# strategy to enhance of time steps

 Register Blogs Members List Search Today's Posts Mark Forums Read

 June 6, 2016, 03:27 strategy to enhance of time steps #1 New Member   faraz Join Date: Nov 2015 Posts: 8 Rep Power: 2 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.

 June 7, 2016, 16:04 #2 Senior Member   Matt Join Date: Aug 2014 Posts: 536 Rep Power: 6 Can you describe how you arrived at your time step and why the total simulation time needs to be one hour?

June 7, 2016, 17:34
#3
New Member

faraz
Join Date: Nov 2015
Posts: 8
Rep Power: 2
Quote:
 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.

June 7, 2016, 17:50
#4
Senior Member

Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,354
Rep Power: 28
Quote:
 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?

June 7, 2016, 18:03
#5
New Member

faraz
Join Date: Nov 2015
Posts: 8
Rep Power: 2
Quote:
 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.

June 7, 2016, 18:27
#6
Senior Member

Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,354
Rep Power: 28
Quote:
 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?

June 7, 2016, 18:53
#7
New Member

faraz
Join Date: Nov 2015
Posts: 8
Rep Power: 2
Quote:
 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.

June 8, 2016, 03:06
#8
Senior Member

Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,354
Rep Power: 28
Quote:
 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.

 June 8, 2016, 06:45 #9 Senior Member   Michael Prinkey Join Date: Mar 2009 Location: Pittsburgh PA Posts: 253 Rep Power: 15 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.

June 8, 2016, 07:08
#10
Senior Member

Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,354
Rep Power: 28
Quote:
 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

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post CKH OpenFOAM Running, Solving & CFD 12 March 21, 2016 14:05 lakeat OpenFOAM Mesh Utilities 57 February 1, 2015 09:25 ernarasimman STAR-CD 2 September 12, 2014 00:01 danny123 OpenFOAM Programming & Development 8 September 6, 2013 02:34 Artex85 OpenFOAM Running, Solving & CFD 9 January 3, 2012 09:06

All times are GMT -4. The time now is 18:07.