Showing papers on "Wave flume published in 1986"

Impulsive breaking wave forces on an inclined pile exerted bi random waves

Abstract: A calculation method of the impulsive breaking wave forces on piles is proposed in this paper. It is derived on the basis of experimental results with some theoretical considerations. Wave forces on a cylinder caused by both regular and irregular trains of waves are measured in a large wave flume. The experimental results are compared with the values predicted by both Wagner and Karman. In the proposed method, it is assumed that the impulsive force acts on the upper half of the pile between the still water level and the wave crest, and that the force distribution along the pile is triangular shape. The peak value in the distribution is expressed by the function of both the breaking force parameter and the inclination angle of the cylinder. The breaking force parameter is the ratio of the bottom slope to wave steepness. The time history of the impact force is given as a triangular pulse with a vertical rise. The duration time is determined as half of the value predicted by Karman's theory.

Measurements of two- and three-dimensional waves in a channel, including the vicinity of cut-off frequencies

Abstract: Measurements of water wave profiles were performed in a rectangular flume equipped with a modular wavemaker. This particular wavemaker could generate both two- and three-dimensional waves. A method is proposed to evaluate quantitatively the deviations of a spacial flow field from the two-dimensional one. Plane propagating waves, as well as pure sloshing waves with their crests parallel to the walls, were generated in the flume. In all cases the measured amplitudes were compared against linear theory predictions.

Velocity and sediment concentration fields across surf zones

Abstract: Wave-induced currents and sediment concentrations in suspension across the surf zone were investigated at prototype scale in the BIG WAVE FLUME in Hannover for combined beach and dune profiles as well during the development phase until quasi-equilibrium stage as after beach equilibrium conditions had been reached. The selected initial profiles were partly investigated in repeated tests with regular and irregular waves. Based upon the obtained experimental data a comparison is presented between the measured and calculated landward displacement of sediment volume through a selected point on the beach into seaward areas. The discrepancies in between the results for measured profile change and for calculated sediment transport volume derived from vertical current and suspension distributions are discussed.

Laboratory study on two-dimensional beach transformation due to irregular waves

Abstract: In the present study, effects of irregular waves on two-dimensional beach transformation and related phenomena were investigated through a series of laboratory experiments. Attempts were made to determine a representative wave of irregular wave trains which controlled individual phenomenon related to the two-dimensional beach profile change. It was found that the representative wave is different for each phenomenon. For the macroscopic beach profile change, it is the mean wave which represents whole incident waves. On the other hand, some of microscopic phenomena, such as initiation of sand movement and sand ripple formation, are controlled by larger waves in the wave train selectively, of which representative wave is the significant wave.

Random breaking waves horizontal seabed

Abstract: According to wave theories the depth limited wave height over a horizontal seabed has a wave height to water depth ratio (H/d) of about 0.8. Flume experiments with monochromatic waves over a horizontal seabed have failed to produce H/d ratios greater than 0.55. However designers still tend to use H/d 0.8 for their design waves. Experiments have been carried out using random wave trains in the flume over a horizontal seabed. These experiments have shown that the limiting H/d ratio of 0.55 applies equally well to random waves.

Wave trapping by breakwaters

Abstract: The refraction of water waves around the heads of breakwaters can lead to large wave heights along the sheltered side of the structure. These waves are shown to be edge waves, trapped on the breakwater, based on the comparison of laboratory data and linear edge wave theory.

Circulation Kinematics in Nonlinear Laboratory Waves

Abstract: A weakly nonlinear solution is presented for the two-dimensional wave kinematics forced by a generic wavemaker of variable-draft. The solution is valid for both piston and hinged wavemakers of variable draft that may be double articulated. The second-order propagating waves generated by a planar wave board are composed of two components; viz., a Stokes second-order wave and a second-harmonic wave forced by the wavemaker which travels at a different speed. A previously neglected time-independent solution that is required to satisfy a kinematic boundary condition on the wavemaker as well as a mixed boundary condition on the free surface is included for the first time. A component of the time-independent solution is found to accurately estimate the mean return current (correct to second-order) in a closed wave flume. This mean return current is usually estimated from kinematic considerations by a conservation of mass principle.

Scale effects on stability and wave reflection regarding armor units

Abstract: In the studies on stability of the armor units and reflection from those, there are some indications on scale effects which are included in the results of small scale experiments. In this study, the fact has been confirmed with large wave flume test, and estimated the critical Reynolds Number where was no scale effect. And by this result on the stability of urmor units, we can evaluated the results in small and middle scale test, and can correct the minimum weight of armor units. So we can design the breakwaters and seawalls rationaly and economicaliy. However, it has not been confirmed the critical Reynolds Number where the influence of scale effect on reflection became negligible.

Nearshore circulations due to wave induced currents by offshore breakwater in finite elements

Abstract: The objective of the present study is concerned with the numerical prediction of wave patterns and wave induced currents adjacent to a breakwater. The wave theory used is that of Berkhoff's (1972) mild slope wave equation with effects of diffraction, refraction and reflection described as Bettess, Liang and Bettess (1984). A finite element model is applied with appropriate boundary conditions. The singularity in the velocity at the tip of the breakwater is modelled effectively using the technique of Henshell and Shaw (1975), originally developed for elasticity. In the case of waves induced currents a potential representation of velocity in the fluid has be€>n used to derive a set of radiation stress expressions based on the theory of Longuet-Higgins (1964, 1970a,b), which are for an arbitrary wave pattern and the bottom variation. These expressions used account for the mean sea level and satisfy Mei's (1973) static balance of momentum flux. The radiation stress is applied to obtain forcing terms for use in a shallow water equation in conjunction with limiting ratio wave breaking where wave height, wave period, wave steepness and beach slope may be considered. Finally, an offshore breakwater on a beach for shore protection has been applied in a complete finite element model to predict both wave pattern and nearshore currents. Two angles of wave incidence are chosen. A series result has been produced.

Wave reflection and transmission by an open-bottom airtight chamber for an active wave control system

Abstract: Wave reflection and transmission by an open-bottom airtight chamber for an active wave control system are investigated with consideration of compressibility of the air in the chamber. The hydrodynamic interaction of incident and scattering waves with the internal air of the chamber is formulated by employing linear potential theory and the equation of ideal gas state. Numerical results are presented to show the effects of internal air pressure upon the characteristics of wave reflection and transmission in various structural configurations. Model tests have been carried out in a two-dimensional wave flume. The experimental results are in good agreement with the numerical predictions. This paper also discusses an active wave control system to attain an optimum running condition by controlling the internal air pressure.