A variationally bounded scheme for delayed detached eddy simulation: Application to vortex-induced vibration of offshore riser

We present a bounded and positivity preserving variational (PPV) method for the turbulence transport equation of Spalart-Allmaras based delayed detached eddy simulation (DDES). We employ the developed solver to simulate the vortex-induced vibration of a slender flexible riser immersed in a turbulent flow. The fluid-structure interface problem is solved by recently proposed partitioned iterative scheme [1], which consists of nonlinear force correction for a stable coupling of the Navier-Stokes equations with the low-mass structure subjected to strong inertial effects from the surrounding incompressible flow. In our fluid-structure model, we consider the flexible cylindrical riser as a long tensioned beam via linear modal analysis. In the present contribution, we first validate the PPV-based DDES solver for the flow past a three-dimensional stationary cylinder at two subcritical Reynolds numbers of Re=3900 and Re=140,000 based on the diameter of the cylinder. We next simulate a three-dimensional flexible riser model under a pinned-pinned condition at Re=4000 and compare our results with that of the experimental measurements. We assess the response characteristics at various locations along the span of the riser and discuss the orbital trajectories at those locations. In particular, the numerical study emphasizes on (i) The effectiveness of the modal analysis in modeling the riser dynamics and a positivity-preserving variational method in DDES for turbulence modeling, (ii) validation of response amplitudes and motion trajectories, and (iii) new insights on the trajectories and vortical structures of the flexible riser undergoing vortex-induced vibrations (VIV). We confirm a standing wave response and complex chaotic response of riser VIV observed in the recent experiments.