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Blood-in-Vessel convergence problem

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Old   December 3, 2016, 09:32
Default Blood-in-Vessel convergence problem
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Lucas Ribeiro
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Hello,

I am trying to simulate a standard FSI case, wave propagation in an elastic tube using ANSYS. The simulation serves to predict blood flow in large arteries [1]. The domain consists of a straight flexible tube with radius r = 0.005 m, length L = 0.05 m and thickness t = 0.001 m. The tube’s wall is a Saint Venant-Kirchhoff material with density ρs = 1200 kg/m3, Young’s modulus E = 3×10^5 N/m2 and Poisson’s ratio ν = 0.3. The tube is clamped in all directions at the inlet and outlet. The fluid is incompressible with a density of ρf = 1000 kg/m3 and a dynamic viscosity of νf = 0.003 Pa s. Both the fluid and the structure are initially at rest. During the first 0.003 s, a uniform overpressure of 1333.2 N/m2 is applied at the inlet and time step size ∆t = 5 × e-5 s. Basically I trying to simulate the same simulation done by Dr. Tukovic, et al, (https://github.com/wyldckat/FluidStr...fsiFoam/3dTube ). They have implemented the solver for OpenFOAM and used it to solve this case, but I am trying to reproduce the same results using ANSYS 16.2.

The problem is that I can't get it to converge. I have uploaded the whole project and I am sharing the link below along with the logs. If anyone could give a clue so solve it, I'd be very happy.

[1] - J. Degroote, K.-J. Bathe, J. Vierendeels, Performance of a new partitioned procedure versus a monolithic procedure in fluid-structure interaction, Computers and structures 87 (2009) 793–801.

Link for whole project: https://drive.google.com/open?id=0B1...HhWak5DbVNUc0U

Thanks
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Old   January 25, 2017, 09:52
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Lucas Ribeiro
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Solved!!

I had to use the Fluent's stabilisation process, since this typical case is VERY unstable.
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Old   December 7, 2017, 09:39
Default Problem with the FSI modeling
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Dear Lucas,

I want to model blood circulation in a VAD device that deals with high deformation of the structure. The problem is the system coupling delivers "highly distorted elements error" every time I run the problem. When I run the transient structutal alone, everything is OK. But, when I run system coupling it return highly distorted elements error for the first iteration, even for very small time steps (1e-6 sec). The loading is a distributed pressure with a magnitude of 4000 Pa. But, even with magnitude of 1 Pa, I still get the same error. Do you know how can I solve this problem?
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Old   December 7, 2017, 09:54
Default Under-relaxation is the key
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Lucas Ribeiro
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Dear Malekan,

The key is to use under-relaxation because it is a very non-linear problem. To stabilize these types of cases Solution Stabilization is available in Fluent in the Dynamic Mesh Zones settings on the Solver Options panel. There, choose the zone that is deforming because it has a "system coupling" type. Finally, change the Scale factor. I remember that I had to use a small under-relaxation parameter value. I think it was 0.001, but this number is problem-dependent.

Tell me if it works, please.

Regards
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Old   December 7, 2017, 10:09
Default Do you think problem is with the Fluent
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Thanks Lucas for your reply. I'll try to follow your suggestion.
But, do you think problem is with the Fluent? since that error was for the first iteration in which was related to the transient structural, since the Fluent part was not started yet.

Regards,
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Old   December 8, 2017, 13:18
Default Relaxation helped
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Dear Lucas,

The relaxation option helped to have some initial results. I'm working on the model and as soon as I get any useful results, I'll let you know.

Regards,
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Old   December 12, 2017, 14:14
Default Convergence problem
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Dear Lucas,

Now, the simulation is working fine. Thanks for your comment.
But, I have another problem. Both data transfer from Structural and Fluent return "bot yet converged" in the convergence part. Do you have any idea why? Is it related to the mesh and element size?
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Old   December 13, 2017, 06:31
Default Convergence result
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Lucas Ribeiro
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Dear Malekan,

Good to hear that you made it. Out of curiosity, how much of relaxation did you use?

Sorry, but I have no ideia about "bot yet converged".

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Old   December 13, 2017, 08:56
Default Not yet converged!
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Dear Lucas,

First, I used smaller values, like 0.01 but I had problems. Then, I used 50 and the solver continued to working. Although, I still didn't get any results that I want. But, I have that "not yet converged" problem for both structural and Fluent parts.

Regards,
Malekan
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2-way fsi, convergence, fluent, fsi 2-way coupling, mechanical

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