FLOW AROUND A CYLINDER - istiarto

– 2.17 –
2.3.3 Comparison to other measurements
Various factors affect the scour depth; those are the fluid, the flow, the sediment, and the
cylinder itself (Breusers et al., 1977; Breusers and Raudkivi, 1991; Graf and Altinakar,
1996). The time may also have an effect (Melville, 1975). A general function relating the
relative scour depth, ds/Dp, to the dimensionless parameters is given as follows (Graf and
Altinakar, 1996):
d s
D p
� f U ��
��
��
U cr
, h
,
Dp d
Dp ��
; �g, �s, ���� ��
(2.19)
in which d is the sediment diameter, �g, �s and �� are dimensionless correction
coefficients due to the sediment grading, pier shape, and pier alignment, respectively. The
influence of those coefficients on the relative scour depth is given in graphical forms
based on various measurement data (see Graf and Altinakar, 1996); it is reproduced here
in Fig. 2.6. For a cylindrical pier aligned with the flow as in the present experiment, there
is not any correction due to neither the shape nor alignment of the pier, �s = �� =1.
Putting the results of the present measurements on these graphs, it can be seen that the
effect of the flow velocity and flow depth agrees reasonably with the other measurements
(see Fig. 2.6a,b). The effect of the sediment diameter in the present measurements,
however, underestimates the scour depth when compared with the other measurements
(see Fig. 2.6c). The coefficient of sediment grading, �g, —taking as �g = 0.9 (see Fig.
2.6d)— may explain partially this underestimation.
2.3.4 Selected cylinder diameter
Based on those preliminary experiments and technical considerations relating to the
ADVP configuration, the D p = 15 [cm] cylinder was selected for the experiment with the
velocity measurements (see Sect. 2.2.4). The flow parameters were modified slightly, by
selecting a smaller discharge, Q = 0.200 [m 3 /s], and accordingly a lower flow depth,
h∞ = 18 [cm] (see Table 2.1). The maximum scour depth with this selected flow
parameters is ds = 25 [cm] (ds/Dp = 1.7), which was obtained after a 5-day of continuous
run. Fig. 2.7 shows the geometry of this scour hole. The ridge formed by the deposition
of the bed material in the downstream section extends up to 5 [m] from the cylinder; the
original bed level was found 0.5 [m] further downstream.