Numerical modelling of scour around complex 3D structures

1. Geology and Geophysics Research Group Fluid Structure Interactions Research Group Numerical modelling of scour around complex 3D structures Greg Melling1, Justin Dix1, Stephen Turnock2 and Richard Whitehouse3 1 Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, UK 2 Engineering and the Environment, University of Southampton, UK 3 HR Wallingford Ltd, UK gjm1v07@soton.ac.uk Motivation Objectives • Scour is understood as the removal of sediment in response to forcing by waves and currents. • Scour is a function of the local hydrodynamics and geotechnical properties of the seabed. It is often associated with marine structures due to locally amplified flow, turbulence and bed shear stresses. • The ability to predict scour around seabed structures is vital since non-treatment of scour can result in expensive structural failures. • In engineering practice often predictive empirical formulae are used for the prediction of scour around simple structure shapes • The development of a CFD-based approach has the advantage of offering a temporally and spatially resolved method for the assessment of scour around structures with complex shapes. • Critically appraise possible CFD-based methods for the simulation of scour • Develop a method within the CFD toolbox openFOAM capable of predicting the hydrodynamically-induced change in morphology of a non-cohesive sediment seabed • Validate the performance of the scour model in light of published physical model data and recently acquired field • Apply the developed method to: • Tidal, bi-directional flow • Real-life consultancy problem • Evaluate maximum lateral and vertical scour extent and time-development of scour in the model Figure 1: Scour around caisson Granular two-phase model 2D Test case and initial results • The chosen two-phase method solves conservation equations for both the fluid and solid phase in the Eulerian framework. Phases are treated as separate but interpenetrating continua. Momentum-exchange between the two phases is considere by additional inter-phase exchange terms consisting of contributions from drag, lift and virtual mass forces • The conservation equations are derived from single-phase flow equations by conditional ensemble averaging: • A test case was set up to evaluate initial performance of the model for scour around a pipeline. • Figure 2 illustrates the process of scour onset for a pipe in contact with a non-cohesive bed. • Following the undertunneling of the pipe the jet scour phase begins as illustrated in Figure 3. Figure 2: Scour onset at a pipe (Mao, 1986) Continuous phase stress tensor. Figure 3: Jet scour phase (Mao, 1986) Dispersed phase stress tensor. Closure models from kinetic theory of granular flow • Figure 4 shows that the vortices which drive the scouring process are captured. • The solid-phase stresses are derived from particle-particle collisions. The intensity of the particle velocity fluctuations determines the stresses, viscosity, and pressure of the solid phase. Closure of the solids stress tensor is achieved by empirical models derived from the kinetic theory of granular flow. • Turbulence closure is achieved using a mixture k-ε model, a slight extension of the standard single-phase implementation. One short-coming of this simple model is that it does not consider turbulence modulation owed to the presence of the particulate fraction: Figure 4: Simulated vortices around the pipe at onset of scour • Figure 5: The erosive forces of the vortices have begun to remove sediment from under the pipe. A large pressure difference inside the soil leads to piping in the upper bed aiding in the eventual breakthrough of a tunnel beneath the pipe. • Figure 6: Once a path between pipe and bed has been created, rapid high-energy jet scour prevails, with high flow velocities in the tunnel and rapid removal of sediment. Figure 5: Initiation of jet scour phase Figure 6: Jet scour develops FSI Away Day 2012 Acknowledgement: This project is supported by funds from the Natural Environment Research Council and HR Wallingford Ltd