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-   -   use of cavitation model (http://www.cfd-online.com/Forums/fluent/30096-use-cavitation-model.html)

gdv July 30, 2002 09:52

use of cavitation model
 
hi everyone,

I used cavitation model in my case but pressure values in some zones remain absurdly under the vaporization pressure value (negative values too). I tried to use different n values and different time steps but nothing seems to change. How can I avoid this?

Thank you

Lanre July 31, 2002 01:58

Re: use of cavitation model
 
Check the Courant number for your problem. You may require an absurdely low time step, e.g., ~1e-6s or less. Try bracketing your pressure by setting max and min pressure limits for your simulation. Use low underrelaxation for pressure, e.g., 1e-3. The bubble density should affect your results so specify a realistic value.

hvn July 31, 2002 03:53

Re: use of cavitation model
 
Hi,

By negative values of pressure, do you speak about absolute or relative pressure?

Regards,

HV

gdv July 31, 2002 04:10

Re: use of cavitation model
 
Thank you for your responce,

I mean, unfortunately, negative absolute pressure.

Regards,

gdv

gdv July 31, 2002 04:18

Re: use of cavitation model
 
Thanks for your responce, I can try to reduce time steps during the unsteady solution but i can't reduce CFL number because i'm using segregated solver and, ,moreover, unfortunately, i can't limit pressure values because my flow in incompressible. I will tell you the new results.

Regards

gdv


Lanre August 1, 2002 01:55

Re: use of cavitation model
 
I meant for you to use the Solve->Controls->Limits... to set min and max absolute pressures to control your solution. Once you have the solution progressing you can relax the min and max limits to be more realistic.

Mark Render August 6, 2002 03:57

Re: use of cavitation model
 
Hi,

reading your discussion the question came to my mind:

Is it really so strange or even impossible that the absolute pressure becomes negative ?

If there is a small nuclei content in the water perhaps the water won't vaporize at the saturation pressure but only at lower pressures.

Any opinions on that ?

Regards,

Mark


gdv August 7, 2002 11:48

Re: use of cavitation model
 
Thank you Mark,

I think that your observation is correct if we want to see qualitatively where cavitation occurs but absolute negative pressures affect the velocity field overestimating local velocity values (in the case one imposes a pressure drop on an incompressible flow) and so possible choking effects on flow rate through an orifice cannot be observed.

Regards


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