[Shayankhan]

Greetings members,
I want to investigate eddy viscosity (m2/s) in Fluent. But, I can find the turbulent viscosity in Fluent with units "kg/m-s".
Is the turbulent viscosity similar to that of eddy viscosity ?? Or should I convert it using the density of fluid ?? Please help. Thank you.

[LuckyTran]

Turbulent and eddy viscosity are the same thing. In your case, it's clear that you need to divide the turbulent viscosity from Fluent by the density to get the thingy that you want. I wouldn't call this the eddy viscosity (because it doesn't have units of viscosity) but eddy diffusivity (having the same units as diffusivity). Anyway, what you decide to call it is up to you.

[Shayankhan]

Originally Posted by LuckyTran

Turbulent and eddy viscosity are the same thing. In your case, it's clear that you need to divide the turbulent viscosity from Fluent by the density to get the thingy that you want. I wouldn't call this the eddy viscosity (because it doesn't have units of viscosity) but eddy diffusivity (having the same units as diffusivity). Anyway, what you decide to call it is up to you.

Thank you for the kind response.

[Santiago]

Originally Posted by LuckyTran

Turbulent and eddy viscosity are the same thing. In your case, it's clear that you need to divide the turbulent viscosity from Fluent by the density to get the thingy that you want. I wouldn't call this the eddy viscosity (because it doesn't have units of viscosity) but eddy diffusivity (having the same units as diffusivity). Anyway, what you decide to call it is up to you.

What you say is not true! Eddy Difussivity refers to scalar turbulence. Eddy viscosity in the Incompressible world is presented as m2/s, corresponding to the kinematic viscosity, which is \nu = \mu/\rho

FLUENT looves putting particular names for things, just to add confusion.

[LuckyTran]

Everything in incompressible world is presented in per rho units. Even mass flux. It's just a preference. It's not Fluent's fault, everybody has their own way of doing things.


Again, eddy viscosity having units of m2/s doesn't have units of viscosity, just like mass flux doesn't have units of mass flux in incompressible world.

[vinerm]

Eddy viscosity models are, well, models. And the way these models model the turbulence (instead of simulate, which is done by LES and DNS) is by adding an extra diffusion due to the turbulence, which is significantly higher (within the turbulent region) than the molecular diffusion. So, the apparent viscosity of a fluid in a turbulent flow is higher than its molecular viscosity. A fluid behaves as a highly viscous fluid when turbulent, hence, requiring extra energy to flow.

Now, eddy viscosity is just a number that quantifies the diffusion of a field, such as, momentum or thermal energy or a component, due to the turbulence. And unit for a diffusion coefficient is same for all fields.

[starz]

Originally Posted by Shayankhan

Greetings members,
I want to investigate eddy viscosity (m2/s) in Fluent. But, I can find the turbulent viscosity in Fluent with units "kg/m-s".
Is the turbulent viscosity similar to that of eddy viscosity ?? Or should I convert it using the density of fluid ?? Please help. Thank you.

I have one more question, so is eddy viscosity = momentum diffusivity?

[LuckyTran]

Originally Posted by starz

I have one more question, so is eddy viscosity = momentum diffusivity?

No. Momentum diffusivity is the molecular viscosity divided by density. It is a material/molecular property.


Eddy viscosity is a flow property. It is a coefficient in the linear relation between the strain rate tensor and the Reynolds stress tensor.

[starz]

Originally Posted by LuckyTran

No. Momentum diffusivity is the molecular viscosity divided by density. It is a material/molecular property.


Eddy viscosity is a flow property. It is a coefficient in the linear relation between the strain rate tensor and the Reynolds stress tensor.

Thanks for your kind answer! Sorry that I did not express it clearly. I think what you mean is molecular momentum diffusivity. I was want to ask if eddy viscosity = turbulent momentum diffusivity?

I just saw this term in a few papers, e.g.:A study of interunit dispersion around multistory buildings with single-sided ventilation under different wind directions. The authors mentioned that: "In the simulation of dispersion using RANS models, the turbulent Schmidt number (Sct) has a significant impact on the calculation of concentration equation (Tominaga and Stathopoulos, 2007). This number is defined as the ratio of turbulent momentum diffusivity to concentration diffusivity."

I suppose "turbulent momentum diffusivity" means "eddy viscosity"?