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Fluid Structure Interaction with *MAT_SOFT_TISSUE and EFG elements

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Presentation on theme: "Fluid Structure Interaction with *MAT_SOFT_TISSUE and EFG elements"— Presentation transcript:

1 Fluid Structure Interaction with *MAT_SOFT_TISSUE and EFG elements
Rudolf Bötticher Introduction This talk features the meshless Element Free Galerkin (EFG) method. This is a talk about LS-DYNA EFG as is in a The EFG method is not explained the EFG implementation in LS-DYNA is not highlighted My interest lies in coupled simulations. Therefore, I chose a fluid structure interaction (FSI) problem. (Only solid EFG elements are possible with 5434a.) For the EFG elements to have a chance I chose a problem with large element distortions and hourglass likelihood: the bending of nearly incompressible *MAT_SOFT_TISSUE membrane. I wanted to try out, whether a analysis of the interaction of multiple fluids with a structure out of EFG solids is possible. It is possible. Even if the structure is out of a composite hyperelastic material. I compare with standard Lagrangian solid elements: this means reduced integration and hourglass control type 6. The type is on the rise in LS-DYNA. Implicit prefers type 6. So let`s look at the pro and cons of EFG. .

2 Motivation Assess whether EFG with FSI and MMALE is possible for *MAT_SOFT_TISSUE. Compare the results with standard elements. Assess whether EFG is more robust. Result will be: EFG is a luxury hourglass control ! And I use an example that originates from LSTC`s FSI_Tutor. A small slice is modeled, because no axisymmetric MMALE is possible. Birmingham,

3 EFG is easy! *CONTROL_EFG *SECTION_SOLID_EFG 5,41 1.4,1.4,1.4
$ in 970 EFG and (dormant) IMPLICIT cards are not tolerated $ in the same deck $ implicit and axisymmetric EFG not implemented! *SECTION_SOLID_EFG 5,41 $ the non-default bigger support helps to have consistent $ EFG simulations for *MAT_SOFT_TISSUE 1.4,1.4,1.4 Birmingham,

4 MM-ALE is easy, if you got a working deck to refine!
*ALE_MULTI-MATERIAL_GROUP Proceedings Birmingham,

5 FSI: tweaking of *CONSTRAINED_LAGRANGE_IN_SOLID
5434a: always use the newest version for consistency and efficiency! TSSFAC NADV METH CTYPE DIREC PFAC ILEAK Birmingham,

6 ELFORM=11 *EOS_GRUENEISEN bulk modulus *MAT_NULL *EOS_IDEAL_GAS AET=4
*MAT_SOFT_TISSUE *EOS_IDEAL_GAS The next slide shows the detailed setup of the biomedical prototype duct. This is an axisymmetric structure, But, because there are no axisymmetric multi-material ALE elements in LS-DYNA, a 3D slice is modeled. Three fluids are in the system. This soft tissue membrane separates a water and an air domain, which are both at atmospheric pressure. The system is triggered by an ambient pressure source here. The rubber membrane is modeled with *MAT_MOONEY-RIVLIN_RUBBER. Here is the domain, where we will compare EFG and standard elements. The fluids` domain is modeled with multi-material ALE solids ELFORM=11. The water has an *EOS_GRUENEISEN and the air an *EOS_IDEAL_GAS. Above is the 30bar uniform ambient pressure source. The parameter AET is set to 4 on *SECTION_SOLID_ALE to model this. The system has constrains to model the outer wall and symmetry axis.. outflow Birmingham,

7 *MAT_SOFT_TISSUE reinforced Mooney-Rivlin rubber C1=C2= 5 MPa
K= 5 GPa (PR=0.4998) reinforcement in tension C5=500 MPa B-direction Birmingham,

8 Why LS-DYNA for this problem?
Curiosity Code is at your disposal Expect the same efficiency as for parallel crash simulation Rely on the advanced material modeling There are various reasons for using LS-DYNA for this problems apart from curiosity. Among them are …….. I do not claim that LS-DYNA is suited best for this problem: other codes may be better. other codes may be better Birmingham,

9 Initial shortening of time step prevents intrusion
*CONTROL_TIMESTEP ,,,,,222 *DEFINE_CURVE 222 Some tweaking of the *CONSTRAINED_LAGARANGE_IN_SOLID card has been documented in former work Downscaling of the very first time steps may be necessary to avoid intrusion. Birmingham,

10 DIREC=3 PFAC=0.1 default *HOURGLASS 1,1,.00001
A default hourglass control is assumed for the MMALE mesh in ELFORM=11. This hourglass control has to be explicitly scaled down to avoid the generation of counter-forces. This is done with *HOURGLASS 1,1,.00001 and the HGEN parameter on the *PART card. The implications are not fully clear. Anyway, the development in the lower fluid shows a difference. Birmingham,

11 near incompressibility and mm dimensions require tiny time step
RI HG6 EFG no implicit, no time step split between tissue/ALE domain, no mass scaling! K=5 GPa Birmingham,

12 FSI, MMALE dependent on time step!
RI HG6 EFG FSI, MMALE dependent on time step! TSSFAC=0.25 Birmingham,

13 Robustness It proves difficult generating an extreme situation where EFG works but standard elements do not! EFG is superior in preventing hourglass modes. In the second part of my talk I want to compare the robustness of EFG and standard Elements. It proves difficult generating a situation in which the solver aborts for standard elements but continues for EFG elements. The standard elements are pretty robust. Therefore, I want to focus on another important feature of EFG. The EFG method does not need a hourglass control. EFG does not introduce non-physical energy in the system to control these hourglass modes. This is hinted in the picture that shows a typical hourglass pattern. Birmingham,

14 EFG performance lack: 10% elements switched, CPU time +15%
ELFORM=1 *HOURGLASS,6 EFG EFG performance lack: 10% elements switched, CPU time +15% 971: *SECTION_SOLID_EFG 1,41 1.4,1.4,1.4,,,3,2 In this simulation the green fluid is now atmospheric air instead of water. The fluid in the pressure source remains water. This amplifies the effect. On the left the results for standard elements are animated; this means reduced integration and hourglass control type 6. Despite this hourglass control a severe hourglass pattern emerges. The deflection of the membrane changes radically. The animation on the right side displays the results with the EFG method. The displacements are smoother. They show no hourglass problem. Apart from that, EFG does not introduce non-physical energy to control hourglass modes. However, this advantage comes at a price to the user. Though, only 10% of the elements are switched to EFG the solution time raises by 20%. Most elements are in the ALE domain and remain the same. There is a performance lack of the EFG method. The 971 version addresses this problem by introducing some approximations that allow for faster calculations. However, the consistency of the results should be checked at the moment. Birmingham,

15 Conclusions FSI simulations with MMALE and *MAT_SOFT_TISSUE EFG solids are possible in LS-DYNA. EFG is superior in hourglass prevention for tricky cases. Time step dependence of FSI needs further investigation! I now come to my conclusions. Well, you can mix it all, all advanced features. Fluid-structure interaction with multi material ALE elements and EFG solids is possible in LS-DYNA. Even for a composite hyperelastic material like *MAT_SOFT_TISSUE: Here, an example was shown, in which EFG is superior in preventing hourglass modes compared to standard elements. However, this requires more CPU time. The time step influence on the fluid structure interaction in LS_DYNA needs further investigation and development for the multi-material ALE elements. This is important, because the time step is small for high Poisson's ratio and small dimensions. At the end, I want to mention once again the decks in the proceedings and on my website. Thank You for listening. Birmingham,


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