Experimental study on the performance of different wave absorbers in a wave flume Maarten Delafontaine Supervisors: Prof. dr. ir. Andreas Kortenhaus, Dr. Varjola Nelko Master's dissertation submitted in order to obtain the academic degree of Master of Science in Civil Engineering Department of Civil Engineering Chair: Prof. dr. ir. Peter Troch Faculty of Engineering and Architecture Academic year 2015-2016 Experimental study on the performance of different wave absorbers in a wave flume Maarten Delafontaine Supervisors: Prof. dr. ir. Andreas Kortenhaus, Dr. Varjola Nelko Master's dissertation submitted in order to obtain the academic degree of Master of Science in Civil Engineering Department of Civil Engineering Chair: Prof. dr. ir. Peter Troch Faculty of Engineering and Architecture Academic year 2015-2016 Copyright © 2016 Maarten Delafontaine De auteur geeft de toelating deze masterproef voor consultatie beschikbaar te stellen en delen van de masterproef te kopiëren voor persoonlijk gebruik. Elk ander gebruik valt onder de bepalingen van het auteursrecht, in het bijzonder met betrekking tot de verplichting de bron uitdrukkelijk te vermelden bij het aanhalen van resultaten uit deze masterproef. The author gives permission to make this master dissertation available for consultation and to copy parts of this master dissertation for personal use. In the case of any other use, the copyright terms have to be respected, in particular with regard to the obligation to state expressly the source when quoting results from this master dissertation. June 7, 2016 i Acknowledgements This Master’s Dissertation is the result of a whole academic year of hard work. However, it would not have been possible to bring it to a good end without the help of several people. I would like to thank the following people sincerely: - My supervisors Prof. dr. ir. Andreas Kortenhaus and Dr. Varjola Nelko for giving me the opportunity of working on this very interesting topic and for assisting me with my work during the whole year. - The technicians of the department of Civil Engineering, Herman, Sam and Dave, for constructing and installing all the different configurations tested during the test program of this Master’s Dissertation. I realise that they had a lot of work on preparing my test program due to the quite high number of configurations. - Ir. Tim Verbrugghe for reviewing one of the last versions of my Master’s Dissertation together with Dr. Varjola Nelko. - Prof. dr. ir. Peter Troch for giving me, with his expert-eye, useful explanations and comments. - Mr. Tom Versluys for helping me every now and then with some practical things while I was standing at the wave flume and for explaining how to obtain a proper digital filter with the active wave absorption system. - Ir. David Gallach Sanchez for introducing me to the WAVELAB-software and for giving me his comments on some generated spectra. - Ir. Maximilian Streicher for helping me a few times with some troubles I experienced with the wave gauges. - Finally I would certainly like to thank my parents, my two sisters and my brother for all the support they gave me during my engineering studies. ii Abstract EXPERIMENTAL STUDY ON THE PERFORMANCE OF DIFFERENT WAVE ABSORBERS IN A WAVE FLUME MAARTEN DELAFONTAINE Supervisors: Prof. dr. ir. Andreas Kortenhaus, Dr. Varjola Nelko Master's dissertation submitted in order to obtain the academic degree of Master of Science in Civil Engineering Department of Civil Engineering Chair: Prof. dr. ir. Peter Troch Faculty of Engineering and Architecture Academic year 2015-2016 This Master’s Dissertation is an experimental study on the performance of different wave absorbers in a wave flume. In wave flumes and wave basins it is of paramount importance to have spending beaches, which reflect the incident waves as little as possible. The efficient design of highly absorbing beaches results in the correct reproduction of both coastal and nearshore phenomena in a laboratory testing facilities. Many different wave absorbers can be used as spending beaches in wave flumes or wave basins. An extensive review of the existing literature was prepared here. The most promising wave absorbers were selected and evaluated from the experimental tests in the small wave flume of the department of Civil Engineering of the Faculty of Engineering and Architecture of Ghent University. The wave absorber(s) with the best absorbing capability were determined. The results of this Master’s Dissertation will be used in the design of the upcoming large wave basin that will be built on the Greenbridge-site in Ostend. Key words: Wave absorber, energy dissipation, reflection coefficient iii Experimental study on the performance of different wave absorbers in a wave flume Maarten Delafontaine Supervisors: Prof. dr. ir. Andreas Kortenhaus, Dr. Varjola Nelko Abstract—In laboratory testing facilities, it is very Inclined Planes Wave Absorber), the vertical mesh, important to dispose of wave absorbers with very good the porous parabola and the sloping beach. wave absorbing properties. For this reason the The first absorber, SIPWA, consists of several objective of this research is to determine which wave superposed and inclined planes as can be seen in absorber one should use when designing a laboratory Figure 1. testing facility in order to achieve the best wave absorbing characteristics. This objective is reached by performing tests on several wave absorbers in a wave flume. A sloping beach is found to show the best performance. However, if no space is available to build a proper sloping beach the designer of a laboratory testing facility can also install a vertical mesh or an absorber based on blue foam. Also with these absorbers quite nice wave energy dissipating characteristics can be obtained. Keywords—Wave absorber, energy dissipation, reflection coefficient INTRODUCTION Figure 1 Side view of the Superposed and Inclined Planes In wave flumes and wave basins it is of paramount Wave Absorber importance to have spending beaches, which reflect Two flows are observed around the planes of the the incident waves as little as possible. The efficient SIPWA: a local flow around the individual planes design of a highly absorbing beach leads to the and an overall flow around the combined planes. The correct reproduction of both coastal and nearshore latter is of importance for the working principle of physical phenomena in a laboratory testing facility. the SIPWA, which is twofold. On the one hand there The objective of this research is to determine which is viscous dissipation, originating from breaking up wave absorber one should use when designing a of the eddies, formed at the edges of the planes where wave flume or a wave basin in order to achieve the the overall flow is passing by. On the other hand, best wave absorbing characteristics. there is the resonance effect. This effect implies that, The research is structured as follows: first, a when the wave crest arrives at the absorber, the water literature review is performed. Second, the most level increases in the confining compartment (space promising wave absorbers are selected and evaluated at the back of the superposed planes) and that, when in a hydraulic test program. Finally, based on the the wave trough arrives, the decrease in water level results, conclusions concerning the performance of at the left side of the absorber is compensated by the wave absorbers are drawn. water accumulated in the confining compartment. As such, the resonance effect contributes to the damping LITERATURE REVIEW of the waves. The combination of both the viscous An extensive literature review was performed in dissipation and the resonance effect leads to the preparation of the test program. During this phase, breaking of the incident waves [1]. four different wave absorbers were selected for the The second wave absorber, the vertical mesh, hydraulic model tests: the SIPWA (Superposed and consists of subsequent vertical perforated plates (see Figure 2). As the water needs to flow through the M. Delafontaine is a master thesis student at the Civil Engineering Department, Ghent University (UGent), Ghent, Belgium. E-mail: [email protected] . iv openings of the consecutive perforated plates the seen in this figure, blue foam is placed behind the wave energy is dissipated and the amplitude of the backside of the parabola in order to achieve reflected wave becomes small which results in a additional wave absorbing capability. small reflection coefficient. Figure 3 Side view of the porous parabola with blue foam Figure 2 Side view of the vertical mesh absorber. The The fourth wave absorber selected from the porosities of the different plates used in the vertical mesh literature is a sloping beach made of stones. Two are mentioned above each plate. Dimensions are in sloping beaches were tested, a sloping beach with millimetres. small stones (diameter of 10-16 mm) and a sloping The third wave absorber, presented in Figure 3, is beach with large stones (diameter of 30-35 mm). the porous parabola. The energy dissipation of this Considering that the space limitation is usually an absorber is reached through the breaking of waves issue in the laboratory, we tested a beach slope of [3]. 2/7, relatively on the steep range. The side view of The fourth and final wave absorber that was these configurations is illustrated in Figure 4. selected was a sloping beach. Also this wave absorber makes use of wave breaking in order to dissipate the wave energy [2]. TEST PROGRAM Tested configurations The tests were conducted in the small wave flume Figure 4 Side view of both sloping beaches of the department of Civil Engineering of the Faculty Finally, two additional wave absorbing materials of Engineering and Architecture of Ghent were tested that weren't investigated in earlier University. The flume has a length of 15 m, a width research but that demonstrated good wave damping of 35 cm and a height of 60 cm. properties. The materials are polyether foam and In the test program a total of eight different hexablocks. configurations were tested, including the four When testing the wave absorbing properties of absorbers selected from the literature review. These polyether foam, two different configurations were four absorbers resulted in five configurations as two tested. First, a vertical wall of polyether foam using spending beaches were tested. Geometrical details of two different porosities in its design and second, a the tested SIPWA and vertical mesh absorber are parabolic shaped surface with only one porosity. The shown in Figure 1 and Figure 2, respectively. two different porosities include rough textured Concerning the vertical mesh, both the porosities polyether foam and fine textured polyether foam. of the different plates and the spacings between them Both configurations are illustrated in Figure 5 and are decreasing in the direction of the incident waves. Figure 6, respectively. These ideas were found to be interesting in the research of Twu and Lin [4] and the research of Tiedeman [3], respectively. The exact values of the spacings were chosen in such a way that the total length of the absorber comes close to 1 m, which is a fine length for a wave absorber in the small wave flume. For the porous parabola, three different cross sections were used in order to have a mild slope at the still water level for each of the three water depths used in the test program (see Performed tests for the Figure 5 Side view of the vertical wall of polyether foam different water depths). In Figure 3 the cross section used at a water depth of 25 cm is shown. As can be v Table 1 Characteristics of the different tests in the test program Test number h [cm] H [cm] S [-] T [s] s p p 1 40 3 0.05 0.6201 2 40 3 0.025 0.8901 3 40 7 0.05 0.9734 4 39 7 0.025 1.5961 5 34 3 0.05 0.6204 6 34 3 0.025 0.9020 7 34 7 0.05 0.9928 Figure 6 Side view of the parabola made out of polyether 8 34 7 0.025 1.6703 foam 9 34 10 0.05 1.2744 Hexablocks are already used for a long time at the 10 25 3 0.05 0.6232 department of Civil Engineering, but its performance 11 25 3 0.025 0.9432 was never studied before. A picture of a hexablock 12 25 7 0.05 1.0533 installed in the wave flume is shown in Figure 7. The 13 25 7 0.025 1.8776 used hexablock has a thickness of 30 cm and behind the hexablock there is a confining chamber of 24 cm. As can be seen in Table 1 three different water Like the SIPWA, the hexablock uses the principle of depths have been used: 40 cm, 34 cm and 25 cm. resonance to initiate wavebreaking, but does not Only test number 4 has been executed at 39 cm have viscous dissipative properties. instead of 40 cm due to wave overtopping at the end of the beach. However, in the discussion of this research this test is also treated as a test performed at 40 cm water depth. RESULTS The test data were analysed using the program WAVELAB 3.675. The reflection coefficients for every configuration were calculated and are shown in Figure 8, Figure 9 and Figure 10. Each graph shows the reflection coefficients for the different configurations at a different water depth (40 cm, 34 cm and 25 cm for Figure 8, Figure 9 and Figure 10, respectively). The results show that the reflection coefficients of the sloping beach with large stones are generally lower than those of the other configurations at water depths of 34 cm and 25 cm. At a water depth of 40 cm, this isn't the exact case, Figure 7 Hexablock installed in the wave flume however, the sloping beach still performs very well. After analyzing the results of all the tests, we can conclude that the sloping beach with large stones is Performed tests the most efficient absorber out of all the wave The full test matrix of the performed tests is given absorbers tested in this study. in Table 1. The parameters h, H, S and T represent s p p respectively the used water depth, the significant wave height, the peak wave steepness and the peak wave period used in the different tests. The wave conditions consist of several irregular waves with a Jonswap spectrum. All thirteen tests have been executed for each of the above-mentioned configurations, resulting in a total of 104 tests. Figure 8 Reflection coefficients at a water depth of 40 cm vi water level changes in order to have a curved surface at the still water level. Figure 9 Reflection coefficients at a water depth of 34 cm Figure 11 Alternative for the configuration depicted in Figure 3 As a consequence, it isn't perfectly clear whether one should choose the blue foam or the vertical mesh when there is no room to install a sloping beach with slope 2/7. It depends on the project itself. When the available space is limited and only tests at high water levels are performed (at which the performance of the blue foam is the best), without changing this level a lot (so that the geometry doesn't need to be changed), the configuration with the blue foam should be selected. If there is an intention of changing the water level very frequently, it is Figure 10 Reflection coefficients at a water depth of 25 cm probably more convenient to work with a vertical mesh. Alternatively, if there is no space to install a A disadvantage of the sloping beach configuration is sloping beach with a slope of 2/7, one can still opt to that it is quite long and sometimes it is not possible build a sloping beach with a steeper slope. When the to have enough space available for this absorber. In smallest feasible slope does not exceed 2/7 too this case, one should look for another wave absorber much, this is probably a better option than placing that can be installed in the facility and still shows blue foam or a vertical mesh as the sloping beach is good wave damping properties. Referring to Figure a lot cheaper (85% and 87.5%, respectively) than 8, Figure 9 and Figure 10, two other options are these options and will still perform quite well. recommended: a porous parabola with blue foam and One could also wonder why the sloping beach with a vertical mesh. Placing a parabola constructed out large stones is performing better than the sloping of polyether foam is not an option as its performance beach with small stones. Two reasons can be brought is not good enough at a water depth of 25 cm. forward. The first one is that the sloping beach with small stones is probably not porous enough as not Concerning the first alternative (placing a porous much pores are present between the small stones. parabola with blue foam), it has been observed The second reason is that the sloping beach with during the test program that hardly any wave is small stones was reshaped under wave attack and breaking on the parabola. Consequently, the porous that as such a slightly steeper slope was created just parabola isn't dissipating much wave energy and can beneath the still water level. The stones of the actually be omitted. As a result, only the blue foam sloping beach with large stones did stay in their remains, and a much shorter evenly efficient wave places (not considering some very small absorber is obtained. This wave absorber is depicted movements). in Figure 11 in the case of a water depth of 25 cm. Besides its shorter length the blue foam is also 16.7% cheaper than the vertical mesh. In addition, the vertical mesh configuration also requires more space in the flume/tank. The blue foam shows also slightly better wave damping properties at a water depth of 40 cm (see Figure 8). However, the disadvantage of the blue foam with respect to the vertical mesh is that the geometry needs to be changed whenever the vii