Detailed Analyses of Flow in Porous Media and around Cylindrical Structures

This thesis investigates the interaction of flow-structure-porous media in marine environment based on numerical simulations and experimental measurements. Scour protection around a mono-pile and permeable seabed around pipelines, as common marine porous structures, are examined, respectively.

To simulate the flow in porous media, a volume-averaged k-ω turbulence model is developed, which is coupled with the Volume-Averaged Reynolds Averaged Navier-Stokes (VARANS) equations. The developed model, implemented in OpenFOAM, is validated against existing experimental results of flow in stone covers under both oscillatory and steady current conditions, showing a good performance. Compared with the volume-averaged k-ϵ and the k-ω SST turbulence models, the modified turbulence model also provides a more accurate estimation of the flow behaviors within the porous media.

Based on the developed model, the 2D and 3D flow behaviors around a mono-pile with scour protection, under both steady and oscillatory flow conditions, are examined. Near the interface between stone cover and free flow, it shows that the
simulated results with parabolic transition fit the measurements better than that with constant and linear porosity transitions. Under the steady current, the velocities, turbulence levels and bed shear stresses upstream of the pile are relatively
high due to the horseshoe vortex penetrating into the protection layer, whereby sinking of the upstream protection layer usually takes place. Exposed to the oscillatory flow, it is found that the horseshoe vortex and attached lee-wake vortices are formed at two sides of the pile, respectively. They penetrate into the entire protection layers, generating high flow velocities and bed shear stresses. The vortices all increase in both size and intensity as the increment of phase in each half-period. Besides, the flow features under oscillatory flow condition are strongly influenced by the Keulegan-Carpenter number. As the number increases, the vortices inclusive of the horseshoe vortex and the lee-wake vortices, both in size and lifespan augments; the bed shear stress both inside and outside the scour protection also amplifies.

Laboratory experiments and numerical simulations complement each other to examine the effect of burial depth and relative positions of twin pipelines on the wave-seabed-structure interaction (WSSI). Measurements show that, in vicinity of
the tested pipes, pore-water pressure decreases gradually towards the seabed bottom; the pressures around the twin pipelines are larger than those around the single one for otherwise identical conditions. The in-house code WSSI is used to simulate the interactions. Modelling results show that the downstream pipeline mainly affects effective stress and pore pressure around the upstream one rather than the shear stress. The seabed stability around the proximity of the pipelines is enhanced when burial depth increases. Wave-induced seabed dynamics in the lee side of the upstream pipeline weakens as the distance between the centers of twin pipelines increases. When the distance is larger than three times of the pipeline diameter, the twin pipelines can be considered as two separated single pipelines.