[farazmardazad]

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.

[fluid23]

Can you describe how you arrived at your time step and why the total simulation time needs to be one hour?

[farazmardazad]

Originally Posted by MBdonCFD

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.

[FMDenaro]

Originally Posted by farazmardazad

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?

[farazmardazad]

Originally Posted by FMDenaro

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.

[FMDenaro]

Originally Posted by farazmardazad

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?

[farazmardazad]

Originally Posted by FMDenaro

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.

[FMDenaro]

Originally Posted by farazmardazad

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.

[mprinkey]

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.

[FMDenaro]

Originally Posted by mprinkey

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