Showing papers on "Wave flume published in 2008"

Analytical Modeling of Landslide-Generated Waves

Abstract: Transient waves generated by a box falling into water are analytically and experimentally investigated (Scott Russell wave generator). Within the frame of linear theory, a new analytical solution for the leading wave is proposed. It is shown that both the impulse pressure of the box’s impact on the free surface and the velocity field generated during the underwater phase (wave generator type boundary condition) have to be taken into account when predicting wave amplitude and period. The analytical solution, in satisfactory agreement with experiments performed in a two-dimensional flume, can be used for validating numerical models and for estimating the order of magnitude of landslide-generated waves.

Green water void fraction due to breaking wave impinging and overtopping

Abstract: The present study uses laboratory measurements to investigate the void fraction of an overtopping flow on a structure. The overtopping flow, also called green water, was generated by the impingement of a plunging breaking wave on the structure following the Froude similarity of an extreme hurricane wave and a simplified offshore structure. The flow is multi-phased and turbulent with significant aeration. A fiber optic reflectometer (FOR) and bubble image velocimetry (BIV) were employed to measure the void fraction and velocity in the flow, respectively, and to determine the water level on the deck. Mean properties of void fraction and velocity were obtained by ensemble-averaging and time-averaging the repeated instantaneous measurements. The temporal and spatial distributions of void fraction reveal that the flow is very highly aerated near the front of green water and has relatively low aeration near the deck surface. The mean void fraction and velocity distributions were also depth-averaged for simplicity and potential use in engineering applications. Using the measured data, similarity profiles for depth-averaged void fraction, depth-averaged velocity, and water level were found. The study suggests that using only the velocity data is insufficient if the flow momentum or the flow rate is to be determined. The accuracy of the void fraction measurements was validated by comparing the directly measured water volume of the overtopping flow with the calculated water volume based on the measured velocity and void fraction.

Infrared measurements of surface renewal and subsurface vortices in nearshore breaking waves

Abstract: [1] When ocean waves reach a surf zone, jets projecting from the breakers splash sequentially, producing horizontal roller vortices beneath the jets and longitudinal counterrotating vortices behind the rollers; these vortices organize into three-dimensional structures that evolve into a turbulent bore with wave propagation. This disrupts any uniform temperature distributions on the surface, creating heterogeneous patterns of surface temperatures. In this study, we extracted surface temperature distributions from infrared measurements in small- and large-scale wave flumes, then used those data to study the renewed surfaces created by subsurface vortices beneath spilling and plunging breakers. In our large-scale experiments, temporal and spatial scales of surface renewal and surface recovery were consistent with earlier work; however, in our small-scale experiments, the spatial scales showed significant deviations from earlier in situ observations. These inconsistencies may be attributed to scale effects for subsurface vortices, and we show that the Froude number (Fr) can be used to characterize the initial formation of longitudinal counterrotating vortices. Further, for turbulent flows fully developed by wave breaking in a bore region, the frequency of surface renewal correlates exponentially with Reynolds number (Re). The computed vorticity on the breaking wave surface exhibits local patterns which correlate strongly with the gravity induced counterrotating vortices, which in turn renew the rear-facing surface of the breaking waves. In contrast the turbulent bore which precedes the wave crest rapidly disturbs and renews the surface in front of the crest. These two different mechanisms for surface renewal, during the nearshore breaking process, lead to modulations in the surface temperature distribution and changes in thermal diffusivity during the propagation of the breaking wave.

Large-Scale Wave Flume Experiments on Highway Bridge Superstructures Exposed to Hurricane Wave Forces

Abstract: Recent failures of United States (US) coastal highway bridges during hurricane events have shown the need for improved modeling and analysis of storm induced wave forces. Failures mostly consisted of bridge superstructures being partially or in some cases completely removed from the supporting elements. Damage was attributed to elevated storm surges that enabled larger waves to reach the superstructure and to inadequate connection designs. Previous research on wave forces for bridge structures used small-scale experiments on the order of 1:15 to 1:20 with essentially rigid bridge models (neglecting realistic fluid-structure interaction). A new innovative large-scale laboratory setup has therefore been developed that enables realistic simulation of storm-induced wave forces on bridge superstructures. The experiment was conducted in the 104 m (324 ft) long wave flume at Oregon State University. The test setup employs guide frames attached to the flume walls that support a 1:5 scale reinforced concrete girder bridge superstructure model. The horizontal stiffness of the bridge substructure (bent columns and cap) can be varied by the guide system to represent different dynamic properties of the supporting structure to enable measurement of fluid-structure interaction. These experiments provide needed large-scale data for wave induced forces on bridges to validate design and analysis methods. First observations and a set of measurements for the rigid case (phase I) and a flexible case (phase II) exposed to a set of regular waves are presented and discussed.

Numerical Simulation on Hydraulic Performances of Quarter Circular Breakwater

Abstract: Quarter circular breakwater (QCB) is a new-type breakwater developed from semi-circular breakwater (SCB). The superstructure of QCB is composed of a quarter circular front wall, a horizontal base slab and a vertical rear wall. The width of QCB's base slab is about half that of SCB, which makes QCB suitable to be used on relatively firm soil foundation. The numerical wave flume based on the Reynolds averaged Navier-Stokes equations for impressible viscosity fluid is adopted in this paper to simulate the hydraulic performances of QCB. Since the geometry of both breakwaters is similar and SCB has been studied in depth, the hydraulic performances of QCB are given in comparison with those of SCB.

Experimental flume simulation of sandbar dynamics

Abstract: Sandbar dynamics is a key feature of coastal hydrodynamics and plays an important role in natural shore protection. It is generally recognised that moderate wave forcing can lead to bar formation whereas storms may induce offshore bar migration. Field experiments nevertheless do not usually allow a detailed description of the evolutions due to the poor temporal resolution between topographic surveys. Here, experiments are carried out in a 36 m long flume equipped with a piston wave generator. The sloping bottom consists of a loose material of low density (1.19 g/cm3) with a median diameter d50=0.6 mm. The Shields and Rouse numbers for the experiments are of the same magnitude as those of natural environments. Time and length scales ratios are roughly 1/3 and 1/10. Irregular waves are generated according to a JONSWAP spectrum. The process of bar formation and evolution at several positions on the profile is described. On the one hand for constant wave climates of very long durations (tens of hours), bars become pitched forward and onshore migration is observed. The bar eventually merges to the berm. On the other hand, a succession of increasing, energetic and decreasing wave conditions that are characteristic of the different phases of storm events are applied. An offshore migration of mega-ripples is observed during the most energetic phase, feeding a large offshore bar. The waning conditions smooth the profile and lead to shore-face accretion. Shoreline positions during the different phases are also discussed.

A coastal area morphodynamics model

Abstract: A coastal area morphodynamics model was developed which is capable of modelling both short-term and long-term bathymetric changes in coastal areas. The model is composed of three main modules: wave transformation module, hydrodynamic module and sediment transport and morphology module. The three main modules run simultaneously where the waves and coastal currents information are passed to the sediment transport module, which calculates the sediment transport fluxes and the corresponding bathymetric changes. The model's output is linked through dynamic plotting libraries, which produces contour plots, vector plots and animation files for the waves, currents, sediment transport fluxes and bathymetry. The model's output compares very well with other commercial models for the case of an offshore breakwater over a sloping beach.

A unique tsunami generator for physical modelling of violent flows and their impact

Abstract: Tsunami waves travel across oceans with quite small vertical displacements, but shoal up dramatically in coastal and nearshore depths, and can cause extensive loss of life and infrastructure. Propagation of tsunami waves in the nearshore, across the shoreline, and then inland is not well modeled by many current techniques. Physical modeling can be used to study flow and force processes, but correct generation of the tsunami wave(s) is essential and conventional wave paddles simply do not have adequate stroke to reproduce the required wavelength. A collaboration between University College London (UCL) and HR Wallingford (HRW) is working to eliminate obstacles to physical modeling of tsunamis and their effects. Within this project, HRW has constructed an innovative Tsunami Generator which is capable of generating multiple waves, an initial draw-down and ensure realistic wavelengths. The Tsunami Generator is mounted within a 45m wave flume and is able to generate tsunami waves which have been previously transformed from deeper water (approx -200m) to shallow water (approx -20 to -50m) using any suitable numerical model. Bathymetry in the wave flume shoals the tsunami waves over a representative coastal slope though the shoreline and inland, covering a suitable inland inundation area. Several stages of modeling are required to validate details of the Tsunami Generator design and control system. The modelling team will then measure flow / wave driven loadings on representative (model) buildings. This paper presents a summary of these and outlines opportunities for international teams to carry out participative testing using this unique facility.

Navier-stokes wave models for investigations of breakwater characteristics

Abstract: The protection of coastal structures is important t o South Africa. The dynamics of breakwaters is a topic that is becoming amenable to numerical study, in terms of the motion of multiple interlocking structures under wave action. In this preliminary s tudy, plunging waves and the loads they exert on semi-submerged structures are investigated with a R eynolds-Averaged Navier-Stokes model using a Volume of Fluid approach to surface modelling. Qualitative comparison of wave properties with experimental observations, including turbulence in surging and plunging waves, is encouraging, but quantitative comparisons are still to be made. Load s on a semi-submerged beam are modelled in preparation for studies of the dolos geometry.

An Irregular Wave Maker of Active Absorption with VOF Method

Abstract: A numerical irregular wave flume with active absorption of re-reflected waves is simulated by use of volume of fluid (VOF) method. An active absorbing wave-maker based on linear wave theory is set on the left boundary of the wave flume. The progressive waves and the absorbing waves are generated simultaneously at the active wave generating-absorbing boundary. The absorbing waves are generated to eliminate the waves coming back to the generating boundary due to reflection from the outflow boundary and the structures. SIRW method proposed by Frigaard and Brorsen (1995) is used to separate the incident waves and reflected waves. The digital filters are designed based on the surface elevation signals of the two wave gauges. The corrected velocity of the wave-maker paddle is the output from the digital filter in real time. The numerical results of regular and irregular waves by the active absorbing-generating boundary are compared with the numerical results by the ordinary generating boundary to verify the performance of the active absorbing-generator boundary. The differences between the initial incident waves and the estimated incident waves are analyzed.

Kinematic Wave Theory for Transient Bed Sediment Waves in Alluvial Rivers

Abstract: Transient bed sediment waves in alluvial rivers have been described using a multitude of hydraulic formulations. These formulations are based on some form of the St. Venant equations and conservation of mass of sediment in suspension and in bed. Depending on the assumptions employed, a hierarchy of formulations is expressed. These formulations in the literature employ uncoupled, semicoupled, or fully coupled transport models treating the sediment waves as either hyperbolic (dynamic wave) or parabolic (diffusion wave). It is, however, hypothesized that the movement of bed sediment waves in alluvial rivers can be described as a kinematic wave. Kinematic wave theory employs a functional relation between sediment transport rate and concentration and a relation between flow velocity and depth. This study summarizes the hierarchy of the formulations while emphasizing the kinematic wave theory for describing transient bed sediment waves. The applicability of the theory is shown for laboratory flume data and hypothetical cases.

Gas transfer under breaking waves: experiments and an improved vorticity-based model

Abstract: In the present paper a modified vorticity-based model for gas transfer under breaking waves in the absence of significant wind forcing is presented. A theoretically valid and practically applicable mathematical expression is sug- gested for the assessment of the oxygen transfer coefficient in the area of wave-breaking. The proposed model is based on the theory of surface renewal that expresses the oxygen transfer coefficient as a function of both the wave vorticity and the Reynolds wave number for breaking waves. Experimental data were collected in wave flumes of vari- ous scales: a) small-scale experiments were carried out us- ing both a sloping beach and a rubble-mound breakwater in the wave flume of the Laboratory of Harbor Works, NTUA, Greece; b) large-scale experiments were carried out with a sloping beach in the wind-wave flume of Delft Hydraulics, the Netherlands, and with a three-layer rubble mound break- water in the Schneideberg Wave Flume of the Franzius Insti- tute, University of Hannover, Germany. The experimental data acquired from both the small- and large-scale experiments were in good agreement with the proposed model. Although the apparent transfer coefficients from the large-scale experiments were lower than those de- termined from the small-scale experiments, the actual oxy- gen transfer coefficients, as calculated using a discretized form of the transport equation, are in the same order of mag- nitude for both the small- and large-scale experiments. The validity of the proposed model is compared to experimental results from other researchers. Although the results are encouraging, additional research is needed, to incorporate the influence of bubble mediated gas exchange, before these results are used for an environ- mental friendly design of harbor works, or for projects in- volving waste disposal at sea.

Statistical modeling of pressure gradients on a barred beach

Abstract: This paper reports the cross-shore variations of a pressure gradient observed on a fixed barred beach in a large-scale laboratory wave flume. The data sets include measurements of the free water su...

A Two-Dimensional Wave Flume Investigation into the Effect of Multiple Vertical Steps on the Form of Breaking Waves

Abstract: A two-dimensional wave flume investigation was conducted to examine the effect of multiple steps on the form of breaking waves. The study was initiated to gain a better appreciation of the design tolerances for artificial surfing reefs. The geotextile bags used to form an artificial surfing reef create multiple steps in the seabed; reducing the size of these steps improves the quality of waves for recreational surfing but comes at considerable economic cost. Sets of steps were placed on a 1 : 15 slope, and image analysis methods were used to quantify the effects of increasing step size on the form and intensity of the breaking waves. The vortex area, angle, and length-to-width ratio were quantified for various step sizes. It is shown that breaking intensity decreases with increasing step size. Variations in wave reflection were measured using a three-element surface-piercing wave gauge. Wave reflection coefficients were found to be unchanged for all step sizes implemented in this experiment, and ...

A Methodology for the Analysis of Physical Model Scale Effects on the Simulation of Wave Propagation up to Wave Breaking: Preliminary Physical Model Results

Abstract: This paper describes the experiments performed at the National Laboratory for Civil Engineering (LNEC) aiming at simulating, in a flume, the wave propagation along a constant slope bottom that ends on a sea wall coastal defence structure, a common structure employed in the Portuguese coast. The objective of these tests is to calibrate the parameters of FUNWAVE, a Boussinesq type model, for wave propagation in coastal regions. This is the first step in the validation of a methodology to combine numerical and physical models in the study of the interactions between beaches and structures. This work is performed in the framework of the Composite Modelling of the Interactions between Beaches and Structures (CoMIBBs) project, a joint research activity of the HYDRALAB III European project.Copyright © 2008 by ASME

Numerical Simulation of Nonliner Wave Propagating in Flume

Abstract: The convention method of testing breakwaters in flume by physical model was replaced by purely numerical model. A Bretschneider/Mitsuyasu spectrum wave was generated in numerical flume, and the design of breakwater is in accordance with Code of Hydrology for Sea Harbour, China ("the Code"). Modified Boussinesq type wave equations were solved including a wave reflection term. By purely simulation method, we have a reflection coefficient of 0.45, and in comparison with the Code, it is concluded numerical models can be used in the pre-design of a breakwater, and safety coefficient is used in the Code.

Enhanced Sediment Transport due to Wave-Soil Interactions

Abstract: The objective of this work is to study the dynamics of enhanced sediment transport due to wave- soil interactions via physical and numerical simulations. A large-scale tsunami scour study has been designed and carried out during the summer of 2007 at the tsunami wave basin at NEES@OSU to study tsunami induced sediment transport and scour. The results showed that majority of the scour occurred during the drawdown due to enhanced sediment mobility as a result of upward pore pressure flux caused by transient wave-soil interaction. Hence, for a constant sloped fine sand beach, positive solitary waves lead to erosion above the shoreline and deposition in the breaking region. Since the large-scale tsunami scour study was limited to the use a fine dune sand from Oregon beach, a separate set of experiments have been designed and it is currently being carried out to study the effects of grain size and upward seepage on sediment transport under current and/or wave loads. Parallel to the experimental studies, the authors also developed and validated the first transient tsunami sediment transport model that accounts for the effects of transient wave dynamics and wave-soil interactions.

Modeling of random wave transformation with strong wave-induced coastal currents

Abstract: The propagation and transformation of multi-directional and uni-directional random waves over a coast with complicated bathymetric and geometric features are studied experimentally and numerically. Laboratory investigation indicates that wave energy convergence and divergence cause strong coastal currents to develop and inversely modify the wave fields. A coastal spectral wave model, based on the wave action balance equation with diffraction effect (WABED), is used to simulate the transformation of random waves over the complicated bathymetry. The diffraction effect in the wave model is derived from a parabolic approximation of wave theory, and the mean energy dissipation rate per unit horizontal area due to wave breaking is parameterized by the bore-based formulation with a breaker index of 0.73. The numerically simulated wave field without considering coastal currents is different from that of experiments, whereas model results considering currents clearly reproduce the intensification of wave height in front of concave shorelines.

Measurements and modeling of cross-shore morphodynamics

Abstract: A lot of processes within process-based models, like Unibest-TC, are still not well understood. Consequently, numerous experiments are carried out to get more insight in these processes. As part of the European HYDRALAB-III project, experiments were carried out in the Gro¼e Wellenkanal (GWK) of the ForschungsZentrum KAuste in Hannover, Germany. Data was obtained from wave heights, °ow velocities, sediment concentrations and bed level measurements. Ruessink et al. (2007) stated that the calibration procedure of Unibest-TC focuses on net sedi- ment transport rates only and free model parameters may compensate for missing or incompletely described processes. Therefore, they suggest to collect concentration and velocity profiles under a wide range of natural conditions, and use these data to validate the temporal and spatial variability of the bedload and suspended sediment transport formulations predicted by the present formulations. This resulted in the following research objective: The validation of the cross-shore profile model Unibest-TC using data collected in a full-scale wave flume. The model is validated step-by-step using wave height, velocity, sediment concentration and bed level data