Numerical and experimental investigation of flow and scour around a half-buried sphere

• Published in: Coastal engineering (Amsterdam) 2013-03, Vol.73, p.84-105
• Main Authors: Dixen, Martin, Sumer, B. Mutlu, Fredsøe, Jørgen
• Format: Article
• Language: English
• Subjects: Bed shear stress; Computational fluid dynamics; Current; Earth sciences; Earth, ocean, space; Effect of turbulence; Engineering and environment geology. Geothermics; Engineering geology; Exact sciences and technology; Experiments; Fluid flow; Geomorphology, landform evolution; Half-buried sphere; k-omega model; Marine and continental quaternary; Mathematical models; Numerical model; Sand; Scour; Sediment transport; Surficial geology; Turbulence; Turbulent flow; Vortices
• ISSN: 0378-3839; 1872-7379
• DOI: 10.1016/j.coastaleng.2012.10.006

The paper describes the results of a numerical and experimental investigation of flow and scour around a half-buried sphere exposed to a steady current. Hot-film bed shear stress and Laser Doppler Anemometer measurements were made with a half sphere mounted on the smooth bed in an open channel. The hydrodynamic model is a 3-D general purpose N–S flow solver. The k-omega SST turbulence model was used for closure. The flow model was used to study the horseshoe vortex and lee-wake vortex flow processes around the sphere. The flow model was coupled with a morphologic model to calculate scour around the half-buried sphere in currents. The morphologic model includes a sediment-transport description, and a description of surface-layer sand slides for bed slopes exceeding the angle of repose. The sediment transport description includes, for the first time, the effect of externally-generated turbulence (induced by the horseshoe-vortex flow and the lee-wake flow processes) on sediment transport. The results show that the scour depth increases and time scale decreases when the effect of externally-generated turbulence is incorporated in the calculations. Empirical expressions representing the numerically obtained data on the equilibrium scour depth and the time scale are presented. The results show that the equilibrium maximum scour depth in the live-bed regime can be approximated by 0.5 D in which D is the sphere diameter. ► Scour description includes, for the first time, effect of externally-generated turbulence. ► This enabled us to switch on and off the externally-generated turbulence in calculations. ► Scour depth increases when the effect of externally-generated turbulence is incorporated. ► Opposite effect for the time scale of scour. ► Increase in scour depth by as much as 35%, and decrease in time scale by as much as O(3).