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Cavitation in a pipe constriction - problems with boundary settings

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Old   December 17, 2014, 09:37
Default Cavitation in a pipe constriction - problems with boundary settings
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Hello Everybody!

I am modeling cavitation in a pipe constriction with Star CCM+ v9.

I have a Question concerning how to set up an Inlet-Boundary to specify velocity and pressure at the same time, is it possible with an velocity inlet?

Current setting: Boundaries:

Pipe - Inlet:
Velocity inlet, 1.8 m/s
Pipe - Outlet: Pressure outlet, 0.0 Pa

Reference Pressure: 1013.25 hPa (Atmospheric Pressure)
Saturation Pressure: 2338 Pa

Is it possible to set a pressure and a velocity at the same time for the pipe-inlet? [for example: v = 1.8 m/s, p = 5 bar_gauge]

I have tried several combinations consisting of an velocity inlet and a transposed pressure outlet, but i m gettin only reveresed backflows on the pipe inlet.

Selected Models:
- Eulerian Multiphase (H20 liquid and H2O gas)
- Multiphase Interaction (H20 liquid --> H2O gas)
- K-Epsilon Turbulence
- VOF

Attached u can find two pictures:
- geometry overview
- result scene for absolut pressure

I would be very thankful for any advice/help
Thank you in advance!

Greetings,
Claudio
Attached Images
File Type: jpg cavitation-model.jpg (21.6 KB, 32 views)
File Type: jpg absolute_pressure.jpg (23.7 KB, 31 views)
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Old   December 18, 2014, 02:49
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Hi,
If I understand correctly, you want to specify pressure as well as velocity at the inlet to model a pressurised pipe. Am I correct?

Have you tried increasing the pressure at the outlet? Since the flow is forced in at the inlet by specifying velocity, the entire pipe will be pressurised.
Also, you might consider using a mass flow inlet instead of a velocity inlet along with a pressure outlet (with higher pressure).

This might work.
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Old   December 18, 2014, 11:49
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Thank you for your fast reply.
Yes you did understand me correctly .

I have tried already a massflow-inlet instead of a velocity inlet, but unfortunately without a sucessfull change in pressure.

Massflow-Inlet:

massflow: 0.1275 kg/s by an Inlet-Area of 0.07 mē
Supersonic pressure: 0.0 Pa


I will try to increase the pressure on the pressure outlet step by step to see how the simulation is reacting. I ll keep you update.
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Old   January 5, 2015, 15:10
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Thanks for all the help.

It s workin with a pressure outlet and a velocity inlet.
Just bit confusing at the start with the absolute pressure and the absolute total pressure, but it s workin now .
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Old   February 2, 2015, 03:08
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Quote:
Originally Posted by ClaudioT View Post
Thanks for all the help.

It s workin with a pressure outlet and a velocity inlet.
Just bit confusing at the start with the absolute pressure and the absolute total pressure, but it s workin now .
Hi, I am simulating sth similar to your case. But mine is not pipe constriction. May I know how much you set ur pressure in inlet? u set it under initial condition as u are working with velocity inlet. How about the pressure outlet? how much u set and do u set static or environmental for the pressure specification.
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Old   February 4, 2015, 15:25
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Hi,

I have used an static pressure at the pressure outlet.
Using a pressure outlet with the mode "environmental" just lead to Reversing Flows in my pipe construction

I ve set the reference pressure to atmospheric pressure (1013hPa), and afterwards I have set the pressure in the outlet pressure to 0.0 bar (so it is equally to the atmospheric pressure).

I ve run several cases to determine cavitation conditions by first increasing the velocity at the velocity inlet and afterwards setting different pressures at the pressure outlet ( 0.0 bar, 0.5 bar, 1 bar...up to 5 bar), to see when the cavitation disappears in the system.

According to my starting post, I had some starting issues with the pressure in the complete pipe construction, when I have set up a too low velocity at the velocity inlet.
I could resolve that problem by either adjusting(lowering slightly) the Under-Relaxation-Factor in the k-e-Turbulence-Modell, or change the Turbulence-Modell to the k-w-Turbulence-Modell, which gave the most stabilzed Residuals.

Under which pressure conditions you are setting ur system?
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