Glass viscosity

marsa27 · September 27, 2018, 10:13

Hello,
I would like to simulate the glass forming with a VOF model. As the viscosity of the glass varies strongly with the temperature, I would know how to implement a viscosity law with the temperature in CFX Pre (I already have this relationship).
Thank you very much

ghorrocks · September 27, 2018, 19:32

You simply define the viscosity as a function and put that in your material model. You can define the function with interpolation tables, CEL expressions or user fortran. For examples of how to do these things and how to modify material models look in the CFX tutorials.

marsa27 · October 8, 2018, 11:22

Quote:

Originally Posted by ghorrocks

You simply define the viscosity as a function and put that in your material model. You can define the function with interpolation tables, CEL expressions or user fortran. For examples of how to do these things and how to modify material models look in the CFX tutorials.

So is it possible to set a CEL expression like a polynomial with the temperaure as variable for the viscosity material property?

Gert-Jan · October 8, 2018, 12:00

Yes that is possible. I would recommend to limit temperature using min & max functions, to avoid unrealistic viscosities outside the range of applicability during the simulation.
Use the Evaluation-fucntion of CFX-Pre when setting up the expression. This allows you to check whether you get realistic values and do not make a typo.
Also, make sure you use enough digitis for your polynomocal coefficients a,b,c, etc. Round off errors can become large if you have a term like a*T^6
In other words: check, check, double check.

marsa27 · March 7, 2019, 20:13

Thank you, how can I limit the function? Can you write an sample?

marsa27 · March 7, 2019, 20:14

Quote:

Originally Posted by Gert-Jan

Yes that is possible. I would recommend to limit temperature using min & max functions, to avoid unrealistic viscosities outside the range of applicability during the simulation.
Use the Evaluation-fucntion of CFX-Pre when setting up the expression. This allows you to check whether you get realistic values and do not make a typo.
Also, make sure you use enough digitis for your polynomocal coefficients a,b,c, etc. Round off errors can become large if you have a term like a*T^6
In other words: check, check, double check.

Thank you, how can I limit the function? Can you write an sample?

ghorrocks · March 7, 2019, 22:16

Have a look in the CFX reference guide for functions available in CEL. Functions which can be used as limiters include step, if and max/min.

Gert-Jan · March 8, 2019, 10:57

min(1e7[Pa s], max(1 [Pa s], function))

where function is your viscosity as function of whatever you want.
If you use one of the predefined viscosity functions like powerlow or Hershel-Bukly, min and max values for shear rate are available as well.

As Glenn mentioned, look for it in the manual.

marsa27 · March 8, 2019, 11:08

Thank you very much.

Quote:

Originally Posted by Gert-Jan

min(1e7[Pa s], max(1 [Pa s], function))

where function is your viscosity as function of whatever you want.
If you use one of the predefined viscosity functions like powerlow or Hershel-Bulkly, min and max values for shear rate are available as well.

As Glenn mentioned, look for it in the manual.

September 27, 2018, 10:13	Glass viscosity	#1
marsa27 Senior Member Davide Join Date: Jul 2015 Posts: 107 Rep Power: 11	Hello, I would like to simulate the glass forming with a VOF model. As the viscosity of the glass varies strongly with the temperature, I would know how to implement a viscosity law with the temperature in CFX Pre (I already have this relationship). Thank you very much

September 27, 2018, 19:32		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	You simply define the viscosity as a function and put that in your material model. You can define the function with interpolation tables, CEL expressions or user fortran. For examples of how to do these things and how to modify material models look in the CFX tutorials. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

March 7, 2019, 20:13	Min max	#5
marsa27 Senior Member Davide Join Date: Jul 2015 Posts: 107 Rep Power: 11	Thank you, how can I limit the function? Can you write an sample?

March 7, 2019, 22:16		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	Have a look in the CFX reference guide for functions available in CEL. Functions which can be used as limiters include step, if and max/min. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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October 8, 2018, 12:00		#4
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,928 Rep Power: 28	Yes that is possible. I would recommend to limit temperature using min & max functions, to avoid unrealistic viscosities outside the range of applicability during the simulation. Use the Evaluation-fucntion of CFX-Pre when setting up the expression. This allows you to check whether you get realistic values and do not make a typo. Also, make sure you use enough digitis for your polynomocal coefficients a,b,c, etc. Round off errors can become large if you have a term like a*T^6 In other words: check, check, double check.

March 8, 2019, 10:57		#8
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,928 Rep Power: 28	min(1e7[Pa s], max(1 [Pa s], function)) where function is your viscosity as function of whatever you want. If you use one of the predefined viscosity functions like powerlow or Hershel-Bukly, min and max values for shear rate are available as well. As Glenn mentioned, look for it in the manual.