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February 19, 2013, 09:26 
wallGradU components

#1 
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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. 

February 20, 2013, 10:14 

#2 
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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 

February 20, 2013, 17:55 

#3 
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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. 

February 21, 2013, 04:07 

#4 
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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


February 21, 2013, 06:14 

#5 
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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 } 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 nonorthogonality or skewness corrections. Hope that helps, Fumiya 

March 1, 2013, 10:36 

#6 
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Amin
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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? 

March 1, 2013, 11:41 

#7 
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Fumiya Nozaki
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Hi Amin,
I think it depends on your simulations:
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 

March 2, 2013, 08:49 

#8  
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Mahdi Hosseinali
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Quote:
So I believe third method Fumiya mentioned would be the best and most reliable 

March 3, 2013, 11:12 

#9 
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Amin
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Hi Fumiya & Mahdi
Thank you. your comments was so usefull. Regards Amin 

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