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1ANSYS, Inc. Proprietary
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Inventory #002678
Wing Flutter Analysis using2-way FSI
Workshop
Wing Flutter FSI
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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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Training ManualStructural Model
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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Training ManualStructural Model
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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Training ManualStructural Model
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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Training ManualStructural Model
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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Training ManualModal Analysis
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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Training ManualModal Analysis
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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Training ManualSteady State Fluid Analysis
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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Training ManualSteady State Fluid Analysis
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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Training ManualSteady State Fluid Analysis
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
and set the following, then click OK:
• Boundary Type = Outlet
• Location = OUTLET
• Mass And Momentum Option =
Average Static Pressure
• Relative Pressure = 0 [Pa]
10. Insert a boundary condition named Sym
and set the following, then click OK:
• Boundary Type = Symmetry
• Location = SYM
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Training ManualSteady State Fluid Analysis
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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Training ManualSteady State Fluid Analysis
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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Training ManualSteady State Fluid Analysis
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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Training ManualSteady State Fluid Analysis
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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Training ManualTransient FSI Analysis
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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Training ManualTransient FSI Analysis
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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Training ManualTransient FSI Analysis
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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Training ManualTransient FSI Analysis
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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Training ManualTransient FSI Analysis
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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Training ManualTransient FSI Analysis
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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Training ManualTransient FSI Analysis
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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Training ManualTransient FSI Analysis
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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Training ManualTransient FSI Analysis
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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Training ManualTransient FSI Analysis
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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Training ManualTransient FSI Solution
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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Training ManualTransient FSI Solution
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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Training ManualTransient FSI Post-processing
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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Training ManualTransient FSI Post-processing
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