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Wall shear stress question

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Old   February 20, 2009, 09:54
Default Hi, All, I am trying to und
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Vivien
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Hi, All,

I am trying to understand how is the utility wallShearStress working, I am stuck by the following part:

dimensionedVector
(
"wallShearStress",
Reff.dimensions(),
vector::zero
)

What is "vector::zero" for?? Can somebody give me ideas?

Thanks a lot!

vivien
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Old   February 20, 2009, 13:08
Default Hi, Another question, for lam
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Vivien
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Hi,
Another question, for laminar flow,I want to calculate the wall shear stress, my code is

wallShear.boundaryField()[patchi] = rho * nu.value() * U.boundaryField()[patchi].snGrad();

rho and nu are density and kinematic viscosity. Does it give the shear stress only tangent to the wall? If so, why in the wallGradU utility, the following code calculate gradient in 3 direction(X,Y,Z)?

wallGradU.boundaryField()[patchi] =
-U.boundaryField()[patchi].snGrad();

so basically I dont really understand how the "snGrad" works...does it only return the normal vector of the wall? could somebody help??
Thanks in advance!

vivien
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Old   February 20, 2009, 13:11
Default Hi, Another question, for lam
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Vivien
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Hi,
Another question, for laminar flow,I want to calculate the wall shear stress, my code is

wallShear.boundaryField()[patchi] = rho * nu.value() * U.boundaryField()[patchi].snGrad();

rho and nu are density and kinematic viscosity. Does it give the shear stress only tangent to the wall? If so, why in the wallGradU utility, the following code calculate gradient in 3 direction(X,Y,Z)?

wallGradU.boundaryField()[patchi] =
-U.boundaryField()[patchi].snGrad();

so basically I dont really understand how the "snGrad" works...does it only return the normal vector of the wall? could somebody help??
Thanks in advance!

vivien
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Old   February 20, 2009, 14:17
Default Hello Vivien, As far as I u
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Jose Luis Santos
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Hello Vivien,

As far as I understand, the wallShearStress utility gives you the 3 components. There are some previous discussions related to this issue, please search for it in the forum.

Regards,
Jose Santos
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Old   March 18, 2010, 08:33
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Claus Meister
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According to defintion of shear stress is

tau=mu*du/dy

in order to obatin wall shear stress in 3d you have to use directionale derivative in flow direction, i.e. tangential to wall. The result is :

tau=mu*(grad U,n)

when n is the surface unit vector and (.,.) the scalar product.
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