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 February 19, 2013, 09:26 wallGradU components #1 Member   Amin Join Date: Mar 2012 Posts: 60 Rep Power: 7 Sponsored Links 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 Senior Member   Fumiya Nozaki Join Date: Jun 2010 Location: Yokohama, Japan Posts: 217 Blog Entries: 1 Rep Power: 11 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 lucasbraganca likes this.

 February 20, 2013, 17:55 #3 New Member   Antonio Join Date: Jan 2013 Posts: 6 Rep Power: 6 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 Senior Member   Mahdi Hosseinali Join Date: Apr 2009 Location: NB, Canada Posts: 271 Rep Power: 11 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 Senior Member   Fumiya Nozaki Join Date: Jun 2010 Location: Yokohama, Japan Posts: 217 Blog Entries: 1 Rep Power: 11 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 00178 Foam::tmp > Foam::fvPatchField::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 cfdsolver1 likes this.

 March 1, 2013, 10:36 #6 Member   Amin Join Date: Mar 2012 Posts: 60 Rep Power: 7 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 Senior Member   Fumiya Nozaki Join Date: Jun 2010 Location: Yokohama, Japan Posts: 217 Blog Entries: 1 Rep Power: 11 Hi Amin, I think it depends on your simulations:Solving laminar flows Solving turbulent flows with low Re trubulence model 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 amin66 likes this.

March 2, 2013, 08:49
#8
Senior Member

Mahdi Hosseinali
Join Date: Apr 2009
Posts: 271
Rep Power: 11
Quote:
 Originally Posted by fumiya 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

 March 3, 2013, 11:12 #9 Member   Amin Join Date: Mar 2012 Posts: 60 Rep Power: 7 Hi Fumiya & Mahdi Thank you. your comments was so usefull. Regards Amin

March 25, 2016, 21:09
#10
Member

RacMat
Join Date: Jan 2013
Posts: 73
Rep Power: 6
Quote:
 Originally Posted by fumiya 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 00178 Foam::tmp > Foam::fvPatchField::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
Hi!

So this means that the first component of GradU is dU/dn ?

Thanks

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