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successful time step for 2D problem failing for 3D case of same problem.

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Old   March 9, 2015, 09:02
Question successful time step for 2D problem failing for 3D case of same problem.
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Manoj Dhiman
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Hello,
I am trying to simulate 3D convective diffusive unsteady problem using SIMPLER technique (Finite Volume Method). I got successful 2D case of the problem. For 3D I am keeping the size of space step same as in 2D, with one extra direction.
The problem is in choosing the time step. When I keep it same as in 2D the simulation diverges and in order to converge it I have to reduce the time step by order of 4.
Is this thing common or I am committing some other problem.
Unluckily if its common than it will take ages for this simulation.
BTW I am running this code with gfortran compilers.
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Old   March 9, 2015, 12:17
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Filippo Maria Denaro
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Quote:
Originally Posted by mandman View Post
Hello,
I am trying to simulate 3D convective diffusive unsteady problem using SIMPLER technique (Finite Volume Method). I got successful 2D case of the problem. For 3D I am keeping the size of space step same as in 2D, with one extra direction.
The problem is in choosing the time step. When I keep it same as in 2D the simulation diverges and in order to converge it I have to reduce the time step by order of 4.
Is this thing common or I am committing some other problem.
Unluckily if its common than it will take ages for this simulation.
BTW I am running this code with gfortran compilers.


yes, the numerical stability involves the relation of dimensionality therefore the constraint is more strict when going from 1D, 2D, 3D.
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Old   March 9, 2015, 22:32
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Though it is true that the stability constraint for 3D is stricter than in 2D, it is difficult to believe that the time step size would have to drop by 4 _orders_. By 4 _orders_ I understand you're saying 10^4 (10,000 times). Otherwise, if the drop is _factor_ 4 (4 times) it is somewhat more reasonable, though it depends (very much) on the problem and what physics are introduced by the addition of the third dimension (and 4x increase doesn't really translate to "ages" of simulation time in added cost)

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Old   March 10, 2015, 09:31
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If you are saying that the time step has to be 10000 times smaller rather than 4 times smaller, I would look for an error in the coding. A factor of 10000 is not reasonable for a jump to 3D, in my experience.
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Old   March 10, 2015, 15:18
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Manoj Dhiman
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by O(4) I mean 10000.
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Old   March 10, 2015, 15:40
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Then you most likely (definitely?) have a bug in the code! What are the 2- and 3-D problems you were solving and what are the corresponding CFL values?

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Old   March 10, 2015, 20:01
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Manoj Dhiman
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Ya!! you guys are right. I found THE "bug", it was resting in expression calculating the difference between pressure and pressure correction.
I was struggling with this problem for last 2 months, and I was so much confident about my code that I thought that its common for 3D problems. But you guys conviction made me doubtful.
I even learned OpenMP and parallelised this code, but there’s no way I could have compensated for O(4) time increase.
Thanks all
I am working on double diffusive convection problem, let me know if you want to know more. You can email me on manoj_d@students.iitmandi.ac.in
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3 dimensional simulation, diverging solution, fortran code, time discretization

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