Vortex shedding from a vibrating cable with attached spherical bodies in a linear shear flow

Abstract

Marine cables often comprise an integral part of a larger structural system, such as an offshore drilling platform. They are also used to support marker buoys, as well as anchored or towed instrument arrays. Consequently, the resonant flow-induced oscillation of these cables, caused by vortex shedding, is extremely undesirable because of the associated damaging phenomena.

The present study, which employs hot wire anemometry as the principal investigative tool, was undertaken to examine the behavior in the near wake of a flexible, helically wound, high aspect ratio (L/d = 107) marine cable in a linear shear flow (steepness parameter β = 0.0053) at centerline Reynolds numbers between 2.0 x 10³ ≤ Re_c ≤ 4.2 x 10⁴.

Particular attention was paid to lock-on or “synchronization” related changes associated with uniform and sheared flow past the cable when it was forced to vibrate in the first mode. The study was extended to include an analysis of the effects on vortex shedding synchronization phenomena generated by placing spherical bluff bodies along the cable span.

The frequency and reduced velocity boundaries of the lock-on regions, for both the cable and sphere-cable combination, as a function of vibration amplitude up to a/d = 0.322, were measured, plotted and analyzed. The critical reduced velocity of the vibrating cable was U*ᵣ = 5.50, and for the sphere-cable combination U*ᵣ = 3. 39. A sudden increase in the vortex formation length in the near wake of the vibrating cable occurred during perfect synchronization (U*ᵣ = 5.50). Shortly thereafter, the formation length returned to stationary cable values.

At Re_c = 2.96 x 10³, the spanwise vortex shedding frequencies behind the stationary and vibrating cable in a linear shear flow (with and without the presence of spherical bluff bodies) were measured, plotted and the results coup a red. A stable cellular vortex shedding frequency structure, strengthened by the synchronous vibration, existed along the span of the cable. The presence of and spacing between the spherical bodies along the cable span significantly affected the sparwise character of the near wake cellular structure. The length of the locked-on region was substantially increased (almost doubled) when the spherical bodies were present along the span of the cable.