
[Sponsors] 
February 19, 2013, 09:26 
wallGradU components

#1 
Member
Amin
Join Date: Mar 2012
Posts: 60
Rep Power: 7 
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: 211
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 

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: 270
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: 211
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<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 
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: 211
Blog Entries: 1
Rep Power: 11 
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  
Senior Member
Mahdi Hosseinali
Join Date: Apr 2009
Location: NB, Canada
Posts: 270
Rep Power: 11 
Quote:
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:
So this means that the first component of GradU is dU/dn ? Thanks 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
getting (0 0 0) wallGradU value when i m probing wallGradU at some points  suryawanshi_nitin  OpenFOAM  1  December 9, 2013 12:36 
how to extract the components of radial and tangential velocities  renyun0511  OpenFOAM Running, Solving & CFD  0  April 8, 2011 07:15 
Transient Statistics for turbo components  Suzzn  CFX  5  May 4, 2010 09:56 
velocity components in cylindrical coordinates  iperten  CFX  2  March 31, 2009 08:22 
reynolds stress components in CEL  Roberto  CFX  0  December 3, 2008 04:29 