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Title: Oscillatory flow hydrodynamics and forces for a bottom-mounted horizontal cylinder over smooth and rough beds up to high KC
Author: Skarlas, Panagiotis
ISNI:       0000 0004 9358 5406
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2020
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High Keulegan-Carpenter number conditions (KC of order 102) occur when largeamplitude oscillatory flows, such as occur under long-period waves, interact with a relatively small-diameter circular cylinder located on the seabed. Under such conditions, the degree of containment of the cylinder within the wave bottom boundary layer, δ0/D, is relatively high (where δ0 is a measure of the wave boundary layer thickness and D is cylinder diameter), which means the hydrodynamic forces on the cylinder are affected by the hydrodynamics within the wave boundary layer, and not wholly determined by the free-stream hydrodynamics as assumed by industry guidelines. This research reports on three experimental series, involving two types of bed roughness – one smooth and one rough – up to KC = 184. Experiments were designed to isolate the bed roughness effect on the hydrodynamic horizontal and lift forces acting on a bed-mounted cylinder. Data obtained from the experiments are considered to make recommendations for the practical calculations of wave-induced hydrodynamic forces on seabed pipelines, accounting for the effect of the bed roughness. Results show that the containment ratio for a cylinder over the rough-bed is approximately a factor two greater than that over the smooth-bed for the same flow conditions. For the highest KC, drag, inertia and lift coefficients over rough-bed are approximately 20%, 16% and 30% lower compared to that over smooth-bed. Relatively large-amplitude oscillations seen in the lift force are shown to be caused by the vortex shedding. By not explicitly accounting for bed roughness, the coefficients for peak horizontal and peak vertical forces on a bottom-mounted cylinder recommended by industry guidelines may be overly conservative by 10% to 20% for horizontal force and up to 30% for vertical force for pipes on rough beds and high KC conditions – such as occur under long-period waves passing over relatively small-diameter pipelines.
Supervisor: O'Donoghue, Tom ; Van der A, Dominic A. Sponsor: Lloyd's Register Foundation Centre for Safety and Reliability Engineering
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Hydrodynamics ; Vortex shedding ; Cylinders--Hydrodynamics