AGARD445_Workshop.pdf

1ANSYS, Inc. Proprietary

© 2009 ANSYS, Inc. All rights reserved.July 2009

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Wing Flutter Analysis using2-way FSI

Workshop

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Training ManualProblem Overview

This problem consists of a mahogany wing at Mach 0.9. The

geometry is based on the AGARD 445.6 wing which has been widely

studied in literature. The structural and fluid meshes used are quite

coarse and are not intended to show best practice.

The response of the wing to an initial

perturbation is analysed using 2-way

FSI between ANSYS CFX and ANSYS

Mechanical. ANSYS Workbench

version 12.1 will be used for the

analysis.

Basic knowledge of Workbench,

CFX and Mechanical is assumed.

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Training ManualWorkflow Overview

• Import the geometry into Workbench and complete the structural

mesh and model

• Perform a Modal analysis to verify the expected mode shapes and

frequencies

• Import the existing fluid mesh which was completed in ICEM CFD

• Setup and solve an initial steady state fluid solution with a fixed wing

• Setup and solve the coupled FSI analysis

• Post-process

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Training Manual

1. Start Workbench 12.1 and save the project as

AGARD445_Workshop.wbpj in a new working directory

– Saving the project at start up sets the working directory

2. Add a Transient Structural (ANSYS) analysis to the Project

Schematic

3. Right-click on the Solution cell (A6) and select Delete

– The solution is performed after completing the fluid setup

4. Right-click on the Geometry cell

and import the file

agard445_wing.agdb

– The geometry has already been

completed

Starting the Project and the Structural Model

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Training ManualStructural Material Properties

1. Double-click the Engineering

Data cell to define the

structural material properties

2. Enter a new material named

Mahogany

3. Double-click on Density under

Physical Properties from the

Toolbox on the left, then enter

the Density value as 381.98 kg

m^-3

4. Under Linear Elastic double-

click on Orthotropic Elasticity

and enter values for the

Young’s Modulus and

Poisson’s Ratio as shown

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Training ManualStructural Model

5. Click Return to Project from the toolbar

Now create the structural mesh and setup

1. Double-click the Model cell (A4) to open

Mechanical and create the structural

mesh and model

2. Select Units > Metric (m, kg, N, s, V, A)

from the main menu

3. Expand the Geometry tree in Outline.

Select wing.

4. In the Details view, under Material,

change the Assignment from Structural

Steel to Mahogany

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Next create a coordinate frame aligned with

the wing corresponding to the orthotropic

material properties

1. Right-click on Coordinate Systems

in the Outline tree and Insert a new

Coordinate System

2. Right-click to Rename the

Coordinate System “Local Ortho”

3. In the Details view, change Define By

to Global Coordinates

4. From the Toolbar select the RY icon

to rotate about the y-axis, and enter

45o under Transformations in the

Details view

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5. Now select wing again under Geometry

and change the Coordinate System to

Local Ortho

The next step is to create the structural mesh

1. Right-click on Mesh and select Generate

Mesh to create the default mesh

– The default mesh is too coarse, particularly

near the leading edge

2. Select Mesh from the Outline tree, then in

the Details view, under Sizing, set Use

Advanced Size Functions to On:

Curvature then re-generate the mesh

– This improves resolution at the highly-curved

leading edge, but produces too many elements

in the swept direction and the elements are too

large away from the leading edge.

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3. Right-click on Mesh and select Insert >

Method

4. Select the wing from the viewer and click

Apply in the Geometry field

5. Set the Method to Sweep, and the Sweep

Num Divs to 20, then re-generate the

mesh

– This gives a more reasonable number of

elements in the swept direction, but the

elements are still too large away from the

leading edge

6. Select Mesh from the Outline tree, then in

the Details view, under Sizing, set Min

Size to 1e-3 m and Max Face Size and Max

Tet Size to 0.02 m. Generate the mesh.

– This completes the structural mesh

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The structural boundary conditions and

analysis settings can now be defined

1. Under Transient (A5) select Analysis

Settings and set the values as shown

– The transient timestep controls are defined

in CFX-Pre. Here we just need to define

how many substeps are needed per

timestep (almost always 1). The Step End

Time is not used. Auto Time Stepping

should be off. Time Integration should be

on for a true transient. Large Deflection

should always be on, even for small

deformations.

2. From the toolbar select Inertial >

Standard Earth Gravity then in the

Details view set the Direction to –Y

Direction

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Training ManualStructural Setup

3. Insert a Fixed Support at the root

surface of the wing

4. From the Toolbar select Loads > Fluid

Solid Interface and apply to the top AND

tip surface of the wing

5. Create a second Fluid Solid Interface

for the bottom surface of the wing

– The Interface Number is shown in the Details

view for each Fluid Solid Interface. This

number is referenced when applying boundary conditions to the fluid side

– In general the interfaces could be combined into a single Fluid Solid Interface.

However, when surfaces meet at a sharp angle, as is the case here at the

trailing edge, it is a good idea to use separate interfaces to avoid any fluid-solid

mesh mapping problems

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Training ManualModal Analysis

This completes the structural setup. At this point you could write out an

input file (Tools > Write Input File) if the case is to be ran on a different

machine (e.g. a cluster). Before proceeding with the fluid setup it is a good

idea to perform a Modal analysis to verify the modal frequencies are as

expected.

1. Return to the main project page (do not close Mechanical)

2. Drag and drop a Modal (ANSYS) system onto the Model cell (A4) of

the Transient Structural analysis

• This creates a new Modal analysis, sharing the Geometry and Model

from the Transient Structural system

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3. Return to the Mechanical window

– A Modal (B5) entry has been added to the

Outline tree

4. Right-click on the Fixed Support under the

Transient (A5) entry and select Copy

5. Right-click on the Modal (B5) entry to Paste

the Fixed Support into the Modal analysis

6. Select the Modal Analysis Settings and

reduce the Max Modes to Find to 4

7. Right-click on Solution (B6) and select

Insert > Deformation > Total

8. Right-click on Modal (B5) and select Solve

– The first four mode frequencies are shown

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9. Select Total Deformation under

Solution (B6) from the Outline

tree, then click the Animation

Play icon. Stop the animation

after viewing.

10. To view the next mode shape,

select Total Deformation again,

then in the Details view change

the Mode to 2. Right-click on

Total Deformation and select

Retrieve This Result.

11. Save the project

12. Close the Mechanical window

after viewing the results and

return to the main Workbench

project page

First Bending Mode at 9.64 Hz

First Torsional Mode at 40 Hz

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Training ManualSteady State Fluid Analysis

Next you will solve a steady-state fluid

solution to provide the starting point for

the transient FSI analysis.

1. Add a Fluid Flow (CFX) Analysis

System to the Project Schematic

(do not connect it to any other

systems)

2. Right-click on the Mesh cell (C3)

and select Import Mesh File

3. Change the File of type: option

to ICEM CFD Output File(*.cfx5)

then select the file

agard_wing3.cfx5 provided with

this workshop

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4. Double-click the Setup cell (C3) to start CFX-

Pre

5. When CFX-Pre opens, edit the Default

Domain and set the following on the Basic

Settings panel:

• Material = Air Ideal Gas

• Reference Pressure = 7703 [Pa] (make

sure the units are correct)

• Buoyancy Option = Buoyant

• Gravity X Direction = 0

• Gravity Y Direction = -g (click the

Expression icon to enter)

• Gravity Z Direction = 0

• Buoyancy Reference Density =

0.0994 [kg m^-3]

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6. On the Fluid Models panel set:

• Heat Transfer Option = Total Energy

• Turbulence Option = Shear Stress

Transport

7. Click OK to complete the domain settings

8. Insert a boundary condition named Inlet

and set the following, then click OK:

• Boundary Type = Inlet

• Location = OPEN

• Mass And Momentum Option = Cart.

Vel. Components

• U = 269.69 [m s^-1]

• V = 0.26969 [m s^-1]

• W = 0 [m s^-1]

• Static Temperature = 269.86 [K]

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The inlet boundary has a slight V velocity

component. This will be removed in the transient

analysis providing a perturbation to the flow and

the wing.

9. Insert a boundary condition named Outlet


• Boundary Type = Outlet

• Location = OUTLET

• Mass And Momentum Option =

Average Static Pressure

• Relative Pressure = 0 [Pa]

10. Insert a boundary condition named Sym


• Boundary Type = Symmetry

• Location = SYM

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11. Insert a boundary condition named WingBtm using the following:

• Boundary Type = Wall

• Location = WINGBTM

• All other settings can remain at their default values

12. Insert a boundary condition named WingTopAndTip using the

following:

• Boundary Type = Wall

• Location = WINGTOP, WINGTIP

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Next, modify the Solver Controls:

1. Edit the Solver Control object from the Outline tree and set:

• Max. Iterations to 50

• Typically you shouldn’t limit the max. iterations too much since

a case may stop before it is converged. In this instance it is

known that the solution is well converged after 50 iterations.

• Timescale Control = Physical Timescale

• Physical Timescale = 0.01 [s]

• Residual Target = 1e-6

2. Click OK

Now create some Monitor Points:

1. Edit the Output Control object from the Outline tree. On the

Monitor tab enable Monitor Options

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2. Create a new Monitor Point named Drag

and set:

• Option = Expression

• Expression Value =

force_x()@WingBtm +

force_x()@WingTopAndTip

3. Create a second Monitor Point named

Lift and set:

• Option = Expression

• Expression Value =

force_y()@WingBtm +

force_y()@WingTopAndTip

4. Save the case then close CFX-Pre

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1. In the Project Schematic double-click the Solution cell (C4)

2. When the Solver Manager opens, enable the Double Precision

toggle, then click Start Run

• The solution will proceed and stop after 50 iterations

3. Check the residuals are converged, the imbalances are

reasonable and the monitor points are showing steady values

4. Close the Solver Manager then save the project

If you wish, examine the steady state fluid results; detailed instructions are

not provided here.

The next step is to create the transient FSI analysis.

You may wish to skip to steps on this page, since the steady-state solution

will take approximately an hour to complete. You can open the project

AGARD445_SteadySolution.wbpj and continue from the next page.

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Training ManualTransient FSI Analysis

1. In the Project Schematic click on

the down-arrow in the corner of the

Fluid Flow system and select

Duplicate. Enter the name for the

new system as Transient FSI.

2. Drag-and-drop the Setup cell of the

Transient Structural system (A5)

onto the Setup cell of the Transient

FSI system (D3)

• This creates the FSI link

3. Right-click on the Setup cell of the

Transient Structural system (A5)

and select Update

• This writes the Mechanical input

file in the background and passes

it to the Transient FSI system

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4. Drag-and-drop the Solution cell

of the Fluid Flow system (C4)

onto the Solution cell of the

Transient FSI system (D4)

• This uses the initial fluid

solution as the starting point for

the transient calculation

5. Double-click the Setup cell of the

Transient FSI system (D3) to edit

in CFX-Pre

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1. In CFX-Pre edit the Analysis Type, set the

following, then click OK:

• Total Time = 0.5 [s]

• Timesteps = 0.0025 [s]

• Analysis Type Option = Transient

2. Edit the Default Domain. On the Basic

Settings tab set the Mesh Deformation

Option to Regions of Motion Specified.

3. Expand the Mesh Motion Model section and

set the Mesh Stiffness Option to Increase

Near Boundaries with a Model Exponent of 2.

• The default value of 10 is often not suitable,

since (1 / Boundary Distance)10 may be

beyond the number range that can be

represented

4. Click OK

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The message window will show a number of errors

because mesh motion boundary conditions now

need to be set.

5. Edit the Inlet boundary. Set the V velocity

component to 0 [m s^-1]

6. The Mesh Motion Option will be set to

Stationary by default. Click OK.

7. Edit the Outlet boundary. The Mesh Motion

Option will be set to Stationary by default.

Click OK.

8. Edit the Sym boundary. Under Boundary

Details set the Mesh Motion Option to

Stationary. Click OK.

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9. Edit the WingTopAndTip boundary.

Under Boundary Details set the Mesh

Motion Option to ANSYS MultiField.

10. Check that FSIN_1 is selected as the

ANSYS Interface

• Total Force will be sent to ANSYS,

Total Mesh Displacement will be

received

11. Click OK

12. Repeat the last 3 steps for the WingBtm

boundary, but use FSIN_2 as the

ANSYS Interface

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13. Now edit the Solver Control object

14. Set Min. Coeff. Loops to 1 and Max.

Coeff. Loops to 4

• These are the CFX iterations per

coupling iteration. In general don’t use

too many – there’s no point in

converging CFX too much if the FSI

boundary displacements are going to

change in the next coupling iteration

15. Set the Residual Target to 1e-4 (RMS)

16. Switch to the Equation Class Settings

tab

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17. Select the Mesh Displacement equation

from the list

18. Enable the Mesh Displacement check

box, the Convergence Control check

box and the Convergence Criteria check

box

19. Increase the Max. Coeff. Loops to 10

20. Set the Residual Type to MAX

These changes set a tighter convergence

target for just the Mesh Displacements

equations and allow for up to 10 loops to reach

that target. It is a good idea to tightly converge

the mesh displacement equations to help

prevent mesh folding.

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21. Switch to the External Coupling tab

22. Set the Min. Iterations to 2

• This is the number of coupling iterations

per timestep. A minimum of 2 is required

for an implicit solution.

23. Set Solve ANSYS Fields to Before CFX

Fields

• For transient cases ANSYS should almost

always be solved first

24. Set the Under Relaxn. Fac. to 1

• Under relaxation slows convergence and is

generally not necessary. There are better

ways to keep the solution stable if required.

25. Click OK

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26. Edit the Output Control object

27. On the Monitor tab create a new Monitor Point called Tip LE

Displacement with the following settings:

• Option = Cartesian Coordinates

• Output Variables List = Total Mesh Displacement

• Cartesian Coordinates = 0.8075 [m], 0 [m], -0.76 [m]

• You can also pick points from the Viewer. To pick a point on

the wing you would need to hide the external boundaries first.

28. Create another Monitor Point called Tip TE Displacement at the

point (1.1775 [m], 0 [m], -0.76 [m])

29. Enable the Monitor Coefficient Loop Convergence check box at

the top of the panel

• This produces monitor point data for each inner CFX iteration and

allows you to judge the stability of the interface solution.

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30. Switch to the Trn Results tab

31. Create a new Transient Results object with the following settings:

• Option = Selected Variables

• Output Variables List = Total Mesh Displacement

• Output Frequency Option = Coupling Step Interval

• Interval = 4

• Note that ANSYS will also write data at this frequency

32. Click OK

33. Select Insert > Solver > Expert Parameter from the main menu

34. On the Discretisation tab enable meshdisp diffusion scheme and

set the Value to 3

• This relates to the numerics of the mesh displacement equation.

This setting can help avoid mesh folding at sharp corners. In this

case the displacements are small, so it likely makes little difference.

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35. On the I/O Control tab enable include pref in forces and set to t

• This includes the CFX reference pressure in the forces sent to

ANSYS. In this case it will make little difference since the wing is a

closed surface.

36. Click OK

37. Close CFX-Pre then save the project

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Training ManualTransient FSI Solution

The next step is to solve the transient FSI

case. It will take some time to solve, so is

best run overnight.

1. Double-click the Solution cell (D4) of

the Transient FSI system to open the

Solver Manager

2. Enable the Double Precision toggle

3. Click Start Run

4. Wait until the solution finishes

Next you will export plot data for use in an

FFT chart in CFD-Post:

1. On the User Points graph, right-click

and select Monitor Properties…

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2. On the Plot Lines tab turn off all points expect the Tip LE Disp

monitor point

3. On the Range Settings tab set the Timestep Range Mode to This

Run Only then click OK

4. Select Workspace > Workspace

Properties from the main menu

5. On the Global Plot Settings tab make sure

Plot Coefficient Loop data is off and set

Plot Data By to Simulation Time. Click OK.

6. Right-click on the monitor plot and

select Export Plot Data… . Save the file

output.csv (note the saved location).

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7. Open the output.csv file in Excel (or any program that can

process comma separate ASCII data)

8. Delete the first column of data, so that Time is in column A and

Displacement in column B, then save the file is csv format (not

Excel format). It should look as shown.

• Only Time and Displacement data is required for the FFT chart

9. Close the Solver Manager,

return to the main project

page and save the project

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Training ManualTransient FSI Post-processing

1. Double-click cell D5 to view the results in CFD-Post

2. Turn on visibility for the Default Boundary in the ANSYS

results

3. Edit this boundary. Set the Colour Mode to Variable, and the

Variable to Total Mesh Displacement.

4. Right-click on a blank area of the Viewer and select

Deformation > Custom

5. Enter a value of 20 to scale the

deformations, then click OK

6. Use the Timestep Selector (in the

Tools menu) to load the results at

different timesteps

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Now create an FFT chart to extract the

flutter frequencies:

1. Select Inset > Chart from the main

menu

2. Set the Type to XY – Transient or

Sequence

3. Enable the Fast Fourier Transform

check box and then the Subtract

mean check-box

4. On the Data Series tab, set the

Data Source to File and browse for

the output.csv file

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5. On the Y Axis tab, set the Y Function

to Magnitude, then click Apply to

create the chart

6. On the X Axis tab change the Axis

Range as necessary and click Apply

to update the chart

Note that an accurate FFT chart may need data

collected over a greater time period than is used here.

The initial start-up transient data can also be removed

to improve the accuracy. The chart shown a signal just

above 14 Hz. The small bump just below 50 Hz is

likely the first torsional mode, but would require a

smaller timestep to resolve accurately. 14 Hz is

somewhat lower than experimental data, but can be

improved with finer meshes.

8. When you have finished post-

processing, close CFD-Post, save

the project then exit Workbench

Documents

AGARD445_Workshop.pdf