# Wall shear stress

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The wall shear stress, $\tau_w$, is given by:
$\tau_w = \mu \left(\frac{\partial u}{\partial y} \right)_{y=0}$
Where $\mu$ is the dynamic viscosity, $u$ is the flow velocity parallel to the wall and $y$ is the distance to the wall.
The SI unit of wall shear stress is pascal ($Pa$), which is identical to $\frac{kg}{m\cdot s^2}$.