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14. 5 Release
Solving FSI Applications Using
ANSYS Mechanical and ANSYS CFX
Lecture 3
Two-way FSI Overview, Geometryand Meshing
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Outline
Coupling OverviewThis lecture starts by describing how the FSI coupling process works.
The main features and capabilities are then discussed along with thecurrent limitations.
Workflow Overview
This section provides a high level view of the FSI workflow for asimple 2-way FSI analysis
Geometry & Meshing
Here well cover what you need to consider when creating the
geometry and mesh for a co-simulation analysis
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Coupling Overview
Simulations involving multiple physics requires solution
of multiple fields
Fields are coupled: solution data from one field is
required by one (or more) other field
Solid Mechanics
Structural
Thermal
Fluid Dynamics
Mass
Momentum
Turbulence
Heat Transfer
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Coupling Overview
CFX solves Mass and Momentum in a single matrix (fully coupled)
Other fields (Turbulence, Heat Transfer, ) are solved in a
segregated manner
Iterations are required to convergence sequentially (segregated)solved fields
Solid Mechanics
Structural
Thermal
Fluid Dynamics
Mass
Momentum
Turbulence
Heat Transfer
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Solid Mechanics
Structural
Thermal
Coupling Overview
MAPDL solver usually solves Structural OR Thermal fieldsMultifield (MFS) solver can couple fields in a segregated
manner in MAPDL
MFS
Fluid Dynamics
Mass
Momentum
Turbulence
Heat Transfer
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Coupling Overview
The MAPDL solver can also couple fields using a fullycoupled approach
Different elements are used to couple different fields
E.g. SOLID 226 can couple thermal and structural fields
Solid Mechanics
Structural
Thermal
Fluid Dynamics
Mass
Momentum
Turbulence
Heat Transfer
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FEA and CFD Element Background
Mechanical/MAPDL use elements that combine mesh andphysics, e.g.
SOLID185 is a structural element
SOLID278 is a thermal element
SOLID226 is a coupled field element
CFX uses mesh elements as a computational stencil
Physics is not associated with elements
Different element types only for different shapes (tet, hex, etc)
Use of multiple elements (for multiphysics) requires: Data transfer between different element types
Sequencing of element solutions
The Multifield solver (MFS) provides an infrastructure for this
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The MFX Solver
The MFXsolver is the external variety of the MFS solver
It couples the MAPDL solver and the CFX solver together
MFX and MFS cannot be combined
Notice the Structural Thermal coupling has been removed below
Solid Mechanics
Structural
Thermal
Fluid Dynamics
Mass
Momentum
Turbulence
Heat Transfer
MFX
MFX
OR
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The MFX Solver
To couple CFX with a Structural Thermal simulation use
Coupled Field Elements MFX can only couple to 1 element type on the MAPDL solver side
Only one SOLVEcommand is allowed in the MAPDL solver
Solid Mechanics
Structural
Thermal
Fluid Dynamics
Mass
Momentum
Turbulence
Heat Transfer
MFX
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Iterative Coupling
Solid Mechanics
Structural
Fluid Dynamics
Mass
Momentum
Turbulence
Heat Transfer
MFX
Iterations are required to converge the quantities
transferred between the MAPDL and CFX solvers
Just like iterations are required to converge segregated fields
within the CFD or FEA solvers
Force/displacement or Temperature/Heat Flow are the
transferred quantities
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Iterative MFX Coupling A transient 2-way FSI simulation has three levels of iterations:
The transient loop each loop/stepmoves forward in time, as in a standard
CFD or FEA transient simulation.
Loads / displacements are
updated between the FEA
and CFD solvers.
The usual inner loop, used to
converge the field(s) within a solver
named Coefficient Loops in CFX and
Equilibrium Iterations in ANSYS.
Time Loop
End Time Loop
End Coupling / Stagger LoopEnd Field Loop
Coupling / Stagger Loop
Field Loop
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Iterative Coupling
Field loop iterations stop when the field
reaches its convergence target (or MaxIterations/Time Step in Fluent)
The CFX field loop does not need to be
converged every Coupling Iteration, only by
the end of the last Coupling Iteration
Coupling loop iterations stop when the forces/ displacements reach their convergence
targets or max number of Coupling Iterations
Ensure the individual field solvers AND the
forces / displacements are converged before
starting the next time step
E.g.: CFX set to 10 iterations/time step,
System Coupling set to 5 Coupling Iterations
and 100 time steps are solved. CFX could
perform a total of 10*5*100 = 5,000
iterations in total if convergence is poor
Time Loop
End Time Loop
End Coupling LoopEnd Field Loop
Coupling Loop
Field Loop
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MFX Key Features
Steady or Transient 1-way and 2-way co-simulationwith surface force/displacement coupling and/or
surface Temperature/Heat Flow coupling betweenCFX and ANSYS Mechanical/MAPDL
Full range of CFX capabilities
FSI interfaces on SOLID, SHELL or SOLSH elements
Restarts supported with CFX and/or MAPDL changes
Integrated post-processing with ANSYS CFD-Post
Parameterization, design exploration and
optimization
Coupling Iterations produce an implicit solution
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MFX Key Features
Load transfer mapping is fully conservative forconserved quantities (Heat Flow, Force)
Both globally and locally at the element level
General Grid Interface (GGI) algorithm for force and profile
preserving algorithm for displacement
Non matching meshes supported
Interface load under relaxation controls
Interface data convergence checking
Supports Large Models CFX can use distributed parallel processing on n machines
MAPDL solver can use local (shared memory) parallel
Remote Solver Manager (RSM) supported
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MFX Key Features
No additional licenses Need any CFX/CFD/CFD-Flo license and an ANSYS Mechanical
or above license
Workbench based setup and execution
Windows 32/64-bit, Linux 64-bit
Command line execution outside of Workbench
Can use separate machines and mix Windows, Linux
Can use MAPDL to create the structural model
Solver data transfer across standard sockets (TCP/IP) Third Party Coupling Scheme Not Required
Efficient, no intermediate files
Takes place in memory (RAM)
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MFX Whats Not Supported
Cannot specify multiple load steps in Mechanical Cannot provide a pre-stressed structural model
Alternative: start with a FSI simulation with some data
transfers suppressed, then restart after unsuppressing data
transfers
Cannot use multi-configurations or remeshing in CFX
Many Mechanical features based on newer contactelements (CONTA17*) are not supported
Work-arounds discussed later
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Comparing MFX/CFX with System Coupling/Fluent
MFX allows 2-way thermal and structural
coupling, including coupled field elementsolutions. This is not available in SC until
version 15.0.
MFX allows forces to be transferred from a
pair of wall boundaries to a set of shell
elements (2-to-1 mapping). SC is limited to
1-to-1 mapping for co-simulation.
Mesh smoothing options with CFX are more
robust and have more control than Fluent.
MFX allows mixed steady/transient couplings
and allows restarts from steady-state to
transient, neither of which are supported
with SC (at 14.5)
Cases requiring transient rotor-stator can be
solved with MFX/CFX but cases requiring FSI
regions in sliding mesh zones cannot be
solved with SC/Fluent
SC/Fluent allows re-meshing whereas MFX is
limited to mesh smoothing.
MFX does not support many features based
on CONTA17* elements. These are fully
supported with SC.
SC support local/shared memory and
distributed parallel for MAPDL, whereas MFXonly support local parallel for MAPDL.
SC has an easier to use WB workflow for
restarts.
Creation of backup points is simpler with SC.
SC requires an "ANSYS Structural" or higherlicense plus any Fluent license. MFX requires
an "ANSYS Mechanical" or higher license plus
any CFX/CFD-Flo license, or a single "ANSYS
Multiphysics" license, or a single "ANSYS
Mechanical CFD-Flo" license.
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Outline
Coupling OverviewThis lecture starts by describing how the FSI coupling process works.
The main features and capabilities are then discussed along with thecurrent limitations.
Workflow Overview
This section provides a high level view of the FSI workflow for asimple 2-way FSI analysis
Geometry & Meshing
Here well cover what you need to consider when creating the
geometry and mesh for a co-simulation analysis
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Workbench Workflow
Standard WB workflow for 2-way FSI Drop a CFXsystem onto the Setup cell of a Transient Structural system
Geometry is shared by default
Fluid and structural meshes are created separately
CFX Solution cell controls the FSI simulation
Structural Solution not used for FSI, but can be used to check the
structural model
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Workflow Overview - Mechanical
Import / create geometry: Extract fluid regions in DM if
necessary
Set up Mechanical model: Very similar to a standard
Mechanical model
Create the FSI interface
region where fluid forces
will be received
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Workflow Overview
Mesh fluid region
Setup CFX model
Define Coupling Timestep controls
Create fluid domain as usual,enabling Mesh Motion
FSI Interface will be a Wall Boundary
where the motion is received from
MAPDL solver
In addition to usual CFX solver
controls, set coupling solver controls
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Workflow Overview
Run in the CFX Solver
Manager Both codes started
automatically
Or can launch one at a time
(different machines, clusters)
Solution output from bothcodes tracked in the CFX Solver
Manager
Both solutions can be post-
processed in CFD-Post
Some limitations forMechanical results in CFD-Post
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Workflow Outside Workbench
Can also run the solutions outside of Workbench
Can be easier when restarts are required
Can use WB for Geometry, Mesh and Mechanical, then export the
CFX mesh/setup and ANSYS Input file and continue outside of WB
Fluid and Sol id
Geometry
Sol id Mesh: supp ressf lu id bodies in here
Struc tural Setup: Write
Input Fi le when com plete
(Tool s > Write Inpu t File)
Export the f lu id m esh, then run CFX
outside of WB, provid ing the f lu id
mesh and the Mechanic al Inpu t File.
Alternat ively, conn ect to a CFX system
then pu l l the .c fx f i le outs ide of WB
Fluid Mesh: supp ress
sol id bodies in here
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Outline
Coupling Overview
This lecture starts by describing how the FSI coupling process works.
The main features and capabilities are then discussed along with thecurrent limitations.
Workflow Overview
This section provides a high level view of the FSI workflow for asimple 2-way FSI analysis
Geometry & Meshing
Here well cover what you need to consider when creating the
geometry and mesh for a co-simulation analysis
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Geometry Considerations
Helpful to use a single CAD file containing both the fluidand solid regions
Ensures that FSI interface region lines up
Fluid geometry/mesh may need to include fillets if present in the
solid
Small mismatch OK ~ half the local element edge length
Can import separate CAD files into DesignModeler and
move / transform as necessary
Consider if the fluid bodies should be split to control meshmotion or if subdomains are needed
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Geometry Considerations
To use shell elements in Mechanical, make a Surfacebody in DM
Given a fluid body, use Create > Thin/Surface with zero thickness
When you select a face, the side with the positive
surface normal is highlighted in green
Force passed to Mechanical is a vector, so the surface normal
direction doesnt matter, but...
Outward pointing normal Inward pointing normal
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Geometry Considerations
Shell element assume the nodes are atthe mid-plane of the true geometry
Forces from a Fluid Solid Interface will be
applied at the mid-plane by default
Negligible error for thin geometries
Correct interpretation of the physics is
given by using Offset Type
Offsets the shell element nodes to the topor bottom of the thickness
In the pipe example shown, assuminginternal flow with outward pointing
normals, use Offset Type = Bottom
Locates the nodes at the true Fluid Solid
Interface
Outward pointing normal
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Meshing
When meshing the fluid, suppress the
solid region in Meshing
And visa-versa
Interface mesh does not need to match
Force still locally and globally conservative
Similar mesh length scales will maintain the
load transfer resolution
Difference in element normal
directions across interface can producetwisting on coarse meshes with surface
curvature
Create a matching mesh if necessary...
FluidSolid
Fluid exerts pressure normal
to fluid boundary elements
Force vector is transferred to the solid,
but does not act normal to the solid
elements results in a twisting force
Solid
Fluid
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Meshing
The structural Model cell contains the fluid & structural bodies in one Part
Mesh both fluid and solid regions in the Mechanical Model, then use File
> Exportto write out a Fluent mesh (.msh)
Suppress the fluid region, continue with the Mechanical Model definition
Import the Fluent mesh into CFX-Pre in a CFX Component System
No automatic mesh update in CFX-Pre
Matching interface mesh mesh the fluid and solid parts
together, using a single Part
Fluid and Sol id
Geometry in a
single Part
Mesh both f lu id and
sol id, export m esh,
then suppress f lu id
Import f lu id mesh
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Meshing
It is also possible to create fluid and solid meshes
outside Workbench (e.g. ICEM)
Import the fluid mesh into CFX-Pre manually
Import the solid mesh into Finite Element Modeler, then connect
to a Mechanical system:
Fluid and solid meshes can be created together ifnecessary so that nodes match at the interface
Meshing the fluid volume then extracting the surface mesh to
use as a Mechanical shell mesh is also possible
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Summary
The MFX solver allows fields solved in different solvers to be
coupled together
For 2-way FSI analyses iterations are typically used between
the solvers within each time step so that the
forces/displacements can converge at that time step
The workflow involves identifying boundary regions inMechanical and CFX that will send/receive data. All coupling
settings are defined in CFX-Pre in Workbench.
The fluid and structural geometries should physically match at
the FSI interface
An offset may be required when using shell elements
Poor mesh resolution on curved surfaces can lead to errors in
the force transfer. Use of a matching mesh avoids this.