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Active Member
ali.moslemi
Posts: 7
Registered: ‎10-12-2012
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Pressure results accuracy

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10-12-2012 01:20 PM

I am modeling a drill bit with several nozzles with specific size. The estimated pressure drop for these nozzles is about 600 psi according to available analytical equation. However, the computed pressure drop is about 900 psi using Autodesk Simulation CFD 2013 . I ran the same model with aaproximately equal number of elemnts with Fluent and the pressure drop is close to 600 psi. Can somebody comment about the accuracy of results in Autodesk Simulation CFD 2013?

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wildej
Posts: 782
Registered: ‎08-25-2011

Re: Pressure results accuracy

10-14-2012 01:03 PM in reply to: ali.moslemi

Hi Ali,

 

This is likely down to a few possible factors:

 

The mesh is not sufficient (unlikely as it sounds like you know what you are doing)

The model is compressible but you are running incompressible (what are the flow speeds?)

You need to use a different advection scheme or maybe material model.

 

Are you able to share a cfz file?



Jon Wilde
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Active Member
akarlsso
Posts: 7
Registered: ‎08-30-2011

Re: Pressure results accuracy

10-15-2012 05:47 AM in reply to: ali.moslemi

I have struggled with this as well but got great help from the support team (Marco Mueller).

 

What could help you is to set boundary conditions as pressure to pressure (dont forget to switch to Adv 2)

 

Disable automatic layer adaption, manually reduce layers and montor one of the wall model varables , Y+.

It should be within 35 to 350. At least in the areas where most pressure drop occurs

 

In my case reducing to only one layer and layer factor to 0,2 took me within 1,5% compared to later lab tests (oil with a pressure drop of approx 1200 psi).

Of course the normal mesh refinement was done too but didnt take me anywhere near these results

 

Do a search for "Y+" in the help files and read all sections, that will explain this far better than I can

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Active Member
ali.moslemi
Posts: 7
Registered: ‎10-12-2012

Re: Pressure results accuracy

10-15-2012 08:50 AM in reply to: akarlsso

Thanks akarlsso for your response. I used Adv5 which is improved version of Adv2 but I am re-running my model with Adv2 to see if I get any different result. My inlet boundary condition is constant flow rate and the outlet is gauge(0) pressure. I don't know about "pressure to pressure" boundary condition and am not sure how to appy that, can you explain more about that?

Regarding reducing the mesh layers in bondary layer, having more layers should increase the accuracy of flow prediction. How does reducing the mesh layer can help it? I know the same thing has been suggested in the manual for wall model function but I am just curious how? 

Thanks

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Active Member
ali.moslemi
Posts: 7
Registered: ‎10-12-2012

Re: Pressure results accuracy

10-15-2012 09:12 AM in reply to: wildej

Hi Jon,

The mesh is very fine near the nozzles so I don't think the mesh refinement can solve this issue. Plus, I ran the same model with some extended nozzles and the pressure drop was about 700 psi. The nozzle extension should not affect the pressure drop that much.

I use water as working fluid and the model is incompressible. I tried to  share the cfz file here but it said it's too large. Its size is about 13 Mb..

Thanks

Ali

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Active Member
ali.moslemi
Posts: 7
Registered: ‎10-12-2012

Re: Pressure results accuracy

10-15-2012 12:30 PM in reply to: akarlsso

I ran my model with Adv2 and I even got higher pressure drop(1100 psi). I reduced the layers to one and layer factor to 0.2 and the result got even worse! now the pressure drop is 1173 psi. I am really lost....

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Product Support
wildej
Posts: 782
Registered: ‎08-25-2011

Re: Pressure results accuracy

10-16-2012 12:57 AM in reply to: ali.moslemi

I am happy to take a look if we can find a way to share the file, Dropbox?



Jon Wilde
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Active Member
ali.moslemi
Posts: 7
Registered: ‎10-12-2012

Re: Pressure results accuracy

10-16-2012 01:59 PM in reply to: wildej

Jon,

I put the cfz file in the folder you sent me. Please let me know how it goes.

Thanks

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Product Support
wildej
Posts: 782
Registered: ‎08-25-2011

Re: Pressure results accuracy

10-17-2012 12:29 AM in reply to: ali.moslemi

Hi Ali, I have it, thanks.

 

I am going to move this to a proper case to work on, but we can report the findings here also once it is solved

 



Jon Wilde
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Active Member
ali.moslemi
Posts: 7
Registered: ‎10-12-2012

Re: Pressure results accuracy

10-23-2012 09:04 AM in reply to: ali.moslemi

Jon,

I increased the outlet pressure drop to 2000 psi according to Appolo and your recommendations. However, I found that I could fail the solver with just doing that! The pressure residuals started acting weird and became constant in some random intervals. Even, I noticed the pressure changed at the outlet boundary condition which should be fixed to 2000 psi. I would appreciate your comment.

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