving zone (ﬂuid-inner) and the
stationary zone (ﬂuid-outer).
Deﬁne −→Grid Interfaces...

(a) Under Grid Interface, enter the name of the interface as sliding-interface.
(b) Select interface-in under Interface Zone 1 and interface-out-1 under Interface Zone 2.
(c) Retain the default values for the other parameters and click Create.
(d) Close the panel.

c Fluent Inc. March 20, 2006                                                                  13
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3. Deﬁne the dynamic mesh parameters.
Deﬁne −→ Dynamic Mesh −→Parameters...

(a) Enable Dynamic Mesh.
The panel expands to show additional controls.

(b) Enable Six DOF Solver.
(c) Disable Smoothing and enable Layering.
(d) In the Layering tab, retain the default values of layering parameters Split Factor
(0.4) and Collapse Factor (0.04).
Note: These settings indicate that while creating new layers, a new layer is
created when the old cell becomes larger than 1.4 (1 + 0.4) times the ideal
height. When the layers collapse, a layer is destroyed when it shrinks to a
height below 0.96 (1 − 0.04) times the ideal height.
(e) In the Six DOF Solver tab, ensure that the Gravitational Acceleration is set to
-9.81 in the X direction.
(f) Click OK to close the panel.

4. Deﬁne the zones for the dynamic mesh.
Deﬁne −→ Dynamic Mesh −→Zones...

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(a) Create rigid body type zones.
i. Select wall-missile under Zone Names, and Rigid Body under Type.
ii. Select launch::libudf in the Six DOF UDF drop-down list.
iii. Under Six DOF Solver Options, select On and deselect Passive.
iv. Click Create.
The wall-missile zone now appears in the Dynamic Zones list.
v. Follow the previous steps to specify rigid body motion for the nozzle-exit and
ﬂuid-inner zones. Enable Passive solver option for these zones.
Note: You must click Create after setting options for each zone.
When a zone is declared Passive, the six DOF solver will not calculate forces
and moments in that zone. However, the zone is still part of the rigid body
motion. In this case, the moving zones ﬂuid-inner, nozzle-exit, and missile-
wall move with the same rigid body motion but the forces are calculated only
on the non-passive zone, missile-wall.

c Fluent Inc. March 20, 2006                                                                  15
Missile Silo Launch

(b) Create stationary type zones.
While ﬂuid-inner moves in a translational rigid body mode, the upper and lower
boundaries (internal-top and internal-bottom) of this ﬂuid zone must be ﬁxed in
order to provide a location for creating and collapsing layers of cells. Thus, these
two zones must be declared stationary explicitly.

i. Select internal-bottom under Zone Names.
ii. Select Stationary under Type.
iii. In the Meshing Options tab, set the value of Cell Height for the adjacent
ﬂuid-inner zone to 1 inch and click Create.
This is the ideal cell height near internal-bottom, and it is set equal to the
iv. Select internal-top under Zone Names.
v. Select Stationary under Type.
vi. In the Meshing Options tab, set the value of Cell Height for the adjacent
ﬂuid-inner zone to 1 inch and click Create.
vii. Close the Dynamic Mesh Zones panel.

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5. Deﬁne a dynamic mesh event that will hold the missile in place until liftoﬀ at t =
0.1 seconds.
Deﬁne −→ Dynamic Mesh −→Events...

(a) Set Number of Events to 2.
(b) For event-1, enable the checkbox under On and set At Time to 0.
(c) Click Deﬁne... for event-1.
The Deﬁne Event panel opens.

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i. Select Change Motion Attribute under Type drop-down list.
ii. Under Attribute, select Moving.
iii. Under Status, select Disable.
iv. Select ﬂuid-inner, wall-missile, and nozzle-exit.
v. Click OK.
(d) Similarly, deﬁne event-2 to enable zone motion at t = 0.1 sec.

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(e) Click Apply and close the panel.

Step 8: Solution Controls

1. Set the solution control parameters.
Solve −→ Controls −→Solution...

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(a) Set the discretization scheme for Flow to First Order upwind.
(b) Set the Courant Number to 0.5.
(c) Click OK.

2. Set the residual monitor parameters.
Solve −→ Monitors −→Residual...
(a) Enable the plotting of residuals by selecting Plot under Options.
(b) Disable the Check Convergence option for all the residuals.
You need to scroll down the list of residuals to view exhaust.

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Step 9: Solution Initialization and Animation Setup

1. Initialize the ﬂow ﬁeld from the far-ﬁeld boundary zone.
Solve −→ Initialize −→Initialize...

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2. Set graphics output parameters.
File −→Hardcopy...

(a) Select JPEG under Format.
(b) Select Color under Coloring.
(c) Under Resolution, set Width to 640 and Height to 480.
These settings will produce output frames that are 640×480 JPEG ﬁles. Each ﬁle
will be around 40 kB in size. The ﬁles can later be animated using the animation
features in FLUENT or using third-party software.
(d) Click Apply and close the panel.

3. Deﬁne commands to be executed at regular intervals during the simulation.
Deﬁne the text commands that generate contour plots on-screen during iteration as
well as output the animation frames. The quantities of interest are the Mach number
and mass fraction of exhaust gas.
Solve −→Execute Commands...

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(a) Set Deﬁned Commands to 3.
(b) Select the checkboxes under On for all the commands.
(c) For command-1 and command-2, set Every to 5 and select Time Step from the
When drop-down list.
(d) Enter the following command in the Command text-entry ﬁeld for command-1:
disp sw 1 cont mach 0 2 hc mach%t.jpg
(e) Enter the following command in the Command text-entry ﬁeld for command-2:
disp sw 2 cont exhaust 0 1 hc exhaust%t.jpg
These two commands will instruct FLUENT to do the following every 5 iterations:
• Set the current window (disp sw X)
• Display a ﬁlled contour (cont variable min max)
• Write a hardcopy of the ﬁlled contour (hc f ilename)
(f) For command-3, set Every to 50 and select Time Step from the When drop-down
list.
(g) Enter the following command in the Command text-entry ﬁeld for command-3:
This command will write compressed case and data ﬁles to the working directory
every 50 time steps.
(h) Click OK to close the panel.

4. Initialize the graphics settings by creating initial contour plots.
(a) Open the Contours panel and the Display Options panel simultaneously.
Display −→Options...
Display −→Contours...

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(b) In the Display Options panel, set Active Window to 1 and click Set.
Graphics window 1 opens. It is the current active window.
(c) In the Contour Plot panel, select Velocity and Mach Number from the Contours of
drop-down list, enable Filled under Options, and click Display.
The contour plot of Mach number appears in graphics window 1.
(d) Restore the saved view, view-0 in graphics window 1.
Display −→Views...
i. Under Views, select view-0 and click Restore.
(e) Set Active Window to 2 in the Display Options panel and click Set.
Graphics window 2 opens. It is the current active window.
(f) In the Contour Plot panel, select Species and Mass Fraction of exhaust from the
Contours of drop-down list, enable Filled under Options, and click Display.
(g) Restore the saved view, view-0 in graphics window 2.
Display −→Views...
(h) Close the Display Options, Contours, and Views panels.

5. Save the case and data ﬁles, silo-setup.gz.

Step 10: Calculate the Initial Solution
For this high-speed compressible ﬂow case, you will perform iterations in two stages. First,
you will calculate several iterations at a low Courant number in order to stabilize the solu-
tion. Then you will increase the Courant number and calculate the remainder of the solution
to convergence. Performing calculations in this way ensures that the solution is stable and
has an acceptable convergence rate.
Solve −→Iterate...

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1. Set the Time Step Size to 0.0005, Number of Time Steps to 0, and Max Iterations per
Time Step to 300.

2. Click Iterate.
FLUENT may give an error message on some systems when you iterate with Number
of Time Steps set to 0. In that case, set Number of Time Steps to 1.

3. Set the Courant number to 1.5.
Solve −→ Controls −→Solution...

4. Set Max Iterations per Time Step to 1000 and click Iterate to continue the solution.
The initial solution is completely converged at the end of 1300 iterations.

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5. Save the case and data ﬁles, silo-init.cas.gz and silo-init.dat.gz.

Residuals
continuity
x-velocity         1e+02
y-velocity
energy
nut                1e+00
exhaust

1e-02

1e-04

1e-06

1e-08

1e-10

1e-12
0     200     400     600      800     1000    1200     1400

Iterations

Scaled Residuals (Time=0.0000e+00)                                                 Mar 20, 2006
FLUENT 6.2 (axi, dp, coupled exp, dynamesh, spe, S-A, unsteady)

Figure 5: Residual Plot After Initial Solution

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Step 11: Calculate the Unsteady Solution

1. Open the Iterate panel.
Solve −→Iterate...
(a) Set Time Step Size to 0.0005, Number of Time Steps to 550, and Max Iterations
per Time Step to 20.
(b) Click Iterate.
While FLUENT is iterating, you can monitor the progress of solution in the two graph-
ics windows. As the solution time reaches t = 0.1 seconds (after 200 time steps), the
thrust will begin to move the missile. The motion of the missile will be seen in both
the graphics windows.

2. Save the case and data ﬁles, silo-unsteady.cas.gz.
File −→ Write −→Case and Data...

Step 12: Postprocessing
During the the simulation, animation frames are generated every 0.0025 seconds (5 time
steps). These frames are saved in the working directory under the ﬁle names machxxxx.jpg
and exhaustxxxx.jpg where xxxx stands for the time step number. Also, data ﬁles are
saved every 0.025 seconds (50 time steps) as silo-unsteadyxxxx.dat.gz.
You can read the saved case and data ﬁles at a particular time step to view the results for
postprocessing.

1. View the results at the moment the rocket is released (t = 0.1 seconds).
0.1 seconds corresponds to 200 time steps.
(b) Display contours of Mach number at t = 0.1 seconds (Figure 6).
(c) Restore the saved view, view-0.
Display −→Views...
(d) Display contours of mass fraction of exhaust gas at t = 0.1 seconds (Figure 7).

2. Display the results at t = 0.15 seconds (Figure 8 and Figure 9).
The results at 0.15 seconds can be viewed by reading the data ﬁle, silo-unsteady0300.dat.gz.

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2.94e+00
2.79e+00
2.65e+00
2.50e+00
2.35e+00
2.21e+00
2.06e+00
1.91e+00
1.77e+00
1.62e+00
1.47e+00
1.32e+00
1.18e+00
1.03e+00
8.84e-01
7.37e-01
5.90e-01
4.43e-01
2.96e-01
1.49e-01
1.80e-03

Contours of Mach Number (Time=1.0000e-01)                                         Feb 15, 2006
FLUENT 6.2 (axi, dp, coupled exp, dynamesh, spe, S-A, unsteady)

Figure 6: Contours of Mach Number at t=0.1 s

1.00e+00
9.50e-01
9.00e-01
8.50e-01
8.00e-01
7.50e-01
7.00e-01
6.50e-01
6.00e-01
5.50e-01
5.00e-01
4.50e-01
4.00e-01
3.50e-01
3.00e-01
2.50e-01
2.00e-01
1.50e-01
1.00e-01
5.00e-02
0.00e+00

Contours of Mass fraction of exhaust (Time=1.0000e-01)                                Feb 15, 2006
FLUENT 6.2 (axi, dp, coupled exp, dynamesh, spe, S-A, unsteady)

Figure 7: Contours of Mass Fraction of exhaust at t=0.1 s

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2.98e+00
2.83e+00
2.68e+00
2.53e+00
2.38e+00
2.23e+00
2.08e+00
1.94e+00
1.79e+00
1.64e+00
1.49e+00
1.34e+00
1.19e+00
1.04e+00
8.95e-01
7.46e-01
5.98e-01
4.49e-01
3.00e-01
1.52e-01
2.97e-03

Contours of Mach Number (Time=2.7500e-01)                                         Mar 20, 2006
FLUENT 6.2 (axi, dp, coupled exp, dynamesh, spe, S-A, unsteady)

Figure 8: Contours of Mach Number at t=0.275 s

1.00e+00
9.50e-01
9.00e-01
8.50e-01
8.00e-01
7.50e-01
7.00e-01
6.50e-01
6.00e-01
5.50e-01
5.00e-01
4.50e-01
4.00e-01
3.50e-01
3.00e-01
2.50e-01
2.00e-01
1.50e-01
1.00e-01
5.00e-02
0.00e+00

Contours of Mass fraction of exhaust (Time=2.7500e-01)                                Mar 20, 2006
FLUENT 6.2 (axi, dp, coupled exp, dynamesh, spe, S-A, unsteady)

Figure 9: Contours of Mass Fraction of exhaust at t=0.275 s

c Fluent Inc. March 20, 2006                                                                                                   29
Missile Silo Launch

3. Link the image sequences created during the simulation into an animation of the
Note: This step requires installation of third-party software.
Animations of the rocket motion are provided along with the tutorial ﬁles. These
animations can be played in any media player that supports the audio/video interleave
(AVI) ﬁle format.

Summary
In this tutorial, you used FLUENT to solve an unsteady problem using the six degree of
freedom solver along with the layering algorithm. While the six degree of freedom solver is
applicable to a three-dimensional ﬂow domain, this tutorial illustrates that it can also be
used to predict pure linear motion.

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