[sita]

Hi all,

Does anyone have some tips on how to limit the volume fraction in a free surface simulation using interFoam + LTS + semi-implicit MULES?

I'm simulating a ship hull at constant (final) velocity, on flat water (OpenFOAM-7, Scientific Linux 7). What I've already tried is:

• Applying a velocity ramp at the start of the simulation
• Increasing nAlphaCorr (number of corrections)
• Increasing nLimiterIter (number of iterations to calculate the limiters for alpha)
• Changing alphaApplyPrevCorr from yes (suitable for slowly varying flows) to no (suitable for highly transient flows)
• Lowering maxAlphaCo (PIMPLE)
• Increasing rDeltaSmoothingCoeff (PIMPLE)
• Setting nNonOrthogonalCorrectors to 3

None of this seems to help, really. As you can see in the log below, alpha.water already starts to go beyond [0 1] during the second PIMPLE iteration in the first time step. It goes up to values around [-40 40], and after a while it starts decreasing. I haven't checked whether it decreases to within [0 1] eventually, but looking at the volume fraction field after 4000 time steps (see attached image; the extreme alpha.water values are constricted to a small number of cells on the hull, but the whole volume fraction field looks extremely messy), I'm afraid more time steps won't lead to a physically correct solution.

I'd greatly appreciate any help with this!

Many thanks,
Sita

Starting time loop

forces forces:
    Not including porosity effects
Flow time scale min/max = 1, 1
Smoothed flow time scale min/max = 1, 1
Time = 1

PIMPLE: Iteration 1
smoothSolver:  Solving for alpha.water, Initial residual = 0, Final residual = 0, No Iterations 1
Phase-1 volume fraction = 0.800151  Min(alpha.water) = 0  Max(alpha.water) = 1
MULES: Correcting alpha.water
MULES: Correcting alpha.water
MULES: Correcting alpha.water
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = 0.800151  Min(alpha.water) = 0  Max(alpha.water) = 1
GAMG:  Solving for p_rgh, Initial residual = 1, Final residual = 0.00265042, No Iterations 2
GAMG:  Solving for p_rgh, Initial residual = 0.00738649, Final residual = 6.70127e-05, No Iterations 4
GAMG:  Solving for p_rgh, Initial residual = 0.00164859, Final residual = 1.27058e-05, No Iterations 5
GAMG:  Solving for p_rgh, Initial residual = 0.000374371, Final residual = 3.73205e-06, No Iterations 7
time step continuity errors : sum local = 4.17722e-06, global = 1.11554e-07, cumulative = 1.11554e-07
GAMG:  Solving for p_rgh, Initial residual = 0.00181009, Final residual = 1.05501e-05, No Iterations 6
GAMG:  Solving for p_rgh, Initial residual = 0.000606215, Final residual = 4.91912e-06, No Iterations 4
GAMG:  Solving for p_rgh, Initial residual = 0.000128742, Final residual = 1.28266e-06, No Iterations 5
GAMG:  Solving for p_rgh, Initial residual = 4.12132e-05, Final residual = 4.67321e-08, No Iterations 34
time step continuity errors : sum local = 5.256e-08, global = 4.63798e-10, cumulative = 1.12018e-07
PIMPLE: Iteration 2
smoothSolver:  Solving for alpha.water, Initial residual = 0.00373314, Final residual = 9.88023e-09, No Iterations 194
Phase-1 volume fraction = 0.80014  Min(alpha.water) = 0  Max(alpha.water) = 1
MULES: Correcting alpha.water
MULES: Correcting alpha.water
MULES: Correcting alpha.water
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = 0.80014  Min(alpha.water) = -0.994681  Max(alpha.water) = 1.26243
GAMG:  Solving for p_rgh, Initial residual = 0.643277, Final residual = 0.0025636, No Iterations 2
GAMG:  Solving for p_rgh, Initial residual = 0.000239485, Final residual = 2.04436e-06, No Iterations 5
GAMG:  Solving for p_rgh, Initial residual = 8.292e-05, Final residual = 6.51857e-07, No Iterations 5
GAMG:  Solving for p_rgh, Initial residual = 2.60221e-05, Final residual = 2.48377e-07, No Iterations 7
time step continuity errors : sum local = 2.63399e-05, global = -2.08929e-06, cumulative = -1.97727e-06
GAMG:  Solving for p_rgh, Initial residual = 0.000181122, Final residual = 1.76238e-06, No Iterations 6
GAMG:  Solving for p_rgh, Initial residual = 3.24913e-05, Final residual = 3.01908e-07, No Iterations 6
GAMG:  Solving for p_rgh, Initial residual = 8.87704e-06, Final residual = 8.24178e-08, No Iterations 9
GAMG:  Solving for p_rgh, Initial residual = 4.60003e-06, Final residual = 4.48455e-08, No Iterations 7
time step continuity errors : sum local = 9.93412e-06, global = 7.66582e-07, cumulative = -1.21069e-06
PIMPLE: Iteration 3
smoothSolver:  Solving for alpha.water, Initial residual = 0.0051518, Final residual = 9.5141e-09, No Iterations 193
Phase-1 volume fraction = 0.800275  Min(alpha.water) = 0  Max(alpha.water) = 8.08354
MULES: Correcting alpha.water
MULES: Correcting alpha.water
MULES: Correcting alpha.water
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = 0.800276  Min(alpha.water) = -40.6607  Max(alpha.water) = 36.5421
GAMG:  Solving for p_rgh, Initial residual = 0.0259583, Final residual = 0.000113245, No Iterations 2
GAMG:  Solving for p_rgh, Initial residual = 0.000229723, Final residual = 2.13199e-06, No Iterations 6
GAMG:  Solving for p_rgh, Initial residual = 3.9345e-05, Final residual = 3.29743e-07, No Iterations 5
GAMG:  Solving for p_rgh, Initial residual = 6.93932e-06, Final residual = 6.35426e-08, No Iterations 7
time step continuity errors : sum local = 3.15815e-05, global = 3.83816e-08, cumulative = -1.17231e-06
GAMG:  Solving for p_rgh, Initial residual = 9.03879e-05, Final residual = 6.14209e-07, No Iterations 5
GAMG:  Solving for p_rgh, Initial residual = 6.38855e-05, Final residual = 5.87189e-07, No Iterations 6
GAMG:  Solving for p_rgh, Initial residual = 4.8023e-06, Final residual = 4.93757e-08, No Iterations 9
GAMG:  Solving for p_rgh, Initial residual = 1.0839e-06, Final residual = 4.14584e-08, No Iterations 2
time step continuity errors : sum local = 7.41179e-06, global = -1.83339e-07, cumulative = -1.35565e-06
smoothSolver:  Solving for omega, Initial residual = 6.41347e-05, Final residual = 6.31364e-06, No Iterations 27
smoothSolver:  Solving for k, Initial residual = 1, Final residual = 0.0949203, No Iterations 5
ExecutionTime = 11.67 s  ClockTime = 17 s

forces forces write:
    sum of forces:
        pressure : (307.111 -1016.58 1924.9)
        viscous  : (0.437865 1.16819 6.12748)
        porous   : (0 0 0)
    sum of moments:
        pressure : (-224.651 35978.2 23389.1)
        viscous  : (1.46676 96.1615 -17.0622)
        porous   : (0 0 0)

Flow time scale min/max = 6.73609e-08, 1
Smoothed flow time scale min/max = 6.73609e-08, 0.00624496

[sita]

Great, I think I found the solution myself: it looks like my initial conditions were off. I've changed some values, and up to now my simulations are running fine.

[shaiashe]

Hi Sita,

what were the changes that you applied to the initial conditions ?

Cheers,

[sita]

Hi Shai,

I'm really-really sorry, but I've changed jobs in the meantime; I'm in bio-fluid dynamics now, so I'm no longer using interFoam, and I can't remember what settings I used back then.

Reading back my post, I'm pretty annoyed with myself for not writing exactly which initial conditions I changed!

As far as I can remember, I gave up using LTS altogether in the end, because it turned out that practically all my simulations would run fine without it.

Sorry I couldn't be of more help!
Sita

[shaiashe]

So you went with regular Euler without compromising the time-step errors ?

[sita]

I did indeed, worked for me most of the time (we used a commercial mesher, Hexpress, by Numeca). I never got to fully testing everything in all sorts of cases, though.

[shaiashe]

Ok thank you very much!, Also for the tip on numeca. I'll check them out

[ufocfd]

thanks - some great suggestions there

for reference its:

rDeltaTSmoothingCoeff 0.05; // up to 0.5
rDeltaTDampingCoeff 0.5;