VGeST - Virtual Geoscience Simulation Tools

Coastal Engineering & VGeST

Sea-level rise and increased storminess present huge challenges to coastal engineers worldwide. The seaward slope of many breakwaters and shoreline defence structures consists of thousands of interlocking units of concrete or rock, making up a massive granular defence against wave attack. The units are placed freely to form an armour layer which is intended to both dissipate wave energy and remain structurally stable.

Design guidance on the mass and shape of these units is based on empirical equations derived from Froude scale physical model tests. The two main failure modes for concrete armour layers are displacement (hydraulic instability) and breakage (structural instability), which are strongly coupled. Breakage mechanisms cannot all be faithfully reproduced under scaled physical models.

Fundamental understanding of the forces governing such wave-structure interaction remains poor and unit breakages continue to perplex the designers of concrete armour units. Designers of breakwaters, coastal structures and wave energy devices predict and optimise performance and survivability in storm conditions using wave tanks as their main methodology. Very large tank facilities approaching prototype scale are expensive and smaller scale models can be unreliable due to force relations not scaling correctly. For a better understanding of wave energy turbulence, block movement, and internal stresses within armour units in breakwaters with the possibility of 15m high waves numerical models have enormous potential.

A growing research effort in coastal structure resilience and performance is turning to numerical modelling of armour layer systems where the main tools are DEM, FEMDEM and CFD. The coupling of the solids modelling (DEM and FEMDEM codes developed within the VGW project) to the generic multiphase adaptive unstructured mesh  code “Fluidity” has recently been achieved and validation results have been excellent. To prepare Fluidity for wave modelling, interface tracking for free surface flows and waves, previously non-existent in Fluidity, were developed first by Julian Mindel. Limitations often found with volume of fluids (VOF) approaches in 3D were overcome by developing a completely different basis for modelling air-water and solid-fluid interfaces. (see interface tracking). The breakage of armour units in a simple drop test simulation was modelled with the Y2D FEMDEM code (Latham et al. 2008). A new 3D FEMDEM code with quadratic tetrahedral elements has been added to VGeST, enabling dynamic stresses to be modelled (Xiang, Munjiza and Latham 2009).  Coupling of the solids and fluids was validated for one way coupling of flow through backed beds and placed in the context of wave structure interaction modelling technology for the testing of coastal structures and wave energy devices (Latham et al. 2009).  Stress chains in particulate systems (Guises et al. 2009) have also been investigated which has implications for how armour unit systems transmit forces and stresses. These can also be seen dynamically in 3D simulations of rock silo filling. Development and application of numerical modelling to coastal engineering is further highlighted on proceeding pages.

References

Latham, J.-P., Munjiza, A., Mindel, J., Xiang, J., Guises, R., Pain, C.C., Gorman, G. and Garcia, X. 2008. Modelling of massive particulates for breakwater engineering using coupled FEMDEM and CFD. Particuology 6,  572–583. doi:10.1016/j.partic.2008.07.010

Xiang, J., Munjiza, A. and Latham, J.-P., 2009. Finite strain, finite rotation quadratic tetrahedral element for the combined finite-discrete element method. International Journal for Numerical Methods in Engineering. 79(8), 946-978. doi: 10.1002/nme.2599

Latham, J.-P., Mindel, J., Xiang, J., Guises, R., Garcia, X., Pain. C.,Gorman, G., Piggott, M., Munjiza, A., 2009. Coupled FEMDEM/Fluids for coastal engineers with special reference to armour stability and breakage.  Geomechanics and Geoengineering, Volume 4, Issue 1, 797-805. dx.doi.org/10.1080/17486020902767362

Guises, R., Xiang, J., Latham, J.-P., Munjiza, A. 2009. Granular packing: Numerical Simulation and Characterization of the Effect of Particle Shape, Granular Matter, Vol 11, 281-292  doi:10.1007/s10035-009-0148-0

Mindel (2008) PhD Thesis