# 3.5: The Friction Velocity or Shear Velocity u*

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 τw is often represented by friction velocity u*, defined by:

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