Showing papers on "Wave flume published in 2000"

Wave Mechanics for Ocean Engineering

Abstract: Periodic Wave Pattern - the approach of differential calculus Periodic Wave Pattern - the control volume approach Wave Effects on Coasts Wind Generated Waves - basic concepts Analysis of the Sea States - the time domain The Wave Climate Design Waves and Risk Analysis Analysis of the Sea States in the Space-Time The Theory of Quasi-Determinism Uses and Consequences of the Quasi-Determinism Theory Analysis of the Wave Forces on Offshore Structures Calculation of the Wave Forces on Offshore Structures Stability Analysis of Coastal Structures Topics Calling for an Overall Overview of Offshore and Coastal Engineering.

Review of Multidirectional Active Wave Absorption Methods

Abstract: Active wave absorption in a physical wave flume or basin requires a wavemaker, a hydrodynamic feedback, and an absorption control system. The feedback provides the information on the waves to be absorbed, and the control system ensures that the paddle of a wavemaker moves accordingly. Different ways of viewing the principle of active absorption are discussed, and basic concepts are examined for the simple case of shallow water waves, starting with the flume case and subsequently including wave obliqueness. A brief review is given of existing 2D systems for active absorption in wave flumes. Quasi-3D systems for multidirectional waves constituted by an array of independent flume systems are discussed, and fully 3D systems, including a coupling between neighboring feedback signals, are reviewed. The diverse appearance of these systems is mainly due to different ideas of accounting for wave obliqueness. An attempt is made to unify the different approaches using the notion of a 2D filter giving the relation between the paddle control and the feedback, regarding each of the two signals as one function of time and space (along the wavemaker), instead of as an array of 1D functions of time. So far, active absorption relies on linear systems. Suggestions are made for the direction of future research, including the prospects for nonlinear extensions.

Waves and breakers in shoaling water

Abstract: Wiegel and Johnson (1950) summarized useable wave theories for deep and shallow water. Mason (1950) discussed waves in shoaling water and compared theoretical predictions with measurements. The theories are shown to apply, within practical limits, to periodic systems of deep water waves, and to periodic waves progressing over a shoaling bottom to wave positions near the breaking point. Near and at the breaking position the wave features are not predicted from theory with desired accuracies and measured characteristics are used to describe breakers. The available measurements are limited and do not show the effects of variables such as the beach slope. Recent work at the University of California has resulted in information on the limits of applicability of the linearized wave theories as applied to wave transformation in shoaling water, and on breaker shapes and motion; including the effect of beach slope.

Free-Wave Energy Dissipation in Experimental Breaking Waves

Abstract: Several transient wave trains containing an isolated plunging or spilling breaker at a prescribed location were generated in a two-dimensional wave flume using an energy focusing technique. Surface elevation measurements of each transient wave train were made at locations before and after breaking. Applying a nonlinear deterministic decomposition approach to the measured elevation, the free-wave components of the transient wave train were derived by excluding the contribution from bound-wave components. The comparison of the amplitude or energy spectra of free-wave components before and after a breaker can accurately reveal the energy dissipation as a function of frequency. It is found that the energy loss is almost exclusively from wave components at frequencies higher than the spectral peak frequency. Although the energy density of the wave components of frequencies near the peak frequency is the largest, they do not significantly gain or lose energy after the breaking. It is also observed that wave components of frequencies significantly below or near the peak frequency gain a small portion (about 12%) of energy lost by the high-frequency waves. These findings are quite different from the empirical formulas presently used for determining wave dissipation due to wave breaking. Hence, they have important implications to the ocean wave energy budget, specially to the energy transfer at frequencies below and near the spectral peak frequency.

Observed and predicted vertical suspended sediment concentration profiles and bedforms in oscillatory-only flow

Abstract: Measurements of hydrodynamic conditions and vertical suspended sediment concentration profiles, C-profiles, have been obtained above rippled sandy beds in a large wave flume. Measured values of wave height and period, water depth and sediment properties are used in well-known formulae to predict sediment dynamics and bed shear stresses due to waves. These data are then used in an existing convective model and a new model to predict C-profiles. Measured C-profiles and C-profiles predicted by the new model are shown to agree well for a range of grain sizes and wave conditions. Grain-scale bed roughness, defined using data from the model, is found to vary with the wave mobility number. Values for the wave mixing coefficient derived using the new model are tested and are found to agree well with published values and with theory.

Modelling sand resuspension by waves over a rippled bed

Abstract: With a two-dimensional k-ϵ model and equations to describe sediment entrainment, diffusion, convection and settling, wave-induced flow velocities and sediment concentration fields over rippled beds have been calculated. Results from the simulations are compared with time-averaged vertical suspended sediment concentration profiles, C-profiles, measured in a large wave flume using the autonomous multi-sensor instrument STABLE. The simulations make it possible to examine in detail the mechanisms of sediment entrainment, vortex formation and advection. The resulting animations of model output allow a quasi-3-D visualization of sediment resuspension processes and aid quantification of the resulting vertical suspended sediment concentration field due to regular waves, asymmetric waves and wave groups. The work contributes therefore, to knowledge about wave-sediment interactions and to understanding of sediment transport mechanics.

Modelling Sand Resuspension by Waves over a Rippled Bed

Abstract: With a two-dimensional k-ϵ model and equations to describe sediment entrainment, diffusion, convection and settling, wave-induced flow velocities and sediment concentration fields over rippled beds have been calculated. Results from the simulations are compared with time-averaged vertical suspended sediment concentration profiles, C-profiles, measured in a large wave flume using the autonomous multi-sensor instrument STABLE. The simulations make it possible to examine in detail the mechanisms of sediment entrainment, vortex formation and advection. The resulting animations of model output allow a quasi-3-D visualization of sediment resuspension processes and aid quantification of the resulting vertical suspended sediment concentration field due to regular waves, asymmetric waves and wave groups. The work contributes therefore, to knowledge about wave-sediment interactions and to understanding of sediment transport mechanics.

Evaluation and Performance Enhancement of a Pressure Transducer under Flows, Waves, and a Combination of Flows and Waves*

Abstract: The performance of a pressure transducer, with its inlet attached to differing hydromechanical front ends, has been evaluated in flow flume and wave flume experiments in which laminar and turbulent flows, and regular progressive gravity waves and combinations of flows and waves, were generated. For steady laminar flows, and for waves propagating on quiescent waters, the transducer’s performance improved when the inlet was at the center and flush with a large, thin, and smooth circular horizontal end plate. This enhancement is likely to have been achieved because of the isolation of the pressure inlet from the separated flows and vortices generated by the transducer housing. Flow disturbances, generated by nearby solid structures, deteriorated the performance of the pressure transducer. However, its performance could be significantly improved by protecting the pressure inlet by a sturdy, curved perforated shield. The dynamic pressure error in this case was 2 mb at 100 cm s−1, compared to 8 mb in t...

Modeling of overturning waves over arbitrary bottom in a 3D numerical wave tank

Abstract: An accurate three-dimensional (3D) Numerical Wave Tank solving fully nonlinear potential ow theory is developed and validated for modeling wave propagation up to overturning over arbitrary bottom topography. The model combines a higher-order 3D-BEM and a Mixed-Eulerian-Lagrangian time updating of the free surface, based on explicit second-order Taylor series expansions , with adaptive time steps. The spatial discretization is third-order and imposes continuity of the inter-element slopes. Discretized boundary conditions at intersections between domain boundary sections (corner/edges) are well-posed in all cases of mixed Dirichlet-Neuman problems. Waves can be generated in the tank by wavemakers, or be directly speciied on the free surface. If required, absorbing layers can be speciied on lateral boundaries. Node regridding to a ner resolution can be speciied at any time step over selected areas of the free surface. Results are presented for both validation tests with a permanent wave propagation over constant depth, and for the computation of a 3D overturning wave over a ridge.

A perforated-ball velocity meter for underwater kinematics measurement in waves and current

Abstract: A perforated-ball velocity meter (PVM) has been developed for measuring all three components of water particle velocities at five different vertical locations simultaneously in regular, long crested as well as short crested free surface wave field in laboratory scale. The device measures the wave induced forces on a cantilevered ball-tube assembly. The wave kinematics is computed from these forces by solution of an inverse problem given by the Morison equation. The drag and inertia coefficients of both the ball and the tube are experimentally determined and the forces on both the ball and the tube are used in the inverse Morison problem. Measurements in wave flume and wave basin have been conducted using the multistage PVM and the measured kinematics has been successfully compared with theory using wave elevation measurements. The device can be used to measure current speed as well as wave kinematics in a wave-cum-current field.

Long waves and the change in cross-shore sediment transport rates on a sheet flow dominated beach

Abstract: In this paper, at first results of experiments in sheet flow conditions under nonlinear asymmetric irregular oscillations with and without long wave components are presented and a method for estimating the net transport rates is proposed. Then the importance of phase lag between free long waves and irregular short waves as well as the effect of steady currents on the magnitude of transport rate are discussed. Implications of the results are further demonstrated by applying the proposed formulation to calculating the cross-shore distributions of sediment transport rates on a sheet flow dominated beach under incident irregular waves with low frequency components. It is found that long waves can significantly enhance the seaward transport of sediments inside the surf zone during a storm. It is also shown that the presence of a standing long wave system may lead to large spatial gradients in the cross-shore transport rates and eventually, to formation of a multiple longshore bar system.

A survey of the generation of ocean waves in a test basin

Abstract: At present stage the analytical design of wave tolerance for floating structures and vessels is still imperfect due to the mutually complex and nonlinear phenomena between structures and waves. Wave tolerance design is usually carried out through iterative evaluations of results from model tests in a wave basin, and this is done in order to reach a final structural design. The wave generation has then become an important technology in the field of the coastal and ocean engineering. This paper summarizes the facilities of a test basin and a wave maker in Japan and also surveys the methodology of the generation of ocean waves in a test basin.

Suspended Sediment Concentrations over Ripples for Waves

Abstract: This paper presents the flow and the suspended sediment movement over ripples for oscillatory flows. A new numerical model system is developed, and applied to a laboratory experimental condition of regular waves and a fictitious condition of irregular waves. The flow field is obtained from a programme proposed by Kim et. al.(1994), which is a modified version of SOLA based on SMAC scheme. The sub-model solves the continuity and Reynolds momentum equations in the x-z plane. The wave orbital velocities, shear stresses, and pressure are all reasonably reproduced by the model. The model results on the vertical velocity component show good agreement with the measurements. The suspended sediment transport sub-model is newly set up to solve the advection-diffusion equation of suspended sediment using a split method, and involving a special shear entrainment from the whole ripple surface. The calculated suspended sediment concentrations for regular waves show reasonable agreement with measurements at Deltaflume. The model results for random waves show that the suspended sediment concentration is higher than those for regular waves and that the sediment diffuses higher than for regular waves with the significant wave height and the peak wave period of the irregular waves.

Analysis of acoustic measurements of suspended sediments collected in a large‐scale wave flume facility

Abstract: Our capability to accurately predict suspended sediment concentration in the marine environment is relatively limited; generally, measuring techniques to obtain suspended concentration have been somewhat primitive. In recent years acoustics has been applied to the measurement of suspended sediments, and through a series of theoretical and experimental studies, a comprehensive description of the interaction of sound with suspensions of marine sediments has been developed. In the present work these developments have been applied to obtain detailed acoustic profiles of suspended sediment concentration under waves in a large‐scale flume facility. Utilizing the acoustic concentration profiles, an assessment of suspended sediment models for predicting sediment concentration has been conducted, and the underlying intrawave mechanisms of sediment entrainment investigated.

Study on function of a l-type curtain wall wave barrier using a numerical wave flume

Abstract: A new wave barrier named ‘L-Type Curtain Wall Wave Barrier’ is developed and its function is studied using a numerical wave flume. The wave barrier comprises a curtain wall and a horizontal flat plate to reflect incident wave effectively; the transmission coefficient of the new wave barrier even with its relatively small width B and small submerged depth d is very low compared with a curtain wall or with a horizontal plate. The calculations were made using the numerical wave flume which is under development by a special research group including the authors. The calculation with its animation can clearly show the mechanism of wave reflection and wave energy dissipation.

Flume simulation of water surface profiles using a general software package

Abstract: The authors describe a method for reproducing pre-specified water surface profiles on a flume, based on a method presented by H. Graveson et al. (1974). Modifications have been introduced to allow for deep waters and to take into account the distance of wave gages from the paddle. The method has been implemented as a module of a software package for sea wave analysis and simulation.