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Old   February 19, 2013, 09:26
Default wallGradU components
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Amin
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Hi all
when I use "wallGradU" command for a curved wall (like a cylinder or sphere), i have 3 components for wallGradU in paraview. what are them?

are ( dUx/dx , dUy/dy , dUz/dz )? if x, y, z been cartesian system....

or if, for the curved wall, we assume normal spatial component to "n" and tangential components to "t1" and "t2" , then 3 components of wallGradU are (dUn/dn , dUt1/dt1 , dUt2/dt2 ) ???

Thanks all
Amin

Last edited by amin66; February 19, 2013 at 09:42.
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Old   February 20, 2013, 10:14
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Fumiya Nozaki
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Hi Amin,

I think the three components of wallGradU are (Ux/Δ , Uy/Δ , Uz/Δ).

where
Δ: distance between cell center and the wall
U = (Ux, Uy, Uz)

The vector wallGradU is the approximation of dU/dn (n: wall normal direction).

Hope that helps,
Fumiya
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Old   February 20, 2013, 17:55
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Antonio
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Hello Friends

fumiya, how can you find the location af separation point at a curvactured wall with this component?

Last edited by Antonio.vucci; February 20, 2013 at 19:33.
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Old   February 21, 2013, 04:07
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Mahdi Hosseinali
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Separation point is where the second derivative becomes zero, there is no way you can find it with first derivative
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Old   February 21, 2013, 06:14
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Fumiya Nozaki
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I have misunderstood the utility "wallGradU" and the "snGrad()" function used in it.
In the utility "wallGradU", the wall normal gradient is calculated using the "snGrad()"
function and this function discretizes the normal gradient using the following first
order approximation:

In fvPatchField.C
Code:
00176    // Return gradient at boundary
00177    template<class Type>
00178    Foam::tmp<Foam::Field<Type> > Foam::fvPatchField<Type>::snGrad() const
00179    {
00180        return (*this - patchInternalField())*patch_.deltaCoeffs();
00181    }
snGrad() = (U[faceI]-U[cellI])/mag(Cf[faceI]-Cn[cellI])
where
faceI: label of the face on which we calculate the normal gradient
cellI: label of the cell adjacent to the faceI
Cf: position vector of face center
Cn: position vector of cell center

As you can see, the surface normal gradient calculated by wallGradU does not apply any
non-orthogonality or skewness corrections.

Hope that helps,
Fumiya
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Old   March 1, 2013, 10:36
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thanks all
Mahdi, as i know separation occurs when the first derivation of velocity near the wall becomes zero.

Fumiya, is there any way to determine separation point with wallGradU on cylinder? or even with other methods?
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Old   March 1, 2013, 11:41
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Fumiya Nozaki
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Hi Amin,

I think it depends on your simulations:
  1. Solving laminar flows
  2. Solving turbulent flows with low Re trubulence model
  3. Solving turbulent flows with high Re turbulence model

In the case of 1 and 2:
You can use the wallGradU to find the positions where wall normal gradient of velocity is zero.

In the case of 3:
You can use the wallShearStress utility to find the positions where the wall shear stress vanishes.

Correct the mistakes, if any, in my post.

Fumiya
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Old   March 2, 2013, 08:49
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Quote:
Originally Posted by fumiya View Post
Solving turbulent flows with high Re turbulence model

You can use the wallGradU to find the positions where wall normal gradient of velocity is zero.

Fumiya
As you know when you capture viscose sub-layer it is a linear profile, so the derivative of it is zero, while the flow is not separated. Flow separates when you have zero shear stress ( You can derive it easily from NS equations, just put non-slip conditions and look for zero pressure gradient )

So I believe third method Fumiya mentioned would be the best and most reliable
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Old   March 3, 2013, 11:12
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Hi Fumiya & Mahdi

Thank you. your comments was so usefull.

Regards
Amin
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