## Fluids

#### CEL expression in CFX

• Zhu
Subscriber

Hi all,

I am trying to write a CEL expression in CFX to describe the viscosity which changes with time as a result of varying concentrations. The equation is attached in a figure as follows:

I have defined an additional variable as a scalar and solved it using a transport equation. However, I am not sure how to get the concentration of this scalar as normalized as [0, 1], and put it into the expression as shown above.

Any suggestions from you would be appreciated and looking forward to hearing back from you.

Yours,

Yu

• DrAmine
Ansys Employee

Just solve it as multicomponent model to avoid worriying about how to normalize. Is this possible?

• Zhu
Subscriber

Although I solve this as multicomponent model, the varying viscosity needs to be applied to one component right? So I will still need to use this expression to describe the viscosity.

• DrAmine
Ansys Employee

The viscosity will be applied for the whole mixture. Though you can define every component's viscosity. The mixing rule will be then used to have the mixture property. So you can define the mixture viscosity or the component viscosity.

• Zhu
Subscriber

Thanks for your response. I realized that I need to model it as a multi-phase model and this expression is used for calculating the viscosity of one fluid, while another fluid is with constant density and viscosity.

So again how can I get this expression work? I tried to use the volume fraction of the fluid to represent the normalized concentration but the simulation failed.

Any further suggestions from you would be appreciated.

• DrAmine
Ansys Employee

I do not understand. The volume fraction is not mass fraction so you need to translate it to that. What do you mean the simulation failed? I cannot comment on customized runs not using built in approaches.

• Zhu
Subscriber

Sorry for confusing you. So basiclly I was asking how to write this expression in CFX since I really need this and can't model it as multicomponent.

• DrAmine
Ansys Employee

Have you tried writing the same but using mass fraction being derived from volume fraction and the solver crashed?

• Zhu
Subscriber

Thanks for your response. I haven't tried that yet but will give it a go. BTW, can I get the same thing (volume fraction) for the scalar passive that is defined as an additional variable?