FLOW AROUND A CYLINDER - istiarto

– 2.16 –
maximum depth was observed. The cylinder was transparent allowing the measurements
be made from inside the cylinder with the help of a periscope. The equilibrium scour
depths obtained after about 120 [hours] for all tests are shown in Table 2.2.
2.3.2 Time evolution of scour depth
The time histories of the scour depth are depicted in Fig. 2.5a,b plotted in normal and
logarithmic scales.
Observation of the scour development within the first 3 to 5 [minutes] revealed that the
scouring process was very active. The scour initiated at two points approximately 90° to
the right and left off the centerline. The initial scouring propagated upstream along the
perimeter of the cylinder and the two scours met at the leading edge of the cylinder. The
scour depth increased rapidly that at the end of the first hour it reached 50% of the
equilibrium depth. The rate of scour slowed down during the next 48 [hours], after which
the scour depth did not show a significant increase. In the fifth day the scour depth did
not change appreciably; the scour geometry thus obtained was considered as a near
equilibrium one.
The equilibrium scour depths from the four test runs are in the range of
1.55 � d s D p � 1.95, which are in the range of the measurements reported in the literature
(see Fig. 2.6 and the discussion given in the next section).
Fig. 2.5 Relative scour depth at clear water scour versus time in (a) the normal scale and
(b) the logarithmic scale