[lehoanganh07]

Dear Fluent user,

I am using mixture model for two phase flow: water-vapor in Nozzle. All boundary value is measure by experiment. But now I have problem with inlet boundary condition.
First simulation, I used pressure inlet BC. The results is very far different from experiment result ( Operation pressure is atmospheric pressure: 100130 Pa, static gauge pressure in let: 5510 pa, velocity: 10.8m/s,so I obtain total gauge pressure: 63770.51 Pa and vapor pressure: 1756 Pa). when I checked result, the pressure inlet rise to 160100 Pa in absolute and velocity reduce to 3 m/s (Note that Pressure outlet fixed 5700 Pa static absolute) and Cavity appare as supercavitation. In real situation, the length cavity just 1/4 Nozzle hole.
I am not really understand why have larger change in static pressure inlet. It may be cause the wrong result in cavity of bubble.
SO now, I change to velocity inlet BC and dont have result yet.

Could you help me to explant this error
Why this phenomenon appare (pressure change too larger)?
and which boundary condition: pressure inlet BC or velocity inlet BC is suitable?

Thank you very much.

[Zaktatir]

Are you sure that you took into account the influence of the operating pressure on your pressure specifications.

[lehoanganh07]

Thank you for your reply, Zaktatir!
But what is the meaning of your question?

[Zaktatir]

Your reference pressure will be added to your pressure specifications. Have u considered that by calculating the inputs of your static and total pressure?.



PS: If your flow is incompressible then your gauge pressure will be ignored and only used if you initialize from the inlet.

[lehoanganh07]

Yes, I know reference pressure will be added to pressure specifications. And also I calculated it to static and total pressure: inlet gauge static pressure:5510 Pa, and absolute pressure (include reference pressure) is 106100 and total pressure (include reference pressure) around 164000 Pa. Also for outlet section.
One more thing, pressure we mention in cavitation option is defaultly absolute pressure, not depent on referent pressure we set, isnt it?

[Zaktatir]

The vapor pressure for cavitation is the absolute one!

[lehoanganh07]

Thank you, Zaktatir
So do you have any suggestion for my simulation. Because I thing the result is wrong!

[Zaktatir]

Please make a screenshot of your pressure inlet BC and the Initialisation Panel!

[lehoanganh07]

http://s15.postimg.org/ws4osv2vf/image.png
http://s15.postimg.org/5g9friy4r/image.png
http://s15.postimg.org/rqxarhvez/image.png

Sorry, I dont know how to upload image to forum, so please see the link above.
For Initialisation I use Hybrid method.

[Zaktatir]

So after seeing your screenshot:

-You said your reference pressure is one atm
-You are giving another values of pressures at inlet from what you have already described above. Please check up this. From direct calculation of that we will get almost 10,82 m/s for the velocity at inlet.
-Please initialize from the Inlet at first. I know Hbyrid provide with the more sophisticated pressure /velocity distribution since we solve a laplacian equation for the pressure but try to do this.
-At last but not least check the convergence and the imbalances of your simulations
-For cavitation simulation i usually use zero reference pressure (picked up an old tutorial for you:http://cdlab2.fluid.tuwien.ac.at/LEH...f/tg/tut17.pdf)

[lehoanganh07]

these value in the screenshot, I did with referent pressure set be 0. In my post at the first just explant the data from experiment