VOF settings

Klavoir · March 28, 2022, 07:41

Hi,

I want to simulate the vortex in a vessel and calculate the resulting difference in water height between the middle and near the wall of the vessel. I want to use VOF model. On top of the water layer is air. I enabled gravity. In the top of the vessel water is coming tangentially in via a pipe, creating the vortex. At the bottom water is pumped out of the vessel and moved upwards (to the same level as the water height in the vessel). For some reason I cannot get the right water height in the vessel (only a small layer on the bottom). I want to run the simulation steady state.
I am confused about specifying operating density, is it necessary and if so, should I take water or air density?
What should I define as the secondary phase?
Other tips to think about more than welcome.

jojoFD · April 7, 2022, 04:47

It depends what kind of model you are using : with the Boussinesq model, operating density should be set to zero.

You can find everything in the Fluent User guide. Take the time to read it.

A few extracts:
Important: In general, you can specify the primary and secondary phases whichever way you prefer. It is a good idea, especially in more complicated problems, to consider how your choice will affect the ease of problem setup. For example, if you are planning to patch an initial volume fraction of 1 for one phase in a portion of the domain, it may be more convenient to make that phase a secondary phase. Also, if one of the phases is a compressible ideal gas, it is recommended that you specify it as the primary phase to improve solution stability.

There is only one specified operating pressure and one specified operating temperature. This means that if you are using the ideal gas law for more than one material, they will share the same operating pressure. If you are using the Boussinesq model for more than one material, they will share the same operating temperature.

Klavoir · April 7, 2022, 10:36

From the manual I understood that with Boussinesq appr. you dont have to define operating pressure at all. Or is that the same as putting it
at zero?

jojoFD · April 7, 2022, 11:10

Depends if Fluent puts a value different than zero by default.

"with the Boussinesq model, operating density should be set to zero."
This sentence is out of the manual as well

From experience, when you set up the multiphase flow case, depending on which model you chose, Fluent tells you what to do for the density in the TUI console.

March 28, 2022, 07:41	VOF settings	#1
Klavoir New Member Join Date: Sep 2021 Posts: 22 Rep Power: 4	Hi, I want to simulate the vortex in a vessel and calculate the resulting difference in water height between the middle and near the wall of the vessel. I want to use VOF model. On top of the water layer is air. I enabled gravity. In the top of the vessel water is coming tangentially in via a pipe, creating the vortex. At the bottom water is pumped out of the vessel and moved upwards (to the same level as the water height in the vessel). For some reason I cannot get the right water height in the vessel (only a small layer on the bottom). I want to run the simulation steady state. I am confused about specifying operating density, is it necessary and if so, should I take water or air density? What should I define as the secondary phase? Other tips to think about more than welcome.

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April 7, 2022, 04:47		#2
jojoFD New Member Join Date: Mar 2021 Posts: 18 Rep Power: 5	It depends what kind of model you are using : with the Boussinesq model, operating density should be set to zero. You can find everything in the Fluent User guide. Take the time to read it. A few extracts: Important: In general, you can specify the primary and secondary phases whichever way you prefer. It is a good idea, especially in more complicated problems, to consider how your choice will affect the ease of problem setup. For example, if you are planning to patch an initial volume fraction of 1 for one phase in a portion of the domain, it may be more convenient to make that phase a secondary phase. Also, if one of the phases is a compressible ideal gas, it is recommended that you specify it as the primary phase to improve solution stability. There is only one specified operating pressure and one specified operating temperature. This means that if you are using the ideal gas law for more than one material, they will share the same operating pressure. If you are using the Boussinesq model for more than one material, they will share the same operating temperature.

April 7, 2022, 10:36		#3
Klavoir New Member Join Date: Sep 2021 Posts: 22 Rep Power: 4	From the manual I understood that with Boussinesq appr. you dont have to define operating pressure at all. Or is that the same as putting it at zero?

April 7, 2022, 11:10		#4
jojoFD New Member Join Date: Mar 2021 Posts: 18 Rep Power: 5	Depends if Fluent puts a value different than zero by default. "with the Boussinesq model, operating density should be set to zero." This sentence is out of the manual as well From experience, when you set up the multiphase flow case, depending on which model you chose, Fluent tells you what to do for the density in the TUI console.