Fluid Solid Interaction----A Publication for ANSYS Users

christy

Modeling Spilt Milk
The Multiphysics capability of ANSYS is crucial to industries where more than one “physics type” is involved. This occurs in many combinations, but we will look at the specific case of Fluid / Structure Interaction.
Sometimes you can not answer your design questions with single-physics modeling.
When interaction between physics is strong, ANSYS Multiphysics capability is of great value.
Fluid flow deflects structure
Structure deflection affects fluid flow
Fluid Solid Interaction in ANSYS
Called “Load Vector-Coupling” in ANSYS
Involves use of FLOTRAN to solve fluid portion
Now a time dependent analysis
Iterate on each time step before advancing
Big Change in 6.1: fluid and solid meshes need not match!

Things to Think About First
Disk Space
You need to store a lot more data
More DOF’s
Larger Model
Model Size & Run Time
Sequential or iterative, run time can kill you
Convergence
Difficulty in converging within a physics will cause problems converging between physics
UNITS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Most common mistake is mismatch of units
The Fluid/Solid Mesh Interface
Meshes usually don’t match up
Fluid and solid mesh requirements are very different
This forces interpolation across boundary
Two interpolation methods:
Interpolation MethodFluid to Solid Load TransferSolid to Fluid Transfer
ConservativeForce in the 3 coordinate directions
Heat rateDisplacements
Mesh velocities
Temperatures
Non-conservativeForce density in the 3 coordinate directions
Heat fluxDisplacements
Mesh velocities
Temperatures

You need two sets of geometric entities and two sets of nodes at boundaries
Do not merge geometry or nodes at interface!
Process for FSI Analysis
Setup the fluid and solid analyses
Do this as if they were separate solutions minus loads at interface
Flag fluid-solid interfaces
Assign FS numbers using SFx family of commands in each model
Specify the fluid-solid interaction solution options
Obtain the solution
Simply use solve, ANSYS handles the rest
Setting Up the FSI Solution
Specify basic information
FSAN to turn on FSI
Set solution order with FSOR
FLUID,SOLID or SOLID,FLUID
Order should be determined by driving physics
Establish static or transient with FSTR
Use FSIN to establish interpolation methods
Specify time controls
FSTI specifies end time
Should be multiple of time steps
FSDT specifies FSI time steps
Must also specify FLUID and SOLID times with DELTIM and FLDATA4,TIME,STEP
These steps must be smaller than the FSI step
Specify iterations
Use FSIT to specify maximum number of stager iterations
Will iterate on FSI solution until number of iterations is met or load values converge
Specify convergence
Use FSCO to specify convergence norm for quantities transferred across fluid-solid interface
Convergence is normalized:
Conv = log (εx/εmin)/(log εmax/εmin)
You specify εmax
Set output frequency
FSOU specifies every step or every Nth step
Specify relaxation
FSRE of 1 does not allow any relaxation
Need to play with this for your problem to get a feel for it
Notes on FSI
Are you sure you need FSI?
Is coupling one-way or not important?
Oftentimes you can run a CFD analysis first, then put the loads on your static structural
Crawl...walk...run
Analyses can get very long, so try to set up simple test cases to get the process down
Get static and fluid parts running by themselves first
Convergence of static and fluid solutions are critical
FSI part won’t solve if static or fluid are unstable
Try to avoid remeshing of fluid area
Can slow down things a lot because new mesh needs to converge
Try to simplify in 2D
Learn more about Multi-Physics analysis!
PADT training classes make things easier
Flotran I (including basic FSI): April 14-16
Flotran II (more FSI): May 19-20

[此贴子已经被作者于2005-7-16 19:29:42编辑过]

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