Vertical Wave Front Research Articles

Experimental study on plunging breaking waves impacting a vertical cylinder with three different model scales

Scale effects, which can influence the reliability and effectiveness of experimental results, are often unavoidable but seldom explored, particularly in the context of plunging waves. To investigate the scale effects of plunging wave impacts on a vertical cylinder, experiments were conducted in a wave flume at model scales of 1:20, 1:30 and 1:40. The wavemaker movements were scaled based on the Froude-similarity law and the pressures on the front surface of the cylinder were analyzed. The results indicated that plunging waves were generated across these three scales, with fine agreement in overall wave elevations and a local waveform similarity of 80–90%. The analysis of pressures showed that the distribution of peak pressures on the cylinders remained consistent across different scales; however, scale effects were closely related to the intensity of wave impacts. Specifically, in cases of relatively weak impact, such as when the wavefront struck the cylinder before breaking or even after an intense wave breaking with intense gas-liquid mixtures, the pressure discrepancies were less than 20% among these different scales. This discrepancy can exceed 50% near the wave peaks during the vertical wave front or early jet formation stages with strong impacts.

Profilometry: a non-intrusive active stereo-vision technique for wave-profile measurements in large hydrodynamic laboratories

Profilometry is proposed as a novel non-intrusive image-based technique to capture the profile of the air–water interface as a dense point cloud. It can be classified as an active stereo-vision method applied to the study of gravity-driven water waves and specifically developed to be used in large hydrodynamic laboratories. As an active vision technique, it relies on the use of light sources, and as a stereo technique, it requires one or more high-speed camera pairs for imaging the same scene synchronously. To enhance the visibility of the laser lights on the wave profile, the water surface is sprayed with water droplets. Profilometry, compared to standard wave probes, can be considered as an alternative source of information that can augment spatial resolution to the identification of the air–water interface to capture nonlinear wave-evolution mechanisms and violent wave–body interactions. Its feasibility and accuracy are examined preliminarily in a small-scale flume and then in a large-scale towing tank using long-crested wave scenarios, including regular, irregular, and focused gravity-driven waves, without the presence of a structure. The values of the wave steepness examined were various and included also quite steep cases with nearly vertical wave fronts. Role played by parameters of the technique, as well as of its setup in capturing the wave features are also analysed, with the aim to provide a useful guidance for other researchers that intent to use and develop further this approach.

Fast wavefront sensing for X-ray optics with an alternating speckle tracking technique.

Advances in accelerator technologies have enabled the continuous development of synchrotron radiation and X-ray free electron laser (XFEL) sources. At the same time, it has been critical to perform in-situ wavefront sensing to aid delivery of high-quality X-ray beams to the end users of these facilities. The speckle-based scanning technique has obtained popularity due to its high spatial resolution and superior sensitivity compared to other wavefront sensing methods. However, these advantages often come at the expense of longer data acquisition times since multiple images have to be collected to derive the necessary wavefront information. Whereas initial speckle tracking techniques could obtain wavefront information relatively quickly, the installation of additional hardware was routinely required to do so. Here, we propose a novel speckle-based approach, termed Alternating Speckle Tracking (AST), to perform fast wavefront sensing within a conventional beamline setup. The wavefront information derived from the new technique has proven to be valuable for many applications that require temporal resolution. Importantly, both horizontal and vertical wavefront information can be simultaneously derived by moving the speckle generator along the diagonal direction. We expect this method will be widely used by the synchrotron radiation and XFEL community in the future.

BREAKING WAVE KINEMATICS, LOCAL PRESSURES, AND FORCES ON A TRIPOD STRUCTURE

This paper presents breaking wave loads on a tripod structure from physical model tests and numerical simulations. The large scale model tests (1:12) are described as well as the validation of the three dimensional numerical model by comparison of CFD wave gauge data and pressures with measurements in the large wave flume inside and outside the impact area. Subsequently, the impact areas due to a broken wave, a curled wave front as well as for wave breaking directly at the structure with a partly vertical wave front are compared to each other. Line forces in terms of slamming coefficients with variation in time and space are derived from CFD results and the velocity distribution is presented at the onset of wave breaking. Finally, the results are briefly discussed in comparison to other slamming studies.

Basin-scale internal-wave tomography using a large-aperture vertical array

Basin-scale acoustic transmissions which are received at a large-aperture vertical line array (VLA), like those deployed for the Acoustic Thermometry of Ocean Climate (ATOC) program, provide several acoustic fluctuation observables that can be used for internal-wave tomography. A single receiver provides observations of wavefront travel-time variance, pulse time spread, and temporal signal coherence. A VLA adds the observations of vertical wavefront coherence and modal time spread and time coherence. The combination of wavefront and modal observations provides tomographic vertical resolution of average internal-wave energies as a function of depth. In addition, vertical and temporal wavefront coherences are complementary observations, since spatial and temporal internal-wave variabilities are related through the dispersion relation. Also, vertical and temporal coherences help test the assumption of a separable internal-wave spectrum in terms of frequency and vertical wave number. First efforts to combine several of these acoustic fluctuation observations from the ATOC North Pacific VLAs in an internal-wave tomographic analysis will be presented. Directions for future research will be discussed.

Modeling of the out-of-plane pulse propagation in the SWARM95 experiment

The acoustical data collected in the shallow water acoustics in a random medium (SWARM95) experiment on the continental shelf off New Jersey in 1995 have shown that the temporal variations of the time series of the airgun signals (center frequency 75 Hz and bandwidth 50 Hz) are both range and azimuthal dependent. A full-wave broadband PE model is used to simulate such shallow water pulse propagation at different horizontal bearings. The time-dependent shallow water internal wave fluctuations constructed with a broad spectrum of excitation and the solitonlike thermocline displacements based on the in situ observations are included in the numerical simulation as well as the bathmetry. The vertical wavefronts and the 2-h time histories of the repeated transmissions from airgun positions above and below the thermocline are calculated and displayed. The simulated data and the experimental data of the receiver arrays at different locations with an approximately 10-km cross-range distance are compared. The results suggest that the azimuthal dependence of the received signals correlate to both the internal wave fields and the bathmetric variations.

Wave impact pressure on vertical walls under breaking waves of various types

Laboratory experiments were conducted to improve our understanding of the physics and characteristics of impact pressures due to collisions of breaking waves against a vertical wall. Measurements of impact wave pressures were performed simultaneously with observation of high-speed video pictures of the violent wave motion at the collision. The physics and characteristics of the impact pressure significantly depend on the colliding conditions of breaking waves. These were studied for the following colliding conditions: flip-through, collision of the vertical wave front, and plunging wave collision. When a small amount of air is entrapped between the breaking wave and the wall at the collision, the impact pressure increases considerably. The highest pressure, of very short duration, is observed when a vertical wave front strikes the wall while trapping a small amount of air in the form of either bubbles or a thin lens-shaped pocket. The impulsive pressure, occurring in the vicinity of the still water level, is transmitted downwards through the water body with the sound velocity. The larger the amount of the entrapped air at impact of the plunging breakers, the lower the magnitude and the longer the rise or compression time of the impact pressures. When plunging and curling of the breaking wave develop well, a thick air pocket is trapped, and damped pressure oscillations, due to the air pocket pulsation, appear immediately after the peak of the impact pressure. The oscillation frequencies are lower the greater the amount of entrapped air, and are almost equal to the resonant frequency of pulsating air pockets. The damping mechanisms, however, still remain unknown. Agreements between the measured and predicted oscillation frequencies suggest that adiabatic processes of the air pocket play an essential role in the physics of high impact pressure.

Middle and upper atmosphere radar observations of ionospheric horizontal gradients produced by gravity waves

By observing simultaneously in multiple directions with the middle and upper atmosphere phased‐array radar, we have been able to measure the horizontal structure produced by waves in the ionospheric density. We find that our data are most clearly ordered on a contour plot of the horizontal gradient of the electron density versus time and altitude. These plots show a periodic oscillation of the gradient with a downward phase progression below 400 km altitude but very nearly vertical wave fronts above that altitude. A simulation of this effect with a simple wave formulation provides a strikingly good comparison with the measurements.

A long-range acoustic experiment with sampling in the internal wave fluctuation band

Transmission at 460 Hz between two moorings centered about 600 km east of Miami is scheduled for August 1988. The 10-day, 260-km experiment has been designed to measure fluctuations induced by ocean internal waves (IW). The use of a 64-element, 150-m vertical receiver array, 6/h pulse rate, and high bandwidth allows measurement of pulse travel time (phase) to a fraction of a carrier period, and allows estimation of phase and intensity coherence times and the vertically lagged correlation function of the acoustic field, including the rms vertical wave front tilt. Path-integral solutions of the parabolic wave equation yield predictions of these statistical properties. For the Garrett-Munk IW model, rms travel-time predictions are 2–6 ms, and rms vertical angle predictions are 0.2–0.7 deg, with the lowest values for shallow turning rays and the largest for near axial rays. Expected phase coherence time scale is 5 min., with intensity time scale varying between 1 and 30 min., indicating the presence of both partially and fully saturated rays. These quantities yield a depth-dependent measure of rms vertical IW displacement, and some information about the IW spectrum.

Normal mode filtering for downslope propagation in a shallow water wedge

Experiments to measure downslope sound propagation in a shallow water wedge with a penetrable bottom have been conducted in an indoor tank. Mode filtering has been applied to the signals received on a vertical array to extract the waveforms of individual normal modes. Results show that the normal modes of the wedge propagate with wave fronts which are curved into arcs of circles centered on the wedge apex. These wedge modes propagate downslope without coupling and are therefore the true normal modes of the wedge. Conventional normal modes with vertical wave fronts show coupling effects for slopes of 2° and higher. The group velocities of the wedge modes are strongly dependent on the slope and water depth and are in agreement with adiabatic normal mode theory. Modes which are not trapped in the water depth at the source can become trapped when the water depth is sufficient to support them.

Model experiments on normal mode propagation in a wedge

Experiments to measure normal mode propagation into and out of a shallow‐water wedge have been conducted in an indoor tank. The wedge has a plane sand bottom and a slope which can be set up to 9°. A seven‐element line source allows individual normal modes of low order to be excited. For downslope propagation, measured attenuation and group velocities are in agreement with simple adiabatic normal mode theory. Dispersion and attenuation are rapidly reduced with increasing slope. For downslope propagation, the usual vertical wavefronts associated with normal modes become curved into spheres centered on the source. Up‐slope propagation is dominated by the disappearance of modes as they reach their cutoff depth. The energy of a mode traveling upslope is rapidly lost to the bottom after cutoff with negligible coupling to lower modes. [Work supported by ONR.]