FLOW AROUND A CYLINDER - istiarto

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which is indicated by the solid line in Fig. 3.6b. The figure confirms also that the
maximum downward velocity is observed at the cylinder face (at r = 10 [cm] or
r D p � 0.67 as the available data closest to the cylinder). This maximum downward
velocity diminishes with angular distance around the cylinder, from w U � = –0.6 at
� = 0° to w U � = –0.3 at � = 135°, and becomes upward velocity, w U � = 0.35 at
� = 180°, as displayed in Fig. 3.6c. The figure shows also the angular variation of the
downward velocity at the original bed level, z = 0 [cm]; it varies in a similar manner as
the maximum value, from w U � ≈ –0.4 in � = 0° to w U � ≈ –0.2 at � = 135°, and
w U � = 0.25 in � = 180°.
Fig. 3.6 Measured downward velocity around the cylinder: (a) vertical distribution along
the cylinder face (r = 10 [cm]) in different planes, (b) radial distribution of the maximum
values in different planes, and (c) angular distribution of the maximum values and of
the values at the original bed level, z = 0.
The fact that the vertical velocity component has largely negative values, except in the
plane � = 180°, suggests that the scour hole attracts the flow. Inside the scour hole, the