Wave dissipation in a permeable structure

Wave dissipation in a permeable structure: Experimental research on the physical processes causing energy dissipation inside an array of cylinders

Jansen, Wiljan (TU Delft Civil Engineering & Geosciences)

Uijttewaal, W.S.J. (graduation committee)
Gijón Mancheño, A. (mentor)
Sirks, Emma (mentor)
Kuiper, C. (graduation committee)
Lanzafame, R.C. (graduation committee)
Winterwerp, J.C. (graduation committee)

Delft University of Technology

Civil Engineering | Hydraulic Engineering | Hydraulic Structures and Flood Risk

2019-10-30

Mangrove forests, which serve as a natural sea defence, have been degraded by human action during the last decades. To protect and recover these areas, different types of permeable structures consisting of bamboo have been proposed and applied. However, these structures are currently based on engineering judgement, as design rules are not available. The goal of this research is to make a step forward in the design optimization of permeable structures and gain a better understanding of the processes causing the wave dissipation inside the structure. In this study, scale experiments are conducted in the wave flume at Delft University of Technology. An array of aluminum cylinders is used as a schematization of the structure, with an element diameter of 4 cm and a minimum spacing of 2 cm. The tested wave height is 0.13 m,with a water depth of 0.6 m for the first set of experiments. These experiments evaluated wave transformation through a selected number of configurations. The water depth was 0.55 m for the second set, where the velocities and forces were also measured inside the structure. As the applied wave periods are short (T = 1 − 2 s), the tested wave conditions are in the range of small KC-numbers (4 < KC < 13). The first part of this research focuses on effect of different configurations and arrangements on the amount of energy dissipation, by measuring the incoming and reflected wave heights in front of and behind the structure. For the short waves, the horizontal arrangements dissipate more energy, as energy is dissipated by both the vertical and horizontal drag forces. However, the effect diminishes with increasing wave period. Considering the total dissipation and the amount of dissipation per element, the placement of the elements in rows perpendicular to the direction of wave propagation is found to be the most effective. To study the effect of the element diameter on the dissipation, the results are compared with previous research by Haage (2018) on a model with a diameter and spacing of 2 cm. As with the change in diameter also the structure porosity changed, a direct comparison was not possible and a comparison based on a simplified drag coefficient is done. No direct effect of the diameter is observed, as the obtained drag coefficients show the same trend and magnitude when plotted against an adapted Keulegan-Carpenter number (KC∗), which is based on the element spacing instead of diameter. Small KC∗-numbers result in large drag coefficients,which decrease when KC∗ increases. The second part of this research focuses on the processes inside the structure that cause the energy dissipation. A force and velocity sensor are applied at three locations inside the structure in separate experiments, to determine the relation of the force and velocity inside the structure with the undisturbed values. Two methods are applied for the analysis, based on two different principles. Method 1 is based on the assumption of a constant pair of force coefficients and an increase in velocity inside the structure, method 2 is based on the assumption of a constant velocity and an increase in force coefficients inside the structure.By comparing both methods, it is found that an increase in velocity is the most important factor for the increase in drag force, which is the driving factor for the energy dissipation. It is also found that the amplification factor for the velocity is dependent on both the structure porosity and frontal porosity.

Building with Nature
BioManCo
Drag coefficients
Wave dissipation
Physical scale model
Dalrymple
Permeable structures
Force measurements
Velocity measurements
Morison's equation
Cylinders
Mangroves

http://resolver.tudelft.nl/uuid:acce330d-968a-46a7-853f-73137b06002a

Student theses

master thesis

© 2019 Wiljan Jansen