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-   -   the problem about"Max(gamma)>1" (http://www.cfd-online.com/Forums/openfoam/73494-problem-about-max-gamma-1-a.html)

zhajingjing March 10, 2010 01:50

the problem about"Max(gamma)>1"
 
Hello everyone,
I use a flap-type wavemaker to make wave, during the calculation, I find the Max(gamma)>1, is this a serious problem which will affect the accuracy of the result?

...
Courant Number mean: 0.001894994 max: 0.4794407
Time = 1.005

DICPCG: Solving for cellMotionUx, Initial residual = 1, Final residual = 8.496246e-07, No Iterations 397
DICPCG: Solving for cellMotionUy, Initial residual = 1, Final residual = 8.863253e-07, No Iterations 58
Execution time for mesh.update() = 1.61 s
time step continuity errors : sum local = 4.29361e-09, global = 1.803651e-10, cumulative = -1.818315e-08
DICPCG: Solving for pcorr, Initial residual = 1, Final residual = 9.335962e-11, No Iterations 222
DICPCG: Solving for pcorr, Initial residual = 2.391854e-11, Final residual = 2.391854e-11, No Iterations 0
DICPCG: Solving for pcorr, Initial residual = 2.391854e-11, Final residual = 2.391854e-11, No Iterations 0
time step continuity errors : sum local = 5.333407e-19, global = -6.352097e-21, cumulative = -1.818315e-08
MULES: Solving for gamma
Liquid phase volume fraction = 0.5000393 Min(gamma) = -1.77805e-30 Max(gamma) = 1.02632
MULES: Solving for gamma
Liquid phase volume fraction = 0.5000393 Min(gamma) = -1.712193e-30 Max(gamma) = 1.02593
DICPCG: Solving for pd, Initial residual = 0.01384289, Final residual = 0.00065049, No Iterations 18
DICPCG: Solving for pd, Initial residual = 0.009080314, Final residual = 0.0003433817, No Iterations 20
DICPCG: Solving for pd, Initial residual = 0.0005947545, Final residual = 2.57424e-05, No Iterations 20
time step continuity errors : sum local = 5.215714e-08, global = 1.521795e-08, cumulative = -2.965199e-09
DICPCG: Solving for pd, Initial residual = 0.07757173, Final residual = 0.003759144, No Iterations 11
DICPCG: Solving for pd, Initial residual = 0.003940919, Final residual = 0.0001762634, No Iterations 16
DICPCG: Solving for pd, Initial residual = 0.0001668971, Final residual = 8.316119e-06, No Iterations 19
time step continuity errors : sum local = 1.606884e-08, global = 1.689781e-10, cumulative = -2.796221e-09
DICPCG: Solving for pd, Initial residual = 0.06889182, Final residual = 0.002239976, No Iterations 11
DICPCG: Solving for pd, Initial residual = 0.0022981, Final residual = 0.0001148254, No Iterations 14
DICPCG: Solving for pd, Initial residual = 0.00011941, Final residual = 8.745692e-08, No Iterations 38
time step continuity errors : sum local = 1.69809e-10, global = 6.533056e-12, cumulative = -2.789688e-09
ExecutionTime = 320.99 s ClockTime = 322 s

Courant Number mean: 0.006729381 max: 0.5210432
Time = 1.01

DICPCG: Solving for cellMotionUx, Initial residual = 0.0005103467, Final residual = 9.866497e-07, No Iterations 16
DICPCG: Solving for cellMotionUy, Initial residual = 0.9999998, Final residual = 8.860694e-07, No Iterations 58
Execution time for mesh.update() = 0.49 s
time step continuity errors : sum local = 1.69809e-10, global = 6.533055e-12, cumulative = -2.783155e-09
DICPCG: Solving for pcorr, Initial residual = 1, Final residual = 9.21361e-11, No Iterations 229
DICPCG: Solving for pcorr, Initial residual = 2.966365e-11, Final residual = 2.966365e-11, No Iterations 0
DICPCG: Solving for pcorr, Initial residual = 2.966365e-11, Final residual = 2.966365e-11, No Iterations 0
time step continuity errors : sum local = 1.638706e-19, global = -2.329411e-22, cumulative = -2.783155e-09
MULES: Solving for gamma
Liquid phase volume fraction = 0.5000523 Min(gamma) = -1.193101e-31 Max(gamma) = 1.02555
MULES: Solving for gamma
Liquid phase volume fraction = 0.5000523 Min(gamma) = -1.149855e-31 Max(gamma) = 1.025178
DICPCG: Solving for pd, Initial residual = 0.282985, Final residual = 0.006764506, No Iterations 3
DICPCG: Solving for pd, Initial residual = 0.006902246, Final residual = 0.000315175, No Iterations 17
DICPCG: Solving for pd, Initial residual = 0.0003561435, Final residual = 1.441456e-05, No Iterations 19
time step continuity errors : sum local = 2.345417e-08, global = 2.280152e-09, cumulative = -5.030029e-10
DICPCG: Solving for pd, Initial residual = 0.1002033, Final residual = 0.004059263, No Iterations 11
DICPCG: Solving for pd, Initial residual = 0.003998678, Final residual = 0.0001579902, No Iterations 14
DICPCG: Solving for pd, Initial residual = 0.0001502544, Final residual = 7.512438e-06, No Iterations 19
time step continuity errors : sum local = 1.186638e-08, global = -3.455044e-10, cumulative = -8.485073e-10
DICPCG: Solving for pd, Initial residual = 0.08480317, Final residual = 0.002404498, No Iterations 11
DICPCG: Solving for pd, Initial residual = 0.002482414, Final residual = 0.000107671, No Iterations 15
DICPCG: Solving for pd, Initial residual = 0.0001129264, Final residual = 7.94363e-08, No Iterations 40
time step continuity errors : sum local = 1.263819e-10, global = 3.338986e-12, cumulative = -8.451684e-10
ExecutionTime = 322.76 s ClockTime = 323 s
...


what can I do?
I modify the write precision or change the nNonOrthogonalCorrectors from 0 to 2, or increase nGammaCorr
But it seems no use.
Any help will be appreciated!

Jingjing

ngj March 10, 2010 04:52

Hi

My experience with moving mesh and interFoam-type solvers is that having mesh-movement courant numbers larger than 1 results in overshooting of gamma. I do not know if that could be the source of your problem?

Best regards,

Niels

zhajingjing March 10, 2010 06:12

Quote:

Originally Posted by ngj (Post 249288)
Hi

My experience with moving mesh and interFoam-type solvers is that having mesh-movement courant numbers larger than 1 results in overshooting of gamma. I do not know if that could be the source of your problem?

Best regards,

Niels

Hi Niels,

The courant number is just around 0.5,never beyond 1.Even after I limit the maxCo below 0.2, Max(gamma) is still beyond 1, sometines even 1.2....I wonder whether I set the boundary condition correctly. I set the flow velocity at the flap in U the same as the velocity of the flap in pointMotion, and set the gamma "zeroGradient" at the flap. Is there any problem?
The abnormal phenomonen didn't happen when I use piston-type wavemaker, it happened when I use flap-type. I don't know whether the result is acceptable.

kathrin_kissling March 11, 2010 06:35

Hi zhajingjing,

what discretisation scheme are you using for the convective term of the gammaEqn?

Can you try to use a first order upwind scheme and check whether the unboundedness still appears? If the error comes from the discretisation of the convective term, this should work.

Best

Kathrin

zhajingjing March 11, 2010 08:18

Hi Kathrin,

I use this :" div(phi,gamma) Gauss vanLeer;" after I change to "div(phi,gamma) Gauss upwind", I find this problem still exist.
I find that the unbounded gamma comes out the area where the flap contacts with the free surface.

zhajingjing April 4, 2010 23:29

gamma>1 also happen in my piston type wavemaker
 
Hi,

In fact, in my piston type wavemaker cases, the maximum gamma value also reach to 1.004. Why, is it the reason of the singularity at the intersection of the wavemaker and free surface?

alberto April 5, 2010 02:33

You said you limited Co at 0.2. Was this limit strictly respected? Try to plot a contour of Co and check where it is large.

Also, keep an eye on the convergence of the pd equation. Switch to GAMG linear solver, and tighten that tolerance to very low (10^-15 or less). You have 10^-8 the correction steps, which might indicate a problem there.

Best


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