[amitray]

Friends I am new to Fluent and I am doing a transient simulation in Fluent. I just need a small help. I want to know how we can ensure that at each time step the simulation converges. I mean as manually I can check that at certain time step the solution converged but suppose there are 2000 time steps and it is not possible to check convergence at each time step manually, so is there any method which can give me the data that at certain time step the solution didn't converge.

[CeesH]

Hi Amit,

You can set convergence criteria (on velocity, temperature, ..., and the residuals of course). If these criteria are met, the simulation will jump to the next timestep. So, set the criteria such that you are sure the relevant parameters are sufficiently converged inside a timestep, and choose the number of iterations per timestep such that the convergence criteria are virtually always met before the number of iterations criterion is fullfilled. That should ensure you are safe.

[amitray]

Originally Posted by CeesH

Hi Amit,

You can set convergence criteria (on velocity, temperature, ..., and the residuals of course). If these criteria are met, the simulation will jump to the next timestep. So, set the criteria such that you are sure the relevant parameters are sufficiently converged inside a timestep, and choose the number of iterations per timestep such that the convergence criteria are virtually always met before the number of iterations criterion is fullfilled. That should ensure you are safe.

Thanks for the reply. I have already set the criteria and have set number of iterations per time step to 200. Initial few iterations it got converged in between 150 to 180 iterations but in between some steps it didn't converge. So I wanted to know if there is any inbuilt method or an UDF that can tell exactly which time step converged and which didn't.

[CeesH]

Hi Amit,

In any case I would reduce your timestep size a bit. Although i don't have the reference at hand, I do recall reading somewhere that transient simulations should target at converging in 20-50 iterations per step to ensure all features are captured (if someone disagrees or has the reference, please throw it in!)

On what basis do you conclude convergence by the way? I often set the targets quite strict, so that even when the criteria are not met, the situation is likely acceptable. Of course it should be possible to write a UDF; the macro N_ITER gives the iteration number so if you export that at the end of each timestep, you know the number of iterations per timestep.

But, you can also simply export a monitor each iteration rather than each timestep (for example, average velocity). You can easily see if that converges per timestep then, in excel, matlab, etc.

[amitray]

Originally Posted by CeesH

Hi Amit,

In any case I would reduce your timestep size a bit. Although i don't have the reference at hand, I do recall reading somewhere that transient simulations should target at converging in 20-50 iterations per step to ensure all features are captured (if someone disagrees or has the reference, please throw it in!)

On what basis do you conclude convergence by the way? I often set the targets quite strict, so that even when the criteria are not met, the situation is likely acceptable. Of course it should be possible to write a UDF; the macro N_ITER gives the iteration number so if you export that at the end of each timestep, you know the number of iterations per timestep.

But, you can also simply export a monitor each iteration rather than each timestep (for example, average velocity). You can easily see if that converges per timestep then, in excel, matlab, etc.

Thanks again. Actually I have set convergence criteria for all residuals to 0.001. I set a time step of 0.001s which I think will capture all details. And why I took 200 iterations because I felt initial iterations will converge in more iterations and later it may converge in less number of iterations. That was my idea as the project involves two phases water and air and is a bit complex. I may also have made a mistake in boundary conditions. By the way thanks for the time you gave. I will do some detailing and then retry the problem.

[Ami24]

Hello,

To have convergence between each time step, you have to switch off convergence criteria in the monitors section as shown in the picture.