Wave Surface Research Articles

Method for reconstruction of axisymmetric capillary wave surface topography using inverse ray-tracing of refracted laser sheet

Capillary waves can be used to measure the fundamental fluid properties such as surface tension as well as, potentially, the viscosity of Newtonian fluids. This requires the measurement of various wave parameters, mainly wavelength, amplitude, and decay coefficient. However, the different scales of magnitudes make it a challenging task. Optical methods are well suited to analyze such problems due to their non-intrusive nature and high dynamic measurement resolution in both space and time. These methods are further categorized as point methods for a single probe measurement and space–time methods for transient measurement of the complete surface. Dynamic space–time methods are preferred despite the associated complex post-processing since they enable reconstruction of the wave surface. Some existing methods are discussed, and an improved method is then proposed to actually solve the associated inverse optics problem. In the method, an axisymmetric wave surface is re-constructed by analyzing the refracted laser sheet. The assumptions, simplifications, and constraints are taken to be compatible with experimental aspects for future validation. It is derived using the fundamental concepts in physics and the only major assumption of the axisymmetric nature of wave surface. The method exploits the underlying symmetry in the topography, making it more versatile, and suited for linear and non-linear capillary waves and waves with planar wavefront. The impact of parameters on the final result is determined through numerical simulations. Very low error (average and maximum) values are observed between reference and reconstructed topography for damped and undamped wave surfaces with a wide range of curvatures. Optimum values of critical parameters and associated reasoning are presented.

Study on the mechanism and load characteristics of secondary cavitation near free surface in underwater explosion

Although the cavitation phenomenon in underwater explosion has been researched for more than 100 years, the phase transition models based on mass and heat exchange between liquid and its vapor phases have only been established in the past decade. In this study, the secondary cavitation phenomenon was first captured by phase transition based on a four-equation system. The bulk cavitation near the free surface induced by underwater explosion was numerically investigated, and three typical bulk cavitation cases were investigated to explore their motion mechanisms and load characteristics on hydrodynamics and phase transition generation. It was found that secondary bulk cavitation will occur only under the condition that both the initial shock wave intensity and the distance between the water surface and the explosion bubble are satisfied in a specific relationship. Producing bulk cavitation was difficult at a relatively deep detonation depth because of the weak rarefaction wave reflected from the water surface by smaller charges. The statistical data under the condition of small charge indicated that the duration of cavitation increases with the increase in charge weight but the growth trend slows down gradually. However, the maximum volume of cavitation increased linearly with an increase in the charge weight. The present results can expand the currently limited database of underwater explosion multiphase fluids and provide insight into the interactions between the shock wave, bulk cavitation, explosion bubble, and water surface.

Frictional instability: A nonlinear mechanism to control shear wave responses in rough contact-based metamaterials

Incorporating nonlinearity in periodic media not only enables enriched wave dynamics but also allows passive tunability of the wave responses. However, studies so far have been mostly focused on nonlinear mechanisms and responses of longitudinal wave propagation whereas shear propagation in the presence of strong nonlinearity is yet to be fully understood. In this talk, we study shear wave propagation through metamaterials with rough contacts including friction. The roughness of contacting surfaces results in structural instability causing the contacts to switch between different regimes—stick, partial slip, and gross slip—giving rise to strong nonlinearity. Moreover, due to the presence of friction, the contacts exhibit hysteretic nonlinearity, i.e., history-dependent response. In this study, we first experimentally evaluate the frictional properties of rough contacts by measuring high-frequency friction hysteresis loops. Then, we develop metamaterial with a periodic arrangement of these rough contacts and use the obtained frictional properties to numerically study nonlinear shear wave signatures. We evaluate higher harmonic generation and demonstrate how they can be tuned through excitation wave amplitude, external precompression, and surface roughness. These fundamental understandings can open new avenues for designing tailored material with memory-dependent nonlinearity for controlling shear wave propagation.

Spatial-temporal measurement of waves in laboratory based on binocular stereo vision and image processing

Acquisition of wave field data is essential for hydrodynamics studies. Current studies show deficiencies in the spatial measurement of wave fields in the laboratory, and a more efficient method is urgently needed. A stereo imaging method was proposed for the spatial-temporal measurement of waves in the laboratory based on binocular stereo vision and digital image processing. Then, a series of regular and focused waves were simulated in a wave flume to examine the feasibility and reliability of the method by comparison with the measurements by wave probes. Floating particles were evenly sprinkled on the water surface as the representation of the water-air interface, providing texture features for wave surface images and allowing the realization of dense reconstruction. Synchronized cameras provided the synchronized stereo pairs sequence for each wave, and temporal reconstruction was achieved by processing frame by frame. The 4D spatial-temporal volume for every wave could be constructed. Subsequently, spatial slices of the volume obtained the instantaneous profiles of the wave field in different directions. Temporal slices obtained the wave time series at a fixed point, which were compared with the measurements of the standard resistive probe to verify the accuracy. The results showed that the proposed method was capable of measuring the spatial-temporal field of waves efficiently and precisely and offered the potential to measure more complex wave fields in nonlinear wave-structure interactions.

Error evaluation of binocular vision method for reconstructing breaking wave surface

Error evaluation of binocular vision method for reconstructing breaking wave surface

Numerical investigation of wave amplitude spectra effects on focusing wave generation

Focusing waves are often considered as an effective simulation of freak waves observed in oceans, or extreme sea states for marine structures design. The understanding of factors in generating prescribed focusing wave is of great importance for its generation in both numerical and physical wave tank. Based on the fully nonlinear potential theory solved by the QALE-FEM, this paper numerically investigates the effect of wave amplitude distribution over the frequency band under a series of the crest elevation parameters and frequency ranges on focusing wave generation, including focusing wave crest elevations and focusing positions. The results suggest that focusing wave crest elevation is significantly affected by the wave amplitude distribution especially for target waves with large wave crests or those are assigned at low frequency domain. Moreover, generation positions of focusing wave are also strongly correlated to amplitude spectra through phase shift of the wave surface and wave nonlinearity denoted by wave steepness.

Experimental investigation on three-dimensional structures of wind wave surfaces

This study experimentally investigated the three-dimensional features of wind wave surfaces in a laboratory. Free surfaces of wind waves under five types of wind velocities at four wind fetches were reconstructed by a stereo camera system. The dispersion law of wind waves was obtained from the spatio–temporal spectrum and showed a Doppler shift owing to the drift current. The wavenumber spectrum in the crosswind and wind direction follow the same power law of k−3 in the equilibrium range. Dynamic mode decomposition (DMD) was used to investigate the three-dimensional structures of the wave surface. Two different modes—the parallel mode and the net mode—were defined to characterize the structure of the wave surface. The parallel mode represents the average wave behavior over time, whereas the net mode represents the three-dimensional characteristics of the wind waves. The ratio of the wave number in the crosswind to the wind direction decreases with increasing wind fetch and converges to approximately 0.5. The wind wave spectrum in the wavenumber space had an elliptical distribution. Using the dispersion law, the wind wave spectrum in the (ω,k) domain can be predicted by a quartic surface.

Study of boat's appendages hydrodynamics by numerical methods

The research was carried out by numerical methods of small scientific trimaran boat with ADCP-profiler as ship's appendages in the range of Froude numbers from 0,19 - 0,316. For тnumerical simulation was used the FineMarineTM software. Numerical modeling was performed for a trimaran without an installed ADCP-profiler and for two mounting options at different locations along the length of the trimaran and in depth. Two types of ADCP-profiler fastening were studied. Mounting the device in a parallelepiped ship trunk has a greater influence on the resistance and moment My. With increasing speed, the area of the wetted surface for "free" mounting does not change much. For mounting "in the trunk" between trimaran's hulls the shape of the wave surface changes quite significantly, which leads to a redistribution of the buoyancy forces along the length of the boat.

Phase-resolved wave prediction for short crest wave fields using deep learning

The phase-resolved wave prediction of short crest waves is important to marine structures for both predicting deterministic motion and assisting decision-making. The present short crest wave phase-resolved prediction methods utilize Fast Fourier Transform (FFT) to process wave elevation fields in a large area, and these methods have shortcomings in calculation efficiency, calculation accuracy, and convenience. This paper proposes a long short-term memory wave prediction model (LSTM-WP model) based on deep learning to achieve a phase-resolved wave prediction of short crest waves. A tank experiment is conducted to verify and analyze the LSTM-WP model. From the results, it can be found that the LSTM-WP model provides high-precision predictions for the short crest wave surface under sea states of levels 4–7. Furthermore, the effects of the direction spectrum, predicted distance, and lead steps are discussed. It can be found that as the direction of the short crest wave becomes more concentrated, the prediction error rises more rapidly with increasing sea states. As the predicted distance increases, the prediction error of the LSTM-WP model increases linearly. As the number of lead steps increases, the prediction error of the LSTM-WP model shows a trend of first decreasing and then increasing.

МНОГОЦЕЛЕВЫЕ ТЕХНОЛОГИЧЕСКИЕ АППАРАТЫ С ГИБКИМ ВОЛНОВЫМ РАБОЧИМ ОБОРУДОВАНИЕМ

The article deals with problematic issues related to the complex processing of materials that are heterogeneous and complex in composition and properties. The necessity of its technological modernization and development is substantiated on the basis of assessment of the technical level of the existing equipment. For the first time, the concept of creating technological machines with a kinematically deformable wave working chamber is proposed and substantiated. Variants of technological solutions for one- and two-wave apparatuses have been developed. They can be used for mixing, grinding and separating materials by size. The main features of the behavior of flexible wave surfaces when interacting with the processed medium are shown. The design of two experimental installations is described, their technical characteristics are given, the composition of the processed materials is stated. The results of experimental studies on the processing of quarry chalk, building mixtures and municipal solid waste are presented. The data obtained make it possible to judge a sufficiently high technological efficiency and the need for further development of devices with wave working equipment, which is expressed in a sufficiently high quality of the products obtained, the ability to control their particle size distribution, the ability to work in conditions of an abrupt change in the properties of the treated medium, including due to self-cleaning elements of the working chamber and the elimination of jamming of pieces of material in the working areas and the combination of various technological functions in one apparatus

Surface wave analysis of the skin for penetrating and non-penetrating projectile impact in porcine legs.

Secondary blast injuries may result from high-velocity projectile fragments which ultimately increase medical costs, reduce active work time, and decrease quality of life. The role of skin penetration requires more investigation in energy absorption and surface mechanics for implementation in computational ballistic models. High-speed ballistic penetration studies have not considered penetrating and non-penetrating biomechanical properties of the skin, including radial wave displacement, resultant surface wave speed, or projectile material influence. A helium-pressurized launcher was used to accelerate 3/8″ (9.525mm) diameter spherical projectiles toward seventeen whole porcine legs from seven pigs (39.53 ± 7.28kg) at projectile velocities below and above V50. Projectiles included a mix of materials: stainless steel (n = 26), Si3N4 (n = 24), and acetal plastic (n = 24). Tracker video analysis software was used to determine projectile velocity at impact from the perpendicular view and motion of the tissue displacement wave from the in-line view. Average radial wave displacement and surface wave speed were calculated for each projectile material and categorized by penetrating or non-penetrating impacts. Two-sample t-tests determined that non-penetrating projectiles resulted in significantly faster surface wave speeds in porcine skin for stainless steel (p = 0.002), plastic (p = 0.004), and Si3N4 ball bearings (p = 0.014), while ANOVA determined significant differences in radial wave displacement and surface wave speed between projectile materials. Surface wave speed was used to quantify mechanical properties of the skin including elastic modulus, shear modulus, and bulk modulus during ballistic impact, which may be implemented to simulate accurate deformation behavior in computational impact models.

VARIABILITY ANALYSIS OF SIGNIFICANT WAVE HEIGHTS AND WIND WAVES IN RIAU ARCHIPELAGO SEA PART ALKI 1

The Riau Archipelago Sea is part of the Indonesian Archipelago Sea Lane (ALKI) 1, with a very high intensity of crossing ships. Analysis of surface wind speed and significant wave height is the most important for the safety and performance of offshore shipping. This research aims to study wave characteristics and wind speed by identifying the main factors that affect significant wave height and surface wind speed. Dominant factors that affect significant wave height and wind speed are needed to decide on the safest path and the best time before crossing in ALKI-1. Temporal and spatial analysis of the seasonal variability of significant wave height and wind speed using ECMWF data for 18 years. The data used are significant wave height data and wind speed every 6 hours during the period 2000 – 2018. Three observation points are used for temporal analysis, it is found that significant wave height and wind speed are influenced by two main factors, namely MJO and Monsoon. Maximum significant wave height and wind speed that occurs in the SON period for points 3 and the DJF period at points 1 and 2. MJO affects directly from phases one to eight for observation points 1, 2, and 3. The 4, 7, and 8 MJO phases affect the value of wave height and wave speed significantly, and the 1, 2, and 5 MJO phases affect wave height and wind speed weakly.
 Keywords: ALKI 1, MJO, MJO phase, Monsoon, Significant wave height, Wind speed.

Wavelet bispectral analysis and nonlinear characteristics in waves generated by submerged jets

Nonlinear interaction dominates the whole process from wave deformation to breaking, and is one of the significant factors that may cause many unstable changes in waves. This study aims to indicate the nonlinear characteristics of waves generated by submerged jets. The time series of those waves under five dimensionless upstream water depths (l = 3.0, 4.0, 4.5, 5.0, 5.5) are measured experimentally. Moreover, the wave surface asymmetry, skewness, spectrum, and wavelet-based bicoherence are obtained. The results show that the amplitude of wave surface varies with the variation of measuring point position and upstream water depth. From the perspective of asymmetry and skewness, the wave surface varies from tilt forward to tilt backward along the propagation path, and the vertical tilt amplitude decreases gradually; meanwhile, except for few measuring points, it is basically negative skewness. It can be seen from the energy spectrum that the wave energy increases with the increase of upstream water depth and decreases with the propagation of wave surface. In addition, the wavelet-based bicoherence distribution shows that the energy transfer caused by the interaction between waves is weak. The findings can provide an improved understanding of the nonlinearity characteristics of waves generated by submerged jets.

Investigations on the second-order transient gap resonance induced by focused wave groups

Two or more marine structures deployed side by side may encounter strong water-body resonances within narrow gaps between them. It may cause significant wave loads on structures and the green water phenomenon on the deck. In this article, the transient fluid motion within a narrow gap formed by two fixed boxes suffered from incident focused wave groups is investigated using a two-dimensional viscous flow numerical wave flume. The focused wave groups adopted have the spectral peak frequency equal to half the fluid resonant frequency inside the gap. The wave fields both inside the gap and around the two-box system, the response/damping time of the transient wave surfaces inside the gap, the maximum wave forces and the ratios of the 2nd-order to the corresponding 1st-order wave surfaces/forces are investigated. It is revealed that the most dangerous place to green water is always the front edge of the two-box system. The damping time of the 2nd-order wave surface is significantly larger than that of the 1st-order one. As the incident wave amplitude rises, the ratios of the 2nd-order to the first-order wave surfaces/forces becomes increases gradually and can even exceed 100% for the wave surface, the horizontal wave force and the moment.

A generalized weak-scatterer approximation for nonlinear wave–structure interaction in marine hydrodynamics

In this paper, a generalized weak-scatterer (GWS) approximation is proposed for solving nonlinear wave–structure interaction problems. In contrast to the original weak-scatterer (OWS) theory, where the approximated free surface boundary conditions (FSBCs) are Taylor-expanded in the vertical direction from the incident wave surface, we apply Taylor series expansion in an arbitrary direction which, in particular, is tangential to the boundary of the floating structure close to the waterline. This leads to generalized kinematic and dynamic FSBCs for the radiated and scattered waves, along with corresponding expressions of the wave loads. Accordingly, an Arbitrary Lagrangian–Eulerian (ALE) approach is adopted to track the free-surface properties. The new GWS method is more consistent than the OWS model in that the wave markers do not separate from the body surface at the waterline for structures with flare. An Immersed-Boundary Adaptive Harmonic Polynomial Cell (IB-AHPC) method is implemented to solve the corresponding boundary value problems (BVPs) for both the velocity potential and the Lagrangian acceleration potential at each time step. The new formulation introduces additional convective terms in the FSBCs, making them similar to the seakeeping problems for ships with forward speed, and this requires special treatment to avoid instability in the time-domain simulations. Based on a matrix-based eigenvalue stability analysis, we illustrate that stable solutions can be achieved by introducing an upwind-biased scheme to discretize the convective terms in the kinematic FSBC. The proposed model is verified by three wave diffraction problems in regular waves, including a submerged circular cylinder, a rounded-corner rectangular ship section, and a trapezoidal section with a large flare angle.

Characterization of diffractive relief structures over large areas by stitching interference microscopic topography

The development of diffractive optical elements (DOEs)enables fascinatingly compact design and chromatic aberration correction. Fabrication errors of DOEs include periodic pattern error, relief profile error and roughness which affect imaging quality and diffraction efficiency. Characterization of the DOE requires thus multi-scale measurement of the surface topography over the full aperture of 10 mm scale. This paper presents a method based on the white light interferometry in combination of subaperture stitching. Firstly, the principle of measurement and subaperture layout design is introduced. The measuring system is easily set up by integrating a scanning platform with the interference microscope. Subaperture stitching algorithm is developed based on the surface height change related to the scanning motion error. The slope of the relief profile is used to model the deviations of overlapping point pairs induced by lateral shift and rotation. Finally, the topography and fabrication error of a Fresnel lens surface are experimentally obtained and characterized over the full aperture through stitching of totally 67 subapertures.

Development of a CFD-based numerical wave tank of a novel multipurpose wave energy converter

The development of new wave energy converters usually involves small-scale experiments in physical wave tanks. The jump to physical models at larger scales is an expensive and time-consuming process that can be supported by computational fluid dynamics (CFD) models. Using the CFD approach, it is possible to numerically simulate complex flows with high accuracy, once validation with experimental data has been carried out. This article describes an approach based on guidelines taken from the literature and adopted to develop and explore the capabilities of a CFD-based numerical wave tank for a novel multipurpose wave energy converter, REEFS. An incremental validation procedure, using experimental data collected with a piston-type wave tank, was adopted; the procedure began with wave-only tests which were followed by wave-structure interaction tests. Snapshots of numerical and experimental approaches were used to analyse fluid flow in the envelope of the REEFS converter. The results demonstrate that the CFD-based numerical wave tank model can adequately simulate the global wave surface profile, as well as local complex phenomena, such as the Venturi aspiration effect, that typically occur near the exterior stay vanes of the device. The results encourage the adoption of this model for future REEFS analysis.

Numerical approach for predicting the slamming loads in oblique long waves considering the wave-body interface

Ship slamming is quite common for ongoing ships, which involves complex wave-body interaction. However, our understanding on the ship slamming is limited to symmetrical cases. In this paper, a numerical approach was established to predict the asymmetric slamming loads in oblique long waves through combining the seakeeping theory and CFD technology. A reference coordinate system located on the wave surface was applied to transfer the ship slamming problems into asymmetrical water entry cases, where the inclination wave-body interface and asymmetrical ship motions were included. Numerical verifications including the CFD method, different water entry models, 3D effects and wave effects were carried out to discuss the applicability. Taking a 21K TEU container ship as an example, the characteristics of slamming loads under oblique long waves were further discussed through combining the free surface evolution and pressure distribution. The results demonstrate that the wave direction has significant influence on the slamming loads. This method can provide a high-efficiency prediction for slamming loads in oblique long waves with reasonable accuracy, which is quite useful at present to serve the impact strength design.

Performance of seal vibrissa-inspired bionic surface in suppressing aerodynamic forces and vortex shedding around a circular cylinder

In this study, circular cylinders with bionic surfaces were developed to control the flow fields around cylindrical structures such as stay cables of bridges. The bionic cylinder geometries were characterized by slanted elliptical governing sections and wave surfaces. Wind tunnel experiments were conducted to identify the surface pressure distributions, aerodynamic forces, and flow field characteristics of bionic circular cylinders at a Reynolds number (Re) of 3.8 × 104. The results show that the aerodynamic forces of the bionic cylinders are sensitive to the wind attack angle (γ) and wave surface wavelength. The control effects were excellent at γ = 0°, 90°, and 180°, whereas they vanished at γ = 45° and 135° compared with a baseline circular cylinder. When the wavelength range of the bionic cylinder is 4D–5D, the maximum reductions in the mean drag and fluctuating lift were 27% and 93%, respectively. The vortex formation region was elongated, and the alternative vortex shedding process was suppressed. Proper orthogonal decomposition was performed to determine the coherent structure of the wake field and the boundary vorticity flux analysis demonstrates the difference in the strength of the wake vortex from the viewpoint of boundary vorticity diffusion between the fluid and wall. Spatial–temporal correlation analysis of the velocity profiles behind different cross-sections shows that the wave surface troughs contribute significantly to disrupting the flow consistency in the spanwise direction.

Experimental investigation on the hydrodynamic response of a moored ship to low-frequency harbor oscillations

Low-frequency harbor oscillations can lead to large motions of moored vessels, resulting in port downtime, damage to ships, and even casualties. Aiming at the operational problems encountered in the Hambantota Port caused by low-frequency oscillations, a series of physical experiments were implemented to investigate the effect of long-wave-induced harbor oscillations on a moored ship. Four natural periods of the harbor were identified by combining the time- and frequency-domain analyses of the wave surface elevations. The spatial characteristics of these natural modes are further analyzed using a mild-slope equation model. Subsequently, the time- and frequency-domain characteristics of the ship motions and the correlations of ship motions with the waves at various frequency bands were determined. The results show that the horizontal motions of the ship (i.e., surge, sway, and yaw) are controlled by the low-frequency wave energy, whereas the vertical motions (i.e., heave, roll, and pitch) are governed by the short-wave energy. Moreover, a significant correlation is observed between the low-frequency waves and horizontal motions, based on which parametric formulas were proposed to estimate the surge and sway motions and to improve the port operation positively.