Learning Forum

[Jason Zhao]

Short question: How do I change μ dynamic viscosity in fluent?

Context: I'm trying to recreate an airfoil simulation. At 0 angle of attack, my result is almost exactly what is expected but when I go to 6 or 12 degrees it's more than 2x off of what's expected. I think my Reynolds number may be incorrect as I don't remember setting up dynamic viscosity. Where do I change this? All the numbers are given to me already so I cant just change Reynolds number by adjusting velocity or density.

[C N]

Hello Jason,

You can change the dynamic viscosity in the fluid properties by selecting the material properties in the fluent . you can choose expression option or user defined option to enter different dynmaic viscosity profiles .

I hope this helps you.

Thanks,

Chaitanya Natraj

[Jason Zhao]

Is the viscosity the same as dynamic viscosity?

[C N]

Hello Jason,

Ansys Fluent provides several options for definition of the fluid viscosity:

• constant viscosity

• temperature-dependent and/or composition-dependent viscosity

• kinetic theory

• non-Newtonian viscosity

• user-defined function

Each of these input options and the governing physical models are detailed in this section. (User-defined functions are described in the Fluent Customization Manual). In all cases, define the Viscosity in the Create/Edit Materials Dialog Box.

 Setup →   Materials

Viscosities are input as dynamic viscosity () in units of kg/m-s in SI units or /ft-s in British units. Ansys Fluent does not ask for input of the kinematic viscosity ().

Please refer 8.4. Viscosity (ansys.com) . 

There are 2 types of viscosities - dynamic and kinematic 

Absolute viscosity - coefficient of absolute viscosity - is a measure of internal resistance. Dynamic (absolute) viscosity is the tangential force per unit area required to move one horizontal plane with respect to an other plane - at an unit velocity - when maintaining an unit distance apart in the fluid.

Kinematic viscosity is the ratio of - absolute (or dynamic) viscosity to density - a quantity in which no force is involved. Kinematic viscosity can be obtained by dividing the absolute viscosity of a fluid with the fluid mass density like

ν = μ / ρ                  

kinematic viscosity= dynamic viscosity/ density.

This should solve your misunderstanding.

Thanks,

Chaitanya Natraj