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-   -   Pressure result wrong!!!!!!! (http://www.cfd-online.com/Forums/fluent/128326-pressure-result-wrong.html)

lehoanganh07 January 9, 2014 05:37

Pressure result wrong!!!!!!!
 
Dear fluent user,
I am modeling cavitating flow in 3D Nozzle with inlet and outlet pressure boundary condition (total pressure inlet: 164650 Pa), but when I check result the static pressure inlet is higher than total pressure. Also the velocity at the inlet Nozzle hole near the cavity reach to 2000 m/s
Any one can solve this problem?
solution method: pressure-velocity: coupling
- gradient: least gauss cell base
- pressure: PRETO!
- all other: QUICK

bharath January 9, 2014 07:13

Hi,

My suggestion is,

1.Try 1st order advection scheme first and converge it fully then check your results.If it is Ok then proceed 2nd order.

2.If you are modelling compressible flow check your initialization gauge pressure at inlet.

3.Give some pictures for BC and Output Results to better understading.

lehoanganh07 January 9, 2014 11:03

Thank you, brahath
Actually, the result I said was nit the final result, it just the result I checked when the program still working.
Also, I read in the Fluent guid, the total pressure inlet will be set as static pressure if have backfliw at inlet and outlet pressure will be not kept as constant, it will be calculated by extrapolartion if flow become supersonic.
So I think it is the reason why I got my reasult like that because the size of Nozzle is small 4 mm, and avarage velocity at inlet is 10m/s
I just cheched the result again, and static pressure at inlet now less than total pressure.
But I still dont understand why when have backflow at inlet, total pressure set as static pressure.

bharath January 10, 2014 07:14

Dear lehoangan,
Generally Backflow at inlet is not advisable.you have to extend your inlet zone to some extent and try to run it again then check your output.

"I read in the Fluent guid, the total pressure inlet will be set as static pressure if have backfliw at inlet"

Could you tell me where this is written in the manual?

Thank you

lehoanganh07 January 10, 2014 09:20

Quote:

Originally Posted by bharath (Post 469431)
Dear lehoangan,
Generally Backflow at inlet is not advisable.you have to extend your inlet zone to some extent and try to run it again then check your output.

"I read in the Fluent guid, the total pressure inlet will be set as static pressure if have backfliw at inlet"

Could you tell me where this is written in the manual?

Thank you

Dear bharath,
For the detail, please type " fluent boundary condition" you may be found it. Also you can type " fluent boundary lecture note" which I found some lecture notes from University about set boundary condition. And also I downloaded them to my PC.
Sorry but right now I am not in my Lab.
Also I think have back flow may be you correct. In my model have change size of gometry right before entran to Nozzle hole so it might be reason.


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