Effect of turbulent pulsating flow around a near-wall submerged cylinder on the potential motion of bed and suspended loads

Abstract

In the present work, the pulsating flow around a submerged cylinder close to the impermeable seabed is examined by means of Large Eddy Simulations (LES). A Reynolds number equal to Re = 5000 is considered, based on the length of orbital motion ao and the maximum flow velocity Uo. This study mainly focuses on the detailed description of the flow characteristics and sediment motion in the vicinity of the seabed and most importantly, in the region between the lower side of the submerged obstacle and the impermeable bottom. The sediment phase, consisting of sand particles, is simulated in a Lagrangian framework using the Maxey-Riley equation (1984) in a 4-way coupling regime, i.e. taking into account momentum exchange between the sediment and the fluid phase as well as inter-particle collisions. The consideration of a layer of sand particles released within an orthogonal region from the sea floor up to a level equal to 0.25αo (in the wall normal direction) provides a realistic representation of submerged structures installed in the coastal environment. The cylinder with a diameter D = 0.5αo is represented using the Immersed Boundary Method (IBM). The domain is discretised in space using Cartesian grids and central finite differences, while for time advancement a two-step time-splitting method is considered using a second order Adams-Bashforth scheme. The subgrid scales of motion are modelled using the simple Smagorinsky model. The turbulence behaviour of the flow in the region between the cylinder and the seabed is discussed along with the effect of the distance of the obstacle and the impermeable seabed for distances up to 1.0D. Potential bed load motion under the submerged cylinder is also investigated by analysing the spatio-temporal variation of wall shear stresses. Particle concentration constitutes the suspended load currently in motion. Regions with increased wall shear stresses are identified in the proximity of the cylinder where intense scouring effects could be initiated. © Copyright 2016 by the International Society of Offshore and Polar Engineers (ISOPE).