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

#1 
Member
Vivien
Join Date: Mar 2009
Posts: 52
Rep Power: 9 
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 

February 20, 2009, 13:08 
Hi,
Another question, for lam

#2 
Member
Vivien
Join Date: Mar 2009
Posts: 52
Rep Power: 9 
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 

February 20, 2009, 13:11 
Hi,
Another question, for lam

#3 
Member
Vivien
Join Date: Mar 2009
Posts: 52
Rep Power: 9 
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 

February 20, 2009, 14:17 
Hello Vivien,
As far as I u

#4 
Senior Member

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 

March 18, 2010, 07:33 

#5 
Senior Member
Claus Meister
Join Date: Aug 2009
Location: Wiesbaden, Germany
Posts: 241
Rep Power: 10 
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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