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214    Handbook of gold exploration and evaluation






















                     4.10 Development of boundary layer in open channel with ideal entrance
                     conditions (Dingman, 1984, modified from Chow, 1959).



              boundary layer. Within the boundary layer, flow possesses a velocity gradient
              that enables it to transmit stress. Viscous forces are negligible in flow outside of
              the boundary layer and hence cannot exert stress on that boundary.
                 A simplified two-dimensional profile of a boundary layer with accompanying
              downward transfer of momentum across a wide open channel section is
              demonstrated in Fig. 4.10. The absence of a vertical velocity gradient denotes
              the absence of friction in the flow entering the horizontal boundary which thus
              has a common velocity of v ˆ V 0 . Friction at the boundary retards the flow
              inducing a downward transfer of momentum and creating a boundary layer of
              thickness  . A laminar boundary layer is developed between 0 and x 1 where
              turbulence arises. Thickness of the boundary layer increases and turbulent flow
              is fully developed at x 2 with a thin zone of laminar flow near the bottom. This
              condition is typical of most streams.


              Significance of Reynolds number
              The parameter known as the Reynolds number `Re' provides a relationship
              between inertial forces and viscous forces for all types of fluid motion. Re has
              the dimensions:
                                        2 ÿ1
                                  2 ÿ1
                     Re = VL=v ˆ L T  =L T   ˆ 0                           4.13
              In open channel (stream) flow the characteristic length is taken as the hydraulic
              radius R ˆ A=P, where A is the cross-section of flow and P is the wetted
              perimeter. Equation 4.13 can be re-written:
                     Re = vR=                                              4.14
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