Showing papers on "Wave flume published in 2007"

A particle image velocimetry investigation on laboratory surf-zone breaking waves over a sloping beach

Abstract: Particle image velocimetry (PIV) measurements were performed in a wave tank under water waves propagating and breaking on a 1/15 sloping beach. The wave transformation occurred in the surf zone over a large domain covering several wavelengths from incipient breaking to swash zone beyond the shoreline. PIV spatial interrogation windows must be small enough to obtain accurate velocities, and one window covers only a small part of the domain. To overcome this problem and to measure the instantaneous velocity field over the whole surf zone area, we have split the full field into 14 overlapping smaller windows of the same size. Local measurements were synchronized with each other using pulsed TTL triggers and wave gauge data. The full velocity field was then reconstructed at every time step by gathering the 14 PIV fields. We then measured the complete space–time evolution of the velocity field over the whole surf zone. We determined also the ensemble-period-average and phase-average components of the flow with their associated fluctuating parts. We used the PIV images and velocity measurements to estimate the void fraction in each point of the surf zone. Special attention was given to the calculation of the spatial derivatives in order to obtain relevant information on vorticity and on the physical terms that appear in the fluctuating kinetic energy transport equation.

Wave breaking onset and strength for two-dimensional deep-water wave groups

Abstract: The numerical study of J. Song & M. L. Banner (J. Phys. Oceanogr. vol. 32, 2002, p. 254) proposed a generic threshold parameter for predicting the onset of breaking within two-dimensional groups of deep-water gravity waves. Their parameter provides a non-dimensional measure of the wave energy convergence rate and geometrical steepening at the maximum of an evolving nonlinear wave group. They also suggested that this parameter might control the strength of breaking events. The present paper presents the results of a detailed laboratory observational study aimed at validating their proposals. For the breaking onset phase of this study, wave potential energy was measured at successive local envelope maxima of nonlinear deep-water wave groups propagating along a laboratory wave tank. These local maxima correspond alternately to wave group geometries with the group maximum occurring at an extreme carrier wave crest elevation, followed by an extreme carrier wave trough depression. As the nonlinearity increases, these crest and trough maxima can have markedly different local energy densities owing to the strong crest–trough asymmetry. The local total energy density was reconstituted from the potential energy measurements, and made dimensionless using the square of the local (carrier wave) wavenumber. A mean non-dimensional growth rate reflecting the rate of focusing of wave energy at the envelope maximum was obtained by smoothing the local fluctuations. For the cases of idealized nonlinear wave groups investigated, the observations confirmed the evolutionary trends of the modelling results of Song & Banner (2002) with regard to predicting breaking onset. The measurements confirmed the proposed common breaking threshold growth rate of 0.0014 ± 0.0001, as well as the predicted key evolution times: the time taken to reach the energy maximum for recurrence cases; and the time to reach the breaking threshold and then breaking onset, for breaking cases. After the initiation and subsequent cessation of breaking, the measured wave packet mean energy losses and loss rates associated with breaking produced an unexpected finding: the post-breaking mean wave energy did not decrease to the mean energy level corresponding to maximum recurrence, but remained significantly higher. Therefore, pre-breaking absolute wave energy or mean steepness do not appear to be the most fundamental determinants of post-breaking wave packet energy density. However, the dependence of the fractional breaking energy loss of wave packets on the parametric growth rate just before breaking onset proposed by Song & Banner (2002) was found to provide a plausible collapse to our laboratory data sets, within the experimental uncertainties. Further, when the results for the energy loss rate per unit width of breaking front were expressed in terms of a breaker strength parameter b multiplying the fifth power of the wave speed, it is found that b was also strongly

Generation of internal solitary wave by gravity collapse

Abstract: Laboratory experiments were conducted to investigate the generation of an internal solitary wave (ISW) in a two-layer free surface fluid system in wave flume (12 m · 0.5 m · 0.7 m), which included wave channel and compartment. There is filled with stratified twolayer fluid system in the channel and small compartment, respectively. These two regions in the wave flume are separated by a movable vertical gate at one end of the flume for generating the ISW (internal solitary wave). An ISW generation is thus caused by gravity collapse upon raising the vertical gate. Given positive potential energies, an elevation-type ISW is followed by an anticlockwise overturning motion at the interface; on the other hand, an ISW of depression-type was generated by clockwise motion. This paper presented physical properties related to wave generation in a stratified fluid in laboratory. In the wave flume, the stable wave propagation, either elevation-type or depression-type ISW, is influenced by environmental condition. An ISW of depression transferred into unstable fluctuation instead of soliton feature whilst the upper layer thickness is greater than the lower layer thickness. Similarly, an ISW of elevation reduces its wave amplitude and causes trailing oscillation followed with the leading wave.

Experimental study of air bubbles and turbulence characteristics in the surf zone

Abstract: [1] A set of laboratory experiments of free-surface elevation, water velocity, void fraction, and bubble distribution was conducted simultaneously for regular wave breaking on a plane slope. The data were collected by using a measurement array of wave gages, a dual-tip resistivity void fraction probe, and an acoustic Doppler velocimeter. The measurements were conducted in two different wave flumes to check the scale effects of air entrainment into the water on the wave breaking. The power law scaling of bubble size distribution was measured, and it was found that the bubble size has a power law of d−1.5∼−1.7 independent of the location and water depth. The relationship between void fraction and turbulent intensity shows a linear relationship between the void fraction and turbulent intensity.

Localized Mixing Due to an Interfacial Solitary Wave Breaking on Seabed Topography in Different Ridge Heights

Abstract: Experiments were carried out in a wave flume on internal solitary wave (ISW) of depression-type propagating over a submarine ridge in triangular shape. Tests were arranged in series from combinations of submarine ridges in different height and ISW in different wave amplitudes. The resultant wave motions were found differing from that of surface gravity waves. Experimental results suggested the blockage parameter ζ can be applied to classify various degrees of ISW-ridge interaction, i.e., ζ < 0.5 for weak encounter, 0.5<ζ<0.7 for moderate encounter, and 0.1< ζ for wave breaking. In addition, three categories of ISW-ridge interaction were also employed by the relationship between the degree of blocking B and dimensionless wave amplitude a i /H 2 .

A Study of Tsunami Wave Fission in an Undistorted Experiment

Abstract: To study tsunami soliton fission and split wave-breaking, an undistorted experiment was carried out which investigated tsunami shoaling on a continental shelf. Three models of the continental shelf were set up in a 205-m long 2-dimensional flume. Each shelf model was 100 m, long with slopes of either 1/100, 1/150, or 1/200. Water surface elevations were measured across the flume, including a dense cluster of wave gages installed around the point of wave-breaking. We propose new methods for calculating wave velocity and the wave-breaking criterion based on our interpretation of time series data of water surface elevation. At the point of wave-breaking, the maximum slope of water surface is between 20 to 50 deg., while the ratio of surface water particle horizontal velocity to wave velocity is from 0.5 to 1.2. The values determined by our study are larger than what has been reported by other researchers.

Current Effects on Nonlinear Wave-Body Interactions by a 2D Fully Nonlinear Numerical Wave Tank

Abstract: Nonlinear wave-current interactions with fixed or freely floating bodies are investigated by a two-dimensional (2D) fully-nonlinear numerical wave tank (NWT). The NWT is developed based on the potential theory and boundary element method (BEM) with constant panels. Mixed Eulerian-Lagrangian (MEL) time marching scheme (material-node approach) is used with fourth-order Runge-Kutta fully updated time integration, regriding, and smoothing techniques, and acceleration-potential formulation and direct mode-decomposition method. Specially devised ϕn −η type artificial damping zones (i.e., numerical beach) are implemented to prevent wave reflection from the end wall and wave maker. Using the developed NWT, nonlinear wave-current interactions (1) without bodies; (2) with a stationary body; and (3) with a floating body for various wave and current conditions have been investigated and some of the NWT simulations are compared with the results of Boussinesq’s equation and perturbation theory. It is seen that the NWT ...

Numerical Simulation of Two-Dimensional Overtopping Against Seawalls Armored with Artificial Units in Regular Waves

Abstract: A new mathematical model for the overtopping against seawalls armored with artificial units in regular waves was established. The 2-D numerical wave flume, based on the Reynolds Averaged Navier-Stokes (RANS) equations and the standard k-e turbulence model, was developed to simulate the turbulent flows with the free surface, in which the Volume Of Fluid (VOF) method was used to handle the large deformation of the free surface and the relaxation approach of combined wave generation and absorbing was implemented. In order to consider the effects of energy dissipation due to the armors on a slope seawall, a porous media model was proposed and implemented in the numerical wave flume. A series of physical model experiments were carried out in the same condition of the numerical simulation to determine the drag coefficient in the porous media model in terms of the overtopping discharge. Compared the computational value of overtopping over the seawall with the experimental data, the values of the effective drag coefficient was calibrated for the layers of blocks at different locations along the seawalls.

Monitoring near-shore shingle transport under waves using a passive acoustic technique.

Abstract: Passive acoustic techniques have been used to measure shingle (gravel) sediment transport in very shallow water, near the wave breaking zone on a beach. The experiments were conducted at 1:1 scale in the Large Wave Flume, Grose Wellen Kanal (GWK) at Hannover, Germany. The frequency spectrum induced by shingle mobilized under breaking waves can be distinguished from other ambient noise, and is found to be independent of water depth and wave conditions. The inverse relationship between centroid frequency and representative grain size is shown to remain valid in shallow water wave conditions. Individual phases of onshore and offshore transport can be identified. Analysis of the acoustic frequency spectrum provides insight into the mechanics of phase-resolved shingle transport.

Experimental investigation on the wave-induced pore pressure around shallowly embedded pipelines

Abstract: A series of regular wave experiments have been done in a large-scale wave flume to investigate the wave-induced pore pressure around the submarine shallowly embedded pipelines.The model pipelines are buried in three kinds of soils,including gravel,sand and silt with different burial depth.The input waves change with height and period.The results show that the amplitudes of wave-induced pore pressure increase as the wave period increase,and decay from the surface to the bottom of seabed.Higher pore pressures are recorded at the pipeline top and the lower pore pressures at the bottom,especially in the sand seabed.The normalized pressure around pipeline decreases as the relative water depth,burial depth or scattering parameters increase.For the silt seabed,the wavelet transform has been successfully used to analyze the signals of wave-induced pore pressure,and the oscillatory and residual pore pressure can be extracted by wavelet analysis.Higher oscillatory pressures are recorded at the bottom and the lower pressures at the top of the pipeline.However,higher residual pressures are recorded at the top and the lower pressures at the bottom of the pipeline.

Irregular wave attenuation and mud mass transport

Abstract: The major aspects of interactive effects between irregular waves and a soft muddy bed, i.e. wave height attenuation and mud mass transport, are examined using numerical simulation and laboratory ex...

Steepness effect on modulation instability of the nonlinear wave train

Abstract: The long-term evolution of a nonlinear wave train in deep water with varied initial wave steepness between 0.1 and 0.3 was experimentally investigated in a super wave flume (300m long, 5m wide, and 5.2m deep). The initial wave train was the combination of one carrier wave and a pair of imposed sideband components which is the most unstable mode, referred to as sideband instability theory. Sixty-six wave gauges were installed downstream along the wave flume to simultaneously measure the evolution of a wave train. Increasing modulation of the wave train was observed due to sideband instability until a critical value which either wave breaking is initiated or maximum modulation is reached. The near recurrence of the initial state of the wave train is exhibited for the nonbreaking case. An effective frequency downshift of the wave spectrum accompanied with wave breaking is observed as the initial wave steepness is larger than 0.11. At postbreaking, the wave train reveals periodic modulation and demodulation, ...

Seiching in a large wave flume

Abstract: INTRODUCTION The Cross-shore Transport Experiment (CROSSTEX) is a multi-investigator, multi-university, scientific research project centered around a coordinated series of near-prototype scale experiments that were conducted at Oregon State University's O.H. Hinsdale Wave Research Laboratory (HWRL) during the summer of 2005 (Maddux et al, 2006). The experiments took place in the 104m long, 3.7m wide and 4.6m deep Large Wave Flume (LWF) on a movable bed consisting of medium sand (^50=0.22 mm). The objective of our portion of the experiments was to obtain high-resolution observations related to onshore and offshore sandbar migration. The analysis of seiching in the LWF was not a primary objective of our experiments; however, our early analysis of the data suggested they contained some unexpected behavior in regards to basin seiching. Herein, we have pursued a detailed analysis of the observed low-frequency motions as an "experiment of opportunity". The experimental conditions consisted of irregular waves generated by the paddle-type wavemaker with a range of peak periods (3-8 sec) and rms wave heights (0.3m to 0.6m) depending on whether erosive or accretionary conditions were desired. Observations of the hydrodynamics were carried out using instruments mounted onto the walls of the flume, suspended from a movable carriage, and remotely observed by video cameras as shown in Figure 1. The present analysis will utilize a limited number of Nortek Vectrino

An experimental study of wave forces on vertical breakwater

Abstract: In this study, a series of hydraulic model tests with regular/irregular waves was carried out in a wave flume to investigate the wave forces acting on a composite-type breakwater. Waves in front of the breakwater, wave pressures on the vertical wall and at the bottom of caisson were measured simultaneously. The maximum horizontal force and uplift force were calculated and compared with Goda's wave force theories. The results had shown that Goda's theories offer higher safety factor. However, the measured uplift force was smaller than Goda's and nonzero at the land-side end of the bottom which might be caused by the path of water flow in the porous media beneath the caisson. It also shows that the results from different irregular wave train with the same spectrum are different, and thus the effectiveness of conventional irregular wave tests with several repeats of the same wave train should be reconfirmed.

Unified Sediment Transport Formulation for Coastal Inlet Application

Abstract: The Coastal Inlets Research Program (CIRP) is developing predictive numerical models for simulating the waves, currents, sediment transport, and morphology change at and around coastal inlets. Water motion at a coastal inlet is a combination of quasi-steady currents such as river flow, tidal current, wind-generated current, and seiching, and of oscillatory flows generated by surface waves. Waves can also create quasi-steady currents, and the waves can be breaking or non-breaking, greatly changing potential for sediment transport. These flows act in arbitrary combinations with different magnitudes and directions to mobilize and transport sediment. Reliable prediction of morphology change requires accurate predictive formulas for sediment transport rates that smoothly match in the various regimes of water motion. This report describes results of a research effort conducted to develop unified sediment transport rate predictive formulas for application in the coastal inlet environment. The formulas were calibrated with a wide range of available measurements compiled from the laboratory and field and then implemented in the CIRP's Coastal Modeling System. Emphasis of the study was on reliable predictions over a wide range of input conditions. All relevant physical processes were incorporated to obtain greatest generality, including: (1) bed load and suspended load, (2) waves and currents, (3) breaking and non-breaking waves, (4) bottom slope, (5) initiation of motion, (6) asymmetric wave velocity, and (7) arbitrary angle between waves and current. A large database on sediment transport measurements made in the laboratory and the field was compiled to test different aspects of the formulation over the widest possible range of conditions. Other phenomena or mechanisms may also be of importance, such as the phase lag between water and sediment motion or the influence of bed forms. Modifications to the general formulation are derived to take these phenomena into account. The (Less)

Direct Measurement of Bottom Shear Stress under Water Waves

Abstract: Wave-induced bottom shear stress is one of the most important parameters in modelling of wave hydrodynamics and coastal sediment transport. A new type of shear plate has been successfully developed to measure instantaneous wave bottom shear stress in a laboratory wave flume. The shear plate measures instantaneous horizontal force by applying the Wheatstone half bridge circuit to detect a tiny horizontal movement of the shear plate. There are about 280 individual test runs carried out over one smooth bed and two roughened beds, respectively. In each test run, instantaneous bottom shear stress was measured at a sampling rate of 10 Hz for about 10 minutes. The measured total horizontal force, which consists of wave-induced bottom shear stress and hydrodynamic pressure, is found linearly proportional to wave height in both laminar and turbulent flows. The wave friction factors measured on the smooth bed are shown to agree excellently with the theoretical values derived in laminar boundary layer flow, but those on the rough beds are affected by the bed roughness length estimated. An empirical formula is also proposed to compute the wave bottom shear stress.

The numerical wave flume of the viscous fluid based on the momentum source method

Abstract: Based on the N-S equation,the VOF method is used to track the free surface and the additive sources in the momentum equation are used to generate and absorb waves.The method of waves generating and absorbing in the viscous fluid is presented.The two-dimentional numerical wave flume which can both generate and absorb waves is obtained by use of the Fluent solver.The validity of waves generating and absorbing is numerically confirmed.The reflection and transmission of water waves past two vertical barriers are further considered,and some phenomenons(such as standing waves) are numerically simulated successfully under the viscous fluid theory.

Representative Waves for Morphological Simulations

Abstract: PLECHA, S., SANCHO, F., SILVA, P. and DIAS, J.M., 2007. Representative waves for morphological simulations. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), 995 – 999. Gold Coast, Australia, ISSN 0749.0208 In the present study, a set of representative waves is derived for the coast adjacent to the Ria de Aveiro, such that it is morphologically equivalent to the complete wave regime. Different schematising methodologies are presented, based on the principles of conservation of wave energy and equivalent longshore sediment transport capacity over the beach profile. Firstly, the sediment transport capacities due to the whole set of waves is computed and then various sets of representative waves are proposed based on the principles above. The most suitable set is selected by taking into account the dominant sediment transport processes at the selected site. The representative waves can be used to significantly reduce computational time in morphological models, by reducing the number of different sea states that characterise the annual wave climate. This simplified nearshore wave regime shall provide the wave effects into the sediment transport formulae and the hydrodynamical model.

External forces of sediment transport in surf and swash zones induced by wave groups and their associated long waves

Abstract: This study investigates the coupling field of grouped wind waves and their associated long waves in the surf and swash zones Based on the calculated wave fields, the contributions of the wind waves and the long waves on the sediment transport efficiency are discussed Spatial variations of the incident grouped wind waves propagating over a plane slope are calculated based on time-dependent mild slope equation Generation of the long waves is reproduced based on a time-varying breakpoint model proposed by Symonds et al [1982] In order to obtain the long wave solutions extending over the landward region from the still water shoreline, calculations using nonlinear shallow water equations are connected to the Symonds' model invoking a moving boundary treatment The Shields parameters under composition of the grouped wind waves and the associated long waves are evaluated to assess the mobility of the bottom sediment The results show that the long waves have greater sediment transport efficiency over the grouped wind waves in the swash zone