Fluent fsi 14.5-lect-03_co_simulation_setup (1)

© 2011 ANSYS, Inc. January 4, 20131 Release 14.5
14. 5 Release
Solving FSI Applications Using
ANSYS Mechanical and ANSYS Fluent
Lecture 3
Co-simulation Setup

Outline
Mechanical Setup
This section will cover the Mechanical setup in the context of a System
Coupling co-simulation. We’ll discuss Analysis Settings, creating Fluid
Solid Interfaces and other topics relevant to System Coupling
simulations.
Fluent Setup
The Fluent setup for System Coupling simulations will be covered next.
In addition to describing how to label Fluent boundaries to
send/receive data from MAPDL, we’ll discuss creating useful monitor
data, solution output, solver controls and other general considerations.
System Coupling Setup
Here we’ll cover the settings in the System Coupling GUI, including
Coupling Steps & Iterations, creating Data Transfers between
participant solvers and solution output controls.

Mechanical Analysis Settings
Set Number of Steps to 1 and Step End Time
to at least the run duration required
• Only 1 load step allowed
– Use restarts to make changes
– Use a table to define time varying loads
• Time duration controlled in System Coupling, but
cannot be larger than the Step End Time set here
Set Auto Time Stepping = Off, Defined By =
Substeps and Number of Substeps = 1
• Gives 1 substep per transient time step (Coupling
Time Step)
• Substepping not supported (but will run)
Don’t use Define By = Time
• It leads to substepping if not set consistently with
the Coupling Time Step in System Coupling

Time Integration
• Set to On for transients
– Accounts for transient effects such as
structural inertia
• Can set to Off to produce a static solution
within the Transient Structural system
– Useful for initialization – see later
– Fluent still needs to be transient
Large Deflection
• Should usually be set to On
• When set to Off, the underlying structural
mesh will not change, so forces from the
deformed Fluent mesh will be applied to the
undeformed structural mesh

Restart Controls
• Generate Restart Points = Program Controlled
– Frequency is set in System Coupling
• Retain Files After Full Solve = Yes
– Must keep this set to allow restarts
Damping Controls
• The fluid often provides much of the damping
• Here we set controls to model the energy loss
in the structure

Output Controls
• The rst file can become LARGE in transient
runs, even for small cases
• Can set Stress and Strain to No to reduce
output if these quantities are not of interest
• Use Calculate Results At to reduce frequency
– All Time Points, Last Time Point
– Equally Spaced Points
• Set the total number of results sets you want
– Specified Recurrence Rate
• i.e. time step frequency
• These Calculate Results At options are
interpreted over the entire run duration
– In a normal Mechanical analysis they are
interpreted per Load Step

Mechanical Fluid Solid Interface
Define non-FSI supports and
loads as usual
Insert a Fluid Solid Interface
for regions that will receive
data from System Coupling
• Can define multiple interfaces

Mechanical Fluid Solid Interface
Split interfaces when faces meet at a small angle
• Avoids mapping problems
A surface can only be included in one Fluid Solid
Interface
• No overlapping interfaces
1
2

Contact Offset
Contact detection can be used to
model contact between a Fluid Solid
Interface and another face
• But this may cause an invalid topology in
Fluent if the mesh is pinched off
Can use a Contact Offset to model
contact without reducing the gap to
zero
• Avoids an invalid mesh topology in Fluent
• But there’s no automated way to block
the flow in the small gap in Fluent

Solution Trackers
Useful to track structural
displacements
Highlight Solution Information, select
Results Tracker > Deformation from
the toolbar
• Select a vertex from the geometry
• Must be a single vertex (node) per tracker
Chart is created after the solution is
complete
History is also stored in the .nlh file
in the Mechanical solution directory

Close Mechanical or Write an Input File
If solving in WB, close Mechanical
• When the Setup cell is updated an input
file is written in the project directory
named ds.dat which will be parsed by
System Coupling
If solving from the command line
outside WB, in Mechanical select
the analysis from the Outline tree
then Tools > Write Input File
• Writes an input file that you will
reference from the command line

Outline
Mechanical Setup
Coupling simulation. We’ll discuss Analysis Settings, creating Fluid
simulations.
Fluent Setup

Setup zones and boundary
conditions as usual
Enable Dynamic Mesh
Set the required Dynamic
Mesh Zones to use the System
Coupling option
• Identifies zones that may receive
displacements from System
Coupling
• Defaults to Stationary when not
connected to System Coupling
• Other zones can use Rigid Body,
Deforming, etc
FSI Interface

FSI Interface – Solution Stabilization
System Coupling Dynamic
Mesh Zones have a Solution
Stabilization option on the
Solver Options tab
• Used to stabilize tightly
coupled FSI cases
• Replaces previous rpvar
commands
• Discussed in detail in the
convergence chapter

Fluent Setup Notes
Make sure mesh motion is consistent at adjacent
boundaries
• E.g. if the edges of an FSI interface are not fixed in Mechanical,
then adjacent boundaries in Fluent should not use Stationary
The forces passed to System Coupling are based on the
relative pressure, not the absolute pressure
• Use an Operating Pressure of 0 Pa if necessary and apply
atmospheric (or other) pressure to non-FSI boundaries in
Mechanical as needed
• Or if an Operating Pressure of 0 Pa is not suitable set an
appropriate Reference Pressure…

Forces sent to System Coupling include
pressure and viscous components
See Fluent doc for formulation
• // Theory Guide :: 0 // 22. Reporting
Alphanumeric Data // 22.2. Forces on
Boundaries // 22.2.1. Computing Forces,
Moments, and the Center of Pressure
The pressure component is based on
(p - pref), where p is the gauge (solved)
pressure and pref is set under Reference
Values
To base the forces on absolute pressure
pref should be the negative of the
Operating Pressure
Forces on FSI Interfaces

Use constant density (incompressible) fluids with care
• Implies an infinite wave speed
• Cannot resolve acoustics / pressure waves (e.g. water hammer)
Using Ideal Gas instead of constant density gases gives a
more stable solution
• For a given interface displacement, an incompressible fluid
responds with a higher pressure change than a compressible one
• Even if the constant density assumption is valid for the converged
solution, it can lead to divergence while iterating
Constant density fluids + transient + closed volume + FSI
does not work!
• Any change in displacement produces an infinite change in
pressure
Fluid Compressibility

When resolving pressure waves in liquids use the
compressible liquid option in Fluent
• May also need to solve the Energy equation for consistency, even
if heat transfer is not of interest
Fluid Compressibility

When using 2.5D remeshing, note that edge nodes on
the ‘source’ face cannot be smoothed or remeshed
MDM with System Coupling
This case works OK with 2.5D remeshing.
The non-FSI edge nodes are fixed but
this does not cause any problems
This case will not work with 2.5D remeshing.
The nodes along the top edge of the fluid zone
cannot be smoothed / remeshed, so as the
structure moves negative fluid volumes will
occur
SOLID
FLUID FLUID
Moving tip of flap is
immersed in the fluid
remeshing zone and does
not approach other
boundaries; works OK
Moving tip of flap
slides along top
boundary, will fail

Fluent uses a default under relaxation factor of 0.1 for
displacements from rigid body motion
• Problematic since you would need many Coupling Iterations to
converge the rigid body motion
• Therefore the rigid body displacement URF is set to 1 when the
Minimum Coupling Iterations is reached
– Likely to get unstable rigid body motion if Minimum Coupling
Iterations is the default of 1
• Will generally need to set Minimum Coupling Iterations > 1 and
also increase the rigid body URF in Fluent.
– Note that Solution Stabilization (discussed later) can be used with
rigid body displacements in Fluent by setting:
(rpsetvar 'dynamesh/sdof-solver-options? #t)

Fluent cannot superimpose mesh motion due to a sliding
mesh or rigid body motion and any other mesh motion (e.g.
from System Coupling) on the same zone
• So you cannot have an FSI interface in a sliding mesh zone
A rotating frame in Fluent should generally be paired
with a Rotational Velocity in Mechanical, neither of
which actually rotate the mesh
• Smoothing or remeshing can be used in Fluent and the FSI
interface behaves just like a stationary frame case
• Rotating the structural model using a Joint or a Remote
Displacement results in Fluent receiving the bulk rotational
motion – generally not a good idea

Always monitor force data on
the System Coupling interfaces
• Integral of Static Pressure may be
suitable
• Directional forces can be obtained
via Custom Field Functions, e.g.:
– Static Pressure * X Face Area
Get Data Every Iteration
• Important to check force is
converging within each time step
• Need iteration data to check this
• Also useful for debugging failed
runs
Monitor Data

Monitor data should
show convergence
within a time step, as
shown.
Further discussion in
the Convergence
chapter.
Monitor Data
1 Time Step

Keep the Fluent UI open (interactive
run) to view monitor plots in Fluent
• If you selected the Plot option
• Future versions will allow plotting in
System Coupling
Track a monitor text file in System
Coupling (beta)
• In Fluent write the monitor data to a file
• See System Coupling section for details on
tracking the file
Monitor Data

Cannot directly monitor
displacements in Fluent
Can monitor mesh
coordinates via a Custom
Field Function
• Define > Custom Field Functions
– Pick the Mesh Coordinate
• Create a monitor for the Custom
Field Function on the deforming
surface
– Point locations do not move with
the mesh
• Could subtract the initial mesh
coordinate to get a displacement
Monitor Data

Output results data from Fluent as
usual
• Can use Autosave and perform post-
processing in Fluent
• Use Automatic Export with the CFD-
Post Compatible format to perform
post-processing in CFD-Post
– Case file should also be written
Restart data is controlled from
System Coupling
Results Data

Transient Formulation
• 1st Order Implicit is the default
• 2nd Order Implicit method (not
bounded) is available when using
Smoothing
• If Remeshing is enabled then only
the 1st Order Implicit method is
available
• The 1st Order method may require a
very small time step for accurate
solutions
Non-Iterative Time
Advancement (NITA) not
supported with System Coupling
Solution Methods
Note the green line, which uses the 1st order transient
formulation with 80 time steps per cycle. Refining to
400 time steps per cycle gives significantly different
results. The 2nd order scheme gives good results at 80
time steps per cycle and further refinement does not
change the results.

Time Step Size & Number of Time Steps
• Not used – controlled by System Coupling
• But Number of Time Steps must be > 0
Max Iterations/Time Step
• Means Fluent iterations per Coupling Iteration
• Use fewer iterations than a normal transient
case
• No point converging each Coupling Iteration
too tightly since the transferred quantities may
change in the next Coupling Iteration
• Use monitor points as a guide; do enough
iterations to get reasonable values for interface
forces, but don’t fully converge the first
Coupling Iteration
Time Step and Iterations

Outline
Mechanical Setup
Coupling simulation. We’ll discuss Analysis Settings, creating Fluid
simulations.
Fluent Setup

After completing the Structural and Fluent setup, the
state of the System Coupling Setup cell will be
• Upstream data is now available for SC Setup
• Refresh the cell and double-click/edit to open the SC GUI

System Coupling GUI
Solution Information
Text Monitors
Chart
Monitors
Outline
Details

Duration Controls
• SC controls the time duration for both
participant solvers
Step Controls
• Set the Time Step Size (if transient) for
both participant solvers
• Set the Min. and Max. # of Coupling
Iterations
– The number of times data is exchanged
between the solvers per (time) step
– …
System Coupling Analysis Settings
Transient
Steady State

• As discussed System Coupling uses iteratively implicit coupling
• The minimum and maximum number of iterations per time
step control the number of iterations between the CFD and the
structural solutions during each time step
• During each time step there are multiple iterations, which
include multiple mesh updates and data transfers between the
solvers
• Convergence for the time step is achieved when the rate of
change in data transfer quantities becomes acceptably small
• Using one iteration per time step is referred to as explicit
coupling, and explicit coupling is generally not accurate for
anything other than one-way or "weakly" coupled cases
System Coupling Analysis Settings

Comparing Explicit and Implicit Coupling
• AGARD 445.6 case
• Standard wing flutter test
case
• Zero angle of attack
• Transonic to low supersonic
speeds
• Traditional linear flutter analysis
tools are not accurate due to
non-linear effects
Bending mode
Torsional mode

Comparing Explicit and Implicit Coupling
• Comparing the predicted flutter
frequency versus time step size
for different Coupling (Stagger)
Iterations
• Explicit coupling (1 Stagger)
requires a smaller time step for
accurate results
• The physics determines the time
step size with implicit coupling
• Large time step with 5 coupling
iterations and small time step
with 1 coupling iteration both
give good results
• Large time step case runs 4 times
faster – implicit coupling reduces
CPU time
• Explicit coupling is unstable at
larger time steps
12
13
14
15
16
1.E-04 1.E-03 1.E-02
Time stepsize [s]
Flutterfrequency[Hz]
5 Stagger
3 Stagger
1 Stagger
-6.E-03
-4.E-03
-2.E-03
0.E+00
2.E-03
4.E-03
6.E-03
0 0.05 0.1 0.15 0.2 0.25
Time [s]
Amplitude[]
dt=0.00025 [s], 1 Stagger

For steady/static runs you can:
• Use 1 step, with many iterations per step
• Use many steps, with 1 iteration per step
Convergence is the same
• Many step approach allows you to
interrupt the run
Intermediate restart and results data is different
• Restart data can be created every n steps
– 1 step --> no restart data possible
• Doesn’t make sense to use Autosave in Fluent
– Number of Fluent iterations completed will vary per step
• In Mechanical results are output at the end of each step by
default
Min. and Max. Coupling Iterations

For transient runs:
• Must make sure convergence is obtained
by the Maximum Iteration number
– Check output files, charts, monitor
points
– If not, adjust settings (see Convergence
section later)
• Setting Min. and Max. Iterations to 1
produces explicit coupling
– Also use this for 1-way transient cases
Min. and Max. Coupling Iterations

Under Participants the systems
connect to SC are shown
For Fluent all Wall regions are
shown
• Walls tagged as System Coupling in
Fluent can be used for 2-way data
transfer
• Other walls can be used for 1-way
data transfer to Mechanical
For Mechanical all Fluid Solid
Interfaces are shown
System Coupling Participants

Use Ctrl key to select a Fluent
and Mechanical region pair,
then right-click and select
Create Data Transfer
• Data transfers can be one-way (i.e.
only transfer force or only transfer
displacement) or two-way
– Can Suppress or Delete one of
the data transfers for one-way
• Can only pick one region from each
participant at once
• Can only use each region once
Creating Data Transfers

Participant, Region & Variable fields
are complete when using the Ctrl key
method to create Data Transfers
Under Relaxation Factor
• Defaults to 1 (no under relaxation)
• For transient cases, relaxation is applied
from the 2nd Coupling Iteration onwards for
each step
– i.e. in the 1st Coupling Iteration of each
step the full loads are always applied
• For steady state cases the URF is always
applied against the previous load
– Could be from the previous Step or the
previous Coupling Iteration
Data Transfer Details

… Under Relaxation Factor
• Reducing the Under Relaxation Factor can
help convergence/stability
• Use of a small Under Relaxation Factor in a
transient case is a red flag
– See the Convergence section
Convergence Target
• Default is usually fine
• See documentation for details on how this
is evaluated
• Monitor data should be used to confirm
convergence

Ramping
• None
– Full data transfer values (but still
under relaxed if applicable) are
passed to the Target from the first
Coupling Iteration
• Linear to Minimum Iteration
– Data transfer values are linear ramped
against the final value from the
previous Coupling Step
– The full load is applied once the
Minimum Iteration (under Analysis
Settings) is reached
– The default Minimum Iteration is 1, so
ramping has no effect unless you
change this to >1

Co-Simulation Sequence
• Who solves first – default is Mechanical
• In most cases it shouldn’t make much
difference
Debug Output
• Discussed later
Intermediate Restart Data Output
• Use this for backup points, not post-
processing data
Execution Control
• Creates Fluent cas & dat files
– In addition to any results requested in Fluent
• Creates restart points in Mechanical
– r00x and rdb files, which are only used for restarts
– no rst data is produced, which is only used for post-processing

Summary
• Mechanical Analysis Settings are generally the same for all FSI
cases since some parameters are not used and a single substep
should be used
• Setting Time Integration to Off is equivalent to a static
analysis. This provides an easy way to initialize a transient.
• If you plan to restart, you must ask to Retain Files After Full
Solve
• Mechanical results files can become very large. Output only
the data you need at a reasonable frequency.
• Create Fluid Solid Interfaces in Mechanical for regions that will
receive forces from System Coupling

Summary
• In Fluent turn on Dynamic Mesh and use the System Coupling
option to identify regions that will receive displacements
• Always create monitors in Fluent to track the forces on the
System Coupling interfaces, with a frequency of Every Iteration
• Also track displacements if possible, or use Solution Trackers
in Mechanical
• Output results files from Fluent at an appropriate frequency
containing only the data needed for post-processing
• In System Coupling create Data Transfers by multi-selecting a
region from each participant solver
• Select a backup frequency in System Coupling, if needed
• Convergence should be monitored and changes made if needed
• This is discussed in detail later

Fluent fsi 14.5-lect-03_co_simulation_setup (1)

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