Learning Forum

[Hilary Egglestone]

Hi All, 

I am new to ANSYS and I have hit a wall with my simulation and I am really hoping someone can help. The situation is as follows: I am 2-D modelling air and water in a cannister that is tubular in shape with a circular area at the bottom. Specifically, I want to show how high pressure air in the bottom of the cannister will eject a column of water out of the tube. I am initializing my solution and then patching both 3 atm of pressure into circular area at the bottom of the cannister and a column of water  into the cannister (see images). Everywhere except the circular area is at 1 atm of pressure. 

Everything looks good until I run my simulation: instead of the water being ejected from the cannister as expected, the high pressure area is dissipating almost instantaneously and the water is dripping down into the circular area. There is no inlet in this system and the only outlet is the outer edge of the square that serves as my domain to capture the ejection. The cannister is completely enclosed except for the opening at the end where the water should be ejected. I have tried patching very high pressures (10 atm) and the result is the same. What could be going on here? 

Other important parameters not shows in screenshot?

General: Pressure Based, Transient Simulation

Models: Multiphase, VOF

Help is much appreciated!!

IMAGES FROM SETUP:

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IMAGES FROM SOLUTION:

[SRP]

 

Hi,

I advise you to employ a density-based solver. After that, carry out initialization and patch the pressure to the specific zones.

 

I hope this solves your problem.

Thanks

Saurabh

 

[Hilary Egglestone]

Thank you SRP for you response. Unfortunately, it looks like when I use the density solver I am unable to use a multiphase model. 

[SRP]

Hi,

1) Your mesh is coarser in cannister. I suggest to make it refine.

2) Can you please share the properties for high pressure air?

3) Which turbulence model you used and what is your time step?

4) During VOF selection, which interface modeling type you selected?

Thank you

Saurabh

[Hilary Egglestone]

1) Thank-you for the comment on the mesh. 

2) The properties for high pressure air were defined only in the patching window as follows: 

and the operating conditions are as follows: 

3) The turbulence model used is k-omega: 

and the time step is as follows:

I also tried running it with the following fixed time step and the results look the same: 

4)  VOF Selection: 

Your help is much appreciated, thank you!

[SRP]

Hi,

 

When you combine high pressure, which is compressible, with another part, water, which is incompressible, the high pressure dissipates almost instantly because in the incompressible area, the speed of the pressure wave becomes infinite.

 

I suggest choosing an ideal gas based on density to modeling both areas as compressible.

 

Thanks

[Hilary Egglestone]

Thank you for your reply - that makes sense!!! I never would have figured that out. 

How exactly do I 'choose an ideal gas based on density'? do you mean that instead of patching with water I should patch that area with a "ideal gas" that is set up to have the properties of water? 

 

[SRP]

Hi,

I suggest you to please refer to user's guide: 8.3. Density (ansys.com)

Thank you