In this chapter the interaction of wave and porous structures has been discussed. The framework for simulating porous materials was introduced by means of the Volume-Averaged
RANS equations, a set of equations in which porosity and other geometrical properties of the porous media, plus linear and nonlinear friction factors representing the force induced by the porous geometry come into play. Turbulent equations need to be volume-averaged as well and closure terms are required to model a set of terms that cannot be simulated. This closure is currently only available for к-s turbulence model, although it can be argued that the enhancement of turbulent dissipation may be included in the friction terms in the lack of turbulent closure.
Two applications of CFD involving porous media have been modelled with the open source model olaFlow, within the OpenFOAM* framework. In the first, the model has been proven to be a useful tool to test new structural typologies. In this sense, the original structure, which was not ready to withstand a certain wave condition has been modified to continue fulfilling its role. This type of problem will be of importance in the near future, when enhancements to existing structures might be needed to mitigate the risks of raising sea levels and higher- magnitude and more frequent storms (Knutson et al„ 2010).
In the second application sediment grains have been represented as moving particles with a DEM model. The movement and interaction of such particles induces local time changes in porosity, which are also incorporated in the equations. Differences with respect to a fixed smooth bottom simulation have been highlighted. The simulation presents interesting swash- sediment interaction processes that agree qualitatively with previous similar references on experiments and real condition observations. Although this application required a very high computational cost, it highlights the advantages of treating sediment with DEM.
Future research needs to increase the efficiency of the DEM simulations to allow a practical use of the model in consultancy companies. Moreover, in the future, the simulation of solid elements larger than the mesh (e.g., concrete units of the primary armour layer) via the DEM and immerse boundary method (Giro et al., 2013), in conjunction with the VARANS equations for the internal layers and core of the structure should be explored. With this method, the stability of coastal structures, especially in challenging areas such as curved layers and in locations of element size transition could be analysed computationally. Furthermore, variables that are difficult or not possible to measure in the laboratory, as for example shear stresses or forces on individual units, may be obtained with the numerical model without perturbing the flow.
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