# 3.5: The Friction Velocity or Shear Velocity u*

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The term * friction velocity *comes from the fact that \(\ \sqrt{( \boldsymbol{\tau}_{\mathrm{w}} / \boldsymbol{\rho}_{\mathrm{l}}}\) has the same unit as velocity and it has something to do with the wall friction force. The wall shear stress

**τ**

**w**is often represented by friction velocity

**u**

*****, defined by:

\[\ \mathrm{u}_{*}=\sqrt{\frac{\tau_{\mathrm{w}}}{\rho_{\mathrm{l}}}}=\sqrt{\frac{\lambda_{1}}{8}} \cdot \mathrm{v}_{\mathrm{ls}}\]