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-   -   avoiding negative absolute pressure!!! (http://www.cfd-online.com/Forums/fluent/33349-avoiding-negative-absolute-pressure.html)

Tania March 30, 2004 06:21

avoiding negative absolute pressure!!!
 
Hello,

I am having problems with negative absolute pressures. does anyone know which parameters should be adjusted to avoid that. Actually, I've already read similar problem in this forum and I tried all the suggestions but it didn't help.

Thanks in advance

Tania.

David March 30, 2004 08:57

Re: avoiding negative absolute pressure!!!
 
hello,

i read your problem all through weeks ago and feel sorry your problem has not been solved.

I read a bit on cavitation -topic unfamiliar to me - and came through the article one of our forum user suggested you.

did you go to : http://cav2003.me.es.osaka-u.ac.jp/C...3-OS-1-004.pdf ?

Particularly they are mentionning far field boundary conditions at upstream and downstream boundaries have you tried them?

let me know if this helps

hope your problem will be solved soon

David


Tania March 30, 2004 11:18

Re: avoiding negative absolute pressure!!!
 
Thanks a lot for your answer.

According to fluent documentation, the farfield bc are used for compressible flow.

And in my case it is uncompressible flow and it is without the cavitation model. Of course my aim is to use the cavitation model but I have to solve this problem first....but it seems complicated !!!!

I have been struggling with this problem for many days. I tried sooooooooooooo many things but they didn't help.

David March 30, 2004 11:33

Re: avoiding negative absolute pressure!!!
 
ok, you've tried with 3 m/s and 1 m/s. have you increased gradually from 3 to 8 m/s to see how it goes? from which velocity do you get the negative value?

i believe whatever small it is there will be compression at the leading edge of your plate right? have you read the article?

have you tried with farfield BC anyway? farfield works for compressible flow but i believe you can try it in your case. incompressible flow is just a condition of compressible flow with so you can apply this condition i believe.

dont give up, i'm sure your problem will be solved soon!

David

Tania March 30, 2004 11:59

Re: avoiding negative absolute pressure!!!
 
Actually, I increased the velocity slowly from 1m/s to 8 m/s. everything seems fine only for 8m/s where I get negative absolute pressure.

What do you mean by "whatever small it is there will be compression at the leading edge of your plate"

Concerning the article, I read other articles from Cav2003. And I 'll try the farfield Bc and see what happens.

michal March 30, 2004 12:10

Re: avoiding negative absolute pressure!!!
 
I had similar problem in case of natural convection. I had a very small velocities (V=0.02 m/s) In my case I changed the reference pressure location to X=0, Y=0. It helped.

michal

David March 30, 2004 12:32

Re: avoiding negative absolute pressure!!!
 
i meant there will be a compression anyway at the leading edge of your plate.

let me know what happens with farfield BC

ensure your operating pressure location as Michal mentionned

David

Tania March 30, 2004 15:37

Re: avoiding negative absolute pressure!!!
 
thanks, I'll tell you the results

Otilia March 30, 2004 18:41

Re: avoiding negative absolute pressure!!!
 
The only way (I can think of) you can get negative absolute pressure is if the operating pressure - reference pressure has been changed from the default value of 101325Pa and your reference location is bad positioned. Check that and I am pretty sure you will solve the problem.


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