# Wall shear stress question

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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: 8 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: 8 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: 8 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     Jose Luis Santos Join Date: Mar 2009 Location: Portugal Posts: 213 Rep Power: 9 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: 9 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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