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-   -   How does Fluent calculate Shear (http://www.cfd-online.com/Forums/fluent/30875-how-does-fluent-calculate-shear.html)

Diana January 30, 2003 14:10

How does Fluent calculate Shear
 
How does Fluent calculate the shear stress on a wall B.C. and at any point in the flow(continuum)? What are the equations used?

Is there any difference in the two cases?


Egorov Maxim (from Russia) February 1, 2003 13:23

Re: How does Fluent calculate Shear
 
For no-slip wall boundary conditions,Fluent use the properties of the flow adjacent to the wall/fluid boundary to predict the shear stress on the fluid at the wall. In laminar flows shear stress calculation simply depends on the velocity gradient at the wall.

shear stress = dynamic viscosity * (partial derivative of velocity by normal at wall).

Shear stress for any point in the laminar flow also define. For turbulent flows shear stress define several difficult.

Excuse me for my English. Best regards, Egorov Maxim.

Diana February 3, 2003 01:02

Re: How does Fluent calculate Shear
 
Thanks Egorov, for your reply.

But how does fluent calculates viscosity in the turbulent flow?

Egorov Maxim (From Russia) February 3, 2003 03:46

Re: How does Fluent calculate Shear
 
For example, for the standart k-e model turbulence the turbulent viscosity , mu_t , is computed by combining k and e as follows:

mu_t=ro*C_mu*(k^2/e) where,

ro - density

C_mu - empirical constant

e - rate of the turbulence dissipation

k - turbulence kinetic energy

You question about the shear stress in the turbulent flows:

shear_stress_turbulence = mu_t * (partial derivative of velocity by normal at wall).

Where the velocity and other cuantities (scalar and vector) of turbulence flow at the near-wall cells define by semi-empirical formulas and functions.

Excuseme for my english. Best regards, Maxim.

J.W.Ryu February 12, 2003 18:14

Re: How does Fluent calculate Shear
 
Hi, I confirmed that shear stress is depending on wall temperature. Sandard k-e turbulence model was used to calculate fluid flow and wall shear stress. I did not use natural convection. Thus, no changing of density was happened. Of course, used viscosity was constant. I only changed heat source so that wall and fluid temperature were changed.

Do you know why shear stress could be changed by changing of temperature?

Thanks,

J.W.Ryu

Egorov Maxim (from Russia) February 12, 2003 19:52

Re: How does Fluent calculate Shear
 
If density and viscouse are constant (but in the phisical density and viscouse is depending on the temperature), then shear stress could be changed on changing of gradient velocity. The gradient velocity is depending of temperature (i.e. of diffusion processes).

Excuseme for my english. Best regards, Maxim.

J.W.Ryu February 13, 2003 12:09

Re: How does Fluent calculate Shear
 
Maxim,

I roughly understood why shear stress could be chanded by temperature.

Thanks so much.

J.W.Ryu

Egorov Maxim (from Russia) February 16, 2003 15:31

Re: How does Fluent calculate Shear
 
Hear stress is depending on the gradient velocity! Is Velocity depending on the temperature?!

p.s. Any difficult answer on the question not know your engineer problem, and my english is very bad. Excuseme...

Best regards, Maxim.


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