Report Forces vs Report Definitions

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  • Last Post 12 April 2020
iakov1703 posted this 06 April 2020

Hi, 

My study is about analysing a rocket with ANSYS Fluent. I am trying to extract Drag Force. I get the value close to predicted through Report -> Forces after the simulation has completed. Usually it is around 600 N Total Drag. 
However, the Drag I compute through Report Definitions -> Drag Force and consequently I observe through Monitors -> Report Plots -> Drag Force is different, around 10'000 N. 
(see picture below)    In both cases I select the same axis and the same wall geometry. I would like to get the Report Definitions working, since then I would be able to see the convergence of drag force with iterations. But also I want both values to be the same, to be confident in the result.   Would you be able to help me with this, please? What could be the reason behind the discrepancy in the results?  

Some of my settings: 2D  Steady Pressure Far Field boundary condition was used with a Gauge Pressure of 86590.5 Pa, high-Mach number. SST model, 30<y+<100

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rwoolhou posted this 06 April 2020

Not sure from your description, please can you post the image? Staff are not permitted to open/download attachments. 

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iakov1703 posted this 06 April 2020

Not sure from your description, please can you post the image? Staff are not permitted to open/download attachments. 

 

I posted this, but please let me know if I should provide you with any other images. Thanks!

abenhadj posted this 06 April 2020

Share how the report definition is defined as well as the whole fore report.

Best regards, Amine

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iakov1703 posted this 06 April 2020

 

"rocket" is half of the rocket geometry, since i am using symmetry. 

Computing Drag Force Definition: 

 

 

While the Force Report:

abenhadj posted this 06 April 2020

Strange. Which version? In my current version I cannot reproduce the behavior.

Best regards, Amine

iakov1703 posted this 06 April 2020

Fluent R2

abenhadj posted this 06 April 2020

I assume 2019R2. right? 

Best regards, Amine

iakov1703 posted this 06 April 2020

Yes, sorry, 2019 R2

abenhadj posted this 06 April 2020

Can you check on your side on a newer version?

Best regards, Amine

iakov1703 posted this 06 April 2020

I can try, but I think I would have to completely rebuild the geometry, mesh, and the solver setup, since I will be using an older version of Fluent...

But generally these two ways of extracting drag force should give the same total drag, right?

iakov1703 posted this 06 April 2020

Sorry I am new here and I just learnt where the “reply” button is

abenhadj posted this 06 April 2020

Yes should be. If you have the latest version just start Fluent outside of the workbench read the case and data file run for one iteration and do the report.

Best regards, Amine

iakov1703 posted this 06 April 2020

Amine,

Sorry did not figure out how to do it. If I have my model in Fluent 2019 R2 and I want to open and test it in Fluent 19.1 (the only other version I have), I should export case & data from Fluent 2019 R2, and then open Fluent 19.1 and import the case and data files? I tried it now but it did not work.. 

Iakov

abenhadj posted this 06 April 2020

I was talking about a new version 2020R1. Reading case and data file in an older version would not always work and we do not give a warranty there. I need to remind here about versions from old to the newest ones:

19.0

19.1

19.2

2019R1

2019R2

2019R3

Current Version is 2020R1

Best regards, Amine

abenhadj posted this 06 April 2020

 

How to access the ANSYS Online Help

 

 

Best regards, Amine

iakov1703 posted this 06 April 2020

Amine, unfortunately I do not have the opportunity to download another version of Fluent now. But thank you very much for the explanation. 

I have just noticed that by performing a 3D stydt, the drag forces using both methods are equal. But I am still not sure why it is not the case for my 2D study. 

abenhadj posted this 06 April 2020

Hm. Strange. The case I am looking into a 2D case and here it is working fine!

Best regards, Amine

iakov1703 posted this 09 April 2020

Rwoolhou, do you maybe know why this could be happening? Any idea or thought could help me a lot. 

I rebuilt my model from scratch, without the axis of symmetry, meshed it differently, ran with different settings, and still I am getting very high and very wrong drag report definitions, while the drag force reports are acceptable.

I am really trying to get the definitions working since I have to run my simulation at many different design points; I set both force report and drag force definition as output parameters, but the definition is overwriting the force report value, not giving me any chances to use Design Points..

Cheers. 

iakov1703 posted this 12 April 2020

Hi,

Could it be because I was using the Planar setting rather than Axisymmetric setting ? 

If Axisymmetric revolves the sketch around the axis, what would fluent do with the Planar 'rocket'? Would it give it a tiny thickness instead of revolving?

Also, how do I select the axis around which I want the geometry to be revolved?

 

Thank you! 

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