Fluid Mechanics and Acoustics Laboratory - UMR 5509

LMFA - UMR 5509
Laboratoire de Mécanique des Fluides et d’Acoustique

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PhD defense ECL

Satria Darmarnegara

Vendredi 29 mars, à 14 h, amphi 201, Bâtiment W1, ECL

Satria Darmarnegara

Scour at the foot of seawalls

Scour at the foot of the seawalls (toe-scour) is one of the major causes of structural damage to sea-walls, involved in 12% of directly-observed damages. For engineering purposes, simple empirical approaches are often used to predict toe-scour but these are often limited. And whilst data from field observations and experimental studies are both very valuable, they both suffer from practical limitations. Experimental work for example is limited by similarity constraints and full scale experiments are expensive, without allowing detailed control of individual parameters. Numerical simulations therefore offer an interesting alternative but they are not without challenges. The first problem is the correct modelling of free surface dynamics including wave breaking rocesses. The second is modelling the scour development process and its feedback on the flow pattern. Finally, to be practically useful, the method should be robust and use reasonably affordable computational resources for full-scale simulation.

This thesis describes the development of an integrated numerical model is based on OpenFOAM – an open source CFD platform – which includes free surface dynamics, sediment transport and bed deformation processes. The free surface dynamics are modeled using the Volume of Fluid (VOF) method with wave generation and absorption capabilities based on the use of a relaxation zone. The sediment transport process is calculated based on the bed load and suspended load approach and solved in the bed boundary using the Finite Area Method. The bed deformation is calculated using the sediment continuity equation, and the mesh is updated to take account the bed hange.

Several calibration tests have been performed to determine the model capabilities. Firstly, a sensitivity analysis was performed to characterize the influence of mesh size and numerical schemes on wave propagation. Secondly, several methods were compared for eliminating unwanted wave reflection. The model was then used to compute the wave-induced mass transport velocity in a closed flume, and the results compared with the theoretical solution and experimental data. It is shown that the failure to model correctly the pressure condition at the free surface leads to an overestimate of the drift close to the surface, which has to be compensated by an excessive negative drift in the body of the fluid. Fourth, the bed shear stress calculation is tested by studying the case of an oscillatory flow boundary layer. Two methods for computing the bed shear stress have been devised and tested with different mesh sizes. Finally, a simulation is carried out using all of these developments, to simulate the problem of erosion induced by the unsteady flow resulting from a dam-break, and the results are compared with an experimental test case. The sediment transport occurs mainly in the form of sheet flow, and a new method of simulating sheet flow has been devised, based on an analogy with dispersion in a fluidized bed. This approach has the advantage over previous models that it does not require any ad-hoc modification of existing sediment transport models. Finally, the complete model is applied to the case of waves impacting on a sea wall, first for horizontal bed case and second for the case of a sloped beach. The hydrodynamic properties of the flow are analyzed for both cases without using the bed deformation module. Then the scour model with movable bed is included in the simulation, for both cases.

Jury :

LI Ming (Senior Lecturer), University of Liverpool, Rapporteur
CONLEY Daniel (Associate Professor) Plymouth University, Rapporteur
RIVIERE Nicolas (Professeur), INSA Lyon-LMFA, Examinateur
SOEMITRO Ria (Associate Professor) Institut Teknologi Sepuluh Nopember, Examinatrice
VINCENS Eric (Professeur), Ecole Centrale de Lyon-LTDS, Directeur de these
PERKINS Richard (Professeur), Ecole Centrale de Lyon-LMFA, Directeur de these


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