Graduation of Marisol Irias Mata

29 September 2021 14:00 till 16:00 - Location: CiTG - Hall G - By: Webredactie

Numerical estimation of wave loads on crest walls on top of rubble mound breakwaters using OpenFOAM

  • Professor of graduation: Dr. ir.  A. Antonini (TU Delft)

  • Supervisors: Prof. dr. ing. M. van Gent (TU Delft), Dr. ir. M. Zijlema (TU Delft), C. van Lem (Royal Haskoning), S. Boersen, (Royal Haskoning), B. Jansen

 

The design of hydraulic structures like breakwaters and crest walls is often based on empirical
formulations, physical models test, numerical models and a fair amount of expert judgement. Each
technique has its own pros and cons. The main limitation of the empirical formulas is that often they
have to be applied outside their range of validity. Physical modelling also has its own shortcomings.
When breaking waves hit the structure, the location of the maximum pressures is still not well known
due to the high spatial variability. By using an array with low spatial resolution, the forces estimated in
the physical flume will usually underestimate to some extent the actual forces experienced by the wall
(Ramachandran et al., 2013). In the last decades, numerical modelling has become an attractive
alternative in simulating wave-structure interactions. Nevertheless, estimating loads on crest walls in
the numerical flume is still at its early stages. On that account, the present work validates the
prediction of wave induced forces on the front face of crest walls on top of rubble mound breakwaters
in CoastalFOAM. A scale model of the Holyhead breakwater, located in Wales, is used.

The key validation topics are the reproduction of wave-structure interaction when heavily breaking
waves reached the wall, the evaluation of the ventilated boundary condition implemented by Jacobsen
et al. (2018), the porous flow inside the armour layer formed by Tetrapods units and whether the
simplification of not using a turbulence model, as done by Jensen et al. (2014) and Jacobsen et al.
(2018), is also valid under heavily breaking waves.

Four validation cases were used to test the capabilities of the numerical flume. The results confirm
that it is possible to accurately reproduce the wave conditions and the wave induced forces from a
physical modelling campaign. Overall, the CoastalFOAM model is able to capture the shape and the
order of magnitude of the force events. A calibrated model predicts the highest wave induced forces
(forces with an exceedance probability of 0.4% and 0.1%) with errors lower than 20%. Moreover, the
results indicate that for practical applications it is not essential to include a turbulence model in the
numerical flume to obtain reliable forces on the front face of crest walls for different wave conditions.
Another outcome of this study is that the implementation of the ventilated boundary condition is
required in the interface between water surface and structural elements to mimic accordingly the airwater
mixture when the structure is subjected to a heavy wave attack. Nonetheless, there is still room
for improvement in this area, where a better understanding of this boundary condition and of the air
entrapment during wave-structure interaction needs further research.

Despite the large computational time required by the numerical flume when large wave trains must
be simulated, a CFD model during design stages of breakwaters and crest walls provides higher spatial
and temporal resolution of the wave induced pressures exerted on the wall than a physical test.
Therefore, a better picture of the forces and pressure distribution in the front face can be obtained.