- **OpenFOAM Running, Solving & CFD**
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- - **Wall shear stress question**
(*http://www.cfd-online.com/Forums/openfoam-solving/57928-wall-shear-stress-question.html*)

Hi, All,
I am trying to undHi, 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 |

Hi,
Another question, for lamHi,
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 |

Hi,
Another question, for lamHi,
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 |

Hello Vivien,
As far as I uHello 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 |

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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