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March 8, 2022, 06:00 
Sudden divergence of elastic pipe FSI

#1 
Member
Merlin Williams
Join Date: Nov 2021
Posts: 71
Rep Power: 4 
Hi, another thread from me...
So I think I have a better understanding of what is wrong with my simulation and I would like peoples opinion on it and any suggestions to improve it. Summary of the problem: I am trying to create an FSI simulation for blood flowing through a brain aneurysm. I have an inlet velocity waveform and specify a pressure of 0 Pa at the outlets. The other BC's are below. What is happening is that I am getting a wave of fluid velocity increase as it moves down the inlet tube, as in the first image. For all of the initial iterations of the FSI sim it converges well (in about 20 FSI corrector iterations) and doesnt have a problem. Then between 0.00025 and 0.0005 suddenly it takes longer to converge in each FSI iteration and eventually after maxing out at 200 FSI corrector iterations it crashes. As it gets slower to converge, it starts to get a lot slower after printing the output: Time = 0.000XXXX, iteration: X After it prints that line there is a long pause, which is longer than it takes to run all of the other calculations in that iteration. Could this indicate what is going wrong? I have also tried to set the coupling time to start at 0.01 seconds. This did roughly the same thing where it was fine with no coupling, then the coupling started and it happily converged for about 0.0002 seconds, then all of a sudden it starts taking longer to converge and finally crashes. The FSI parameters that I am using are: FSI relaxation factor = 0.001 FSI tolerance = 1e06 reuse = 4 Solid density = 1300 Poissions ratio = 0.45 Youngs Modulus = 1.6e6 deltaT = 2.5e6 maxDeltaT = 1e5 maxCo = 0.2 Solid domain relaxation = 0.6 (No relaxation for pressure or velocity in fluid domain) Also what would be the effect of ading relaxation for pressure or velocity in the fluid domain, might this help or produce more problems? All of these sims have used a meshes with nonorthogonalities no greater than 60 and no more than 80000 elements. Some have used boundary layers some havent. The checkMesh for the fluid and solid from image attached is below: Fluid mesh: Code:
Time = 0 Mesh stats all points: 16283 live points: 16283 all faces: 44045 live faces: 44045 internal faces: 39371 cells: 13939 boundary patches: 4 point zones: 0 face zones: 0 cell zones: 0 Overall number of cells of each type: hexahedra: 13765 prisms: 44 wedges: 0 pyramids: 86 tet wedges: 0 tetrahedra: 44 polyhedra: 0 Checking topology... Boundary definition OK. Point usage OK. Upper triangular ordering OK. Face vertices OK. Number of regions: 1 (OK). Checking patch topology for multiply connected surfaces ... Patch Faces Points Area [m^2] Surface topology inlet 106 113 8.04803e06 ok (nonclosed singly connected) fluidInterface 4514 4539 0.000310381 ok (nonclosed singly connected) outlet2 38 42 2.36964e06 ok (nonclosed singly connected) outlet1 16 18 8.41541e07 ok (nonclosed singly connected) Checking geometry... This is a 3D mesh Overall domain bounding box (0.0524018 0.0690049 0.550233) (0.0426186 0.0581131 0.533035) Mesh (nonempty, nonwedge) directions (1 1 1) Mesh (nonempty) directions (1 1 1) Mesh (nonempty, nonwedge) dimensions 3 Boundary openness (1.37959e16 1.15997e16 9.4579e17) Threshold = 1e06 OK. Max cell openness = 2.79196e16 OK. Max aspect ratio = 5.45218 OK. Minumum face area = 5.24083e09. Maximum face area = 1.64157e07. Face area magnitudes OK. Min volume = 2.1492e13. Max volume = 4.08655e11. Total volume = 3.15458e07. Cell volumes OK. Mesh nonorthogonality Max: 38.8545 average: 5.18479 Threshold = 70 Nonorthogonality check OK. Face pyramids OK. Max skewness = 0.544832 OK. Mesh OK. Code:
From function void polyMesh::initMesh() in file meshes/polyMesh/polyMeshInitMesh.C at line 81 Truncating neighbour list at 22506 for backward compatibility Time = 0 Mesh stats all points: 13617 live points: 13617 all faces: 31650 live faces: 31650 internal faces: 22506 cells: 9028 boundary patches: 3 point zones: 0 face zones: 0 cell zones: 0 Overall number of cells of each type: hexahedra: 9016 prisms: 12 wedges: 0 pyramids: 0 tet wedges: 0 tetrahedra: 0 polyhedra: 0 Checking topology... Boundary definition OK. Point usage OK. Upper triangular ordering OK. Face vertices OK. Number of regions: 1 (OK). Checking patch topology for multiply connected surfaces ... Patch Faces Points Area [m^2] Surface topology solidEnds 116 174 6.866e06 ok (nonclosed singly connected) fluidInterfaceSolid 4514 4539 0.000310381 ok (nonclosed singly connected) outerSolidFace 4514 4539 0.000360442 ok (nonclosed singly connected) Checking geometry... This is a 3D mesh Overall domain bounding box (0.0527121 0.0693202 0.550553) (0.0422981 0.0577922 0.532975) Mesh (nonempty, nonwedge) directions (1 1 1) Mesh (nonempty) directions (1 1 1) Mesh (nonempty, nonwedge) dimensions 3 Boundary openness (1.14638e16 3.75844e16 2.4529e16) Threshold = 1e06 OK. Max cell openness = 2.0773e16 OK. Max aspect ratio = 4.89239 OK. Minumum face area = 1.33733e08. Maximum face area = 2.47861e07. Face area magnitudes OK. Min volume = 2.31903e12. Max volume = 4.31077e11. Total volume = 1.07163e07. Cell volumes OK. Mesh nonorthogonality Max: 55.353 average: 8.25151 Threshold = 70 Nonorthogonality check OK. Face pyramids OK. Max skewness = 1.24647 OK. Mesh OK. U: Code:
internalField uniform (0 0 0); boundaryField { outlet2 { type fluxCorrectedVelocity; } inlet { type groovyBC; refValue uniform (0 0 0); refGradient uniform (0 0 0); valueFraction uniform 1; value uniform (0 0 0); valueExpression "(a1*sin(b1*x+c1) + a2*sin(b2*x+c2) + a3*sin(b3*x+c3) + a4*sin(b4*x+c4) + a5*sin(b5*x+c5) + a6*> gradientExpression "vector(0,0,0)"; fractionExpression "1"; evaluateDuringConstruction 0; cyclicSlave 0; variables 25 ( "x=time();" "a1=1.476;" "b1=1.011;" "c1=0.4674;" "a2=1.206;" "b2=1.132;" "c2=3.477;" "a3=0.05853;" "b3=8.393;" "c3=0.2345;" "a4=0.04648;" "b4=16.76;" "c4=0.6503;" "a5=0.00276;" "b5=5.108;" "c5=2.317;" "a6=0.02639;" "b6=33.52;" "c6=1.617;" "a7=0.0228;" "b7=25.14;" "c7=1.522;" "a8=0.01827;" "b8=41.87;" "c8=2.746;" ) ; timelines ( ); lookuptables ( ); } outlet1 { type fluxCorrectedVelocity; } fluidInterface { type newMovingWallVelocity; value uniform (0 0 0); } } Code:
boundaryField { outlet1 { type fixedValue; value uniform 0; } outlet2 { type fixedValue; value uniform 0; } inlet { type zeroGradient; } fluidInterface { type extrapolatedPressure; value uniform 0; } Code:
internalField uniform (0 0 0); boundaryField { solidEnds { type fixedDisplacement; value uniform (0 0 0); } fluidInterfaceSolid { type solidTraction; traction uniform ( 0 0 0 ); pressure uniform 0; value uniform (0 0 0); } outerSolidFace { type solidTraction; traction uniform ( 0 0 0 ); pressure uniform 0; value uniform (0 0 0); } } In my very novice opinion it seems that as all of the simulations are failing after roughly the same amount of time after being coupled, there must be some sort of numerical error that starts insignificant but then takes 0.000250.0005 seconds to increase to an extent where it affects the solution, and the time it takes to crash the simulation is proportional to the refinement of the mesh, ie more refined takes less time to break. If this is correct how might I find out what is happening? I also think it must be to do with the solid settings or mesh as I can run a CFD sim with the same fluid parameters and it will converge perfectly. Here is a log file for a simulation which used a slightly more refined mesh, but which crashed in a very similar fashion. https://drive.google.com/file/d/10J8...ew?usp=sharing EDIT: I have also just realised that there is a missing '}' at the end of the pressure BC for the fluid. Could this tiny error be a problem, becuase it is also missing on the 3dTube tutorial case which I built my fsi case from. EDIT2: here is a link to the most recent case file: https://drive.google.com/drive/folde...mu?usp=sharing Last edited by MFWilliams; March 8, 2022 at 07:07. 

June 16, 2022, 11:35 

#2 
Member
Join Date: Mar 2020
Posts: 66
Rep Power: 6 
Hi,
have you solved your problem? i have the exact same block in the simulation. 

Tags 
convergance, divergence, fsi coupling, solid domain 
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