Learning Forum

[Lennart]

Hello everyone,

I have a problem with the Design Point function in the Ansys Workbench. I'd like to simulate a 2D pipe flow with various mass-flows at the inlet. I used the Design Points for calculating various mass-flows. I set the inlet pressure as an output parameter, but when I add more Design Point input values (mass-flow) and compute, the output value of my inlet pressure does not change.

[Keyur Kanade]

Please check the pressure drop.
Regards Keyur
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[Lennart]

What do you mean with checking the pressure drop? Where can I do this?

[Rob]

Open the design point up and have a look at the flow field. What is the outflow condition?

[Lennart]

I hope you meant this window. Outflow condition is a constant gauge pressure of 130000 Pa

[Amine Ben Hadj Ali]

Ensure that every design point is starting from the Initialization you are taking and not starting from precious dp or cached data. In the "Solution" Cell in the WB, if you show the properties you will be able to change that. If you click on parameter Set and show the properties please let every DP to start / initiate from "Current".

I have a feeling that your solution is not deeply converged. For that reason if you now select one DP make it actual and run it: Which value is computed? Judge convergence of it.

[cfdasdf]

Is it possible for each DP to initialize itself based on its respective flow parameters?I have Design Points with the same geometry but sometimes very different flow parameters. Updating all Design Points ("Update Selected Design Points") yields in many diverged solutions. Running each Design Point on its own (starting the Fluent GUI and initializing it manually) yields converged solutions without any problems...

 

Thanks in advance for your reply!

[cfdasdf]

@DrAmine: do you have an answer to my question?

[Lennart]

Ok under which property do I change the initialization in "Solution"?
When I select one DP and click on Solution to run it and then calculate it, the calculation stops at 30 iterations, fluent closes and there's a message: "Collection has been changed. Enumeration operation may not be executed"
Where's the problem?

[Amine Ben Hadj Ali]

First of all: forget about your DP's. Just focus on one particular set of operating conditions, disable Convergence Check by Residual and run for a say 1000 iterations. Paste the Residual plot here.

[Lennart]

Ok, I deleted the Design Points and set a constant input mass-flow and output gauge pressure.
Viscous = Laminar
Check Convergence = all three hooks off
Method = SIMPLE
Initialization -> Initial Values -> Gauge Pressure = 130000 Pa (because my output gauge pressure is 130000Pa)

[Amine Ben Hadj Ali]

Why you do not set an operating pressure equal to 130000 Pa and just use gauge pressure at outlet set to 0 Pa: that is best to avoid round off errors and very important for a couple of other cases.
Now if you compute the area average of Absolute pressure at Inlet what do you get?

[Lennart]

I'm an absolute beginner in fluent, so I didn't know such things sorry :D
To compute the absolute pressure at inlet, I set the Area-Average Pressure at inlet as an output parameter. In the Workbench parameters there's p_inlet = 2,3133E+05 Pa.

[Amine Ben Hadj Ali]

If you are beginner: I recommend that you start first of all doing a couple of tutorials and perhaps attend one of the self paced trainings on the Ansys Learning Hub or check any other resources you might have access to.
To set the output parameter correct: Under Report Definition create a surface integral->Area-Average of abolsute pressure at inlet and tick "Create Output Parameter".
So now correct the operating pressure in Fluent, in the Workbench under View tick "Properties". Now if you click on Parameter set, tell WB to initiate every DP from current. Do not forget in Fluent to enable convergence by residual again (please enable local scaling under Scale), under Initialization please select "Hybrid" and let it go.

[Lennart]

Maybe I approached my project wrong. To better understand how I come to this problem, I explain it:
I have a laminar 2D-pipe flow with a constant mass-flow at the inlet and a constant pressure outlet. I'd like to get to know what inlet pressure I'll get, when I change my mass-flow values. I think the easiest way is to do it with Design Points and then set the inlet mass-flow as an input parameter and the inlet pressure as an output parameter. Then I change the mass-flow values in the Design Points.
Do you think that works with this method?

[Rob]

That works, and given the above I'd turn off convergence criterion and set 500-1000 iterations for each run. That avoids any issues if Fluent thinks it's converged before it's really started the run.

[Lennart]

Ok and do I have to click on the retain data checkbox? The strange thing now is, that the inlet pressure for 10 kg/s is lower than the pressure for 1 kg/s :D where's the problem? Or is it important which DP is selected to get logical values?

[Rob]

Only if you want to retain the data. Otherwise Fluent will run, output the parameters and not retain the results. I suspect you want to retain data for further analysis but it's for you to decide.
There's not much of a pressure drop change in the system at all so check convergence and what the flow is doing.

[Lennart]

How can I check convergence? Is it reached only if Fluent says it's reached (when you enabled check convergence) or do I see this with the residuals? Is it converged when the residuals are constant?

[Rob]

Open the model and have a look at the residuals, fluxes and monitors. If you're not sure how to check convergence read through (and do) a few of the tutorials as they cover the basics.

[Lennart]

Where do I find the basic tutorials? I have only the student version cause I'm a student.

[Rob]

Click on Help: we bundle some with the documentation.