Showing papers on "Wave flume published in 2012"

Eulerian-Lagrangian analysis for particle velocities and trajectories in a pure wave motion using particle image velocimetry.

Abstract: This paper investigates the velocity and the trajectory of water particles under surface waves, which propagate at a constant water depth, using particle image velocimetry (PIV). The vector fields and vertical distributions of velocities are presented at several phases in one wave cycle. The third-order Stokes wave theory was employed to express the physical quantities. The PIV technique's ability to measure both temporal and spatial variations of the velocity was proved after a series of attempts. This technique was applied to the prediction of particle trajectory in an Eulerian scheme. Furthermore, the measured particle path was compared with the positions found theoretically by integrating the Eulerian velocity to the higher order of a Taylor series expansion. The profile of average travelling distance is also presented with a solution of zero net mass flux in a closed wave flume.

Experimental Rotations of a Pendulum on Water Waves

Abstract: The rotations of a parametric pendulum fitted onto a suitable floating support and forced to move vertically under the action of water waves have been studied on the basis of a dedicated wave flume laboratory experiment. An extended experimental campaign has been carried out with the aim of providing insight into the mechanics of the pendulum’s response to the wave forcing and data useful as a benchmark for available theories. A large number of time histories of the pendulum’s angular position have been collected. Rotations have been detected for different values of the frequency and of the amplitude of the excitation, showing the robustness in parameter space, and for different initial conditions, showing the robustness in phase space. This experiment, suggested by the recently developed concept of extracting energy from sea waves, constitutes preliminary experimental proof of that concept’s practical feasibility.

A Semi-Infinite Numerical Wave Flume Using Smoothed Particle Hydrodynamics

Abstract: This paper presents the new boundary condition implemented in the LNEC (Laboratório Nacional de Engenharia Civil) SPH (Smoothed Particle Hydrodynamics) numerical model based on the SPHysics model and on a standard SPH formulation: This piston-type wave-maker now includes dynamic wave absorption and allows the simulation of a semi-infinite flume. Verification of the active wave-maker absorption is carried out through the simulation of the interaction between a regular incident wave and an impermeable vertical breakwater. Results show that the active wave-maker allows outgoing waves to be absorbed and reflection at the wave-maker to be avoided.

Steady streaming and sediment transport at the bottom of sea waves

Abstract: The flow and sediment transport in the boundary layer at the sea bottom due to the passage of surface waves are determined by considering small values of the wave steepness and of the ratio between the thickness of the boundary layer and the local water depth. Both the velocity field and the sediment transport rate are determined up to the second order of approximation thus evaluating both the steady streaming and the net (wave-averaged) flux of sediment induced by nonlinear effects. The flow regime is assumed to be turbulent and a two-equation turbulence model is used to close the problem. The bed load is evaluated by means of an empirical relationship as function of the bed shear stress. The suspended load is determined by computing the sediment flux, once the sediment concentration is determined by solving an appropriate advection‐diffusion equation. The decay of the wave amplitude, which is due to the energy dissipation taking place in the boundary layer, is taken into account. The steady streaming and the sediment transport rate at the bottom of sea waves turn out to be different from those which are observed in a wave tunnel (U-tube), because of the dependence on the streamwise coordinate of the former flow. In particular, in the range of the parameters presently investigated, the sediment transport rate at the bottom of sea waves is found to be always onshore directed while, in a water tunnel (U-tube), the sediment transport rate can be onshore or offshore directed.

Experimental investigation on the evolution of the modulation instability with dissipation

Abstract: An experimental investigation focusing on the effect of dissipation on the evolution of the Benjamin–Feir instability is reported. A series of wave trains with added sidebands, and varying initial steepness, perturbed amplitudes and frequencies, are physically generated in a long wave flume. The experimental results directly confirm the stabilization theory of Segur et al. (J. Fluid Mech., vol. 539, 2005, pp. 229–271), i.e. dissipation can stabilize the Benjamin–Feir instability. Furthermore, the experiments reveal that the effect of dissipation on modulational instability depends strongly on the perturbation frequency. It is found that the effect of dissipation on the growth rates of the sidebands for the waves with higher perturbation frequencies is more evident than on those of waves with lower perturbation frequencies. In addition, numerical simulations based on Dysthe’s equation with a linear damping term included, which is estimated from the experimental data, can predict the experimental results well if the momentum integral of the wave trains is conserved during evolution.

On mooring design of wave energy converters: the seabreath application

Abstract: The design of a mooring system of a Wave Energy Converter is a challenging process that points out several unsolved technical problems, mostly related to the highly non-linear hydrodynamic phenomena occurring when high waves (e.g. 8 m high with 200 m wavelength) propagate in relatively shallow waters (e.g. 20 m). The aim of this note is to point out the relevance of the non-linear response of a WEC anchored in relatively shallow waters (shallow in the “non-linear” sense) in terms of loads applied to the mooring lines. Further, the effects of this cyclic load on the anchors is investigated. Note that to some extent it is like checking the importance of geotechnical and coastal engineers in the design process of the WEC structure and its mooring system (typically carried out by naval architects). The whole mooring design process is first outlined and then it is schematically applied to a specific case, namely a promising Italian device named SeaBreath (www.seabreath.it), in view of a possible deployment in the Adriatic Sea. The main concern of mooring designers is related to resonance effects induced by the second order drift. Therefore specific tests have been carried out in the 36 m long x 1.0 m wide x 1.3 m high wave flume of Padova University. Tests focused on the forces on the mooring lines induced by the sum of two regular waves of similar frequency. The mooring design is still far from complete: the physical model proved the relevance of the aforementioned effects but a numerical investigation (not yet performed) is required to draw final conclusions.

Wave groups and sediment resuspension processes over evolving sandy bedforms

Abstract: High resolution measurements of the water velocity, bedforms and suspended sediment concentration were made using an Acoustic Doppler Velocimeter, acoustic bedform scanners and an Acoustic Backscatter System, under irregular free-surface waves. The waves were generated in a large scale flume facility above a number of bedform types. These data were analysed in (i) the frequency domain in order to examine the frequency at which sediment suspensions occurred in the oscillatory bottom boundary layer and the free stream; and (ii) the time domain in order to examine the instantaneous entrainment and vertical transport of sediment at intra-wave, wave average and wave group time scales. During the course of the experiments the significant wave height was systematically incremented enabling the character of sediment suspensions to be studied under a number of flow and bedform regimes. Wave groups were identified as an important control over sediment suspensions in both the wave boundary layer and free stream, with fluctuations in the suspended sediment concentration occurring at low, wave group, frequencies. However, the initial entrainment process, within the wave boundary layer, occurred at intra-wave frequencies. In contrast, in the free stream, sediment suspensions were dominated by the vertical transport of sediment at wave group time scales. During wave groups the sediment suspension field was characterised by the upward transport of sediment due to the continual injection of turbulence under a series of waves which generated a wave pumping effect. The character of a wave group is considered to be an important control over sediment suspensions in the free stream. Four distinct types of wave group were identified and the instantaneous sediment suspension field below each type examined. Such comparisons were possible using the high resolution Acoustic Backscatter System which enabled both intra-wave and wave group processes to be resolved up to 0.8 m above the bed.

Sand transport under full-scale progressive surface waves

Abstract: The morphology of coastal areas is constantly changing under the influence of sediments being transported to, from and along the coast. Under storm conditions with high waves and flow velocities, bed forms are being washed out and large quantities of sand are transported in a thin, mm to cm thick layer close to the bed called the sheet-flow layer. Since sand transport under storm conditions is primarily controlled by small scale near-bed processes, development of well-founded methods for predicting near-bed sand transport are critical for estimating sand budget in coastal areas. Various transport models have been developed to predict both the quantities and directions of sediment transport under storm conditions. The majority of the existing models are based on data obtained from oscillatory flow tunnel experiments. Even though oscillatory flow tunnels provide a good approximation of the flow experienced at the sea bed, theory and former experiments indicate that flow differences between full scale progressive surface waves and oscillatory flow tunnels may have a substantial effect on the net sand transport. The research presented in this thesis focuses on the influence of surface wave effects on sand transport under sheet-flow conditions. For the first time, detailed measurements of wave boundary layer flow and sheet-flow layer transport processes under full scale surface waves are presented and analysed. These results give new insights and provide quantitative data of wave boundary flow, sheet-flow layer concentrations, sediment fluxes and net transport rates under velocity skewed surface waves for different wave conditions and types of sediment.

An Experimental Study of Changes of Beach Profile and Mean Grain Size Caused by Tsunami-Like Waves

Abstract: Chen, J.; Huang, Z.H.; Jiang, C.B.; Deng, B., and Long, Y.N., 2012. An experimental study of changes of beach profile and mean grain size caused by tsunami-like waves. Devastating tsunami waves can mobilize a substantial amount of coastal sediments and change the coastal morphology considerably. In this study, laboratory experiments were performed in a wave flume to investigate the changes of beach profile and mean grain size caused by tsunami-like waves that have a front similar to that of a solitary wave and an undulating tail following the wave front. A composite slope was constructed using a mixture of two sizes of sand. In addition to recording the wave rush-up and rush-down processes using video cameras, measurements were also made on water surface elevation, bed profile and the final size distribution of the sand mixture for three water depths. The results showed that sand suspension and erosion were caused mainly by sheet flows when water retreats from the beach, whereas the offshore sand...

Breaking wave kinematics, local pressures, and forces on a tripod structure

Abstract: This paper presents breaking wave loads on a tripod structure from physical model tests and numerical simulations. The large scale model tests (1:12) are described as well as the validation of the three dimensional numerical model by comparison of CFD wave gauge data and pressures with measurements in the large wave flume inside and outside the impact area. Subsequently, the impact areas due to a broken wave, a curled wave front as well as for wave breaking directly at the structure with a partly vertical wave front are compared to each other. Line forces in terms of slamming coefficients with variation in time and space are derived from CFD results and the velocity distribution is presented at the onset of wave breaking. Finally, the results are briefly discussed in comparison to other slamming studies.

Loading of vertical walls by overtopping bores using pressure and force sensors - A large scale model study

Abstract: This study is based on the data obtained from tests carried out in the Large Wave Flume (Grosser Wellenkanal (GWK)) in Hannover in the frame of a joint research project of Ghent University (Belgium) and Forschungszentrum Kuste (FZK, Germany). The goal of the research project is to determine the wave induced loads on vertical storm walls located at the end of overtopped dike, which are designed to protect coastal cities from overtopping and floods. The loads resulting from waves overtopping the dike and impacting the vertical wall as a bore are measured by means of both force and pressure sensors. This paper describes the results of pressure and force records at the vertical wall, including a comparative analysis of the overall forces obtained by pressure integration and force sensors for two different wall setups: Fully blocked wall and partially blocked wall.

Simulation of Wave Overtopping of Maritime Structures in a Numerical Wave Flume

Abstract: A numerical wave flume based on the particle finite element method (PFEM) is applied to simulate wave overtopping for impermeable maritime structures. An assessment of the performance and robustness of the numerical wave flume is carried out for two different cases comparing numerical results with experimental data. In the first case, a well-defined benchmark test of a simple low-crested structure overtopped by regular nonbreaking waves is presented, tested in the lab, and simulated in the numerical wave flume. In the second case, state-of-the-art physical experiments of a trapezoidal structure placed on a sloping beach overtopped by regular breaking waves are simulated in the numerical wave flume. For both cases, main overtopping events are well detected by the numerical wave flume. However, nonlinear processes controlling the tests proposed, such as nonlinear wave generation, energy losses along the wave propagation track, wave reflection, and overtopping events, are reproduced with more accuracy in the first case. Results indicate that a numerical wave flume based on the PFEM can be applied as an efficient tool to supplement physical models, semiempirical formulations, and other numerical techniques to deal with overtopping of maritime structures.

Secondary waves in coastal zone: physical mechanisms of formation and possible application for coastal protection

Abstract: The formation of secondary wave in a coastal zone was investigated on the base of field, laboratory and numerical experiments. It was found that formation of secondary waves is essentially part of weakly nonlinear-dispersive wave transformation and determined by a periodic exchange of energy between the first and second harmonics. The formation of secondary waves depends on a stage of wave transformation and defined by amplitude of secondary harmonic and by phase shift between first and second harmonics. On the base of numerical modeling and laboratory experiments an idea of combination of underwater structures with floating breakwater is investigated. Waves propagating above submerged bar generate secondary waves that decrease the mean period of waves. Each additional bar reinforces and stabilizes this effect. Behind the bars the floating breakwater can be applied, because it suppresses successfully only short waves. Advantages and disadvantages of this idea are discussed.

Design and construction of an optimum wave flume

Abstract: This paper describes the design, construction, and testing of a small wave flume and associated equipment. The wave flume is equipped with a flap-type wave generator, capable of producing both regular and irregular waves. Flap paddle is used to examine deep water and floating structures. Instrumentation includes three capacitive wave gauges. The wave-maker is controlled by a desktop computer, which also provides data-logging capability. In order to design a wave absorber of restricted length, all the different mechanisms able to dissipate wave energy studied. Finally from the survey of the wave absorbers used in different laboratories, a gravel beach designed for absorbing the waves coming from wave maker. By sealing leaks around paddle the error of measuring height-to-stroke ratios versus relative depth is decreased by 3%. The main advantage of this system over the previous ones is reduction of the amount of effort in the set up with optimum deflection and low level cost utility.

Wave run-up observations on revetments with different porosities

Abstract: Wave run-up plays an important role in the design of coastal protection structures. However, none of the existing formulae for wave run-up predictions explicitly considers the effect of revetment porosity. Recently, two revetments have been tested in the Large Wave Flume (GWK) of Forschungszentrum Kuste (FZK), a new type of highly porous polyurethane bonded (PBA revetment) revetment and a smooth interlocked pattern placed concrete block revetment (IPPB revetment), which is considered as “weakly permeable” for the present study. Wave run-up is evaluated by video data analysis based on timestack image processing. The results derived from the timestacks are compared to run-up data measured with conventional wire gauges and the good agreement demonstrates the accuracy and reliability of the video data analysis. The effect of the porosity of the revetment is incorporated into the EuroTop wave run-up formula, showing that for the present case it may reduce the relative run-up heights Ru,2%/Hm0 by about 25 % to 50 % as compared to a smooth impermeable slope.

Applicability of CADMAS-SURF to evaluate detached breakwater effects on solitary tsunami wave reduction

Abstract: Detached breakwaters, made with wave-dissipating concrete blocks such as Tetrapods, have been widely applied in Japan, but the effectiveness of such kinds of detached breakwaters on tsunami disaster prevention has never been discussed in detail. A numerical wave flume called CADMAS-SURF has been developed for advanced maritime structure design. CADMAS-SURF has been applied mainly to ordinary wave conditions such as wind waves, and little attempt has been made for expanding its application to tsunami waves. In this study, the applicability of CADMAS-SURF for evaluating the effectiveness of detached breakwaters on a solitary tsunami wave reduction is investigated by comparing the calculated results with those from hydraulic experiments. First, the effectiveness of a detached breakwater on the reduction of wave height and wave pressure was confirmed both by hydraulic experiments and numerical simulations. Finally, CADMAS-SURF has been found to be a useful tool for evaluating the effects of detached breakwaters on tsunami wave height and pressure reduction, as a first step in a challenging study.

Contact line dynamics and boundary layer flow during reflection of a solitary wave

Abstract: In this paper we present a set of wave flume experiments for a solitary wave reflecting off a vertical wall. A particle tracking velocimetry (PTV) technique is used to measure free-surface velocity and the velocity field in the vicinity of the moving contact line. We observe that the free surface undergoes the so-called rolling motion as the contact line moves up and down the vertical wall, and fluid particles on the free surface almost always flow toward the wall except at the end of the reflection process. As the contact line descends along the wall, wall boundary layer flows move in a downward direction and therefore the boundary layer acts like a conduit through which the surface-rolling-induced flow escapes from the meniscus. However, during the last phase of the reflection process flow reversal occurs inside the wall boundary layer. An approximate analytical solution is developed to explain the flow reversal feature. Very good agreement between the approximate theory and measured data is obtained. Because of the flow reversal, boundary layer flows collide with the surface-rolling-induced flows. The collision gives rise to a jet ejecting from the meniscus into the water body, which later evolves into a small eddy. It is noticed that the fluid particles in different regions such as the free stream, the free-surface boundary layer and the wall boundary layer, can be transported to other regions by passing through the meniscus.

Modelling of the impact of a wave farm on nearshore sediment transport

Abstract: This paper presents the results from an integrated modelling system investigating the effects of a wave farm on nearshore sediment transport. Wave Hub project is a large scale demonstration site for the development of the operation of arrays of wave energy generation devices located at the southwest coast of the UK where multiple field measurements took place. The two-way coupled SWAN and ROMS models with nested modelling system were set up at the Wave Hub site and run with and without a wave farm. The model results show that the presence of the wave farm has significant impacts on the nearshore circulation, bed shear stresses and sediment transport. The morphological changes are also altered by the wave farm. The study is the key element for the wave resource characterization and environmental impact assessment of the wave farm.

The morphological response of beaches protected by different breakwater configurations

Abstract: A series of experimental tests on a 2D movable-bed physical model of the protected sandy beach of Gabicce Mare, on the central Adriatic Sea in Italy, is described. Three emerged and three submerged configurations of rubble-mound detached breakwaters, for beach protection, placed at different positions, were tested in the Ancona (Italy) wave flume and the obtained results were compared with those found for a structure-free configuration. Both wave dissipation and reduction of beach erosion efficiency under various wave conditions were obtained and, in particular, the short-term hydro-morphodynamic response of the different tested breakwater configurations.

Experimental and numerical investigation of the internal kinematics of a surf-zone plunging breaker

Abstract: Over the last couple of decades both the qualitative and quantitative understanding of breaking waves in the surf zone have greatly increased. This is due to the advances in experimental and numerical techniques. However, few comparisons between these two different investigative techniques for surf-zone breaking waves have been reported. In this study, a comparison is made between the experimental and numerical investigation of the internal kinematics of a surf-zone plunging breaker. The full-field velocity measuring technique known as Particle Image Velocimetry (PIV) is used in the experiments. In the hybrid numerical scheme, the main model solves the Navier–Stokes equations using a Finite Volume method and the free-surface is simulated using a Volume of Fluid (VOF) method. An important feature of this work is that, unlike in most other comparisons between numerical and experimental results, the exact geometry of the physical wave flume and the exact motion of the physical wavemaker are duplicated in the numerical wave tank. To achieve this, an additional numerical model using a Boundary-Integral Method (BIM) is employed to generate the input conditions for the Navier–Stokes solver. Very good agreement was found for all comparisons: free-surface elevations, velocity vector maps, velocity profiles and velocity-magnitude contours. However, some small discrepancies were observed. In the free-surface elevation comparisons, a slight time lag was observed in the numerical results and it is suggested that this was due to the small amount of smoothing applied in the BIM to enable it to continue to supply input data to the Navier–Stokes solver well beyond the breaking of the wave. In addition, some small differences were also found between the numerically predicted velocity distributions and those measured in the experiments. These disagreements occurred mostly in the aerated region and it is proposed that they could be caused by errors in the PIV velocity data due to air bubble effects. However, they could also be attributed to the fact that no turbulence model is used in the numerical scheme and it is these aerated areas where the turbulence levels are the highest.

Geosynthetics in hydraulic and coastal engineering: filters, revetments and sand filled structures

Abstract: The paper deals with 2 applications of geotextiles in coastal and hydraulic engineering: Geotextiles in filters and revetments, and in sand filled structures. Geotextiles are often replacing granular filters. However, they have different properties than a granular filter. For a the application of geotextiles in revetments, the consequences of the different properties will be shown: How permeability is influenced by a geotextile and what can be the consequences of the weight differences between granular and geotextile filters. In another application, the filter properties of geotextiles are only secondary. In geotextile tubes and containers the geotextile is used as ‘wrapping material’ to create large units that will not erode during wave attack. The structures with geotextile tubes and containers serve as an alternative for rock based structures. The first of these structures were more or less constructed by trial and error, but research on the shape of the structures, the stability under wave attack and the durability of the used material has given the possibility to use design tools for these structures. Recently also the morphological aspects of these structures have been investi-gated. This is of importance because regularly structures with geotextile tubes fail due to insufficient toe protection against the scour hole that develops in front of the structure, leading to undermining of the structure. Recent research in the Delta Flume of Deltares and the Large Wave Flume in Hannover has led to better understanding what mechanisms determine the stability under wave attack. It is shown that therefore also the degree of filling is of importance and the position of the water level with respect to the tube has a large influence.