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
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Transcript
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 asilo. The motion of the missile is predicted using the six degree of freedom (6DOF) solverin 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 havea good understanding of the basic setup and solution procedures. In this tutorial, you willuse 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 predictthe 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 6DOFsolver applies Newton’s law to determine the acceleration of the missile.
Thrust, gravity, and pressure forces are considered in the calculation, whereas the drag onthe missile is neglected. The missile engine starts to fire at t = 0 seconds but the missile isheld fixed in the silo and is not allowed to move until t = 0.1 seconds. The flow is assumedto 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.
File −→ Read −→Case...
2. Scale the grid to inches.
Grid −→Scale...
(a) Under Units Conversion, select in from the Grid Was Created In drop-down list.
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. Donot 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-topand 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 atinternal-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.
The Information dialog box is displayed. Read the instructions carefully and click OK.FLUENT creates the appropriate directory structure, makefiles, and compiles the codefor 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 theworking directory.
3. Click Load.
Step 3: Models
1. Enable the Coupled solver with Explicit formulation, Unsteady time condition, andAxisymmetric space.
(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 iscreated when the old cell becomes larger than 1.4 (1 + 0.4) times the idealheight. When the layers collapse, a layer is destroyed when it shrinks to aheight 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.
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 andfluid-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 forcesand moments in that zone. However, the zone is still part of the rigid bodymotion. 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 onlyon the non-passive zone, missile-wall.
While fluid-inner moves in a translational rigid body mode, the upper and lowerboundaries (internal-top and internal-bottom) of this fluid zone must be fixed inorder to provide a location for creating and collapsing layers of cells. Thus, thesetwo 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 adjacentfluid-inner zone to 1 inch and click Create.
This is the ideal cell height near internal-bottom, and it is set equal to thecell height of the quadrilateral mesh adjacent to the internal-bottom zone.
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 adjacentfluid-inner zone to 1 inch and click Create.
(c) Under Resolution, set Width to 640 and Height to 480.
These settings will produce output frames that are 640×480 JPEG files. Each filewill be around 40 kB in size. The files can later be animated using the animationfeatures 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 aswell as output the animation frames. The quantities of interest are the Mach numberand mass fraction of exhaust gas.
(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 ofdrop-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 theContours 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 solutionto convergence. Performing calculations in this way ensures that the solution is stable andhas an acceptable convergence rate.
(a) Set Time Step Size to 0.0005, Number of Time Steps to 550, and Max Iterationsper 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), thethrust will begin to move the missile. The motion of the missile will be seen in boththe 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 timesteps). These frames are saved in the working directory under the file names machxxxx.jpgand exhaustxxxx.jpg where xxxx stands for the time step number. Also, data files aresaved 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 forpostprocessing.
1. View the results at the moment the rocket is released (t = 0.1 seconds).
0.1 seconds corresponds to 200 time steps.
(a) Read the data file, silo-unsteady0200.dat.gz
File −→ Read −→Data...
(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.
3. Link the image sequences created during the simulation into an animation of theunsteady flow and rocket motion.
Note: This step requires installation of third-party software.
Animations of the rocket motion are provided along with the tutorial files. Theseanimations 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 offreedom solver along with the layering algorithm. While the six degree of freedom solver isapplicable to a three-dimensional flow domain, this tutorial illustrates that it can also beused to predict pure linear motion.