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Initializing transient analysis using static analysis in twoway FSI simulation 

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July 9, 2014, 10:52 

#41 
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I'm not sure why the continuity residuals would be high. If other residuals are low then it could just be the way the residuals are normalized. The default residual normalization in Fluent is not the best. You can change to local scaling to get a better residual normalization. Under Monitors > Residuals, pick "compute local scale" then set the reporting option to "local scaling".
For smoothing the Diffusion smoothing is the most robust, but Spring should work OK too for this case. Setting the reference pressure to the min transient pressure sounds good. In the data transfers the actual force values are passed to the structure. 

July 9, 2014, 19:27 

#42 
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Daniel
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"You can change to local scaling to get a better residual normalization"
Velocity residuals are normal, it is only the continuity. Nevertheless, residual reporting is now OK with your suggestion about local scaling. "Setting the reference pressure to the min transient pressure sounds good. In the data transfers the actual force values are passed to the structure." Changing reference pressure alters the force sending to structure part. The mean pressure in my case is 13784.46 Pa, however minimum pressure is 10964.52 Pa. These different values will change the final result, as they change force magnitude on the structure.  how can I set monitor points for force, displacement on FSI interface in fluent? Thanks in advance Last edited by Daniel_Khazaei; July 10, 2014 at 07:13. 

July 10, 2014, 08:53 

#43 
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Can you assume the min pressure of 10964.52 Pa is consistent with the MRI scan geometry? Then you could adjust your pressure levels so that you always pass a positive pressure.
For force I find it easiest to monitor the Data Transfer Sum in System Coupling. Alternatively you can monitor the integral of Static Pressure in Fluent, which will be close to the force. For displacement you'll have to create a Results Tracker in Mechanical. If you use the Show Mesh option in Mechanical then you can pick a mesh node, otherwise you have to pick a geometry vertex. 

July 10, 2014, 16:40 

#44 
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Daniel
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"Can you assume the min pressure of 10964.52 Pa is consistent with the MRI scan geometry?"
You mean that the MRI image based geometry has its current shape (initial) due to the min pressure of 10964.52 Pa? They have only provided the mass flow waveform at the inlet of CCA. I have no pressure related information on my geometry. I am using pressure waveform presented in the publications with the same geometry. "Then you could adjust your pressure levels so that you always pass a positive pressure." How can I make this adjustment? When I set the reference pressure to mean blood pressure, I get pressure in range of: (almost) [3000 Pa, 3000 Pa] However, setting reference pressure to min blood pressure gives pressure in range of: (almost) [0 Pa, 6000 Pa] Last edited by Daniel_Khazaei; July 11, 2014 at 20:02. 

July 11, 2014, 20:18 

#45 
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Daniel
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Would you please clarify a few things for me:
1) How many coupling iteration is normal in tightly coupled simulation? I have read that I should keep number of coupling iteration below 10! Ansys recommendation for transient: min:1 and max:5 2) I have set the RSM convergence target in system coupling to 1E04, should I still expect the simulation to converge within 10 coupling iteration? 3) The fluent part hardly converges below 1E4, should I expect that? Fluid only transient simulation converges below 1E5 in 6 iteration over each time step. Last edited by Daniel_Khazaei; July 12, 2014 at 08:05. 

July 14, 2014, 09:57 

#46 
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1) The number of Coupling Iterations can vary from case to case, but it should be possible to converge just about any case in 10 or less Coupling Iteration. The key point is not to use too much underrelaxation, since this will slow convergence too much. If a case is unstable from a coupling perspective (i.e. force/displacement oscillate and diverge within a coupling step) then don't reduce the underrelaxation. You should use Solution Stabilization in Fluent instead.
2) If this is the Data Transfer convergence target then 1e4 is a very tight tolerance. I would expect it to take more coupling iterations to reach this target. So yes, more than 10 coupling iterations may be needed. I doubt you need that tight of a tolerance though. Monitor your quantities of interest and see when they are converged. 3) I assume these values are with the local scaling option. I'm not sure why Fluent wouldn't converge as well with FSI when compared to fluidonly. 1e4 isn't too bad. As long as your quantities of interest are converged then this convergence level sounds OK. 

July 16, 2014, 07:06 

#47 
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As you suggested, very low under relaxation was needed to get convergence. Hence, I have decided to use stabilization in dynamic mesh setup on system coupling zone.
With the following setting, I was able to get convergence within 5 coupling iteration in the first time step:  Fluent convergence target based on local scaling: 5E05  Force convergence target in system coupling: 1E03  Displacement convergence target in system coupling: 1E03 Also I am just testing the first step to determine the optimal coefficientbased scaling factor: Scale factors higher than 7 prevent crashing and huge oscillations, however the solution does not converge within 10 coupling iteration until I set a very large value of 450. Solver output for scale factor = 450: Code:
====================================================================== +====================================================================+    Solution    +====================================================================+ ====================================================================== ++  MAPPING SUMMARY  ++  Data Transfer    Diagnostic  Source Side Target Side  +++  Data Transfer 2    Percent Nodes Mapped  N/A 100   Data Transfer    Percent Nodes Mapped  100 100   Percent Area Mapped  100 100  ++ +====================================================================+  COUPLING STEP = 1 SIMULATION TIME = 2.00000e003    Solver  Solution Status   Data Transfer    Diagnostics  Source Side Target Side  +====================================================================+  COUPLING ITERATION = 1  ++  Transient Structural  Converged                   +                   Data Transfer  Not yet converged...   Change:RMS  1.00000e+000 1.00000e+000    transient FSI Fluent  Not yet converged...                   +                   Data Transfer 2  Not yet converged...   Change:RMS  1.00000e+000 1.00000e+000  ++  COUPLING ITERATION = 2  ++  Transient Structural  Converged                   +                   Data Transfer  Not yet converged...   Change:RMS  1.22517e002 1.22517e002    transient FSI Fluent  Converged                   +                   Data Transfer 2  Not yet converged...   Change:RMS  4.32637e002 4.32637e002  ++  COUPLING ITERATION = 3  ++  Transient Structural  Converged                   +                   Data Transfer  Not yet converged...   Change:RMS  1.41697e003 1.41697e003    transient FSI Fluent  Converged                   +                   Data Transfer 2  Not yet converged...   Change:RMS  6.29899e003 6.29899e003  ++  COUPLING ITERATION = 4  ++  Transient Structural  Converged                   +                   Data Transfer  Converged   Change:RMS  2.48035e004 2.48035e004    transient FSI Fluent  Converged                   +                   Data Transfer 2  Not yet converged...   Change:RMS  2.17488e003 2.17488e003  ++  COUPLING ITERATION = 5  ++  Transient Structural  Converged                   +                   Data Transfer  Converged   Change:RMS  5.81177e005 5.81177e005    transient FSI Fluent  Converged                   +                   Data Transfer 2  Converged   Change:RMS  2.10873e004 2.10873e004  +====================================================================+ ====================================================================== +====================================================================+    Shut Down    +====================================================================+ ====================================================================== System Coupling Service shut down... System coupling run completed successfully. Now I have question about overdamping, how can I check that I am not overdamping the solution? here is the force summery: Code:
*** FORCE SUM ACROSS TARGET INTERFACE . . . . .Fluid Solid Interface (FSIN_1) RECEIVING FORCE FX SUM = 0.69983E02 RECEIVING FORCE FY SUM = 0.13517E03 RECEIVING FORCE FZ SUM = 0.87715E02 COUPLING ITERATION. . . . . . . . . . . . . . . 2 *** FORCE SUM ACROSS TARGET INTERFACE . . . . .Fluid Solid Interface (FSIN_1) RECEIVING FORCE FX SUM = 0.62144E02 RECEIVING FORCE FY SUM = 0.29081E03 RECEIVING FORCE FZ SUM = 0.88670E02 *** FORCE SUM ACROSS TARGET INTERFACE . . . . .Fluid Solid Interface (FSIN_1) RECEIVING FORCE FX SUM = 0.63194E02 RECEIVING FORCE FY SUM = 0.18824E03 RECEIVING FORCE FZ SUM = 0.88334E02 COUPLING ITERATION. . . . . . . . . . . . . . . 4 *** FORCE SUM ACROSS TARGET INTERFACE . . . . .Fluid Solid Interface (FSIN_1) RECEIVING FORCE FX SUM = 0.62649E02 RECEIVING FORCE FY SUM = 0.23105E03 RECEIVING FORCE FZ SUM = 0.88437E02 COUPLING ITERATION. . . . . . . . . . . . . . . 5 *** FORCE SUM ACROSS TARGET INTERFACE . . . . .Fluid Solid Interface (FSIN_1) RECEIVING FORCE FX SUM = 0.62679E02 RECEIVING FORCE FY SUM = 0.22082E03 RECEIVING FORCE FZ SUM = 0.88426E02 

July 16, 2014, 10:43 

#48 
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To avoid overdamping it is best to start with the unstable solution and increase the coefficient until you approach the converged solution. The unstable solution will have oscillations that grow in magnitude. As you increase the coefficient you'll move to oscillations that decrease in magnitude, then to a critically damped response (i.e. a slight overshoot then convergence), then to overdamped (no overshoot, steadily approaching the converged solution).
A case that has a slight overshoot in the force/displacement monitor values, then approaches convergence, is the ideal response. Look at the force/displacement values obtained with this case. If you use a very large coefficient of 450 you may be so overdamped that the solution is almost stationary, which may give "converged" residual values, but the wrong answer. If you haven't reached the same force/displacement value as the case that has a slight overshoot, then you have too much damping. I would focus more on the monitor point convergence than on reaching specific residual targets. Residuals can be misleading. 

July 20, 2014, 19:52 

#49 
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Daniel
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Hello again
I have tried to find the optimal scale factor for my case, starting with unstable solution while slowly increasing the scale factor. I have set 2 different result trackers in Structural to monitor displacement. After reaching to scale factor equal to 1.5, one of these monitor points shows the behavior you have described, however the other one looks unstable. Also I am monitoring integral of static pressure on the arterial wall, however it's stable no matter what. When I use small scale factors, continuity equation never reaches the residual criteria with very slow convergence.  I am also working on a much simpler 2D case involving an elastic flap in a channel. The exact same problem with continuity equation is also there. (Without local scaling continuity residual goes beyond 1e+1) However, velocity components converge smoothly under 1E5. I have noticed that the continuity converges well when I disable the solution stabilization in dynamic mesh, but structural crashes at the second coupling iteration. Best wishes Last edited by Daniel_Khazaei; July 21, 2014 at 00:28. 

January 17, 2017, 05:03 

#50 
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Johannes Hall
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Sorry for opening this thread again but I noticed that you have discussed the initialization of "steady" FSI results with system coupling, Fluent and Mechanical. I was wondering if it's possible doing this and then changing to the pressurebased solver when the initialization is done with the densitybased solver?
Regards! Edit: I see, pressuredensity coupling, not density based solver. My bad. Another question though, when trying to do this initialise I have to run quite many time steps in order to get a good convergence in fluent. I tried to use a large time step but when doing that the RMS in system coupling didn't converge well. I actually have a hard time getting the RMS down for smaller time steps as well. Any ideas on how to deal with this? Last edited by yonpanman; January 17, 2017 at 09:00. 

September 12, 2017, 10:56 

#51  
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YK
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Quote:
Did you manage to get this work? Please let me know. Thanks. 

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