## Fluids

#### How to get system convergence

• trippleD
Subscriber

Hello everybody,

I'm doing a simulation of the flow in an annular gap. The inner cylinder is modelled as a rotating wall and the outer one is a stationary wall. The left and right side are stationary walls as well. All in all, it is a closed system. I'm using the realizable k-epsilon model with enhanced wall treatment.

For the convergence, the resiudals should fall by a magnitude of 10^-3 respectively 10^-6. I do a steady state simulation with 2000 iterations. But i don't reach the values to say, that my system is converged (please see the attached picture).

What can i do for reaching a converging system?

CFD_Simulation_model

• DrAmine
Ansys Employee

Please use coupled solver with pseudo-transient. We do not look into attachments as ANSYS Staff.

• trippleD
Subscriber

Thanks for your answer. I will try it. The options for Pseudo Transient Options stay Default?

Okay, i didn"t know that. But here it is explained how to add a project. For what reason?

• DrAmine
Ansys Employee

that applies for non ANSYS Staff: other community members. If you want ANSYS Staff to help you need to add description and screenshots.

• trippleD
Subscriber

Ok. I have an additional question. Why do you reommend the coupled solver with the Pseudo Transient?

• DrAmine
Ansys Employee

That is written in the documentation: Momentum equation + pressure-velocity corrections are done in coupled fashion. It converges quicker and uses an additional implicit under relaxation factor for all cells by using low order time marching approach to solve steady state problem.

• trippleD
Subscriber

I've just perfomed round about 400 iterations.

I there a way to accelerate the convergence behaviour.When looking at the picture it will take forever to reach convergence.

• DrAmine
Ansys Employee

Run for more iterations and we will see. You can start by using fixed time scale corresponding to 0.1/angular velocity or 0.01/angular velocity

• trippleD
Subscriber

I have done a few more iterations as you can see in picture below:

So can i say that the system is converged. The simulation was running with a timescale factor of 1. Or should i try a new simulation with the recommendation from you? What did you mean exactly. Time Scale factor = 0.1 or = (0.1/agular velocity)

• DrAmine
Ansys Employee

To judge convergence you better monitor some key variables or integral quantities as well as check the imbalances under flux report. If all are good then you can say that it is converged.

• trippleD
Subscriber

Okay, i check some variables and they look like they make sense.

It is a close system, so i think the zeros are good an the value for interior fluid is the mass flow due to rotation. The inner cylinder is rotating with 52,36 rad per second. the inner radius is 0,025m. The maximal velocity is then 1,309m/s. So the maximum velocity i get from the reports is:

Is it normal?

So do you think the system is converged? I want to hear your opinion.

Thanks again, you are helping me a lot.

• DrAmine
Ansys Employee

You need to monitor the values while the solution evolves: Say like volume average of (tangential) velocity or velocity at certain point (or temperature or wall shear stress or torque or anything relevant for your applicaiton).

You need to build up your opinion. I am helping you to do that.

• trippleD
Subscriber

Okay, so i have to do the simulation again and monitoring the values you have mentioned?

• DrAmine
Ansys Employee

Yes.

• DrAmine
Ansys Employee

Or continue running after creating the required reports.

• trippleD
Subscriber

Okay, i have done the report for the tangential velocity and start the simulation. I will let you know when it is finished.

During the meantim. Let"s say the system is converged and i start my simulation with the data of the converged system. How many iteration do i need than for getting good results? Or is the simulation finished when the system is converged?

• DrAmine
Ansys Employee

You define when the run finishes. The software just finishes run to the condition you set: Residual tolerance, monitor convergence or whenever you hit stop. So that is you who needs to care about all that.

I recommend doing some standard tutorials.

• trippleD
Subscriber

I think i was misunderstood. but now it is clear. I have done some tutorials, but the cfd is such a big topic

• trippleD
Subscriber

I continue the simulation. It is still running, but i want to give you an intermediate status.

As you can see, the volume average of tangential velocity isn't changing. So i think, the system is converged, right?

• DrAmine
Ansys Employee

You need to make your judgment on your own. IMHO the profile looks okay. Check if you have any mass imbalances in your domain. Report mass of the whole volume and examine its evolution during iteration.

• trippleD
Subscriber

I do the judgement on my own, but i want to double check it with someone who has experience with it. Okay. so all in all i can say, if the values aren't fluctuating anymore, the system is converged.

Is there a possibility to export the values of the residuals ?

• DrAmine
Ansys Employee

You want to export them into file or you want to visualize them as contour variable?

• trippleD
Subscriber

It would be the best to export them into file to read them in Matlab

• DrAmine
Ansys Employee
You require an UDF for that. Alternative is to store the non scaled residuals and export to ASCII. Scaled residuals as workaround can be exported if you write transcript of Fluent run. You need later to remove all unnecessary lines.
• trippleD
Subscriber

Can you explain it more in detail how it works (export the scaled resiudals)? Thanks a lot.

• DrAmine
Ansys Employee

• trippleD
Subscriber

Thanks a lot Amine, that works very well

Because the simulations are very time consuming, is there way to extract the from resiudals from saved simulation which are already done?

• DrAmine
Ansys Employee
As i write earlier you require an UDF for that. Alternative is to store the non scaled residuals and export to ASCII: I would go for that. Scaled residuals as workaround can be exported if you write transcript of Fluent run. You need later to remove all unnecessary.
• trippleD
Subscriber

Can you show me both? The UDF and the export of the non scaled residuals.

• DrAmine
Ansys Employee

UDF: Sorry I cannot show.

To export non scaled residual you first enable them to be stored:

In the the Fluent console you type /solve/set/expert and then when it is asked for Save residuals for post-processing you confirm with yes. You ran one single iteration and you will the non-sacled residuals available. Afterwards you go (GUI) File>Export you select type ASCII and then the residual variables.

• DrAmine
Ansys Employee

You can alternatively go unde menu plot (TUI)>residuals-set then plot-to-file. Afterwards you ran whatever iterations you want

then you under menu plot type res and then answer with yes for all equations.

• trippleD
Subscriber

You can alternatively go unde menu plot (TUI)>residuals-set then plot-to-file. Afterwards you ran whatever iterations you want

then you under menu plot type res and then answer with yes for all equations.

I don't have this possibility. With TUI, you mean Tree User Interface,right? Where do you mean exactly ?

• DrAmine
Ansys Employee

Text User Interface in the Fluent Console not the Tree. You need definitely to make some more exercise with Fluent.

• trippleD
Subscriber

If you give me a short expression i've never seen before, it's completly normal that you don"t know what it should mean.

Can you give me the exact commands. I always get an invalid command error.

You can alternatively go unde menu plot (TUI)>residuals-set then plot-to-file. Afterwards you ran whatever iterations you want

then you under menu plot type res and then answer with yes for all equations.

• DrAmine
Ansys Employee

Check Fluent Text Command List Manual.

• Rob
Ansys Employee

Click in the text window in Fluent and press the key. You'll see some options, type in "display" and hit twice: first to complete the command and second to display the new menu list. Continue to see what happens.  To move up a menu level use "q".

An example command would be (and assuming the image is available and surfaces set)

/display/save-picture my-image.jpg

• trippleD
Subscriber

Okay, thanks a lot for your really good help. I have learned a lot

• DrAmine
Ansys Employee

Please mark as "Is Solved" so other might learn too if they have same questions!