Submerged cantilevered flexible pipe discharging flow in post-critical dynamic regime: an experimental-numerical assessment

Abstract This work presents experimental results and a comparison with a numerical model for the dynamics of a cantilevered flexible pipe with a rigid ballast attached to its free end, submerged in still water, discharging fluid in the post-critical regime. The experiments were carried out using an optical tracking system in order to avoid interference with both the fluid and the structure, while enabling direct measurement of the displacements of multiple cross-sections along the model length. The results demonstrate that the numerical model is able to predict the existence of a critical discharging flow velocity above which the trivial vertical equilibrium configuration becomes unstable and the system develops a steady state nonlinear oscillatory response, characterizing a Hopf bifurcation. However, the experimental dynamics proved to be significantly richer than those predicted numerically. In particular, the experiments revealed the emergence of a 2:1 internal resonance between orthogonal directions, broadband spectral content with multiple harmonic components, and complex three-dimensional figure-eight trajectories. In contrast, the numerical model predicted a predominantly monochromatic and synchronized response associated mainly with the unstable bending mode. The observed discrepancies indicate that the structural formulation combined with a classical Morison drag representation is insufficient to reproduce the full phenomenology of the experimental response. The results therefore suggest the existence of additional fluid–structure interaction mechanisms, as well as the influence of axial dynamics, which are not captured by the present mathematical formulation and require further investigation through more comprehensive numerical models.