Learning Forum

[LI KUAN-LIN]

I am using non-Newtonian fluids for VOF-DPM simulation. For the viscosity of the liquid, I use the Carreau Model, and when the viscosity of the particles is set to a constant value and VOF-DPM is turned on, the following dialog box pops up in the system. I think the viscosity of the particles also needs to use the viscosity properties of non-Newtonian fluids, but I am not sure how to change it.

[Prashanth]

Hello, you can try to input the viscosity-shear rate curve using a polynomial expression, for both particle and fluid viscosities. The strain rate variable is available in expressions.

[LI KUAN-LIN]

Thank you for your reply. I have successfully written out the sticky expression (function of shear rate) and set the viscosity of the fluid to this expression. As shown in the figure below.

But when setting the viscosity of the particle, there is no ‘expression’ option available for selection. How should I set it?

I use the k-omega SBES turbulent model, while the Carreau Model uses functions related to shear rate.

I also tried to write the relationship between viscosity and shear rate as a UDF and import it into Fluent, hoping to represent the viscosity of particles using UDF. The UDF is as follows:

The fluid part can be successfully modeled using UDF. 

However, the following window pops up when modeling the particle part.

I think the UDF I wrote is only limited to fluid, so the particle cannot read this UDF. What I want to ask is, how can I modify the viscosity of the particle to match that of the fluid?

[Prashanth]

Try this: create an expression, make it input parameter. Both the fluid and particle viscosities has the common "Input parameter" option. 

Also, for particle viscosity from UDF, DPM_PROPERTY macro is required.

[LI KUAN-LIN]

I used the expression in the above figure and tried to import the input parameter, but an error window popped up. It seems that only numbers can be entered as parameters.

 

[Prashanth]

It looks like the parameters must return a constant value. This leaves you with the option to use the UDF definition for particle viscosity.

Or, you can just switch off the breakup model to avoid this mismatch.

[LI KUAN-LIN]

If according to my definition above, the viscosity of a non-Newtonian fluid is a function of shear rate, then what about DPM particles? How should shear rate be expressed for DPM particles? By the way, do particles have the concept of shear rate?

[Prashanth]

The particle visosity does influence the breakup mechanism. But considering the point mass approach of DPM, it is not a good idea to use the same shear-dependent law. The point masses does not enclose flow within them, that causes shear.

Actually, the code check makes sure one uses the same viscosity definition for both VOF and discrete phase, when using the transition model.

[LI KUAN-LIN]

Okay. Based on the results of our discussion, let me summarize my question:

If I want to continue using non-Newtonian fluids to perform VOF-to-DPM and open secondary breakup, and express the fluid viscosity through UDF, in order to make the program determine the same viscosity definition for both VOF and discrete phase, how do you think the UDF for particle viscosity should be written?

[Prashanth]

Link to a very similar forum thread where the questions has been addressed: /forum/forums/topic/viscosity-settings-for-non-newtonian-particles-in-dpm-models-using-expression/

I cannot guarantee that it'll work as I haven't tried this combination for VOF-DPM. Again, you need DEFINE_DPM_PROPERTY for particle viscosity.