tutorial: store separation from a 3d delta wing introduction...ansys fluent 13.0 tutorial guide, and...

Tutorial: Store Separation from a 3D Delta Wing

Introduction

This tutorial illustrates the setup and simulation of store separation from an airplane wing.

The flow is inviscid and compressible. The objective of this simulation is to model the

motion of the store using the six degree of freedom (6DOF) solver in ANSYS FLUENT. The

results of the ANSYS FLUENT simulation are compared with the results computed from

a series of wind tunnel tests. For the details about the wind tunnel testing, refer to the

Appendix B and Appendix C.

This tutorial demonstrates how to do the following:

• Use the DEFINE SDOF PROPERTIES macro to specify the mass matrix and any

external forces/moments.

• Use the dynamic mesh (DM) feature in ANSYS FLUENT.

• Set up a compressible, transonic flow (Mach 1.2) in ANSYS FLUENT.

• Set the boundary conditions.

• Set up dynamic adaption.

• Obtain a first order solution using the density-based implicit solver.

Prerequisites

This tutorial is written with the assumption that you have completed Tutorial 1 from the

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.

You should be familiar with the dynamic mesh model. For more information refer to Section

11.6, Using Dynamic Meshes in the ANSYS FLUENT 13.0 User’s Guide.

Note: If you are using a single processor machine with a clock speed of 2.5 GHz, this

tutorial will require:

• Two hours to work through it.

• An additional 8-12 hours for the calculations.

© ANSYS, Inc. April 13, 2011 1

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Store Separation from a 3D Delta Wing

Problem Description

The problem considers the flow around a store attached to the clipped delta wing of an

airplane. The flow is three dimensional and inviscid.

Figure 1 shows the geometry of the wing and store. A sting is attached to the base of a

finned store. The sting was used as a part of the wind tunnel experiment. The root chord

length is 25 ft and the store diameter is 1.67 ft. The length of the store is approximately 10ft,

without the sting. The domain extends approximately 100 store diameters in all directions

around the wing and store. The wing root is coplanar with the symmetry plane.

Figure 1: Schematic of the Problem

Setup and Solution

Preparation

1. Copy the files (delta.msh.gz, six_dof_property.c) to your working folder.

2. Use FLUENT Launcher to start the 3D version of ANSYS FLUENT.

For more information about FLUENT Launcher see Section 1.1.2 Starting ANSYS FLU-

ENT Using FLUENT Launcher in the ANSYS FLUENT 13.0 User’s Guide.

2 © ANSYS, Inc. April 13, 2011

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Step 1: Mesh

1. Read the mesh file (delta.msh.gz).

File −→ Read −→Mesh...

As the mesh file is read, ANSYS FLUENT will report the progress in the console.

The mesh consists of 212064 tetrahedral cells.

Step 2: General Settings

1. Define the solver settings.

General −→ Density-Based

(a) Select Density-Based from the Type list.

The density-based solver is recommended for applications that use compressible

aerodynamics. Implicit solver converges much faster than the explicit solver, but

uses more memory.

2. Check the mesh.

General −→ Check

3. Display the mesh.

General −→ Display...



(a) Deselect all the surfaces from the Surfaces list and select boattail, store, and

wall-9.

(b) Click Display and close the Mesh Display dialog box.



(d) Set the display options.

Graphics and Animations −→ Options...

i. Enable Titles, Axes, and Colormap in the Layout group box.

ii. Click Apply and close the Display Options dialog box.

(e) Set the view of the model.

Graphics and Animations −→ Views...

i. Click Auto Scale and close the Views dialog box (or use shortcut Ctrl-A).

ii. Rotate, translate, and zoom the visible geometry using the mouse (see

Figure 2).



Figure 2: Mesh Display

Step 3: Models

1. Set the viscous model to Inviscid.

Models −→ Viscous −→ Edit...



Step 4: Materials

Materials −→ Create/Edit...

1. Select ideal-gas from the Density drop-down list. This automatically enables the energy

equation.

2. Click Change/Create and close the Create/Edit Materials dialog box.



Step 5: Boundary Conditions

Boundary Conditions

1. Set the boundary conditions for farfield.

Boundary Conditions −→ farfield −→ Edit...



(a) Enter 1.2 for Mach Number.

(b) Enter -1 for X-Component of Flow Direction.

(c) Enter 216.65 K for Temperature in the Thermal tab.

(d) Retain the default values for the remaining parameters.

(e) Click OK to close the Pressure Far-Field dialog box.

2. Set the boundary conditions for outflow.

Boundary Conditions −→ outflow −→ Edit...

(a) Retain 0 P a for Gauge Pressure.

(b) Enter 216.65 K for Backflow Total Temperature in the Thermal tab.

(c) Retain the default settings for the remaining parameters.

(d) Click OK to close the Pressure Outlet dialog box.



Step 6: Operating Conditions

Boundary Conditions −→ Operating Conditions...

1. Enter 20646 Pa for Operating Pressure.

This corresponds to an altitude of 38000 ft or 11600 m.

2. Enable Gravity from the Gravity group box.

3. Enter 9.807 m/s2 for Gravitational Acceleration in the Z direction.

The Z axis points towards the ground (see Figure 2).

4. Click OK to close the Operating Conditions dialog box.



Step 7: Initial Solution

1. Set the solution method parameters.

Solution Methods

(a) Select First Order Upwind from the Flow drop-down list in the Spatial Discretization

group box.

2. Set the solution control parameters.

Solution Controls

(a) Retain 5 for the Courant Number.



3. Enable the plotting of residuals during the calculation.

Monitors −→ Residuals −→ Edit...

(a) Disable Check Convergence for all the residuals.

(b) Click OK to close the Residual Monitors dialog box.



4. Initialize the domain.

Solution Initialization

(a) Select the zone farfield in the “Compute from” section.

(b) Click Initialize.

5. Apply full multigrid (FMG) initialization using the TUI command.

solve/initialize/fmg-initialization

Enable FMG initialization? [no] yes

FMG initialization often facilitates an easier startup, where CFL ramping is not nec-

essary thereby reducing the number of iterations required for convergence.



6. Run the solution for 250 iterations.

Run Calculation

(a) Enter 250 for Number of Iterations.

(b) Click Calculate (see Figure 3).

This calculates the steady flow.

Figure 3: Scaled Residuals

7. Save the case and data files (delta-steady.cas/dat.gz).

File −→ Write −→Case & Data...



Step 8: User-Defined Functions

Define −→ User-Defined −→ Functions −→Compiled...

1. Click Add... for the Source Files.

2. Select six_dof_property.c and click OK in the Select File dialog box.

3. Click Build to build the library.

ANSYS FLUENT displays a Warning dialog box warning you to ensure that the UDF

source files are in the same folder that contains the case and data files. Click OK in

the Warning dialog box.

ANSYS FLUENT sets up the folder structure and compile the code. The progress of

the compilation is displayed in the console.

4. Click Load to load the library.

The UDF uses a macro (DEFINE_SDOF_PROPERTIES) to define the mass matrix

and any external forces/moments.



Step 9: Transient Solver Setup

1. Define the transient solver settings.

General −→ Transient

(a) Select Transient from the Time list.



Step 10: Dynamic Mesh Setup

1. Set up the dynamic mesh.

Dynamic Mesh

(a) Enable Dynamic Mesh.

(b) Enable Six DOF from the Options group box.

(c) Enable Smoothing and Remeshing from the Mesh Methods group box and click

Settings....



i. Click Smoothing tab and set the Spring Constant Factor to 0.25.

You can control the smoothening stiffness by adjusting the value of Spring

Constant Factor. A value of zero indicates that there is no dampening (the

presence of the object is felt throughout the domain) and 1 is the default level

of dampening.

ii. Click Remeshing tab.



A. Enable On in Sizing Function group box.

B. Set the values as shown in the table:

Parameters Values

Variation 100

Minimum Length Scale (m) 7e-3

Maximum Length Scale (m) 24

Maximum Cell Skewness 0.8

Size Remesh Interval 1

The values of Minimum Length Scale and Maximum Length Scale should

be equal to those of the existing mesh, which can be checked by clicking

the Mesh Scale Info... button.

• The value of Size Function Variation controls the size of an interior cell

with respect to the boundary cells those are nearest to it.

• A value of 0.4 indicates that the interior cell size can be at the most

1.4 times the size of closest boundary cells.

• A value of -0.3 indicates that the size of a given interior cell must be

larger than 0.7 times the size of the closest boundary cells.

• A value of 100 indicates that the size of a given interior cell can be

quite large compared to the resolution of the boundary mesh.



• The value of Size Function Rate indicates the variation of cell size from

the boundary. It can vary from -0.98 to 0.98, and has a default value

of 0.7.

• A value of zero indicates a linear variation. Negative values indicate

a faster transition whereas positive values indicate a slower

transition.

• Transitioning is done slowly to keep the cell density near the store

relatively high.

C. Click OK to close Mesh Method Settings dialog box.

2. Set up the moving zones.

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

(a) Select store from the Zone Names drop-down list.

(b) Ensure that Rigid Body is selected in the Type group box.

(c) Click the Motion Attributes tab.



i. Ensure that delta missile::libudf is selected from the Six DOF UDF drop-down

list.

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

iii. Retain the default settings for the Center of Gravity Location group box.

The CG orientation is not important here because only theta is required to

move the object. If you have experimental data of orientation for comparison,

enter the correct initial orientation. Otherwise, you need to add an offset

for the comparison.

iv. Enter the following values for Center of Gravity Velocity.

Parameters Values

V X (m/s) -1.353393e-01

V Y (m/s) -9.516563e-02

V Z (m/s) 2.641932

v. Enter the following values for Center of Gravity Angular Velocity.

Parameters Values

Omega X (rad/s) 1.705652e-01

Omega Y (rad/s) 7.203959e-01

Omega Z (rad/s) 2.159020e-01

(d) Click the Meshing Options tab and set the Cell Height to 0.05 m.

The approximate size of the cell faces at the midsection of the store is 0.05 m.

(e) Click Create to create the zone.

(f ) Similarly, create the boattail zone by enabling Passive from the Six DOF Solver

Options group box.

Enabling Passive allows boattail to move with the store but it does not contribute to

the force calculation. The boattail is included in the computational fluid dynamics

(CFD) model only to replicate the support structure used in the wind tunnel tests.

For the details about the wind tunnel testing, refer to Appendix B.

(g) Close the Dynamic Mesh Zones dialog box.

Step 11: Mesh Preview

1. Save the case and data file (delta-unsteady-init.cas/dat.gz).

Mesh preview changes the mesh permanently. For the flow calculations, read these

files into ANSYS FLUENT after the mesh preview.

File −→ Write −→Case & Data...

2. Display the mesh (see Figure 2).

Display −→Mesh...



3. Preview the motion.

Dynamic Mesh −→ Preview Mesh Motion...

(a) Set the Time Step Size to 0.002 seconds.

(b) Enter 30 for the Number of Time Steps.

(c) Click Preview to see the motion (see Figure 4).

You can see that the motion is acceptable.

Figure 4: Mesh Motion at t=0.06 s

4. Exit ANSYS FLUENT without saving.



Step 12: Solution

1. Start the 3D version of ANSYS FLUENT and read the case and data file

(delta-unsteady-init.cas/dat.gz).

2. Enter 10 for the Courant Number.

Solution Controls

3. Save the case and data files every 100 time steps.

Calculation Activities

(a) Enter 100 for Autosave Every (Time Steps) and click Edit....

i. Enter delta-unsteady-init.gz for the File Name.

ii. Click OK to close the Autosave dialog box.

4. Create an animation sequence to view the motion of the free-falling store.

Calculation Activities (Solution Animations)−→ Create/Edit...



(a) Set Animation Sequences to 1.

(b) For sequence-1, set Every to 5 and select Time Step from the “ When” drop-down

list.

(c) Click the Define... for sequence-1 to open the Animation Sequence dialog box.

i. Set Window to 2 and click the Set.

ii. Retain the default selection of Mesh from the Display Type group box.

iii. Click Edit... to open the Mesh Display dialog box.

A. Disable Edges from the Options group box.

B. Ensure that boattail, store, and wall-9 are selected from the Surfaces list.

C. Click Display and close the Mesh Display dialog box.

iv. Click OK to close the Animation Sequence dialog box.

Active for sequence-1 is enabled in the Solution Animation dialog box.

(d) Click OK to close the Solution Animation dialog box.

5. Zoom in and rotate the view to set the display as shown in Figure 5.



Figure 5: Mesh Display for Animation

6. Run the calculation for 400 time steps.

Run Calculation

(a) Set the Time Step Size to 0.002 seconds.

(b) Set the Number of Time Steps to 400.

This will allow the store to fall for 0.8 s.



Figure 6: Position of the Store at t = 0.02 s

(c) Set Max Iterations/Time Step to 15.

(d) Click Calculate.

You can also run this tutorial in viscous mode using the realizable k-epsilon model

with enhanced wall treatment. Obtain the steady solution with first order dis-

cretization using 100 iterations with a Courant number of 5, followed by 200

iterations at a Courant number of 20. For the second order solution, perform

150 iterations at a Courant number of 5, followed by 500 iterations at a

Courant number of 10.

Step 13: Postprocessing

1. Play back the animation.

Graphics and Animations −→ Solution Animation Playback −→ Set Up...

(a) Set the Reply Speed slider to Fast.

(b) Click the play button.

The animation starts playing and you can see the movement of the store.

(c) You can save the animation in MPEG movie format.

i. Select MPEG from the Write/Record Format drop-down list.

ii. Click Write.

You need to rewind the animation by sliding the frame counter to the left.

The animation will play again and create an MPEG file.

You can also read the case and data files saved at different time steps and note the

store position. Figures 6—9 show the position of the store relative to the wing at every

100 time steps.




Figure 10 shows the positions of the store at all the four times superimposed with the

locations measured during the experiment. The comparison between the experimental

analysis and CFD results is shown in Figure 11.

Figure 10: Store Positions Superimposed with the Experimental Locations



Figure 11: Comparison: Experimental and CFD Analysis



Appendix A: Pressure Gradient Based Adaptation

Note: When you use this step of adapting the gradient, the calculation will take a very long

time to complete (approximately 100 hours). It also uses up more memory. Hence,

ensure that you have more than 6GB memory.

1. Adapt the gradient.

Instead of using the default adaptation criteria of ANSYS FLUENT you can do pressure

gradient based adaptation to capture the bow shock wave and the expansion wave

around the wing and the store accurately. Doing the adaptation at every time step

will help in increased accuracy.

Adapt −→Gradient...

(a) Select Gradient in the Method group box.

(b) Select Scale in the Normalization group box.

(c) Enable Dynamic in the Dynamic group box.

(d) Enter 1 for the Interval.

(e) Retain the default selection of Pressure... and Static Pressure from the Gradients

of drop-down list.

(f ) Enter 0.3 for Coarsen Threshold and 2 for Refine Threshold.

The dynamic gradient adaption approach executes the gradient adaption

automatically. In contrast with the static gradient adaption, dynamic gradient

adaption is a fully automated process. For time dependent as well as steady

state problems, you can perform the entire solution without changing the initial

settings. You can let the solver periodically perform adaptions without

changing/entering any parameter. In this case, dynamic adaption is performed

to adapt the grid dynamically during the calculation and to capture the flow field

and the trajectory.



For more information about dynamic adaption, refer to Section 29.4.1, Dynamic

Gradient Adaption Approach, of ANSYS FLUENT 13.0 User’s Guide.

(g) Click Apply and close the Gradient Adaption dialog box.

2. Display the pressure contours along with the adapted grid.

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

(a) Disable Filled and enable Draw Mesh from the Options group box.

i. Select boattail, store, and wall-9 from the Surfaces selection

list in the Mesh Display panel, which popped up. Close the

Mesh Display panel.

ii. Select boattail, store, and wall-9 from the Surfaces selection

list in the Contours panel.

(b) Click Display and close the Contours dialog box (Figure 12).

Figure 12: Grid Showing Adaption in High Pressure Gradient Region

(c) Zoom the visible geometry to view the adapted grid as shown in Figure 13.





Figure 13: Zoomed View Showing Adaption in High Pressure Gradient Region

(d) Display the pressure contours on a symmetry plane.

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

i. Select symmetry from the Surfaces selection list.

ii. Ensure that boattail, store, and wall-9 are selected from the Surfaces selection

list.

iii. Click Display and close the Mesh Display dialog box.

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

i. Select symmetry from the Surfaces selection list.

ii. Click Display and close the Contours dialog box.

iii. Zoom the visible geometry to view the adapted grid close to the wing as

shown in Figure 14.



Figure 14: Grid Showing Adaption Close to Wing

The case and data files for the calulations with adapting the gradient are stored in dm-store3d-adapt.cas.gz and dm-store3d-adapt.dat.gz.

Appendix B: Captive Tra jectory Support (CTS) Method of Wind Tunnel Testing

The wind tunnel test, commonly called the capture trajectory support (CTS) method, is

conducted by measuring the forces and moments on the store at discrete locations along

the calculated trajectory path of the store. A sting is attached to the end of the store to

provide support and to channel the instrumentation wires back to the control room thus

preventing the store from actually being dropped from the underside of a wing.

The test is conducted by measuring the force and moments on the store at a fixed location

after it is released from the underside of the wing. These forces/moments are used in

conjunction with Newton’s second law to get the linear and angular accelerations of the

store only. The effect of the support structure (i.e. boattail and sting) is factored out of the

calculations. A The accelerations acting on the store are integrated over a small time step

to get a new position/orientation of the store. The store is repositioned at this new location

and additional wind tunnel measurements are taken. The process is repeated to calculate

a trajectory path of the store over a given distance from the underside of the wing.



Appendix C: Comparison of ANSYS FLUENT Results with the Wind Tunnel Test

This tutorial compares the trajectory of a store simulated by ANSYS FLUENT with the

trajectory computed by CTS method. The experiment consists of a series of wind tunnel

tests (and not a 6DOF calculation) to emulate the trajectory of a free-falling store.

• The geometry under consideration is of a store that has a boattail attached to the aft

end. The boattail replicates the support structure used to hold the store in the wind

tunnel when the flow measurements are taken. The effect of the supporting structure is

factored out of the test results while computing the trajectory of the store. Therefore,

a boattail is added to the CFD model to replicate the support structure used in the

wind tunnel tests. However, it is not included in the force calculations to be consistent

with the data reported from the wind tunnel tests.

• The forces and moments on the store are computed and used to predict the location

of the store at another instance of time. Thus, the reported trajectory of the store is

computed and not measured.

• The ANSYS FLUENT analysis is a 6DOF calculation of the store. The ANSYS FLU-

ENT calculated trajectory path for the store matches very well with the computed

trajectory path from the wind tunnel tests.


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