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strategy to enhance of time steps

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Old   June 6, 2016, 03:27
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faraz
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Hi all,
I try to simulate an unsteady multiphase issue with vof model, but time step size for continuity convergence is order of e^10-6, and flow time is about an hour, so i can't simulate it because of very high computational expense.

Are there a strategy to enhance time step size?

thank you for answers.
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Old   June 7, 2016, 16:04
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Can you describe how you arrived at your time step and why the total simulation time needs to be one hour?
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Old   June 7, 2016, 17:34
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Originally Posted by MBdonCFD View Post
Can you describe how you arrived at your time step and why the total simulation time needs to be one hour?
I use variable time step with courant number=2, time step automatically change to near 10^-6. i simulate a big tank fill process that have about one hour filling time.
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Old   June 7, 2016, 17:50
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Filippo Maria Denaro
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Originally Posted by farazmardazad View Post
I use variable time step with courant number=2, time step automatically change to near 10^-6. i simulate a big tank fill process that have about one hour filling time.

what about the physical dimensions of the tank, the size of the computational domain and the velocity?
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Old   June 7, 2016, 18:03
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Originally Posted by FMDenaro View Post
what about the physical dimensions of the tank, the size of the computational domain and the velocity?
volume of tank =5 cubic meter
number of mesh = 170000, but it's low
mass flow rate = 0.15 kg/s.
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Old   June 7, 2016, 18:27
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Originally Posted by farazmardazad View Post
volume of tank =5 cubic meter
number of mesh = 170000, but it's low
mass flow rate = 0.15 kg/s.

well, consider that in each direction ci = ui dt/ hi but, depending on the scheme you are using, the numerical stability does not depend only on the courant number but is it a combination with the diffusive constraint.
Are you using an explicit time integration?
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Old   June 7, 2016, 18:53
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Originally Posted by FMDenaro View Post
well, consider that in each direction ci = ui dt/ hi but, depending on the scheme you are using, the numerical stability does not depend only on the courant number but is it a combination with the diffusive constraint.
Are you using an explicit time integration?
No, i am using implicit formulation, because of pressure-based solver.
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Old   June 8, 2016, 03:06
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Originally Posted by farazmardazad View Post
No, i am using implicit formulation, because of pressure-based solver.
that should allow higher time step.... I suggest to check during the solution to write the values of max(ci) and max(alphai), this latter being the diffusive stability parameter.

But with your small time step the solution is physically acceptable after a certain amount of steps? I always suspect a bug in the solver.
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Old   June 8, 2016, 06:45
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It is not clear to me if this is a commercial code, community code, or something you've developed. The time step will be determined by the smallest cell in the mesh. If you have a inlet tube and some transition cells feeding into the tank or, if you have applied boundary layer stretching near walls, some of those cells could have a very small length scale with a typical velocity value, and that makes the CFL number time step very small.
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Old   June 8, 2016, 07:08
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Originally Posted by mprinkey View Post
It is not clear to me if this is a commercial code, community code, or something you've developed. The time step will be determined by the smallest cell in the mesh. If you have a inlet tube and some transition cells feeding into the tank or, if you have applied boundary layer stretching near walls, some of those cells could have a very small length scale with a typical velocity value, and that makes the CFL number time step very small.

I agree ... but he wrote of being using an implicit formulation and that makes me having some doubts...for that reason I suggested to write the resulting max cfl in the computation
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