[Hgholami]

Dear foamer.
I try to model Soti et al. article "Flow-induced deformation of a flexible thin structure as manifestation of heat transfer enhancement". Their geometry are similar of Hron-Turek model but larger deflection of plate. I change dynamicMeshDict and add top and bottom wall to patches to handle this deformation. In running, the plate start to oscillate proper, and the deformation amplitude increase with time. but after some oscillation period, the solid deformation start to expand the plate, as below. and finally the velocity increases and floating point occur.

My question is which parameters that transmit this error to solver? are there due to mesh size decreasing?

[Daniel_Khazaei]

Without knowing the solid material properties and your fsiProperties dictionary details, I can only guess:
The implicit coupling iteration (inner fsi iteration) doesn't converge in each time step causing the velocity field to blow up.

• How many iteration does it take for the solver to reach convergence in each time step?
• What is your fsi convergence tolerance?

[Hgholami]

the fsiProperties is

solidPatch plate;
solidZone plateZone;

fluidPatch plate;
fluidZone plateZone;

relaxationFactor 0.005;

interfaceDeformationLimit 0.002;

outerCorrTolerance 1e-4;
nOuterCorr 60;

interpolatorUpdateFrequency 0;

//couplingScheme Aitken;
couplingScheme IQN-ILS;

couplingReuse 0;

coupled yes;
predictor yes;

and solid properties is

planeStress no;

rheology
{
type linearElastic;
rho rho [1 -3 0 0 0 0 0] 10000;
E E [1 -1 -2 0 0 0 0] 1.4e6;
nu nu [0 0 0 0 0 0 0] 0.4;
}

[Daniel_Khazaei]

Do you get convergence within the prescribed nOuterCorr of 60?
Although I think that is too much inner iteration for the simulation time to be reasonable.

• Try to keep it below the default value of 30.

Why using such a low relaxation factor for the first two iterations? (relaxationFactor)

• Does the default value of 0.05 cause instability?

I'm afraid that the interface deformation tolerance of 1e-4 is two light for such a strongly coupled simulation:

• Use 1e-6 or 1e-7 to be sure that interface variables are fully converge before heading to the next time step.

Regarding the interface deformation limit, you are skipping solving the mesh motion until the accumulated displacement reaches the prescribed value and although this will speed up your simulation it can also cause the simulation to diverge due to the lose of accuracy.

• For strongly coupled simulation keep it at 0.

You didn't answer the main question, does the simulation converge properly in each time step?

[Hgholami]

Dear Khazaee
The solution converge properly in each time step, before reach to nOuterCorr. But it need to use low relaxation factor, because increasing relaxationFactor to 0.05, diverge solution.
The problem starts when in a time Step the converge isn't appear, then the max velocity increases in next time steps and the solution diverge.
So, it isn't clearly, which parameter introduce the diverge.
Now I try to use interface deformation tolerance of 1e-7 and interface deformation limit 0.

[Hgholami]

the results log was

Current fsi relative residual norm: 0.237396
Alternative fsi residual: 0.000451543

Time = 0.0904, iteration: 53
Maximal accumulated displacement of interface points: 0.00287293
GAMG: Solving for cellMotionUx, Initial residual = 0.284575, Final residual = 8.25916e-07, No Iterations 9
GAMG: Solving for cellMotionUy, Initial residual = 0.00633877, Final residual = 8.60538e-07, No Iterations 4
GAMG: Solving for cellMotionUx, Initial residual = 0.00251555, Final residual = 4.99283e-07, No Iterations 5
GAMG: Solving for cellMotionUy, Initial residual = 9.75429e-05, Final residual = 4.5803e-07, No Iterations 2
GAMG: Solving for cellMotionUx, Initial residual = 0.000216841, Final residual = 7.5024e-07, No Iterations 3
GAMG: Solving for cellMotionUy, Initial residual = 4.78647e-06, Final residual = 9.52308e-08, No Iterations 2
GAMG: Solving for cellMotionUx, Initial residual = 2.45077e-05, Final residual = 2.78931e-07, No Iterations 2
GAMG: Solving for cellMotionUy, Initial residual = 5.75566e-07, Final residual = 2.52936e-08, No Iterations 2
GAMG: Solving for cellMotionUx, Initial residual = 2.58656e-06, Final residual = 6.38896e-08, No Iterations 2
GAMG: Solving for cellMotionUy, Initial residual = 8.15413e-08, Final residual = 6.81777e-09, No Iterations 2
Evolving fluid solver: simpleFluid
Courant Number mean: 0.00509794 max: 0.0718506 velocity magnitude: 2.13169
DILUPBiCG: Solving for Ux, Initial residual = 1.69255e-05, Final residual = 2.04556e-10, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 3.87838e-05, Final residual = 5.12499e-10, No Iterations 1
DILUPBiCG: Solving for T, Initial residual = 2.60503e-06, Final residual = 7.08107e-19, No Iterations 3
GAMG: Solving for p, Initial residual = 0.463702, Final residual = 8.96859e-07, No Iterations 166
GAMG: Solving for p, Initial residual = 5.29002e-07, Final residual = 4.72136e-07, No Iterations 2
GAMG: Solving for p, Initial residual = 4.72136e-07, Final residual = 4.21393e-07, No Iterations 2
time step continuity errors : sum local = 1.0671e-11, global = 1.00739e-12
Courant Number mean: 0.00509605 max: 0.0718506 velocity magnitude: 2.13165
DILUPBiCG: Solving for Ux, Initial residual = 0.00034187, Final residual = 1.31826e-09, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000647049, Final residual = 4.43769e-09, No Iterations 1
DILUPBiCG: Solving for T, Initial residual = 1.64656e-07, Final residual = 4.07583e-19, No Iterations 3
GAMG: Solving for p, Initial residual = 0.00470111, Final residual = 9.5084e-07, No Iterations 70
GAMG: Solving for p, Initial residual = 9.51131e-07, Final residual = 8.46786e-07, No Iterations 2
GAMG: Solving for p, Initial residual = 8.46786e-07, Final residual = 7.53965e-07, No Iterations 2
time step continuity errors : sum local = 1.90879e-11, global = -1.63793e-12
Courant Number mean: 0.00509605 max: 0.0718506 velocity magnitude: 2.13166
DILUPBiCG: Solving for Ux, Initial residual = 8.49261e-05, Final residual = 2.9928e-10, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000162847, Final residual = 1.10464e-09, No Iterations 1
DILUPBiCG: Solving for T, Initial residual = 4.69454e-09, Final residual = 1.72592e-20, No Iterations 3
GAMG: Solving for p, Initial residual = 0.000448699, Final residual = 9.6565e-07, No Iterations 44
GAMG: Solving for p, Initial residual = 9.65837e-07, Final residual = 8.5836e-07, No Iterations 2
GAMG: Solving for p, Initial residual = 8.5836e-07, Final residual = 7.63068e-07, No Iterations 2
time step continuity errors : sum local = 1.9316e-11, global = -1.59423e-12
Courant Number mean: 0.00509605 max: 0.0718506 velocity magnitude: 2.13166
DILUPBiCG: Solving for Ux, Initial residual = 2.10751e-05, Final residual = 2.8395e-10, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 4.09195e-05, Final residual = 2.77616e-10, No Iterations 1
DILUPBiCG: Solving for T, Initial residual = 1.11622e-09, Final residual = 3.26274e-21, No Iterations 3
GAMG: Solving for p, Initial residual = 0.000111793, Final residual = 9.7267e-07, No Iterations 24
GAMG: Solving for p, Initial residual = 9.72714e-07, Final residual = 8.56298e-07, No Iterations 2
GAMG: Solving for p, Initial residual = 8.56298e-07, Final residual = 7.55912e-07, No Iterations 2
time step continuity errors : sum local = 1.91344e-11, global = -1.45064e-12
Setting traction on solid patch
Total force (fluid) = (-5.04694 -22.6577 1.92893e-13)
Total force (solid) = (4.77552 22.2434 -1.94685e-13)
Evolving solid solver: unsTotalLagrangianSolid
DICPCG: Solving for D, Initial residula = 0.000161116, Final residual = 1.9265e-07, No outer iterations = 20
Max relative residual = 0.091944, Relative residual = 9.12633e-05, enforceLinear = 0
Current fsi relative residual norm: 1
Alternative fsi residual: 0.00320077

Time = 0.0904, iteration: 54
Maximal accumulated displacement of interface points: 0.0417414
GAMG: Solving for cellMotionUx, Initial residual = 0.644976, Final residual = 7.91795e-07, No Iterations 10
GAMG: Solving for cellMotionUy, Initial residual = 0.0229824, Final residual = 4.80133e-07, No Iterations 6
GAMG: Solving for cellMotionUx, Initial residual = 0.00277923, Final residual = 5.50974e-07, No Iterations 5
GAMG: Solving for cellMotionUy, Initial residual = 0.000126275, Final residual = 5.73838e-07, No Iterations 2
GAMG: Solving for cellMotionUx, Initial residual = 0.000287842, Final residual = 9.94785e-07, No Iterations 3
GAMG: Solving for cellMotionUy, Initial residual = 6.24588e-06, Final residual = 1.14872e-07, No Iterations 2
GAMG: Solving for cellMotionUx, Initial residual = 3.26168e-05, Final residual = 3.7052e-07, No Iterations 2
GAMG: Solving for cellMotionUy, Initial residual = 7.43987e-07, Final residual = 2.96932e-08, No Iterations 2
GAMG: Solving for cellMotionUx, Initial residual = 3.46038e-06, Final residual = 8.51121e-08, No Iterations 2
GAMG: Solving for cellMotionUy, Initial residual = 1.03042e-07, Final residual = 7.7279e-09, No Iterations 2
Evolving fluid solver: simpleFluid
Courant Number mean: 0.00774371 max: 0.722412 velocity magnitude: 20.3402
DILUPBiCG: Solving for Ux, Initial residual = 0.000510444, Final residual = 2.70013e-07, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.0019864, Final residual = 1.16873e-09, No Iterations 2
DILUPBiCG: Solving for T, Initial residual = 4.38472e-05, Final residual = 9.18841e-14, No Iterations 3
GAMG: Solving for p, Initial residual = 0.926829, Final residual = 9.80306e-07, No Iterations 200
GAMG: Solving for p, Initial residual = 2.89641e-07, Final residual = 2.58609e-07, No Iterations 2
GAMG: Solving for p, Initial residual = 2.58609e-07, Final residual = 2.30905e-07, No Iterations 2
time step continuity errors : sum local = 9.80875e-11, global = -9.42611e-12
Courant Number mean: 0.00781173 max: 0.569187 velocity magnitude: 26.6585
DILUPBiCG: Solving for Ux, Initial residual = 0.0146298, Final residual = 7.35807e-07, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.00898921, Final residual = 3.71578e-09, No Iterations 2
DILUPBiCG: Solving for T, Initial residual = 6.64144e-06, Final residual = 4.72606e-15, No Iterations 3
GAMG: Solving for p, Initial residual = 0.0559703, Final residual = 9.43765e-07, No Iterations 98
GAMG: Solving for p, Initial residual = 9.75612e-07, Final residual = 8.69077e-07, No Iterations 2
GAMG: Solving for p, Initial residual = 8.69078e-07, Final residual = 7.74239e-07, No Iterations 2
time step continuity errors : sum local = 3.06496e-10, global = 2.679e-11
Courant Number mean: 0.0078112 max: 0.542143 velocity magnitude: 25.3919
DILUPBiCG: Solving for Ux, Initial residual = 0.00386819, Final residual = 7.63941e-07, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.00355023, Final residual = 8.43555e-07, No Iterations 1
DILUPBiCG: Solving for T, Initial residual = 8.79361e-07, Final residual = 8.75165e-16, No Iterations 3
GAMG: Solving for p, Initial residual = 0.00463734, Final residual = 8.98097e-07, No Iterations 74
GAMG: Solving for p, Initial residual = 8.97377e-07, Final residual = 7.98972e-07, No Iterations 2
GAMG: Solving for p, Initial residual = 7.98973e-07, Final residual = 7.1143e-07, No Iterations 2
time step continuity errors : sum local = 2.82385e-10, global = 2.42679e-11
Courant Number mean: 0.00781099 max: 0.541624 velocity magnitude: 25.3676
DILUPBiCG: Solving for Ux, Initial residual = 0.000975643, Final residual = 1.63967e-07, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000985652, Final residual = 3.47318e-07, No Iterations 1
DILUPBiCG: Solving for T, Initial residual = 1.494e-07, Final residual = 1.89168e-16, No Iterations 3
GAMG: Solving for p, Initial residual = 0.00123167, Final residual = 9.63733e-07, No Iterations 50
GAMG: Solving for p, Initial residual = 9.63516e-07, Final residual = 8.56075e-07, No Iterations 2
GAMG: Solving for p, Initial residual = 8.56075e-07, Final residual = 7.60856e-07, No Iterations 2
time step continuity errors : sum local = 3.02212e-10, global = 2.47939e-11
Setting traction on solid patch
Total force (fluid) = (189.946 266.482 -2.23025e-12)
Total force (solid) = (-158.971 -326.598 2.14702e-12)
Evolving solid solver: unsTotalLagrangianSolid
det: -0.909054, 1.001
DICPCG: Solving for D, Initial residula = 0.00883054, Final residual = 5.6837e-06, No outer iterations = 50
Max relative residual = 1.13098, Relative residual = 0.00110747, enforceLinear = 0
Current fsi relative residual norm: 1
Alternative fsi residual: 0.0728413

Time = 0.0904, iteration: 55
Maximal accumulated displacement of interface points: 9.85912
GAMG: Solving for cellMotionUx, Initial residual = 0.707085, Final residual = 9.02776e-07, No Iterations 10
GAMG: Solving for cellMotionUy, Initial residual = 0.0370538, Final residual = 7.71471e-07, No Iterations 6
GAMG: Solving for cellMotionUx, Initial residual = 0.00297136, Final residual = 5.99365e-07, No Iterations 5
GAMG: Solving for cellMotionUy, Initial residual = 0.000111731, Final residual = 5.09348e-07, No Iterations 2
GAMG: Solving for cellMotionUx, Initial residual = 0.000378415, Final residual = 5.6832e-07, No Iterations 4
GAMG: Solving for cellMotionUy, Initial residual = 5.60756e-06, Final residual = 1.02387e-07, No Iterations 2
GAMG: Solving for cellMotionUx, Initial residual = 4.33015e-05, Final residual = 4.41035e-07, No Iterations 2
GAMG: Solving for cellMotionUy, Initial residual = 6.64832e-07, Final residual = 2.64947e-08, No Iterations 2
GAMG: Solving for cellMotionUx, Initial residual = 4.56358e-06, Final residual = 8.64536e-08, No Iterations 2
GAMG: Solving for cellMotionUy, Initial residual = 9.22762e-08, Final residual = 6.90697e-09, No Iterations 2
Evolving fluid solver: simpleFluid
Courant Number mean: 0.161806 max: 179.4 velocity magnitude: 659.511
DILUPBiCG: Solving for Ux, Initial residual = 0.517266, Final residual = 3.11769e-07, No Iterations 4
DILUPBiCG: Solving for Uy, Initial residual = 0.865463, Final residual = 5.02855e-07, No Iterations 4
DILUPBiCG: Solving for T, Initial residual = 0.252526, Final residual = 3.39017e-08, No Iterations 5
Floating point exception (core dumped)