VORTEX-INDUCED OSCILLATIONS 615
14.12.2 In-line oscillationsExperimental studies on the in-line response of flexible cylinders exposed to
steady flow exhibit two peaks, as shown in Fig. 14.22. The peak at the higher
value of the reduced velocity V/fnDis associated with the alternate shedding
of the vortices and the one at the lower reduced velocity with the vortices
shed symmetrically from the cylinder. The critical value of V/fnDat which
oscillations are initiated is found to be a function of Ks(Fig. 14.23). It has also
been found that no excitation in the in-line direction occurs for Ks1.8.
Fig. 14.22 Typical response of a vertical pile in a steady current
Fig. 14.23 Criterion for onset of instability with respect to in-line oscilla-
tions (Hallam, Heaf and Wootton, 1978)