NonNewtonian Modeling Coefficients

dreamchaser · October 9, 2014, 01:15

Hi,

I am modeling the flow of grease over a body. I know that grease is a "shear-thinning" fluid so I am trying to apply the Bird Carreau Non-Newtonian model. You can see the equation describing the model here:

http://www.arc.vt.edu/ansys_help/cfx_mod/i1299063.html

However, I am confused about the parameters for the time constant and power-law index. I was reading that you have to do a curve fit to get these? Any advice on how to do this would be appreciated.

I currently have the viscosity at high and low shear viscosity. I also have some values of the shear rate and corresponding viscosity at different temperatures. However, I am not sure how to graph these to get the time constant and power-law index.

Thanks!!

ghorrocks · October 9, 2014, 05:50

If solving the equation for TC and index is too hard, then guess a TC and index and see what viscosity it gives you. Then adjust your TC and index guesses until it gives you the right viscosity.

You can do this in Excel using "goal seek"... or any other numerical program (python, matlab, etc)

October 9, 2014, 01:15	NonNewtonian Modeling Coefficients	#1
dreamchaser Member Mike Join Date: Jun 2012 Posts: 58 Rep Power: 13	Hi, I am modeling the flow of grease over a body. I know that grease is a "shear-thinning" fluid so I am trying to apply the Bird Carreau Non-Newtonian model. You can see the equation describing the model here: http://www.arc.vt.edu/ansys_help/cfx_mod/i1299063.html However, I am confused about the parameters for the time constant and power-law index. I was reading that you have to do a curve fit to get these? Any advice on how to do this would be appreciated. I currently have the viscosity at high and low shear viscosity. I also have some values of the shear rate and corresponding viscosity at different temperatures. However, I am not sure how to graph these to get the time constant and power-law index. Thanks!!

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October 9, 2014, 05:50		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	If solving the equation for TC and index is too hard, then guess a TC and index and see what viscosity it gives you. Then adjust your TC and index guesses until it gives you the right viscosity. You can do this in Excel using "goal seek"... or any other numerical program (python, matlab, etc)