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2.1: Definitions

  • Page ID
    29269
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    2.1.1 The Friction Velocity or Shear Velocity u*

    The term friction velocity comes from the fact that √(τ12l) has the same unit as velocity and it has something to do with the friction force. The bottom shear stress τ12 is often represented by friction velocity u*, defined by:

    \[\ \mathrm{u}_{*}=\sqrt{\frac{\tau_{12}}{\rho_{\mathrm{l}}}}=\sqrt{\frac{\lambda_{\mathrm{l}}}{\mathrm{8}}} \cdot \mathrm{v}_{\mathrm{l}_{\mathrm{s}}}\]

    2.1.2 The Thickness of the Viscous Sub Layer \(\ \mathrm{\delta}_{\mathrm{v}}\)

    Very close to the pipe wall the flow is laminar in the so called viscous sub layer. The thickness of the viscous sub layer is:

    \[\ \delta_{\mathrm{v}}=11.6 \cdot \frac{v_{1}}{\mathrm{u}_{*}}
    \\ \delta_{\mathrm{v}}^{+}=\frac{\delta_{\mathrm{v}} \cdot \mathrm{u}_{*}}{v_{\mathrm{l}}}=11.6 \]


    2.1: Definitions is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Sape A. Miedema via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.