Wave Attenuation Performance Research Articles

Hydrodynamic Behaviour of Floating Polygonal Platforms under Wave Action

The hydrodynamic behaviour of floating regular polygonal platforms under wave action was studied by conducting parametric studies. Considering triangular, square, hexagonal, and circular platforms of similar size and draft, the results show that their added mass, radiation damping, and RAOs are similar. However, the wave exciting forces are slightly different, particularly the horizontal forces. The polygonal platforms oriented with one of its corners in line with the prevailing wave direction can lead to a reduction in the horizontal force on the platform, a feature that helps in reducing the cost of a mooring system. Moreover, such oriented platforms are able to disperse the waves better in multiple directions and hence will not pose problems for ships or marine vessels passing by the platform on the weather side. Thus, the orientation of a polygonal platform is an important design consideration. From the comparison study among different polygonal platforms, their wave attenuation performances are slightly similar. The hydrodynamic analyses performed herein for the parametric studies were sped up considerably by using a significantly lesser number of Fourier coefficient sets for the series functions that define the velocity potentials when compared to those used by previous researchers in their analytical approaches. The adoption of the radius function defined by cosine-type radial perturbation does not only generate the geometric boundaries of polygonal platforms, but it also simplifies the formulation and quickens the computations.

Effect of engineered aggregate configuration and design on stress wave attenuation of metaconcrete rod structure

Metaconcrete is a new type of concrete made by partially replacing natural aggregates with locally resonant engineered aggregates to create frequency bandgaps to resist target impulsive loads. The engineered aggregate consists of solid core coated with a relatively soft material to activate the resonant behaviours of inclusion at the desired frequencies, thus trapping wave energy to attenuate stress wave propagation. Several studies on the characteristics and behaviours of particular metaconcrete have been reported in the literature, but there is a lack of systematic study on how the configurations of engineered aggregates influence the bandgap properties and the stress wave attenuation performance of metaconcrete structures subjected to impulsive loading. In this study, bandgap regions in metaconcrete with various configurations of engineered aggregates are calculated by using software COMSOL Multiphysics, and the influences of geometric and material parameters of engineered aggregates, e.g., core size, coating thickness, core density and coating modulus on the bandgap region are discussed. The influences of engineered aggregates configurations on the stress wave attenuation of metaconcrete rod structure are numerically investigated by using finite element code LS-DYNA. A flowchart to design engineered aggregates with the integration of multiple resonant frequencies is proposed to enhance the attenuation performance of metaconcrete rod structure. The results from this work facilitate the design of metaconcrete with broader bandgap regions and improve the damage mitigation effectiveness when subjected to impulsive loading.

Experimental investigation on the hydrodynamic performance of a multi-chamber OWC-breakwater

The multi-chamber Oscillating Water Column (OWC) device has recently become more attractive due to its potential high efficiency. In this paper, the hydrodynamic performance of a single-, dual- and triple-chamber OWC-breakwater are investigated experimentally. In the first instance, quantitative comparisons are implemented to understand the hydrodynamic performance of multi-chamber OWC-breakwaters. Specific attention has been dedicated to the hydrodynamic performance of capture width ratio (CWR), reflection coefficient, transmission coefficient, dissipation coefficient and effective frequency bandwidth. The investigation identified various findings that can be summarized as follows: i) hydrodynamic interactions between chambers in the multi-chamber OWC device has improved wave power extraction characteristics; ii) comparing with the conventional pontoon breakwater, the multi-chamber OWC-breakwater showed better wave attenuation performance in longer waves; iii) wave steepness is important for evaluating the performance of the multiple-chamber OWC-breakwater device; and iv) the implementation of the multi-chamber scheme broadens the effective frequency bandwidth (satisfied the condition of KT < 0.5 and η > 0.2) of OWC-breakwater.

Experimental study on the dynamic response of a water ballast type floating breakwater

Water ballast type floating breakwater has a wide range of applications due to its good wave attenuation performance. In this study, a series of physical model tests have been carried out to get insights into the dynamic response characteristics quantitatively of the floating breakwater, and tension of the mooring lines. The external and inner water pressure and its distribution on the floating body are presented. The results show that the sway and roll motions of the floating body exhibit strongly nonlinear characteristics as a result of “water-on-deck” and “sloshing” occurrence. The heave motion maintains a downward drifting and the greatest heave amplitude occurs when the wave period is close to the heave natural period. At both waveward and leeward side of the floating body, the mooring lines give almost comparable amplitudes, but opposite phase, of tension in terms of both upper and lower. The maximum water pressures appear at the waveward side, which is near the still water level. At the corner between the floating box and the vertical plate, the pressure changes sharply, leading to a “snap'' value. Overall, the results in this study provide a database for detailed design for the water ballast type floating breakwaters in future structural and material selection.

Representative Transmission Coefficient for Evaluating the Wave Attenuation Performance of 3D Floating Breakwaters in Regular and Irregular Waves

Wave attenuation performance is the prime consideration when designing any floating breakwater. For a 2D hydrodynamic analysis of a floating breakwater, the wave attenuation performance is evaluated by the transmission coefficient, which is defined as the ratio between the transmitted wave height and the incident wave height. For a 3D breakwater, some researchers still adopted this evaluation approach with the transmitted wave height taken at a surface point, while others used the mean transmission coefficient within a surface area. This paper aims to first examine the rationality of these two evaluation approaches via verified numerical simulations of 3D heave-only floating breakwaters in regular and irregular waves. A new index—a representative transmission coefficient—is then presented for one to easily compare the wave attenuation performances of different 3D floating breakwater designs.

Bandgap coupling effects between hybrid nonlinear synchronized switch damping and linear two-order resonant bandgaps in piezoelectric meta-structures

To study the bandgap coupling mechanism between nonlinear synchronized switch damping and multi-order linear resonant bandgaps, this research investigates the bandgap coupling effects between nonlinear synchronized switch damping and two-order linear resonant bandgaps in a piezoelectric meta-structure. Specifically, a piezoelectric meta-structure with hybrid nonlinear synchronized switch damping and linear two-order linear resonant electrical networks is proposed. Based on the theoretical modeling of the proposed structure, band structure and vibration transmittance performance are investigated in detail. Results showed that bandgap coupling can produce a trade-off with the advantages of both nonlinear and two-order linear resonant bandgaps. The new bandgap can not only realize broadband wave attenuation performance but also greatly enhance the attenuation in selected frequency bands, which provides a new method to manipulate elastic waves in a diversified way.

Experimental and analytical investigation on hydrodynamic performance of the comb-type breakwater-wave energy converter system with a flange

In this paper, the hydrodynamic performance of the comb-type breakwater-wave energy converter (CTB-WEC) system with a flange was investigated. Based on the linear potential flow theory, a semi-analytical model for wave interaction with the CTB-WEC system equipped with the flange was developed using matching eigenfunction method. In particular, Chebyshev polynomial was adopted to handle the singularity of velocity at the flange edge. Successful validation of the semi-analytical model was achieved by theoretical examination and comparing with the experimental data. The influence of wave resonance behavior in the confined water region (surrounded by caissons and the flange) was emphasized. It was found that: 1) the wave resonance behavior in the confined water region is modified due to the presence of the flange; 2) the hydrodynamic efficiency and wave attenuation performance of the CTB-WEC system is improved by properly configuring the flange; 3) the presence of piston and sloshing mode wave resonance in the gap between the WEC device and the flange led to the increment of hydrodynamic efficiency.

Semi-analytical study on the hydrodynamic performance of an interconnected floating breakwater-WEC system in presence of the seawall

• A semi-analytical model for the dual pontoon system with the presence of the partial reflected seawall was developed. • Hydrodynamic performance of the interconnected floating breakwater-WEC system was analyzed theoretically. • The effect of the seawall on the performance of the system was revealed. Based on linear potential flow theory, a semi-analytical model is developed using the matching eigenfunction method for a dual pontoon system in the presence of partially reflective seawall. To study the motion characteristics of an interconnected floating breakwater-wave energy converter system, the equations of motion in the frequency-domain are studied using the Lagrange-multiplier method. The model is verified using wave flux conservation law. A thorough investigation of the hydrodynamic performance of the integrated system is conducted, and the optimal power take-off damping of the system is derived theoretically. The results show that the proposed system exhibits a qualified wave power extraction performance and wave attenuation performance, and the presence of the seawall affects the performance of the integrated system significantly.

Influence of Wave-Absorbing Chamber Width on the Wave Attenuation Performance of Perforated Caisson Sitting on Rubble-Mound Foundation

A Jarlan-type perforated caisson consisted of a perforated front wall, a solid rear wall, and a wave-absorbing chamber between them. The wave-absorbing chamber was the main feature of the perforated caisson, and its width had a great effect on wave attenuation performance. In this study, a larger range of the wave-absorbing chamber width was observed in model experiments to investigate the effect on wave attenuation performance including the reflection coefficients and the horizontal wave forces of a perforated caisson sitting on a rubble-mound foundation. A resistance-type porosity numerical model based on the volume-averaged Reynolds-averaged Navier–Stokes (VARANS) equations was validated by comparing the present results with those of previously reported and present experiments. The validated numerical model was then used for extended research. It was found that the reflection coefficients, the total horizontal wave force, and its components all tended to oscillate in a decrease ⟶ increase ⟶ decrease manner with increasing the wave-absorbing chamber width. The reflection coefficients and wave forces acting on both sides of the perforated front wall were found to be synchronized regardless of perforation ratio or the rubble-mound foundation height.

A comparative study of the hydrodynamic characteristics of permeable twin-flat-plate and twin-arc-plate breakwaters based on physical modeling

To explore the similarities and differences between permeable twin-flat-plate (TFP) and twin-arc-plate (TAP) breakwaters in terms of their wave attenuation performance and structural behavior, the two breakwaters are compared in terms of the wave transmission coefficient, reflection coefficient, wave pressure, wave force, and flow field characteristics by physical model tests. The results show the wave transmission coefficients of the TAP breakwater are 34.5% and 25% smaller than those of the TFP breakwater in the emerged state and at the still water level, respectively. The wave reflection coefficient of the TFP breakwater shows a maximum increase of approximately 66.7% compared with the TAP breakwater. Except for the upper surface of the upper plate in the emerged state, the wave pressures on the TAP breakwater are smaller than those on the TFP breakwater by up to 62.4%. The vertical wave forces on the TAP breakwater are less than those on the TFP breakwater by up to 40.2%. The characteristics of the flow field around the TFP and TAP breakwaters are markedly different. The results of this study provide necessary theoretical guidance for engineering applications of permeable plate-type breakwaters.

Hybrid lattice metamaterials with auxiliary resonators made of functionally graded materials

This paper aims to enhance and tune wave-propagation characteristics of periodic architected structures by adding auxiliary resonators made of functionally graded materials (FGMs). For this purpose, cantilever FGM beams are added to periodic metamaterials with square and hexagonal topologies, and the effects of the material distribution of the added resonators on their wave-attenuation performance are analyzed. More specifically, a combination of locally resonant and Bragg-type bandgaps is formed as a result of adding FGM resonators, while the conventional structures have no bandgap in the considered region. The studied low-frequency region is of high importance, and the appearance of wide bandgaps there opens horizons for new structural and acoustic applications. Further, these bandgaps depend on the material parameters of the resonators, and their location and width are changed, systematically as functions of the elastic modulus ratio, density ratio, and non-negative power-law exponent of the resonators. For the numerical analysis, a finite element formulation is developed for an FGM beam, and the wave propagation is studied using Bloch’s theorem. According to the results, with increasing the elastic modulus contrast of the auxiliary FGM resonators, the locations of the bandgaps move higher, while increasing the density ratio contrast moves them to lower frequencies. Additionally, the effect of adding auxiliary FGM resonators on the directionality of the wave propagation is studied using the iso-frequency contours of the first dispersion branches of each structure. The results of the present study can be a starting point for using FGM resonators to design tunable elastic/acoustic metamaterials with the ability to filter waves in predefined frequency ranges.

Aquaculture farms as nature-based coastal protection: Random wave attenuation by suspended and submerged canopies

As the frequency and intensity of storms increase, a growing need exists for resilient shore protection techniques that have both environmental and economic benefits. In addition to producing seafood, aquaculture farms may also provide coastal protection benefits either alone or with other nature-based structures. In this paper, a generalized three-layer frequency dependent theoretical model is derived for random wave attenuation due to presence of biomass within the water column. The biomass can be characterized as submerged, emerged, suspended and floating canopies that can consist of natural aquatic vegetation with potential aquaculture systems of kelp or mussels. The present analytical solutions can reduce to the solutions by Mendez and Losada (2004), Chen and Zhao (2012) and Jacobsen et al. (2019) for submerged rigid aquatic vegetation. The present theoretical model incorporates the motion of these canopies using a cantilever-beam model for slender components and a buoy-on-rope model for elements with concentrated mass and buoyancy. Analytical results are compared with existing laboratory and field datasets for submerged and suspended canopies. The theoretical model was then used (in a case study at a field site in Northeastern US) to investigate the capacity of suspended mussel farms with submerged aquatic vegetation (SAV) to dissipate wave energy during a recent storm event. Compared to a dense SAV meadow in shallower water, the suspended aquaculture farms more effectively attenuate random waves with a smaller peak period and the higher frequency components of wave spectrum. The performance of suspended aquaculture farms is less affected by water level changes due to tides, surge and sea level rise, while the wave attenuation performance of SAV decreases with increasing water level due to decreased wave motion near the sea bed. Incorporating suspended aquaculture farms offshore significantly enhance the coastal protection effectiveness of SAV-based living shorelines and extend the wave attenuation capacity over a wider wave period and water level range. The combination of suspended aquaculture farms and traditional living shorelines provides a more effective nature-based coastal defense strategy than the traditional living shorelines alone.

The effect of the coastal reflection on the performance of a floating breakwater-WEC system

In this paper, the hydrodynamic performance of a floating breakwater-Wave Energy Converter (WEC) integrated system was investigated under the framework of the linear potential flow theory, with a focus on the effect of the coastal reflection. The reflection coefficient and capture width ratio were verified using the rule of wave energy flux conservation. The water wave resonances, including the piston- and sloshing-mode, between the device and the coastal wall, were excited. By comparing to the cases without the coastal wall, the presence of the coastal wall led to the significant modifications of the hydrodynamic characteristics, efficiency and wave attenuation performance of the system. Specifically, the occurrence of the piston- and sloshing-mode resonance accompanies the increment of the efficiency of the system. The null efficiency was found when the device was located at the node of standing waves, which was formed due to the presence of the coastal wall. The effect of the coastal wall in irregular waves was found to be relatively milder when compared to that in regular waves.

Hydrodynamic performance of a Comb-Type Breakwater-WEC system: An analytical study

Abstract A breakwater-WEC system combining heaving body Wave Energy Converters (WEC) and Comb-Type Breakwater (CTB) was investigated. The traditional CTB consists of a distributed array of separated bottom-mounted caissons and wave chambers are located between two caissons. Heaving bodies provide the power take off (PTO) principles that are arranged at the wave chamber of the CTB. The interaction of the CTB and WEC was investigated based on the linear potential flow theory. An analytical model has been developed to examine the hydrodynamic performance of CTB-WEC system. The analytical model is validated with results from an experimental study. Results show that an increase in conversion efficiency is observed when the device is located in the aft end of the wave chamber. A high efficiency (i.e., 77.4%) and qualified wave attenuation performance of the integrated system are achieved for the proposed CTB-WEC system. The wave resonance along the incident wave direction in the wave chamber is beneficial for wave energy capturing. Furthermore, it was found that the critical value kc corresponds to the wave resonance, perpendicular to the incident wave direction, out of the wave chamber. The property of efficiency mitigation at regions of k > kc should be avoided while designing such a system.

Numerical investigations and optimizations of typical submerged box-type floating breakwaters using SPH

Box-type floating breakwaters have been studied extensively. However, the examinations of the performances of them with different shapes or different numbers of boxes are limited. In addition, if it is possible to optimize the floating breakwaters by combining the optimized parameters in separated testing groups is still unclear. Consequently, in this study, four typical box-type floating breakwaters, i.e., dual rectangular breakwater, single rectangular breakwater, dual circular breakwater, and single circular breakwater, are selected to do extensive investigations using SPH. The coupling between DualSPHysics and MoorDyn is implemented and verified, and the coupled code can be applied to simulate the interactions between waves and moored floating structures. A total of 88 cases are modeled, and the effects of the geometry parameters on wave-attenuation performances of the breakwaters are clarified. The numerical results show that the performances of the breakwaters are greatly sensitive to the immersion depth. In general, the larger and lighter the breakwaters, the better the performances will be. Meanwhile, the dual rectangular breakwater performs the best in wave attenuations, and its motions and forces are smaller than single breakwaters. Importantly, combining the optimized parameters in each separated testing group is found to be effective to optimize the floating breakwaters.

Synthesis and characterization of BiFeO3/RGO composites for promising microwave absorption materials

In this paper, BiFeO3 (BFO) and reduced graphene oxide (RGO) composites were synthesized by ultrasonic assisted hydrothermal reduction method. The structures of BFO and BFO/RGO composites were analyzed by XRD, Raman and TG methods. With the aid of SEM and TEM, we observed the slice layer structure of the composites. All the composites have excellent electromagnetic(EM) wave absorption capacity, the maximum reflection loss (RL) value of sample BFO/RGO-2 reached up to − 32.3 dB at a thickness of 2 mm, and the effective bandwidth was 4.1 GHz (< − 10 dB). The 3 mm wave attenuation performance of sample BFO/RGO-3 was up to 10.3 dB, which is higher than that of pure BFO (5.5 dB). The impressive EM absorption performance of the composites was attributed to the combination of BFO and RGO, which greatly enhanced the electrical conductivity and dielectric loss. We claim that BFO/RGO composite is a promising material which can be applied to microwave absorption.

Numerical studies of submerged moored box-type floating breakwaters with different shapes of cross-sections using SPH

The wave-attenuation performances and hydrodynamics of moored box-type floating breakwaters with the same weight, same wall thickness, and six different cross-sections, i.e. circular, triangle-down, triangle-up, rectangular, trapezoid-down, and trapezoid-up, are investigated numerically using the smoothed particle hydrodynamics (SPH) method. Coupling between DualSPHysics and MoorDyn is implemented in the coupled SPH-mooring model. The effects of the key parameters, such as the breakwater density, immersion depth, ballast-water gravity, and wave condition, on wave-attenuation performance are examined systematically. The results reveal that the performances of floating breakwaters are significantly affected by the immersion depth and wave condition, however, they are hardly affected by density and ballast-water gravity. Furthermore, the top deck is found to be an extremely effective part of a well-designed breakwater, and asymmetrical designs such as longer top-faces and shorter bottom-faces should be adopted for achieving reasonable material utilizations. Irrespective of the shapes of the cross-sections, the best performances of the different breakwaters are almost at the same level after careful designs are implemented. Importantly, a strong relationship is identified between the motions, mooring tensions, and wave-attenuation performances, which implies that the motions are directly proportional to the wave-attenuation performances and the mooring tensions.

Wave Attenuation Performance and the Influencing Factors of A Lower Arc-Plate Breakwater

Comprehensive experimental and numerical studies have been undertaken to investigate wave energy dissipation performance and main influencing factors of a lower arc-plate breakwater. The numerical model, which considers nonlinear interactions between waves and the arc-plate breakwater, has been constructed by using the velocity wavegenerating method, the volume of fluid (VOF) method and the finite volume method. The results show that the relative width, relative height and relative submergence of the breakwater are three main influencing factors and have significant influence on wave energy dissipation of the lower arc-plate open breakwater. The transmission coefficient is found to decrease with the increasing relative width, and the minimum transmission coefficient is 0.15 when the relative width is 0.45. The reflection coefficient is found to vary slightly with the relative width, and the maximum reflection coefficient is 0.53 when the relative width is 0.45. The transmission and reflection coefficients are shown to increase with the relative wave height for approximately 85% of the experimental tests when the relative width is 0.19−0.45. The transmission coefficients at relative submergences of −0.04, −0.02 and 0 are clearly shown to be greater than those at relative submergences of 0.02 and 0.04, while the reflection coefficient exhibits the opposite relationship. After the wave interacts with the lower arc-plate breakwater, the wave energy is mainly converted into transmission, reflection and dissipation energies. The wave attenuation performance is clearly weakened for waves with greater heights and longer periods.

Topological design of square lattice structure for broad and multiple band gaps in low-frequency range

Several studies have demonstrated that the design of square lattice is an efficient approach to obtain band gap characteristics. However, there are still some challenges in the design of phononic crystals capable to prohibit the propagation of low-frequency waves. In the present paper, aiming to broaden and manipulate the low frequency band gaps, an innovative structure with filling materials in a square lattice is proposed, where the spatial distribution of the materials has been optimized using an improved genetic algorithm. By analyzing the band structure and vibration modes of the designed structure, it becomes evident that the low-frequency band gaps are obtained due to the local resonance of the constituent materials. The transmission spectrum of the finite designed structure confirms an efficient wave attenuation performance in the low-frequency range. Furthermore, the effect of ligament thickness on band gaps was also investigated. The designed phononic porous plate, featuring prominent band gap properties, can be used as an example to guide the topological design of phononic crystal structures for a wide range of engineering implementations.

Interplay of local resonances and Bragg band gaps in acoustic waveguides with periodic detuned resonators

This letter is concerned with acoustic wave propagation and transmission in acoustic waveguides with periodically grafted detuned Helmholtz resonators. The interplay of local resonances and Bragg band gaps in such periodic systems is examined. It is shown that, when the resonant frequencies of the resonators are tuned close to a Bragg band gap, the behavior of the Bragg band gap can be affected dramatically. Particularly, by introducing appropriately tuned resonators, the bandwidth of a Bragg band gap can be reduced to zero, leading to a very narrow pass band with great wave attenuation performance near both band edges. The band formation mechanisms of such periodic waveguides are further examined, providing explicit formulae to locate the band edge frequencies of all the band gaps, as well as the conditions to achieve very narrow pass bands in such periodic waveguides.