Showing papers on "Wave flume published in 2005"

Sheet flow and suspended sediment due to wave groups in a large wave flume

Abstract: A series of sand bed experiments was carried out in the Large Wave Flume in Hannover, Germany as a component of the SISTEX99 experiment. The experiments focussed on the dynamic sediment response due to wave group forcing over a flat sand bed in order to improve understanding of cross-shore sediment transport mechanisms and determine sediment concentrations, fluxes and net transport rates under these conditions. Sediment concentrations were measured within the sheet flow layer (thickness in the order of 10 grain diameters) and in the suspension region (thickness in the order of centimetres). Within the sheet flow layer, the concentrations are highly coherent with the instantaneous near-bed velocities due to each wave within the wave group. However, in the suspension layer concentrations respond much more slowly to changes in near-bed velocity. At several centimetres above the bed, the suspended sediment concentrations vary on the time scale of the wave group, with a time delay relative to the peak wave within the wave group. The thickness of the sheet flow changes with time. It is strongly coherent with the wave forcing, and is not influenced by the history or sequence of the waves within the group. The velocity of the sediment was also measured within the sheet flow layer some of the time (during the larger wave crests of the group), and the velocity of the fluid was measured at several cm above the sheet flow layer. The grain velocity and concentration estimates can be combined to estimate the sediment flux. The estimates were found to be consistent with previous measurements under monochromatic waves. Under these conditions, without any significant mean current, the sediment flux within the sheet flow layer was found to greatly exceed the sediment flux in the suspension layer. As a result, net transport rates under wave groups are similar to those under monochromatic waves.

Suspended sand transport in surf zones

Abstract: [1] Three tests were conducted in a wave flume to investigate time-averaged suspended sediment transport processes under irregular breaking waves on equilibrium beaches consisting of fine sand. Free surface elevations were measured at ten locations for each test. Velocities and concentrations were measured in the vicinity of the bottom at 94 elevations along 17 vertical lines. The relations among the three turbulent velocity variances are found to be similar to those for the boundary layer flow. The vertical variation of the mean velocity, which causes offshore transport, is fitted by a parabolic profile fairly well. The vertical variation of the mean concentration is fitted by the exponential and power-form distributions equally well. The ratio between the concentration standard deviation σC and the mean varies little vertically. The correlation coefficient γUC between the horizontal velocity and concentration, which results in onshore transport, is of the order of 0.1 and decreases upward linearly. The offshore and onshore transport rates of suspended sediment are estimated and expressed in terms of the suspended sediment volume per unit area. A time-averaged numerical model is developed to predict as well as the mean and standard deviation of the free surface elevation and horizontal velocity. The bottom slope effect on the wave energy dissipation rate DB due to wave breaking is included in the model. The computation can be made well above the still water shoreline with no numerical difficulty. Reflected waves from the shoreline are estimated from the wave energy flux remaining at the shoreline. The numerical model is in agreement with the statistical data except that the undertow current is difficult to predict accurately. The measured turbulent velocities are found to be more related to the turbulent velocity estimated from the energy dissipation rate Df due to bottom friction. The suspended sediment volume expressed in terms of DB and Df can be predicted only within a factor of about 2. The roller effect represented by the roller volume flux does not necessarily improve the agreement for the three tests.

Extreme wave forces and wave run-up on offshore wind- turbine foundations

Abstract: This paper presents studies of wave run-up and wave forces on offshore wind- turbine foundations. The analyses are based on numerical investigations using a Navier- Stokes solver. The free surface is resolved with a Volume of Fluid technique (VOF). Waves approach the foundation from deep water to shallow water over a slope. This initiates breaking for high waves. The breaking process is shown to have a major influence on the run- up and wave forces.

Measurements of wave modulation and breaking

Abstract: To investigate the evolution of wave modulation and wave breaking, a series of elaborate experiments were conducted in a super wave flume (300 m × 5.0 m × 5.2 m) at Tainan Hydraulics Laboratory. Two wave trains of initial regular waves and initial imposed sidebands were generated by a wave maker with active wave absorption using an electro-hydraulic server feedback control system. The wave profiles were recorded by 66 high-resolution capacitance-type wave gauges positioned along the wave flume, and connected to a specially designed data acquisition system to collect a large number of parallel inputs in the long flume. Experimental data were obtained on wave modulation and related wave breaking. The long time evolution of the nonlinear wave train and the corresponding wave spectra are demonstrated for both breaking and non-breaking cases. Specifically, the periodic modulation and demodulation of wave train is observed at post-breaking stage.

Laboratory experiment and three-dimensional large eddy simulation of wave overtopping on gentle slope seawalls

Abstract: Wave overtopping on gentle slope seawalls for both smooth and stepped front faces was investigated in a laboratory wave flume. Overtopping rates were measured by a catchment basin placed behind the...

A numerical wave flume to study the functionality and stability of coastal structures

Abstract: Adaptation of COBRAS VOF model for typical breakwater applications and worked out examples of water movement in and around breakwater cross sections.

Laboratory study of sand bed forms induced by solitary waves in shallow water

Abstract: [1] The surface profile of water waves propagating in shoaling water approaches the solitary waveform before wave breaking. The effect of the high non-linearity of solitary waves may be very significant on bed forms induced in the nearshore zone. In this study, experiments on bed form generation beneath solitary waves are carried out in a 10-m-long flume used in resonant mode. Solitary waves are generated in shallow water on the background of a standing harmonic wave. One solitary wave (soliton) propagates in each direction of the flume on the time period of the flow, above an initially flat sandy bed. Ripples form rapidly on the bed and a strong interaction with the free surface occurs. The amplitude of the soliton and the phase shift between the soliton and the harmonic wave decrease with time, while the ripple amplitude increases. The amplitude of the harmonic wave is not affected by the ripples. The final ripple wavelength is about 1000 times the sand median diameter. Bars with superimposed ripples appear, with bar crests being positioned beneath the nodes of the standing wave, when bars form with crests beneath the antinodes of surface wave for standing waves without solitons. The Eulerian drift distribution in the flume is not affected by the propagation of solitons. We propose an energy balance for solitons propagating in shallow water above flat beds in which a term for the dissipation due to sand ripples is introduced, which defines a coefficient of interaction between solitons and ripples.

Breaking Wave Impinging and Greenwater on a Two-Dimensional Offshore Structure

Abstract: The measurement of velocity fields of a plunging wave impacting on a structure in a two-dimensional wave flume was investigated experimentally. As the wave impinged and overtopped the structure, a large highly aerated region was created in front of the structure and water splashed on top of the structure. The broken wave in front of the structure and associated greenwater on top of the structure are highly aerated containing not only a large number of bubbles but also very large sizes of bubbles. A modified PIV method incorporating the traditional PIV method with the shadowgraphy technique was introduced to obtain the velocity in the highly aerated region and the splashing water on the deck by correlating the “texture” of the bubble images. It was found that the maximum fluid particle velocity in front of the structure during the impinging process is about 1.5 times the phase speed of the wave, while the maximum horizontal velocity above the deck is less than the phase speed. It was also found that the dam breaking solution does not work well in predicting the greenwater velocity.

Threshold for Ripple Formation on Artificially Roughened Beds: Wave-Flume Experiments

Abstract: The lack of systematic studies of the influence of bottom irregularities on ripple initiation led to a series of wave-flume experiments with the purpose of exploring a general relationship. Three types of sand beds with different degrees of roughness were prepared for the experiments: flat, notched, notch-mounded beds. Three kinds of well-sorted sands with different diameters were used for the bed material: fine, medium, and coarse sands. With combinations of waves, water depth, sand, and bed roughness, 248 experimental runs were carried out. Data were analyzed considering (1) the mobility of sediment, expressed in terms of the mobility number, M, (2) the degree of bed roughness, represented by the Reynolds number, Re, and (3) the asymmetry of flow field due to nonlinearity of waves, represented by the relative water depth, kh (= 2πh/L; h = water depth and L = wavelength). The result showed that the threshold for rippling is described by M = 2 + A exp B, where A = 5.7 (3.79/(kh + 0.65) − 1) and B...

The application of a numerical model to coastal surface water waves

Abstract: Based on the Navier-Stokes Equations (NSE), numerical simulation with fine grids is conducted to simulate the coastal surface wave changes, including wave generation, propagation, transformation and interactions between waves and structures. This numerical model has been tested for the generation of the desired incident waves, including both regular and random waves. Some numerical results of this model are compared with available experimental data. In order to apply this model to actual cases, boundary conditions are considered in detail for different shoreline types (beach or breakwater, slope or vertical wall, etc.). Finally, the utility of the model to a real coastal area is shown by applying it to a fishing port located in Shidao, Rongcheng, Shandong Province, P. R. China.

Interaction of Waves and Reef Breakwaters

Abstract: Reef breakwaters can provide shoreline protection with low environmental impact. They offer a sensitive engineering solution where a competent economical and functional design method needs the knowledge of relationships linking basic parameters such as freeboard, crest width, wave transmission and set-up behind the structure.

Numerical simulation of water wave impacts using a navier-stokes solver

Abstract: The calculated results for the violent impact of water waves onto rigid stationary coastal structures are presented in this paper. The simulation is performed in a numerical wave flume using our recently developed in-house Navier-Stokes solver([4]) named LVOF. It is a VOF finite volume approach that incorporates the surface tension effects, coupled with a dynamic subgrid-scale (SGS) turbulence model. Test cases concern a combination of wave propagation, shoaling, reflection, diffraction, breaking and overtopping after impact. Additionally, the effects of a current on wave-structure interactions are investigated, including the study of the influence of viscosity on the wave boundary layer under breakwater and the 3D effects. Our results demonstrate that our solver can describe most of the significant features of motions induced by regular and irregular waves. In particular, the shape of the free surface agrees well with measurements under grid refinementscaptured and even during lengthy computations.

Validation of Wave Propagation in Numerical Wave Tanks

Abstract: During the last few years there has been a strong growth in the availability and capabilities of numerical wave tanks. In order to assess the accuracy of such methods, a validation study was carried out. The study focuses on two types of numerical wave tanks: 1. A numerical wave tank based a non-linear potential flow algorithm. 2. A numerical wave tank based on a Volume of Fluid algorithm. The first algorithm uses a structured grid with triangular elements and a surface tracking technique. The second algorithm uses a structured, Cartesian grid and a surface capturing technique. Validation material is available by means of waves measured at multiple locations in two different model test basins. The first method is capable of generating waves up to the break limit. Wave absorption is therefore modeled by means of a numerical beach and not by mean of the parabolic beach that is used in the model basin. The second method is capable of modeling wave breaking. Therefore, the parabolic beach in the model test basin can be modeled and has also been included. Energy dissipation therefore takes place according to physics which are more related to the situation in the model test basin. Three types of waves are generated in the model test basin and in the numerical wave tanks. All these waves are generated on basin scale. The following waves are considered: 1. A scaled 100-year North-Sea wave (Hs = 0.24 meters, Tp = 2.0 seconds) in deep water (5 meters). 2. A scaled operational wave (Hs = 0.086 meters, Tp = 1.69 seconds) at intermediate water depth (0.86 meters) generated by a flap-type wave generator. 3. A scaled operational wave (Hs = 0.046 meters, Tp = 1.2 seconds) in shallow water (0.35 meters) generated by a piston-type wave generator. The waves are generated by means of a flap or piston-type wave generator. The motions of the wave generator in the simulations (either rotational or translational) are identical to the motions in the model test basin. Furthermore, in the simulations with intermediate water depth, the non-flat contour of the basin bottom (ramp) is accurately modeled. A comparison is made between the measured and computed wave elevation at several locations in the basin. The comparison focuses on: 1. Reflection characteristics of the model test basin and the numerical wave tanks. 2. The accuracy in the prediction of steep waves. 3. Second order effects like set-down in intermediate and shallow water depth. Furthermore, a convergence study is presented to check the grid independence of the wave tank predictions.Copyright © 2005 by ASME

Flow Measurements and Numerical Simulation on Low-Crested Structures for Coastal Protection

Abstract: A series of laboratory and prototype experiments have been performed to measure and model the flow around and inside low-crested structures and submerged breakwaters. The results of the tests are used to improve the understanding of the wave-structure interaction and its effect on the near-shore dynamics. The database become the core to develop an empirical model to determine dynamic pressure and velocity profiles for design purposes, and to assess the laboratory scale effects on coastal structures. Finally, the large amount of measured information is used to calibrate and validate COBRAS, a 2DV, VOF-type, RANS numerical model, proven to be a powerful tool for the functional and structural design of low-crested and submerged breakwaters, as well as a numerical wave flume to increase the applicability of empirical models.

Velocity Field Measurement of Impinging Waves on a Structure

Abstract: As the wave impinges on and overtops the structure, a large highly aerated region is created in front of the structure and water splashs on top of the structure. The broken wave in front of the structure and associated green water on top of the structure are highly aerated containing not only a large number of bubbles but also very large sizes of bubbles. In this paper, the velocity field of the highly aerated region and the splashing water on the top is measured using a modified PIV method incorporating the traditional PIV method with the shadowgraphy technigue by correlating the ` texture ` of the bubble images. The velocity fields of a plunging wave impacting on a structure in a two-dimensional wave flume is measured. It is found that the maximum fluid particle velocity in flout of the structure during the impinging process is about 1.5 times the phase speed of the wave, while the maximum horizontal velocity above the top is less than the phase speed, It is also found that the dam breaking solution does not work well in predicting the green water velocity.

The calculation of horizontal wave forces on perforated caissons with top cover

Abstract: In this paper, the investigation of the interaction of irregular waves with perforated caissons with top cover is carried out in wave flume. The method of correlation analysis is used to study the relationship between the total horizontal wave forces and their arms on the perforated caissons and their major influencing parameters then the correlation equations for the calculation of wave forces and their arms are obtained by Least Square Method.From the comparison of the total horizontal forces on perforated caissons with top cover between regular wave test data and irregular wave measured data,we know the total horizontal forces about the irregular waves acting on perforated caissons with top cover are usually larger than that about the regular wave acting on perforated caissons.So for real engineering practice,to use the irregular wave forces on perforated caissons is recommended.

Issues at the frontiers of coastal morphodynamics modelling

Abstract: The morphodynamic model developed by DHI Water and Environment has been tested against several field and laboratory data sets. Modelled and measured sea bed elevations, currents, water level gradients and sediment transport rates were compared using three different data sets. The sea bed elevation comparisons used the field data sets from the Tweed River sand bypassing project which included wave, tide and sea bed measurements. This data set was combined with sediment size, river flow, rainfall and water level measurements from government agencies and developed into morphodynamic model covering the Tweed entrance ebb tide delta and the surrounding coastline. The model results show limited agreement with the measured sea bed levels. To improve the model stability, useability and results, several improvements were implemented, including a new stable bed updating scheme, an accelerated sediment transport rate tabulation, improved boundary conditions and refined spatial resolution. Further, it is shown that dramatic changes in forcing neither significantly improve the results nor change them. This lack of response to changes in the forcing is attributed to underestimations of gradients in water levels, currents and waves, ultimately leading to sediment transport gradient being underestimated. This is confirmed by detailed water level gradients obtained from Moreton Island rip current experiment, where the current model underestimated water level gradients by at least a factor two. However, it is also shown that these gradients can be well predicted when wave-current and current-current (cross-shore current mixing the longshore momentum) interactions are included in the model. Unfortunately, at this time, these two features require significant computational resources and are not practical with current computers and broad area models such as the Tweed River. Finally, the sediment transport model was compared to field scale laboratory experiments from the Hannover wave flume. These measurements differ from the traditional wave tunnel tests in that the near-bed velocity exhibits acceleration-asymmetry. The sediment transport model compares well to the measurements if driven by the measured near-bed velocities, as does a new simple formulation by Nielsen and Callaghan [2003]. The robustness of predictions diminishes when the model is used to estimate the near-bed velocity. In particular, the model predictions show that different near-bed velocity formulations are required to closely reproduce the measurements. Again, this indicates that when the near-bed velocity formulation is held constant, as required in the morphodynamic model, incorrect sediment transport gradients will results. This study also provides practical methods to; reduce the computation effort involved in calculating sediment transport rate tables; incorporate complex cross-shore boundaries; and stably incorporate sediment sinks and sources. The study also provides a thorough review of bed updating schemes and the key parameters in terms of numerical practicalities that make up the sediment transport model. Finally, the study generates a list of morphodynarnic model modifications and parameter changes that do not significantly improve morphological results. The model review and testing indicated that including; wave-current interactions (wave modelling); and improved sediment transport formulation offer two possible model improvements that would provide morphodynamic predictions. The model review and testing indicated that including; wave-current interactions (wave modelling); and an improved sediment transport formulation offer two possible model improvements to achieve improved morphodynamic predictions.

Kinematic Wave and Diffusion Wave Theories

Abstract: A wide range of phenomena, natural as well as man-made, in physical, chemical, and biological hydrology exhibit characteristics similar to those of kinematic or diffusion waves. The wide range of phenomena suggests that these waves are very pervasive. This study presents theories that are considered fundamental to advancing the state of the art of hydrology. The term “wave” implies a disturbance traveling upstream, downstream, or remaining stationary. We can visualize a water wave propagating where the water itself stays very much where it was before the wave was produced. We witness other waves that travel as well, such as heat waves, pressure waves, and sound waves. There is obviously the motion of matter, but there can also be the motion of form and other properties of matter. Keywords: diffusion wave theory; flood hydrograph; flow routing; flux law; hydrology; kinematic shock; kinematic wave theory; nonuniform flow; numerical scheme; steady-state flow; uniform flow; unsteady-state flow; wave

Experimental investigations on tsunami runup height on permeable beachs

Abstract: Tsunamis may be generated by earthquake-triggered movement of the sea bottom, landslides and collapses. With the change of nearshore bathymetry these waves progress towards inland and causes large damage. Prediction of runup height will play an important role in dimensioning coastal structure as tsunamis are more effective in the runup area at the shoreline. In the literature research on runup height, tsunami wave is liken to solitary wave and produced by horizontal movement of a vertical plate, which is a proposed by Goring (1978). In this study, for the simulation of sudden motion of the ocean bottom, tsunami wave generation system is developed by piston attached to an horizontal plate. The piston moves vertically. Experiments were carried out in the glass-side wall wave flume of 22.5 m length, 1 m width, and 0.50 m depth at the Hydraulics Laboratory, Civil Engineering Faculty, Istanbul Technical University. The beach was formed by natural beach sand and had a slope of 1 vertical to 5 horizontal. The specific gravity of sand was 2.63 g/cm 3 and the diameter of sand was 0.35 mm. In the experiments it is observed that the waves are not broken. Water surface profiles and velocity values of the waves calculated and generated are compared. Runup height of tsunami waves on permeable beach is analysed and the experimental results, for impermeable slopes are compared with the runup law and it is seen that they are in consistency. Empirical equation are proposed for permeable beach by defining parameters effecting runup height, wave height, slope, water depth, and the characteristics of the material at the slopes.

Boundary Layer Processes in the Surf Zone and Inner Shelf in CROSSTEX and MISO

Abstract: : Long term goals are to observe and model turbulent stresses in the bottom boundary layer (BBL) arising from wave forcing and low frequency currents, and the resulting sediment transport, and bedform evolution in the inner shelf and surf zone. The presence of mobile sandy beds across continental shelves and surf zones results in complex interactions and strong feedbacks between fluid motion in the bottom boundary layer and sediment movement both as bed load and suspended sediment flux. A primary goal is to develop parameterizations of the formation of different bedform types to different aspects of wave and current forcing and to improve models of wave-forced sediment transport. The observations made in this project are being used to understand and model sediment transport and effective bed roughness and BBL wave dissipation in response to these evolving bedforms. A primary scientific objective of this project is to measure turbulent stresses, shear and sediment fluxes in the bottom boundary layer in both inner shelf and surf zone field experiments and large scale tank experiments. The field observations represent the more complex superposition of forcing factors including cross-shore and long-shore low frequency currents, and broad band wave forcing. These long term (months to years) timeseries provide a range of these forcing parameters. Inner shelf field BBL observations from the 12m depth cabled MISO observatory in Monterey bay and surfzone measurements from RIPEX and NCEX are being used in these analyses. Data sets from these field programs are being complimented by measurements using the same instrument systems deployed in the large scale wave flume at the OSU Hinsdale Large Scale Wave Flume during CROSSTEX in the fall of 2005. This provided an opportunity to measure the BBL under controlled, repeatable, programmable wave conditions with negligible longshore mean currents.

Influence of Relative Water Depth on Wave Run-up Over Costal Structures; Smooth Slopes

Abstract: This paper describes an experimental study carried out in a laboratory wave flume to quantify the influence of the relative water depth on the wave run-up over a smooth sloping structure. The run-up measurements were carried out over practically important ranges of the wave steepness, the relative water depth and the structure slope. The results indicate an increase in the wave run-up at shallow depths compared to deep water conditions. This increase is up to about 20% for plunging breakers and is as much as 65% for surging breakers.