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Chapter 2
1-way FSI
Solving FSI Applications
using Workbench, ANSYS
Mechanical and CFX
1-way FSI
2-2
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Training ManualOne-way Data Transfer
• In Workbench both thermal and structural surface loads
can be transferred from CFX to ANSYS
– Temperature
– Wall Heat Transfer Coefficient
– Pressure
• This includes both normal (Pressure) and tangential (Shear) loads
• In fact force data that comes directly from the solution of the
momentum equations is used
• CFD-Post performs the interpolation in the background
• A beta-feature also allows volumetric thermal loads to be
transferred from CFX to ANSYS
1-way FSI
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Training Manual
CHT Mesh
Integrated Process in Workbench
CFD CHT SolutionGeometry
Thermal Loads Pressure Loads Thermal Stress Solution
Example Project
Schematic
1-way FSI
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Training ManualConnecting Systems in Workbench
• Workbench supports two types of connections
– Connections that share data
• Used when inputs and outputs of two connected cells are identical
• Can only be created between two cells of same type
– Connections that transfer data
• Used when output of one cell is input to a connected cell
• Usually created between two cells of different types
– Both connection types are used for FSI systems in Workbench
Shared Data Connection
Transfer Data Connection
1-way FSI
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Training ManualConnecting Analysis Systems for FSI
• Highlight desired
ANSYS Analysis
System in Toolbox and
drag on top of Solution
cell (A5) in CFX Fluid
Flow system
• Red outlines show cells
that will Share and
Transfer data with new
system
• Connections are
automatically created
between both systems
Drag
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Training ManualConnecting Analysis Systems for FSI
• Alternatively, right-click on Solution cell in Fluid Flow system, select
Transfer Data To New, then select the appropriate target system
• Connections are automatically created between both systems
1-way FSI
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Training Manual
• In the Customs Systems toolbox the FSI: Fluid Flow (CFX) > Static
Structural system can be added to the Project Schematic
– This transfers pressure loads from CFX to Mechanical
• Any project schematic can be saved/added to Custom Systems
toolbox
• To add a new workflow to Custom Systems, right-click on schematic
white space and select Add to Custom from displayed pop-up menu
– Workbench will prompt for a custom template name to save
Using Custom Systems
=
1-way FSI
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Training ManualGeometry and Mesh Considerations
• Fluid and Solid geometries should physically match
– Helpful for both geometries to come from a single CAD file
– DM can easily extract a fluid volume from a CAD model of the
structure
• Mesh does not need to match 1-to-1
– But similar mesh resolution will improve interpolation accuracy
– Can write an ANSYS Input File and load into CFD-Post, along with
the fluid mesh, to check interface mesh
• Must be a .inp file written from Workbench (not Mechanical APDL)
• CFD-Post recognizes .dat files as FLUENT files
– Check the Imported Load contour/vector plots in Mechanical
• These are automatically created after the import
1-way FSI
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Training ManualImported Loads
• In Mechanical an Imported Load entry will
automatically be added to the tree when
solution data is transferred from CFX
• There are two Scoping Methods for
imported loads
– Geometry Selection: Select faces on solid
geometry to transfer CFD loads
– Named Selection: Select existing named
selections to transfer CFD loads
• All boundary condition names in CFX
results file will be exposed via drop-down
1-way FSI
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Training ManualData Transfer to Mechanical Systems
• CFX data is transferred to Mechanical Systems via CFD-Post
– Launched in background
• Only surface loads can be transferred automatically in
Workbench
– Temperature transfer as a body load is a manual process (beta)
• CFD-Post performs interpolation process for vector and scalar
quantities
– Mechanical nodal values are transferred by linear interpolation
from the surrounding CFD surface nodes
– If interpolation process cannot find a face to map to, then closest
point is chosen
– Not conservative
– Interpolation process can be slow for large cases
• Planning to enhance interpolation process in a future release
1-way FSI
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Training Manual
• Surface Temperature or Heat Transfer Coefficient (HTC) data is
transferred from CFX to Mechanical via a Thermal system
– Static Thermal, Transient Thermal, Thermal Electric supported for
Mechanical analysis systems
– Solid temperature field will be solved in Mechanical
– May have already solved as Conjugate Heat Transfer (CHT) in CFD
• Engineering Data used to specify material properties for
ANSYS Mechanical systems
– Need to make sure thermal properties are consistent with CFD
• Specific Heat, Thermal Conductivity, Density
Thermal Transfer to Mechanical Systems
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Training Manual
1. Right-click on Imported Load (Solution) object in Outline panel
2. Select Insert > Temperature or Insert > Convection Coeff.
3. Choose a scoping method and corresponding CFD surface
4. For Convection Coeff., choose Near Wall Temperature or Ambient
Temperature (see next slide)
5. Highlight Imported object to see contour and range
Thermal Transfer to Mechanical Systems
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Training ManualThermal Transfer to Mechanical Systems
• If CHT has been solved in CFX, Temperature will be
available on the solid surface
– Preferable to transfer Temperature in this case
– If transferring HTC, check that calculated surface temperatures
in Mechanical match those in CFX
• When importing HTC data (Convection Coeff. in
Mechanical), can choose Near Wall Temperature or
Ambient Temperature
– Method chosen should be consistent with the way HTC was
calculated in CFX
– Wall Heat Transfer Coefficient is the variable imported from CFX
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Training ManualThermal Transfer to Mechanical Systems
• By default CFX will calculate Wall HTC using the Near
Wall Temperature
– Temperature in the wall adjacent control volume
– See CFX doc for details
• HTC is often calculated using a specific reference
temperature
– E.g the average fluid inlet temperature
• In these cases the parameter tbulk for htc is set in
CFX
– Wall Heat Transfer Coefficient is then calculated using this
reference temperature instead of the Near Wall Temperature
• Should then select Ambient Temperature in ANSYS and
provide the same reference temperature
1-way FSI
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Training ManualPressure Transfer to Mechanical Systems
• Pressure data is transferred
from CFX to Mechanical
through a structural analysis
system
– Static Structural, Transient
Structural, Shape
Optimization
• An Imported Pressure entry
will appear by default
• To add additional CFX
Pressure loads right-click on
Imported Load (Solution) and
select Insert > Pressure
1-way FSI
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Training ManualThermal Stress Workflow
Workflow 1: Thermal Data Only Transferred to Mechanical
Workflow 2: Thermal and Pressure Data Transferred to Mechanical
(A5 to C5 link added manually)
1-way FSI
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Training ManualThermal Stress Workflow
1. Solve CFD conjugate heat transfer (CHT) analysis for
combined fluid and solid
2. Transfer surface temperatures/HTC to a Thermal
analysis system
– Solve temperature field
3. Connect to a Structural analysis system
– WB will apply 3D temperature field as a body load automatically
– Optionally apply Pressure load from CFD results
– Solve for deformations (stresses)
1-way FSI
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Training Manual
• Automotive Manifold
– Full conjugate heat transfer
solution in FLUENT
– Cast iron solid material
properties
– Free stream external convection
CFD Mesh ANSYS Mesh
Thermal Stress Example
1-way FSI
2-19
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Training ManualThermal and Structural Loads
Solid Temperature (CFD) Solid Temperature (ANSYS)
Surface Pressure (CFD) Surface Pressure (ANSYS)
1-way FSI
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Training ManualANSYS Stress Solution
Equivalent Stress Total Deformation
Solution Metrics: 5 minutes to process all loads and solve ANSYS stress problem
1-way FSI
2-21
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Training ManualAlternative Thermal Stress Workflow
1. Solve CFD conjugate heat
transfer (CHT) analysis for
combined fluid and solid
2. Connect to a Structural
analysis system
– Optionally apply Pressure
load from CFD results
3. Export a .cdb file from the Structural analysis
4. Use CFD-Post to write 3D solid Temperature field onto
the .cdb file (beta feature)
5. Read .cdb file back into the Structural analysis system
and solve for deformations (stresses)
1-way FSI
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Training ManualAlternative Thermal Stress Workflow
• To export the .cdb file from Mechanical:
– Insert a Command Snippet similar to that
shown and Solve
– The example shown refers to a Named
Selection of solid bodies called SolidPart
– This writes the .cdb
file without solving
the model due to the
/EOF command
– Use View > Files
from the Project
page to locate the
.cdb file
/PREP7
CMSEL,S,SolidPart
NSLE,S,1
CDWRITE,DB,Export,cdb
ALLS
/EOF
1-way FSI
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Training ManualAlternative Thermal Stress Workflow
• In CFD-Post:
– First, set Edit > Options as shown
– Restart CFD-Post
Note: The Interpolation Tolerance controls the area or layer outside of the domain that is treated
as part of the domain for the purpose of interpolation. The default is 0.5%. Some nodes from
the .cdb file will lie just outside of the CFX CHT domain, but as long as they lie within this layer
data will be interpolated successfully. Spurious values in the exported load file (e.g. 1e38) may
indicate that the Interpolation Tolerance should be increased.
1-way FSI
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Training ManualAlternative Thermal Stress Workflow
• In CFD-Post:
– Load fluid results
– File > ANSYS Import/Export > Import
ANSYS CDB Surface
– Select file exported from Mechanical
– Don’t specify an associated boundary
– A User Surface object is created, but
will not be visible
– Edit the User Surface and scope the
Domains to only the solid domains
– Ensures that the fluid temperature is not
used when interpolating
– Can visualize the User Surface by
creating a Point Cloud, with Sampling
= Vertex and Reduction Factor = 1
1-way FSI
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Training ManualAlternative Thermal Stress Workflow
• In CFD-Post:
– File > ANSYS Import/Export > Export
ANSYS Load File
– Select the Unit System you are using
in Mechanical
– Export Temperature to a .sfe file and
place in the directory where the .cdb
file came from
– Check the data in this file for expected
units
• To read in the .sfe file in the
Structural system use a Command
Snippet
• See EX1 in the examples directory
/PREP7
/inp,export,sfe
/SOLVE
1-way FSI
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Training ManualTime-averaged Data Transfer
• Time-averaged data is
useful in a number of
cases, e.g.
– Averaged pressure loads
from transient CFD
simulations
• LES, DES, SAS
– Averaged HTC from
transient CFD
• Turbine blade cooling
• No built-in support for
time-averaged data
transfer
1-way FSI
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Training ManualTime-averaged Data Transfer – Thermal
• Variables transferred from CFX to
ANSYS are Wall Heat Transfer
Coefficient or Temperature
• To transfer a time-averaged
quantity:
– Use Transient Statistics in CFX to
generate the time-averaged quantity
– In CFD-Post use the Replace with
Expression feature to overwrite Wall
HTC or Temperature with the time-
averaged value
• Original values are retained in the res
file – recover at any time
– Transfer data as usual to ANSYS
1-way FSI
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Training ManualTime-averaged Data Transfer – Structural
• Transferring structural (pressure) data can use the same
approach, but requires some additional steps
• Boundary Flow data from the .res file will be used if
available, so this must be explicitly turned off
– Enable the toggle Output Boundary Flows on the Results panel of the
Solver Control form, and then setting the boundary flows to "None"
• The CFX variable used is Absolute Pressure, minus the
Reference Pressure
– So Absolute Pressure.Trnavg should be generated
• Overwrite Absolute Pressure with the transient average
• Close the .res file, then re-open in CFD-Post and transfer
data as usual
– You must close and re-open the res file in this case
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Training ManualTransient Data Transfer
• Transient data can be transferred to steady state and
transient Mechanical systems
– Source time can be selected from a drop-down list when Imported
Load is highlighted in Outline panel
– By default, CFD-Post always loads last time step in sequence
– Load step “End Time” must match “Analysis Time” for Imported
Load
Select time from drop-down list
1-way FSI
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Training ManualTransient Data Transfer
• However, not practical for many transient cases
– Need to set up an Imported Load for each timestep
– Load mapping repeated n times – slow
• Use 2-way framework for 1-way transients
– Automated transient load transfer
– Faster and more accurate interpolation
– But must run both codes simultaneously with a common time step
– Steady-state cases can also be done 1-way
• To use the GGI-based interpolation
– See 2-way FSI chapter
1-way FSI
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Training ManualData Transfer from ANSYS to CFX
• No automated transfer in Workbench yet
• In Mechanical APDL use EXPROFILE command to
generate profiles for CFX
– Use Command Snippets or Mechanical APDL
• Read into CFX-Pre like any other Profile Boundary
• Surface quantities
– Displacements (steady-state mesh morphing)
– Temperature or heat flux
• Volume quantities
– Heat generation and potentially B-field for
magnetohydrodynamics (MHD)
• See ANSYS documentation, section 6.1 in the Coupled-
Field Analysis Guide, or search for “EXPROFILE”
1-way FSI
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Training ManualData Transfer from ANSYS to CFX – Example
• Solve modal
analysis in ANSYS
• Export the mode
shape
• CFX: transient
analysis with
prescribed mesh
motion
ANSYS Mode Shape
Apply as Mesh
Deformations in
CFX-Pre
CFD Results
1-way FSI
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Training ManualData Transfer from ANSYS to CFX – Example
• Can use superposition method to combine mode shapes
1st mode 683 Hz 4th mode 3707 Hz2nd mode 1707 Hz 3rd mode 2248 Hz
1-way FSI
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Training ManualData Transfer from ANSYS to CFX – Example
• Can use superposition method to combine mode shapes
iiidisp tAx .sin
Ai : constant amplitude for i
th mode
i : frequency for i th mode
i : i th mode shape
Deformation, scaled by
factor 200
31
32
33
34
35
36
37
0 0.001 0.002 0.003 0.004
Time [s]
F
o
rc
e
[
N
]
Amplitude 1
Amplitude 2
Normal force on blade