# Eddy viscosity VS Turbulent viscosity

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 February 26, 2019, 21:24 Eddy viscosity VS Turbulent viscosity #1 New Member   Naveed Join Date: Dec 2016 Location: Saitama, Japan Posts: 11 Rep Power: 7 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.

 February 27, 2019, 09:53 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,861 Rep Power: 58 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 and AbdelFree02 like this.

February 27, 2019, 20:52
#3
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Naveed
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Quote:
 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.

February 20, 2020, 10:37
#4
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Santiago Lopez Castano
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Quote:
 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.

 February 20, 2020, 10:44 #5 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,861 Rep Power: 58 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.

 February 20, 2020, 11:50 Viscosity #6 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 34 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. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

April 25, 2021, 06:26
eddy viscosity? momentum diffusivity?
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Quote:
 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?

April 25, 2021, 18:47
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Lucky
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Quote:
 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.

May 2, 2021, 09:00
#9
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 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"?

 Tags eddy viscosity, turbulent viscosity