[reza_gharib1369]

hi all,
i have a problem. when i use vof model, i must set courant number in this model. But this error is displayed.
primitive error @ node 2: global courant number is greater than 250.0. the velocity field is probably diverging.please check the solution and reduce the time-step if necessary.
Meanwhile time step=0.01,what can i do to solve this error?

[jamalf64]

Originally Posted by reza_gharib1369

hi all,
i have a problem. when i use vof model, i must set courant number in this model. But this error is displayed.
primitive error @ node 2: global courant number is greater than 250.0. the velocity field is probably diverging.please check the solution and reduce the time-step if necessary.
Meanwhile time step=0.01,what can i do to solve this error?

You must select a smaller time step. For example I use time step=0.000001 in my problem

[reza_gharib1369]

thank you for your answer

[stefenbink]

hi, in my case, I want to fill the tub until it is full with water in 2D. if I use time step size low = 0,1; number of time step = 100; auto save every(time step) = 10

i need time until 60 s, until the tub is full but in interation (before the tub is full) i got error message in 2 s, . the tub is not full yet

what things should i change?


ps: error "Global courant number is greater than 250.00 The
velocity field is probably diverging. Please check the solution
and reduce the time-step if necessary."


thanks alot, if you can help me, this now.

Attachment 27667

[jamalf64]

You must reduce time step,too. trust me

[stefenbink]

Originally Posted by jamalf64

You must reduce time step,too. trust me

thanks,, i have tried and it worked.
btw
is it can solve Divergence detected in AMG solver: pressure correction or x-momentum too? may calculation is end with divergence lately

[jamalf64]

Originally Posted by stefenbink

thanks,, i have tried and it worked.
btw
is it can solve Divergence detected in AMG solver: pressure correction or x-momentum too? may calculation is end with divergence lately

Sorry, I dont know

[enass massoud]

Mesh Requirements: Create uniform mesh. In regions where the mesh is refined, ensure that there is a gradual transition to the coarser mesh. Avoid sudden changes in cell size. The maximum skewness of the volume mesh should be less than 0.95 and maximum aspect ratio of tetrahedral cells should be less than 5. In compressible phase calculations, use of non-conformal interfaces can leads to solution instability and divergence. You should avoid non-conformal interfaces in the region of liquid-air interfaces. This is one limitation of VOF with compressible calculations. This limitation becomes magnified when you use MDM ( Explicit mesh update) with explicit VOF.

Phase: Use compressible phase as primary phase.

Viscous model: Check the Reynolds number and use Turbulence model if needed.

Specified Operating density: Switch on the specified operating density and specify the density of lightest phase.

Implicit body force: Turn on if dificulty in convergence. You should turn off when surface tension force is important and with small body forces.

P-V Coupling: Use SIMPLEC/SIMPLE

Spacial Discretization: Least Spquare Cell Based/ Green Gauss Cell Based

URF: Use small values. Pressure-0.2, Momentum- 0.3, Turbulent kinetic energy- 0.5, Turbulent dissipation rate – 0.5.

Use this command for better patching: (rpsetvar ‘patch/vof ? #t)

If you face divergence at the beginning of the simulation, start the simulation with very small time step size, and increase after a few time steps if Global courant number is under control. The global courant number is printed in the Fluent console window ( with explicit VOF) at every time step.

The Global courant number depends on the mesh size, velocity field, and the time step size used for the transport equations. If CFL exceeds 2 and keeps on increasing, that means your velocity field is increasing or/and the interface is moving through dense cells, and the time step size used is too high. You need to reduce the time step size to bring the Global courant number under control.

For VOF calculations (using the Explicit scheme), FLUENT allows you to use variable time stepping in order to automatically change the time-step when an interface is moving through dense cells or if the interface velocity is high. If there are frequent velocity jumps in your problem, it is better to use the variable time stepping method to control the CFL under limit. The solution will be stable with the variable time stepping method.

If you use the fixed time step, the CFL may exceed the value 2 whenever there is a velocity jump or when the interface is moving through dense cells, and your results will be time step size dependent. If you continue with the same time step size, the results will not be accurate, and this may even lead to divergence. It is better to use variable time stepping method for this type of problems and for compressible VOF calculations.

Variable time stepping method: Here the input will be CFL. The global courant number is constant and the time step size varies with the velocity field. You should give appropriate value for Global courant number (CFL). Because, the time step size for transport equations are calculated from this CFL. You need to specify the Global courant number, minimum time step size, maximum time step size, minimum step change factor and maximum step change factor.

Global courant number: The default value of the Global Courant number is 2, but smaller value may be required for more accurate solution and more stable numerical calculation. In some cases, you need to reduce this up to 0.5 for accurate results and . This is because the time step size (so, the CFL) should be small enough to get the accurate results. In some cases you may use CFL greater than 2 depending on the problem.

Maximum Time step size: minimum grid size / maximum velocity in the domain

Minimum Time step size: It should be greater than 1e-10. You cannot use time step size less than 1e-10. This is the limitation of VOF Explicit scheme.

Minimum step change factor: The default value is 0.5.

Maximum step change factor: The default value is 5. It is better to reduce this value to 1.5-2 to avoid the sudden increase in time step size.

If the Explicit VOF with variable time stepping does not work, try the Implicit VOF scheme with Bounded Second Order Transient scheme.

If still there is a divergence, check your mesh quality, boundary conditions and physics of the problem

[thasneem]

Hello,
In my analysis the fluid has to move against the gravity on heating in a Pulsating heat pipe(PHP) from the evaporator section in the bottom to the condenser in the top(of PHP).The movement of the fluid starts after a good 10s.But before it could reach 10s it shows courant no. error.Is it possible to give a larger step size say 10s initially and then reduce the time steps (Variable time stepping method) so as to get the lead?

[soumitra2102]

Thank You for detailed explanation

[hellaadouni]

Hey M.ennas,

I want it to know why you think it is important in this case using "operating density".
Is it because the fluid is compressible?

Im orking with air water flow and both are considered incompressible, im not using the "operation density" Is it wrong?

[hitzhwan]

Originally Posted by jamalf64

You must select a smaller time step. For example I use time step=0.000001 in my problem

But it will take a long time to calculate it .

[hitzhwan]

Originally Posted by enass massoud

Mesh Requirements: Create uniform mesh. In regions where the mesh is refined, ensure that there is a gradual transition to the coarser mesh. Avoid sudden changes in cell size. The maximum skewness of the volume mesh should be less than 0.95 and maximum aspect ratio of tetrahedral cells should be less than 5. In compressible phase calculations, use of non-conformal interfaces can leads to solution instability and divergence. You should avoid non-conformal interfaces in the region of liquid-air interfaces. This is one limitation of VOF with compressible calculations. This limitation becomes magnified when you use MDM ( Explicit mesh update) with explicit VOF.

Phase: Use compressible phase as primary phase.

Viscous model: Check the Reynolds number and use Turbulence model if needed.

Specified Operating density: Switch on the specified operating density and specify the density of lightest phase.

Implicit body force: Turn on if dificulty in convergence. You should turn off when surface tension force is important and with small body forces.

P-V Coupling: Use SIMPLEC/SIMPLE

Spacial Discretization: Least Spquare Cell Based/ Green Gauss Cell Based

URF: Use small values. Pressure-0.2, Momentum- 0.3, Turbulent kinetic energy- 0.5, Turbulent dissipation rate – 0.5.

Use this command for better patching: (rpsetvar ‘patch/vof ? #t)

If you face divergence at the beginning of the simulation, start the simulation with very small time step size, and increase after a few time steps if Global courant number is under control. The global courant number is printed in the Fluent console window ( with explicit VOF) at every time step.

The Global courant number depends on the mesh size, velocity field, and the time step size used for the transport equations. If CFL exceeds 2 and keeps on increasing, that means your velocity field is increasing or/and the interface is moving through dense cells, and the time step size used is too high. You need to reduce the time step size to bring the Global courant number under control.

For VOF calculations (using the Explicit scheme), FLUENT allows you to use variable time stepping in order to automatically change the time-step when an interface is moving through dense cells or if the interface velocity is high. If there are frequent velocity jumps in your problem, it is better to use the variable time stepping method to control the CFL under limit. The solution will be stable with the variable time stepping method.

If you use the fixed time step, the CFL may exceed the value 2 whenever there is a velocity jump or when the interface is moving through dense cells, and your results will be time step size dependent. If you continue with the same time step size, the results will not be accurate, and this may even lead to divergence. It is better to use variable time stepping method for this type of problems and for compressible VOF calculations.

Variable time stepping method: Here the input will be CFL. The global courant number is constant and the time step size varies with the velocity field. You should give appropriate value for Global courant number (CFL). Because, the time step size for transport equations are calculated from this CFL. You need to specify the Global courant number, minimum time step size, maximum time step size, minimum step change factor and maximum step change factor.

Global courant number: The default value of the Global Courant number is 2, but smaller value may be required for more accurate solution and more stable numerical calculation. In some cases, you need to reduce this up to 0.5 for accurate results and . This is because the time step size (so, the CFL) should be small enough to get the accurate results. In some cases you may use CFL greater than 2 depending on the problem.

Maximum Time step size: minimum grid size / maximum velocity in the domain

Minimum Time step size: It should be greater than 1e-10. You cannot use time step size less than 1e-10. This is the limitation of VOF Explicit scheme.

Minimum step change factor: The default value is 0.5.

Maximum step change factor: The default value is 5. It is better to reduce this value to 1.5-2 to avoid the sudden increase in time step size.

If the Explicit VOF with variable time stepping does not work, try the Implicit VOF scheme with Bounded Second Order Transient scheme.

If still there is a divergence, check your mesh quality, boundary conditions and physics of the problem

Thank you. Can you tell me how to set the "Bounded Second Order Transient scheme"?

[hitzhwan]

I find that when I tried it, it given the errors belew:
Warning: Second order time discretization not available with explicit VOF.
Switching to first order time discretization.

[hitzhwan]

Originally Posted by hitzhwan

Thank you. Can you tell me how to set the "Bounded Second Order Transient scheme"?

I find that when I tried it, it given the errors belew:
Warning: Second order time discretization not available with explicit VOF.
Switching to first order time discretization.

[vinerm]

Second order time-discretization scheme, either pure 2nd order or bounded one, are not available for Explicit VOF. This is because 2nd order requires a larger stencil of time, i.e., n-1, n, and n+1, which is not available while using explicit VOF (data at time n-1 is unavailable).

If mesh is good and time-step is small enough, first-order accuracy is as good as second-order.

[hitzhwan]

Originally Posted by vinerm

Second order time-discretization scheme, either pure 2nd order or bounded one, are not available for Explicit VOF. This is because 2nd order requires a larger stencil of time, i.e., n-1, n, and n+1, which is not available while using explicit VOF (data at time n-1 is unavailable).

If mesh is good and time-step is small enough, first-order accuracy is as good as second-order.

But I see many papers use the second-order discretization scheme in the explicit VOF

[vinerm]

Spatial discretization can make use of second-order or even higher-order. However, time-discretization does not have data for higher-order.

[hitzhwan]

Originally Posted by vinerm

Spatial discretization can make use of second-order or even higher-order. However, time-discretization does not have data for higher-order.

Now I am doing some simulation on the falling film. If I choose the variable timestep, and set the fixed courant number =0.5, the real timestep will decrease to 1e-6, which will take a long time to calculate one case, which I cannot bear. But if I use a fixed timestep=1e-5, the continuity Residuals cannot decrease below 1e-3, which will make the results incorrect, and even divergence. I have tried many ways, such as changing the boundary condition and mesh model, but it does not work. Can you help me deal with it?

[hitzhwan]

Originally Posted by jamalf64

You must select a smaller time step. For example I use time step=0.000001 in my problem

Your timestep is too small, how long does it take to calculate a case?