VOF and multiphase segregated flow

natalie26 · February 9, 2012, 12:51

Hi,

i hope someone can help me

i took the VOF model to simulate a gas flow above a constant water level.
Additionally i want to simulate a water injection at the inlet.
As far as i understood the tutorial i need to use the segregated flow for a continuous-dispersed phase? Or is it possible to define a massflow of 50% water and 50% air while using the VOF model?

thanks for help!

natalie

Ladnam · February 9, 2012, 14:12

Usually it is better to use the multiphase segregated model when air is dispersed in water, like for a inlet with fractional flow. Use VOF when you have an interface between water and air that is important to capture, e.g. boat on free surface. Be patient with the multiphase segregated model, it is harder to get it to converge than the VOF model.

natalie26 · February 9, 2012, 15:38

yes i do have to capture the interface between the water on the bottom of a valve and the gasflow streaming over it. so far it works (with the vof model

)
but now i don´t know how to generate the water droplets in the airflow at the inlet...
in the hibiki bubble tutorial they defined the inlet with air and bubbles with different velocities and air as the continous phase. but there the multiphase segregated flow is choosen and no additional water surface is present

can i define such an inlet with the vof model?
thanks!

abdul099 · February 18, 2012, 06:34

Have a look on the user guide, it explains the limits of VOF.
In order to capture bubbles or droplets with VOF, you need a mesh resolution significantly smaller than the bubbles or droplets. That's too much effort for most applications, better to do that with an Eulerian / Eulerian approach (multiphase segregated flow in ccm+).

Water at the inlet can be specified by setting an appropriate volume fraction at an appropriate position, therefore you might use a field function. But don't expect a nice result as long as your mesh resolution doesn't meet the requirements.

natalie26 · February 21, 2012, 15:01

so...the results with the vof model are quite good! Indeed the mesh is very very fine!
I was thinking about using the lagrangian-model to specify an injector at the inlet. Isn´t this easier than using a field function?
There i can specify water droplets with a specific diameter and velocity.
I don´t know if this will work...? Can i combine these models?
Until now i always get a floating point exception, but it could be the initial conditions...
thanks for your answers

February 9, 2012, 12:51	VOF and multiphase segregated flow	#1
natalie26 New Member Join Date: Feb 2012 Posts: 8 Rep Power: 14	Hi, i hope someone can help me i took the VOF model to simulate a gas flow above a constant water level. Additionally i want to simulate a water injection at the inlet. As far as i understood the tutorial i need to use the segregated flow for a continuous-dispersed phase? Or is it possible to define a massflow of 50% water and 50% air while using the VOF model? thanks for help! natalie

February 9, 2012, 15:38		#3
natalie26 New Member Join Date: Feb 2012 Posts: 8 Rep Power: 14	yes i do have to capture the interface between the water on the bottom of a valve and the gasflow streaming over it. so far it works (with the vof model) but now i don´t know how to generate the water droplets in the airflow at the inlet... in the hibiki bubble tutorial they defined the inlet with air and bubbles with different velocities and air as the continous phase. but there the multiphase segregated flow is choosen and no additional water surface is present can i define such an inlet with the vof model? thanks!

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February 9, 2012, 14:12		#2
Ladnam Member Join Date: May 2010 Posts: 40 Rep Power: 15	Usually it is better to use the multiphase segregated model when air is dispersed in water, like for a inlet with fractional flow. Use VOF when you have an interface between water and air that is important to capture, e.g. boat on free surface. Be patient with the multiphase segregated model, it is harder to get it to converge than the VOF model.

February 18, 2012, 06:34		#4
abdul099 Senior Member Join Date: Oct 2009 Location: Germany Posts: 636 Rep Power: 21	Have a look on the user guide, it explains the limits of VOF. In order to capture bubbles or droplets with VOF, you need a mesh resolution significantly smaller than the bubbles or droplets. That's too much effort for most applications, better to do that with an Eulerian / Eulerian approach (multiphase segregated flow in ccm+). Water at the inlet can be specified by setting an appropriate volume fraction at an appropriate position, therefore you might use a field function. But don't expect a nice result as long as your mesh resolution doesn't meet the requirements.

February 21, 2012, 15:01		#5
natalie26 New Member Join Date: Feb 2012 Posts: 8 Rep Power: 14	so...the results with the vof model are quite good! Indeed the mesh is very very fine! I was thinking about using the lagrangian-model to specify an injector at the inlet. Isn´t this easier than using a field function? There i can specify water droplets with a specific diameter and velocity. I don´t know if this will work...? Can i combine these models? Until now i always get a floating point exception, but it could be the initial conditions... thanks for your answers