Learning Forum

[iakov1703]

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

[Rob]

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

[iakov1703]

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!

[DrAmine]

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

[iakov1703]

 

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

Computing Drag Force Definition: 

 

 

While the Force Report:

[DrAmine]

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

[iakov1703]

Fluent R2

[DrAmine]

I assume 2019R2. right? 

[iakov1703]

Yes, sorry, 2019 R2

[DrAmine]

Can you check on your side on a newer version?

[iakov1703]

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]

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

[DrAmine]

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.

[iakov1703]

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

[DrAmine]

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

[DrAmine]

 

How to access the ANSYS Online Help

 

 

[iakov1703]

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. 

[DrAmine]

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

[iakov1703]

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]

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!