CDis related to the Reynolds number, which for cylindrical members and normal water
temperatures is given by the equation:
Re 9.3VD 105 in sec/m^2 units (8.11)Section 5 of BS6349 includes graphs relating CDfor cylindrical members to their surface
roughness and Reynolds number. They show that CDfor rough members is in the range of
0.4 to 0.6 for Reynolds numbers between 10^5 and 10^6. The code gives values for CDand C 1
(CMin equation 8.7) for square section piles as shown in Figure 8.13 and Table 8.2.
If piles or other submerged members are placed in closely spaced groups, shielding of current
forces in the lee of the leading member will occur. Shielding can be allowed for by modifying
the drag coefficient. Values of the shielding coefficient have been established by Chappelaar(8.11).
Where currents are associated with waves it may be necessary to add the current velocity
vectorially to the water-particle velocity uto arrive at the total force on a member. Also, the
possibility of an increase in the effective diameter and roughness of a submerged member
due to barnacle growth must be considered.
Having calculated the current force on a pile it is necessary to check that oscillation will
not take place as a result of vortex shedding induced by the current flow. This oscillation
occurs transversely to the direction of current flow when the frequency of shedding pairs of
vortices coincides with the natural frequency of the pile.
Determination of the critical velocity for the various forms of flow-induced oscillation of
cylindrical members is given in BS6349-1, Clause 38.3, by the equation:
Vcrit KfNWs (8.12)where Kis a constant equal to
1.2 for onset of in-line motion
2.0 for maximum amplitude of in-line motion
3.5 for onset of cross-flow motion
5.5 for maximum amplitude of cross-flow motion
fN natural frequency of the cylinder
Ws diameter of the cylinder.412 Piling for marine structures
Figure 8.13Flow conditions for determining drag conditions.RFlow direction(a) (c)ys(b)