Learning Forum

[sbecker]

When I run a part with Ansys AIM 19.0 and water, it works fine.  When I add a custom material, A100, it gives me the error, "Dynamic viscosity has not been defined for materials associated with some fluid physics regions."  Any idea what I am missing? 
 

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[Gaurav Sharma]

Hi SBecker

For a basic fluid analysis, density and viscosity are the must have properties. I can see you have specified "first Normal Viscosity" in the custom material. Let me check with the fluids expert and see if this still needs any further specification to complete the material definition. Will get back to you soon.

Regards,

Gaurav

[Gaurav Sharma]

Hi SBecker

I have learnt that even for defining a viscoelastic fluid, it is required to add viscosity as a separate material property. Please add this property to your user defined material and this would do away the problem.

Please note that total viscosity has 2 components, pure viscous component and the contribution coming from viscoelastic definition. In your case while the viscoelastic part is well defined, pure viscous component still remains to be specified.

I hope that answers your question.

Thanks & Regards,

Gaurav

[sbecker]

Gaurav,

I have specified the viscosity as a function of the shear rate.  I don't understand what you are asking for by stating "pure viscous component".   Is that a constant viscosity, or is it a function of something else?  If it is a function, what is the format?

Thanks

[Gaurav Sharma]

SBecker Pure viscous component is nothing but the Viscosity property required for your simulation to solve. For example, the default Air material in AIM has following properties: Similarly, it is essential for your custom material to have viscosity defined in order to completely define the material. Thanks & Regards, Gaurav

[sbecker]

Gaurav_ANSYS 

Our fluid is shear-thinning, so the viscosity is constantly decreasing as the shear rate increases.  The curve has been fully characterized by the low-shear horizontal asymptote, the high-shear asymptote, the slope and location of the middle of the curve, so it doesn't seem necessary to add another fixed viscosity point.  I can simply add this point to get the software to work.  At what shear-rate do you suggest I select the viscosity point? 

[Gaurav Sharma]

SBecker 

I am checking with the expert and will get back to you soon. Thanks for your patience.

Regards,

Gaurav

[Gaurav Sharma]

SBecker 

The CFD simulations in AIM do not support viscoelastic models. Unfortunately, the current AIM architecture doesn't reflect this and it isn't intuitive. Dynamic viscosity is the material property that is valid for CFD simulations and once defined, it is being taken into consideration by the solver. Any viscoelastic definitions, even if present, will be ignored. 

If you have equation for Carreau Yasuda law in terms of strain rate, dynamic viscosity can be specified as an expression based on this equation.

I hope the explanation provides the required clarification. My apologies for the confusion caused by my previous response.

Thanks & Regards,

Gaurav