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How does Fluent calculate Shear

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Old   January 30, 2003, 14:10
Default How does Fluent calculate Shear
  #1
Diana
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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?

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Old   February 1, 2003, 13:23
Default Re: How does Fluent calculate Shear
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Egorov Maxim (from Russia)
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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.
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Old   February 3, 2003, 01:02
Default Re: How does Fluent calculate Shear
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Diana
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Thanks Egorov, for your reply.

But how does fluent calculates viscosity in the turbulent flow?
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Old   February 3, 2003, 03:46
Default Re: How does Fluent calculate Shear
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Egorov Maxim (From Russia)
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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.
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Old   February 12, 2003, 18:14
Default Re: How does Fluent calculate Shear
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J.W.Ryu
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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
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Old   February 12, 2003, 19:52
Default Re: How does Fluent calculate Shear
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Egorov Maxim (from Russia)
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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.
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Old   February 13, 2003, 12:09
Default Re: How does Fluent calculate Shear
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J.W.Ryu
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Maxim,

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

Thanks so much.

J.W.Ryu
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Old   February 16, 2003, 15:31
Default Re: How does Fluent calculate Shear
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Egorov Maxim (from Russia)
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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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