What is the detail definition of wall shear stress

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Dear Sir or Madam,

I am sorry to trouble you now, however I can not find the accurate definition of wall shear stress, X-wall shear stress, Y-wall shear stress and Z-wall shear stress. I am wondering that if the X, Y, Z-wall shear stress is equal to the product of fluid viscosity and the shear rate of X, Y, Z-direction and wall shear stress is equal to the product of fluid viscosity and the shear rate at the wall? If it is true then the square of wall shear stress shall be equal to the sum of the square of X, Y and Z –wall shear stress. However it is not abide by the Fluent when I check some points.

Therefore would you please to give me some information about the definition of wall shear stress ,X,Y,Z-wall shear stress?

Thanks a lot! Zhang Junmei