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miya March 26, 2004 09:21

CAVITATION
 
Hello,

I would like first to say that I am a beginner in fluent. So, please don't mind my questions. here they are :

1- in cavitation modeling, Does Fluent compare the absolute pressure in each point in the domain to the vapour pressure ???or Fluent compares Cp to the cavitation number?

2- So, if I want to simulate a cavitating flow around an hydrofoil. Knowing only the velocity inlet (10 m/s). How should I set the Pressure to have a certain value of the cavitation number?

Thanks

Miya

mateus March 29, 2004 03:29

Re: CAVITATION
 
Hi!

Fluent uses a cavitation model that is based on Rayleigh equation:

the mass transfer between the phases is = +/- sqrt(delta p/rho).

delta p has to be treatet absolut. If p is lower than p_v you have evaporation, if delta p is greater than p_v you have condensation.

That's the basis of all cavitation models in Fluent. The model in version 6.1 is slightly more complicated and works much better than the old one. Check out the Fluent documentation also.

The cavitation number is usually defined in one point in your domain (where you measure the pressure). In the simulation that is usually at the inlet or at the outlet of your domain. If you use the outlet definition it's easy, since you only have to define the pressure outlet bc. If you define it (cavitation number) on inlet, you have to determine the pressure outlet by iteration, since there are pressure losses in your domain...

Hope this helps

MATEUS

miya March 29, 2004 05:00

Re: CAVITATION
 
thanks Mateus for you answer.

So, to define the cavitation number in a point (reference point) in my domain. I should specify the absolute pressure in that reference point. And this has to be done in the operating pressure . Is it right???

But I know from fluent documentation, that fuent uses the operating pressure only to calculate the absolute pressure (Pabsolute= Pstatic(gauge)+ operating pressure).

So, to calculate the absolute pressures, fluent will add to the static (gauge) pressures, the reference pressure (set in the operating pressure) and this pressure is different from the Patomspheric.

So, since the operating or reference pressure is different from Patm , one can expect negative absolute pressures???

Miya

miya March 30, 2004 03:57

Re: CAVITATION
 
Hope someone can answer to my question !

mateus March 31, 2004 10:49

Re: CAVITATION
 
Hi!

I hope this will clear your problems. You define the cavitation number in reference point with absolut pressure, which is the sum of the operating and gauge (the one you define at pressure outlet) pressure. If you get negative pressures there must be something wrong with your problem (convergence). Try another mesh. A good trick to do is also to define the p_op and p_gauge so that they are the same in size (p_op=p_gauge).

Hope you suceed.

MATEUS

miya March 31, 2004 13:33

Re: CAVITATION
 
thank you very much for your help !

miya


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