PhD defence

PhD Defence Tuesday 10th June: Wave Interaction with Porous Coastal Structures

Thursday 22 May 14

Contact

Erik Damgaard Christensen
Head of Section, Professor
DTU Mechanical Engineering
+4545 25 13 98

The defence takes place in Building 101, room S09, DTU, 10th June at 14:00.

Abstract
Porous breakwater structures are widely used as protection against waves for ports and harbours as well as for general coastal protection. The structures differers depending on the exact purpose e.g. harbour protection, detached breakwaters, groins, submerged breakwaters etc. Typical types of structures are rubble mound breakwaters and berm breakwaters where common structural elements are core material, filter layers and armour layers.

The armour stones serves as the main protection of the filter and core material against wave action. Therefore the armour stones must maintain stable when exposed to waves. The general design methods are based on a long tradition of experimental investigations in scale models. This has resulted in empirical design formulas which in combination with physical model tests during the design phase constitutes the typical approach to breakwater design. Numerical models are also applied as part of investigating and designing breakwaters.

The models can provide more detailed information on some topics, such as pressure attenuation through the porous core material, while it is more difficult to simulate the direct destabilisation and movements of individual stones. The present study seeks to extend the methods currently applied to gain more insight into the physical processes involved with armour layer destabilisation. Both experimental and numerical methods are treated.

In Chapter 2 the flow and turbulence around armour layer stones as well as the shear stresses are investigated based on physical experiments. A detailed methodology was applied which takes a different approach than normally seen for breakwater experiments.
The physical processes related to generation of turbulence were separated by means of a series of experiments with increasing complexity. Hereby the contribution to generation of turbulence, and destabilizing shear stresses, from the wave breaking, the armour layer, and the porous core was singled out. In Chapter 3 a similar detailed approach was taken towards experimental investigation of the pressure induced forces in the filter layers below the main armour stones. Here it was shown how pressure gradient in the filter layer can contribute to the destabilization of the structure. In Chapter 4 the numerical approach towards breakwater investigations was treated in terms of resistance type porosity models solved with the Navier-Stokes equations.

The method was based on adding the effect of the porous media via the Darcy-Forchheimer equation to the momentum equation. This is a well know method that has been applied for several decades. A detailed derivation was presented of the volume averaged Reynolds averaged Navier-Stokes (VARANS) equations that forms the basis for the model. With this derivation it was possible to show the origin of the resistance terms which are eventually modelled with the Darcy-Forchheimer equation.

The model was calibrated by including several calibration cases that has not previously been applied. Hereby several flow regimes were included giving a better understanding and applicability of the calibrated coefficients. In Chapter 5 the porosity model was extended to be coupled with an immersed boundary method (IBM). This method provides a simple mean of including complex geometries in the numerical model without the need for complicated mesh generation. Hereby parts of the structure, such as the armour layer, can with relative ease be resolved directly. This can provide results such as flow and turbulence around the armour stones and the direct forces on the stones for evaluation of stability. An example was shown with the simulation of a rock toe structure on a rubble mound breakwater.

The stones in the toe structure were resolved directly in the model while the rest of the breakwater was included with the porosity model. In Chapter 6 both experimental and numerical topics are included. The physical experiments includes the first results from the experiments on flow and turbulence around armour stones. These results are presented in greater details in Chapter 2. The numerical simulations includes the flow around the idealised armour layer in terms of spheres which are compared to the measurements from the experiments.

Principal supervisor: Professor Erik Damgaard Christensen
Co supervisor: Professor B. Mutlu Sumer