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The back-bent duct buoy (BBDB) is a floating oscillating water column wave energy converter, which has good environmental suitability, survivability, energy-harvesting capability, and smaller mooring forces. In this study, a BBDB OWC model with a generic shape was designed and tested using regular wave scenarios in a wave flume. Four combinations of the degree of freedom (DOF) were employed in the experimental tests: the zero DOF (fixed, ZDOF) mode, the two DOFs (heaving and pitching, TDOFs) mode, and the single DOF (only pitching or heaving, SDOF-P and SDOF-H) modes. The time histories of key performance parameters, kinematic responses, hydrodynamic, aerodynamic, and energy-harvesting performances were reported and analyzed. The resonance migrations of the air pressure and pneumatic power were observed for different DOF conditions. In addition, the pitching and heaving motions are found to have positive and negative effects on the energy-harvesting performance of the BBDB model. The peak values of capture width ratios for the ZDOF, SDOF-P, SDOF-H and TDOFs conditions are 0.57, 0.51, 0.10, and 0.26, while the average values are 0.19, 0.19, 0.03, and 0.11, respectively. [ABSTRACT FROM AUTHOR]

The back-bent duct buoy (BBDB) can be used as a floating oscillating water column (OWC) wave energy converter, showing its potential and reliability in most previous prototype sea trials. In this study, an experimental test on a generic BBDB OWC model was conducted in a wave flume using irregular wave scenarios. By using a specific mechanism, the degree-of-freedom (DOF) of the model can be preset in four modes: the ZDOF (the model is fixed with zero DOF) mode, the TDOFs mode (the model conducts the pitching and heaving motions with two DOFs), the SDOF-P mode (the model conducts the pitching motion with the single DOF), and the SDOF-H mode (the model conducts the heaving motion with the single DOF). The kinematic, hydrodynamic, and aerodynamic responses and energy-harvesting performances of the BBDB model were presented and analyzed. It was found that the pitching and heaving motions made positive and negative contributions to wave power capturing of the model, resulting in a moderate energy-harvesting performance for the TDOFs mode. Furthermore, the analysis indicated that strengthening the pitching motion and restraining the heaving motion will benefit the wave energy conversion of the BBDB device. • A generic BBDB OWC model was tested experimentally using irregular wave scenarios. • Four typical combinations of the degree of freedom were employed. • The heaving motion was found to restrain the pitching motion. • Two spectral peaks in the pitching motion result in a broader responding bandwidth. • An innovative optimization strategy was proposed for the BBDB device. [ABSTRACT FROM AUTHOR]

Marsh plants protect the coasts and coastal communities by dissipating wave energy. The dissipation of wave energy depends on the geometric (leaf and stem dimensions) and mechanical (material rigidity) properties of the plants. First, flexible plants bend in response to hydrodynamic drag (called reconfiguration), which diminishes wave decay relative to a rigid plant of the same morphology. Second, mutual sheltering between plant leaves and stem can reduce the drag, and thus wave damping. This study connects the prediction of drag on an individual plant and the measurement of wave decay over a meadow of plants to build a physically-based wave-damping model that includes leaves, the impact of reconfiguration, and the impact of sheltering between plant elements. Model plants were constructed to be both geometrically and dynamically similar to Spartina alterniflora. The wave damping model assumed linear wave conditions, but was validated using a wide range of weakly nonlinear wave conditions within model plants, over a patch of live Spartina anglica in the laboratory, and over a meadow of Spartina alterniflora in the field. After validation, the model was used to explore the variation in wave decay over a range of wave parameters and plant geometric and bio-mechanic parameters, including stem length, stem diameter, number of leaves per plant, leaf length, leaf width, the rigidity of the stem and the leaf. • A physics-based prediction for wave dissipation by flexible marsh plants with leaves was developed and validated with laboratory experiments. • The dependence of wave dissipation on water depth, wave amplitude, and wave period was explored in detail with model predictions. • The impact of leaf and stem geometry and mechanical properties on wave dissipation were revealed by systematic model predictions. [ABSTRACT FROM AUTHOR]

Self-rectifying air turbines, acting as a power take-off facility, can convert the incident pneumatic power to the shaft torque in the oscillating water column devices. The radial-flow impulse turbine (RFIT) is becoming popular because of its excellent operating performance. A generic RFIT model was designed and tested in constant and reciprocating airflows. The incident and torque coefficients are reported with the scatter diagrams of the turbine efficiencies. The steady-state efficiency in the constant airflow and the average efficiency in the reciprocating airflows peak at 0.48 and 0.49, respectively. The RFIT model has a significant difference in operation performance under the centrifugal and centripetal modes, and the varying frequency of the sinusoidal reciprocating airflows has a more obvious influence on the centripetal mode. In the comparison between two types of airflows with the same pneumatic power, the RFIT model is found to obtain a better performance in the reciprocating airflows. • A radial-flow turbine model was tested in reciprocating airflows experimentally. • Significant differences were observed under centrifugal and centripetal modes. • The varying frequency of airflow has a greater influence on the centripetal mode. • The turbine model can obtain a larger average efficiency in reciprocating airflows. [ABSTRACT FROM AUTHOR]

Salt marshes are a common feature in coastal regions and have been noted for their ability to attenuate wave energy, providing an important first line of coastal defense. Marsh plants usually consist of multiple leaves distributed along a central stem. This paper constructed a model predicting wave force on a marsh plant by modeling the reconfiguration of both the leaves and stem in waves. The individual leaf and stem models and the full plant model were validated with experimental measurements of drag and plant motion using both live and dynamically-similar model plants under a range of wave conditions. Although the leaves exhibited greater reconfiguration than the stem, they contributed more than 70% of the plant drag. Plant reconfiguration produced a drag force that had a weaker than quadratic dependence on wave velocity. A simplified model, which combines scaling laws for the stem and individual leaves, is proposed and validated. Wave drag on a variety of marsh species with different morphology and rigidity were estimated and compared. [ABSTRACT FROM AUTHOR]

Abstract Oscillating water column (OWC) is the most widely used wave energy converting technology for onshore structures. The interest in OWC wave energy converters (WECs) has been rising in the last decade because of its capability of long-term operation with high reliability and survivability. The impulse and Wells turbines, acting as axial-flow self-rectifying turbines and rotating unidirectionally in reciprocating airflows generated by the air chamber, play a key role in the pneumatic power conversion of OWC facilities. Although these turbines were developed based on theoretical analysis and steady-state laboratory tests, computational fluid dynamics (CFD) technologies have become a popular and convenient desktop tool for their design and optimization. This review aims to bridge the knowledge gap between engineering applications and CFD achievements for this type of turbine. Recent developments in the CFD modeling, geometry optimization, and performance enhancement have been summarized. Perspectives on the CFD model for OWC axial-flow turbines and its future trends are also proposed. Highlights • A comprehensive review of CFD studies on axial-flow OWC turbines are presented. • Detailed aspects of CFD models for both Wells and impulse turbines are summarized. • CFD achievements of geometry optimization and performance enhancement are reported. • Parameters of CFD models for steady-state analysis are recommended. • Future trends of CFD models for OWC turbines are discussed. [ABSTRACT FROM AUTHOR]

Highlights • Self-starting performance of an OWC impulse turbine is studied comprehensively. • A fully passive fully-driven numerical model is used for unsteady simulations. • The peak efficiency of 0.44 is achieved under the constant flow and load conditions. • The self-starting speed of impulse turbine is mainly determined by the incident air-flow velocity. • The moment of inertia has limited effects on the turbine performance at the stable stage. Abstract In recent times, self-rectifying axial-flow air turbines are being widely employed in oscillating water column (OWC) wave energy converters (WEC). The steady performance of air turbines has been systematically investigated in previous studies. However, there still exists a lack of information on their unsteady performance, such as in the self-starting characteristics and subsequent running behavior. In this study, the unsteady behavior of impulse turbine under various constant-flow conditions is investigated. Experimental studies were conducted to investigate the effects of constant-load on the variations in the rotation speed, the pressure drop and the torque output of the turbine starting from rest. A fully passive flow-driving numerical model is employed for further detailed analysis of the flow and pressure fields. Followed by a well-agreed validation using the corresponding experimental data, the three dimensional (3D) transient model is used to study the effects of the air-flow velocity magnitude and the rotors' moment of inertia on the self-starting performance of the turbine. Except for the variations in the rotation speed, the pressure drop and the pneumatic torque, the distributions of the flow-field and the pressure over the blades at specific time-points are analyzed. [ABSTRACT FROM AUTHOR]

The oscillating water column is the most widely used wave energy conversion technology for long-term operations, which has the potential to play a critical role in massive applications of wave energy. In this study, an isolated OWC model employing a generic cubic air chamber and an impulse turbine model was tested in a wave tank using regular and irregular wave scenarios. The wave distribution characteristics surrounding the isolated OWC model in oblique waves with various incident angles and other testing conditions were reported. As the incident waves were forward, the symmetrical wave-height distributions could be observed with enhancement and reduction zones in front of the skirt wall and in the sheltered area of the model. Furthermore, the wave distributions were asymmetrical and more complicated in oblique waves. In addition, the effects of the incident wave angle, height, period, and rotation speed of the turbine model on the staged and overall energy-harvesting performances were analyzed. The OWC model obtained the peak energy-harvesting performance, which reduced as the incident angle decreased. As the incident angle decreased to 60° and zero, the overall capture width ratio reduced to approximately two-thirds and a quarter of the value for the incident angle of 90°. Based on experimental results, the annual energy production of a prototype isolated OWC plant in a typical sea state in North China was predicted. The annual energy production of the plant in the actual sea condition was approximately two-thirds of that only in the forward wave incidence. • A generic OWC model with a real impulse turbine was tested using oblique waves. • Hydrodynamic interactions between the OWC and oblique waves were investigated. • The wave field distributions surrounding the OWC model were presented. • Reduction effects of incident wave angle on operating performances were evaluated. • The AEP of a prototype OWC plant in semi-omnidirectional waves was predicted. [ABSTRACT FROM AUTHOR]

Most previous experimental studies on the hydrodynamic and aerodynamic performances of an oscillating water column (OWC) wave energy converter were conducted in a wave flume. In this study, an experimental study on an isolated OWC plant is carried out in a wave tank. The interactions between the OWC system and incident regular and irregular waves are analyzed by measuring the surrounding wave fields using a wave gauge array. The wave distribution patterns and reflection ratios of the OWC model are presented. In addition, the free-surface elevation, air pressure varying amplitude, turbine torque output, and relative capture width ratios and efficiencies in regular and irregular wave scenarios are measured and analyzed. The operating performances of the OWC model are analyzed and compared to the results derived from the flume tests. An overall performance reduction of over 20% can be observed in the wave tank tests. In addition, the representative results of the overall capture width ratio of the OWC model derived from irregular wave tests are corrected from the scaling effects caused by the similarity contradiction. • An isolated OWC model was tested experimentally in a wave tank. • The wave field distribution patterns around the OWC device were presented. • The reflection ratios of the isolated OWC model were reported. • The operating performances in regular and irregular wave scenarios were analyzed. • The performance comparisons between wave tank and flume tests were conducted. [ABSTRACT FROM AUTHOR]

This work is a response to the discussion paper by Ghisu, T. and Cambuli, F. on our recent review paper (Ciu er al., 2019) published in Ocean Engineering. Two main reasons are given for the exclusion of parts of their papers from our review. First, our literature retrieval process was not able to use any published work from after our submission date, nor any work with considerable and inappropriate self-duplications under the academic criterion of citing references. Second, we are skeptical of Ghisu's "transient" model, which employed a multiple reference frame model to coordinate with unsteady Reynolds-Averaged Navier-Stokes equations and was unconvincing due to the lack of sufficient validation. [ABSTRACT FROM AUTHOR]