Hydrodynamic Efficiency Research Articles

Array buoys with nonlinear stiffness enhance low-frequency wave attenuation and energy capture

Extraction of energy and elimination of ocean waves at low frequencies are challenges facing current wave energy devices. A recent idea based on reducing the equivalent stiffness has been applied to such devices for low-frequency wave attenuation and energy capture. This study investigates a model of an array of buoys with an additional nonlinear stiffness mechanism to this end. The problems of hydrodynamic interaction between multiple floating bodies and interactions among nonlinear wave structures are solved by a semi-analytical method that combines the eigenfunction matching expansion method with the multi-harmonic balance method. The physical mechanism of the proposed nonlinear system of multiple buoys was explored, and it was found to deliver good performance in terms of power capture and wave elimination due to its “phase control” feature. Bragg resonance occurred in the arrayed buoys, which was not conducive to hydrodynamic efficiency. The properties of the multi-buoy system were evaluated, and it was found to be superior to a single buoy of equal volume. The results of this study indicate that an attached mechanism with nonlinear stiffness can be beneficial both for exploiting wave energy and reducing transmitted waves.

Numerical simulation of unsteady hydrodynamic performance of CRP

Strong hydrodynamic interference between forward and aft contra-rotating propellers (CRP) can result in load fluctuations, local blade vibrations, and noise. This study adopts a Reynolds-averaged Navier–Stokes solver (RANS) and a shear stress transport (SST) k−ω turbulence model to investigate the unsteady hydrodynamic performance of CRPs during uniform flow. The influences of time step and grid scale on the numerical simulation are discussed first. The numerical results of the open-water hydrodynamic performances of the CRPs agree well with experimental data. The fluctuation patterns of the hydrodynamic loads (i.e., force and torque) and propulsion efficiency of CRPs having different advance coefficients induced by the mutual hydrodynamic interference between the forward and aft propeller are investigated. A series of probes that rotate with the propellers are set upon the surface of the blades to analyze the unsteady local pressures under the hydrodynamic interference between the forward and aft propellers.

Numerical investigation of a dual cylindrical OWC hybrid system incorporated into a fixed caisson breakwater

In this paper, a hybrid Oscillating Water Column (OWC)system combining with a heaving floater Wave Energy Converters (WEC) was investigated. The traditional cylindrical-type breakwater consists of dual cylinders with an opening inlet located at the outer wavefront wall, allowing a ring-type wave chamber formed between two cylinders. The oscillating buoy OB is hinged at the front of the OWC device. The WAVE Chamber formed between the two breakwater cylinders is to be used as OWC. Based on the Computational Fluid Dynamics (CFD) software Star CCM+, A three-dimensional numerical wave tank is developed to investigate the hydrodynamic performance of the hybrid system, and the numerical model is validated with published experimental results. The power take-off (PTO) damping performance and the hydrodynamic efficiency affected by water conditions and geometrical dimensions of the device are discussed. Results show that the after combining the floater to the breakwater-type WEC, a great improvement on frequency bandwidth was achieved and the efficiency increase to 83.28% compared to the case of a single device, which was of 57%. Furthermore, the effects of the opening inlet height and the volume ratio of the OWC chamber were studied to optimize the certain geometrical parameters. Lower height ratio of the opening inlet of the OWC chamber should be avoid for larger water depth condition while designing such a hybrid system.

Research on Size Optimization of Wave Energy Converters Based on a Floating Wind-Wave Combined Power Generation Platform

Wind energy and wave energy often co-exist in offshore waters, which have the potential and development advantages of combined utilization. Therefore, the combined utilization of wind and waves has become a research hotspot in the field of marine renewable energy. Against this background, this study analyses a novel integrated wind-wave power generation platform combining a semi-submersible floating wind turbine foundation and a point absorber wave energy converter (WEC), with emphasis on the size optimization of the WEC. Based on the engineering toolset software ANSYS-AQWA, numerical simulation is carried out to study the influence of different point absorber sizes on the hydrodynamic characteristics and wave energy conversion efficiency of the integrated power generation platform. The well-proven CFD software STAR CCM+ is used to modify the heaving viscosity damping of the point absorber to study the influence of fluid viscosity on the response of the point absorber. Based on this, the multi-body coupled time-domain model of the integrated power generation platform is established, and the performance of the integrated power generation platform is evaluated from two aspects, including the motion characteristics and wave energy conversion efficiency, which provides an important reference for the design and optimization of the floating wind-wave power generation platform.

Automated manufacture of optimised shape-adaptive composite hydrofoils with curvilinear fibre paths for improved bend-twist performance

Composite marine propellers improve hydrodynamic efficiency by inducing bend-twist coupling and allowing for passive pitch changes. One critical limitation, however, is the extent to which a composite propeller blade can deform and cause a pitch change without incurring structural failure. Recent numerical studies showed that curvilinear tows could improve the structural response of a composite blade by lowering its deflection or stress and strain required to induce a pitch change, but no experimental validation has been carried out before. The current study, thus, presents the manufacture of composite sandwich hydrofoils made with steered tows using automated fibre placement and validates the curvilinear tow benefits. Two hydrofoils were optimised with straight and curved fibre path layups, respectively and were manufactured for mechanical testing. The manufacturing complications arising from steering curvilinear tows in a three-dimensional convex mould are also discussed in the paper. The study found that significant tow buckling occurred near the tool cavity edge due to excessive steering radius during manufacture. The follow-up structural cantilevered tests showed that the experimental results were consistent with the FE predictions despite the presence of some manufacturing defects. The experiment agreed that the hydrofoil manufactured with curved tows achieved a similar tip twist but a significant reduction in deflection and critical principal strains compared to the hydrofoil made with straight tows. The use of a foam core reduced the overall weight of the sandwich hydrofoils by about 25% compared to that of a fully-carbon composite hydrofoil, and the numerical analysis showed that the core shear failure induced by transverse shear stresses was unlikely to occur.

Numerical investigation of hydrodynamic performance and efficiency of a dual-chamber oscillating water column under different damping and chamber breadth ratio combination

The dual-chamber oscillating water column (OWC) is a modified wave energy converter that has received considerable attention owing to its high capture efficiency and adaptability to wave conditions. In this study, the hydrodynamic performance of a land-based dual-chamber OWC was numerically investigated, focusing on the effect of damping combinations and chamber breadth on hydrodynamic efficiency. The commercial software ANSYS Fluent was employed to solve the Reynolds-averaged Navier–Stokes equation, and the solution was verified through a comparison with experimental data. The variations in water surface elevation, pressure, and efficiency of a dual-chamber OWC with different orifices and chamber breadth ratios were investigated. The results indicate that an OWC with different opening ratios is more efficient and offers advantages in adapting to variable wave environments compared with an OWC comprising two chambers with the same orifice dimensions. Furthermore, it was found that incident waves with a high frequency result in a higher absorption efficiency for the combination of smaller damping of the front chamber and larger damping of the rear chamber; by contrast, waves at lower frequencies have higher absorption efficiencies with the opposite combination. The breadth ratio has different effects on the hydrodynamic efficiency for different damping conditions. The findings can serve as a reference for the practical application of dual-chamber OWCs.

Hydrodynamic performance of dual-chamber Oscillating Water Column array under oblique waves

A multiple Oscillating Water Column (OWC) device may provide better wave absorption over a wider frequency bandwidth than a single-chamber OWC due to multiple resonances. The scattering and radiation of three-dimensional oblique waves by an array of periodic dual-chamber OWCs are considered here along a coastal cliff. A semi-analytical model was developed based on potential flow theory and matching eigenfunction method to investigate the oblique wave interaction with a dual-chamber OWC array system. The velocity singularity at the tip of a chamber wall is resolved by introducing the Galerkin technique to accelerate the convergence. The semi-analytical solution is verified by the Haskind relation and energy conservation law. Hydrodynamics of the proposed system and the influence of wave and geometric parameters were investigated. Theoretical results indicate that a dual-chamber OWC array has a broader capture bandwidth than a single-chamber OWC array for both normal and oblique waves. The presence of the along-shore and cross-shore sloshing resonance is theoretically confirmed in each subchamber of OWC unit, which decreases the hydrodynamic efficiency and increases the wave reflection drastically. Although the wave loading on the chamber wall decreases with increasing incident wave angle θ, the wave loading on chamber/partition wall may increase sharply due to sloshing resonance at critical frequency kc. To our knowledge, this is the first attempt to investigate the hydrodynamics of dual-chamber OWC array under oblique waves. The present theoretical results indicate the potential risks of structural damage and total wave reflection due to sloshing resonance, which should be an important design consideration.

Hydrodynamic Performance of a Floating Offshore Oscillating Water Column Wave Energy Converter

A floating oscillating water column (OWC) wave energy converter (WEC) supported by mooring lines can be modelled as an elastically supported OWC. The main objective of this paper is to investigate the effects of the frequency ratio on the performance of floating OWC (oscillating water column) devices that oscillate either vertically or horizontally at two different mass ratios (m = 2 and 3) through two-dimensional computational fluid dynamics simulations. The frequency ratio is the ratio of the natural frequency of the system to the wave frequency. Simulations are conducted for nine frequency ratios in the range between 1 and 10. The hydrodynamic efficiency achieves its maximum at the smallest frequency ratio of 1 if the OWC oscillates horizontally and at the largest frequency ratio of 10 if the OWC oscillates vertically. The frequency ratio affects the hydraulic efficiency of the vertical oscillating OWC significantly stronger than that of the horizontal oscillating OWC, especially when it is small. The air pressure and the volume oscillation in OWC is not affected much by the horizontal motion of the OWC but is significantly affected by the vertical motion, especially at small frequency ratios.

Investigation of power generation from river surface runoff with a novel tilted axis hydrokinetic turbine for off-grid sites

In this study, a novel tilted axis hydrokinetic turbine system is presented and tested to generate electricity from surface water streams in off-grid locations. This system consists of a turbine, differential, reducer, and alternator, and resembles a Pelton turbine. It is used in shallow waters near river banks with the free flow velocity. This study reports significant hydrodynamic efficiencies with field tests consistent with theoretical estimates obtained at the design and fabrication stage, for different flow velocities (0.5–3 m/s in 0.5 m/s internal) and blade numbers (7, 9, and 11 blades) on the Namnam Stream in the Muğla province/Turkey. On the same turbine system, 7, 9, and 11 blades can be disassembled in the center of the turbine. In order to facilitate testing at different water flow velocities, the flow velocity is adjusted by excavation operations in the stream bed. The results of the study indicate that as the water speeds increase, the turbine power, speed, and efficiency of all blade turbines also increase. In terms of hydrokinetic energy potential, the water flow velocity and turbine speed are 2 m/s and 3 r/min, respectively. Because of parasitic drag and lift-induced drag, 7- and 11-blade turbines have higher mechanical losses, lower turbine efficiency, and increased cost due to weight. A 9-blade turbine is preferred for such a prototype due to torque, load power (800 W), and efficiency (15.9%) compared to the others. As a result, this study reports that such a prototype can be used in unpowered riverbanks or free surface currents.

Numerical simulation of a stationary offshore multi-chamber OWC wave energy converter

This paper aims to study the improvement in hydrodynamic efficiency of an OWC device by dividing a single chamber into multiple chambers. The two-dimensional incompressible Reynolds-Averaged Navier-Stokes equations are solved to simulate the water motion due to waves and a compressible aerodynamic model is used to calculate the airflow through the turbine. Simulation of a dual-chamber-two-turbine (2C2T) case was first conducted using a range of turbine coefficients to identify the best turbine coefficient to be used for the rest of the study. The dual-chamber-dual-turbine configuration was found to increase the efficiency more than the case of two chambers sharing a single turbine (2C1T). The triple-chamber-triple-turbine (3C3T) configuration further increases the efficiency when compared with the 2C2T configuration. Despite achieving greater changes in free surface elevation, when compared with multi-turbine multi-chamber configurations, shared turbine configurations achieve lower overall efficiency due to phase differences in surface elevation between chambers.

Experimental study of the hydrodynamic performance of a U-oscillating water column wave energy converter

The U-Oscillating Water Column (U-OWC) is one of the most promising wave energy converters for wave energy extraction. In this study, the hydrodynamic performance of a U-OWC device was tested experimentally in a wave flume. Typical time series of key parameters are illustrated, including the free surface elevation, the air pressure variation, and the hydrodynamic efficiency. Energy capture over a range of wave conditions is investigated in order to determine the effects of geometrical parameters, including the front wall opening height, the U-duct length, and the U-duct height. The results show that a relatively high front wall opening height is beneficial for the U-OWC device. The length and height of the U-duct have a significant influence on the converting efficiency, and the optimal dimensionless values were evaluated. Moreover, the incident wave period also significantly affects the performance of the device.

Hydrodynamic analysis of floating tunnel with submerged rubble mound breakwater

The wave interaction with a Submerged Floating Tunnel (SFT) of two different shapes (rectangular and circular) in the presence of a submerged rubble mound breakwater (SRMB) is analyzed using Multi-Domain Boundary Element Method (MDBEM). Furthermore, three typical SFT cross-sections (rectangular, trapezoidal, and circular) of equal area and structural height in the presence of SRMB under similar operating conditions are investigated as comparative study to analyse the influence of SFT shape on hydrodynamic performance. The performance of the tunnel configurations is analyzed as a (a) measurement in terms of hydrodynamic efficiency and (b) criterion for tunnel structure safety. In both shallow and intermediate water depth regions, the critical wave number and the critical angle of incidence followed by resonant wave reflection are identified, and suitable structural parameters of SRMB such as structural porosity in the armour layer, relative crest width, relative gap width between the SFT and the SRMB, structural width and position (relative draft of tunnel structure measured from the free water surface) of SFT are investigated. The present parametric investigation of SFT with SRMB reveals an improved wave transformation properties for a specific range of water depth. The coupling of SRMB has resulted not only in a reduction of wave-induced force acting on SFTs, but also in improved performance in wave transformation characteristics as a coastal protection structure, which is substantially determined by SRMB structural properties. Due to the presence of SRMB, the SFT's safety is improved, which may also add stability to the SFT. A comparative study of different distinct cross-sections of SFTs indicates that, due to its shape, the circular SFT has a reduced reflection capability and lower wave-induced force with nearly the same wave transmission as the rectangular and trapezoidal SFT. The study performed on the coupled SFT and rubble mound breakwater may be useful in determining the suitability of breakwaters not only for maintaining shore dynamics but also for protecting important floating structures for underwater transit.

Squirmer locomotion in a yield stress fluid

An axisymmetric squirmer in a Bingham viscoplastic fluid is studied numerically to determine the effect of a yield stress environment on locomotion. The nonlinearity of the governing equations necessitates numerical methods, which are accomplished by solving a variable-viscosity Stokes equation with a finite element approach. The effects of stroke modes, both pure and combined, are investigated, and it is found that for the treadmill or ‘neutral’ mode, the swimmer in a yield stress fluid has a lower swimming velocity and uses more power. However, the efficiency of swimming reaches its maximum at a finite yield limit. In addition, for higher yield limits, higher stroke modes can increase the swimming velocity and hydrodynamic efficiency of the treadmill swimmer. The higher-order odd-numbered squirming modes, particularly the third stroke mode, can generate propulsion by themselves that increases in strength as the viscoplastic nonlinearity increases to a specific limit. These results are closely correlated with the confinement effects induced by the viscoplastic rigid surface surrounding the swimming body, showing that swimmers in viscoplastic environments, both biological and artificial, could potentially employ other non-standard swimming strategies to optimize their locomotion.

A numerical approach for hydrodynamic performance evaluation of multi-degree-of-freedom floating oscillating water column (OWC) devices

A numerical approach has been developed in this study for hydrodynamic analysis of complex-shaped oscillating water column (OWC) converters. Physically, in the circumstance when the device is free to move or moored in waves, the airflow movement inside the chamber is coupled tightly with the changing of the internal fluid surface and the hydrodynamic response of the device. By taking these coupling effects into account, appropriate boundary integral equations are formulated with supplemental theoretical relations. The boundary value problem is then solved using a higher-order boundary element method (HOBEM). After obtaining the wave force and dynamic air force, the coupled motions and hydrodynamic efficiency are evaluated by integration. In particular, unlike conventional methods limited to fixed OWCs, the proposed method is proved applicable to those floating with coupled rigid-body motions. An optimal turbine parameter is hence mathematically derived to maximise the wave power absorption. Besides the linear quantities, nonlinear wave drift loads are evaluated via a newly derived formulation that accounts for the far-field contribution and the oscillating air pressure over the internal fluid surface. Based on the above methodology and the resulting tool, numerical studies are carried out for three cylindrical OWC scenarios that are floating in waves, attached by a reflector and connected to a submerged caisson, respectively. Different modes of body motion and typical free-surface oscillation modes in the chamber are analysed and discussed. It is found that, for an OWC with an attached arc-shaped reflector or connected to a submerged caisson, its wave power absorption can be characterised by a series of apparent peaks which are associated with the resonant internal fluid and in close proximity to the resonance frequencies of body motions involving surge, heave or pitch. Numerical results also indicate the benefits of floating OWCs from the coupled rigid-body motion modes and OWC’s resonance, expanding the frequency range of efficient conversion and improving the device’s adaptability to variable oceanic environments.

FLUID FLOW INDUCED BY AN ELASTIC PLATE IN HEAVING MOTION

We perform three-dimensional simulations of an elastic plate heaving in a sinusoidal motion with different frequencies to explore the effects of the forcing heaving frequency on the hydrodynamic force generation and flow structures. It is aimed to simulate the motion of a rectangular elastic plate, representing a simplified flexible wing or fin, placed in an open water tank as in a corresponding experimental study. The top edge of the plate oscillates sinusoidally in water at rest. Simulations are conducted for the fluid structure interaction of an elastic plate with a special version of the open-source library C++ OpenFOAM, foam-extend-3.2. The plate oscillates at different heaving frequencies in a range of 3.5 Hz and 4.5 Hz. Experiments are carried out to validate our simulations. The results are in a good agreement with the experiment in terms of the representation of the resonant frequency and the induced hydrodynamic forces. It is found that the hydrodynamic force and propulsive efficiency are mainly affected by the elastic deformation and forcing heaving frequencies. The generated thrust is observed to be significantly enhanced at the resonant frequency, while the propulsive efficiency is increased at the heaving frequency which is greater than the resonant frequency. The results of our simulations point to the importance of resonant flapping frequency for considering the optimal heaving frequency to achieve the best performance and to get the improved thrust force, which are crucial for the locomotion of birds, insects, fishes, flapping-based micro air and underwater vehicles.

Experimental and statistical analysis of the hydrodynamic performance of planing boats: A Comparative study

Hydrofoil-supported catamarans (Hysucats) have emerged in the last two decades as competitors against monohulls in terms of efficiency in high-speed planing regimes. Nonetheless, according to the literature, they have not been compared experimentally. In this work, a monohull, a catamaran and a Hysucat hull were compared experimentally in order to evaluate the geometry with the best hydrodynamic behavior. The hulls were towed along a body of water with a test bench and variables such as drag, trim angle, wetted area, wetted keel length, and sinkage were measured both with sensors, installed in the boat and in the test bench, and multimedia data. These geometries had the same length, width, draft, submerged volume, and weight distribution. The equations derived by Savitsky were used as a precursor for the experimental design and for a deeper understanding of the planing phenomena. It was found that the Hysucat and the catamaran boats present up to 50% higher drag in planing speeds than the monohull, basically due to their much higher wetted area and lower trim angle. However, some potential alternatives for improving the hydrodynamic efficiency of the catamaran and the Hysucat are proposed.

Estimation of cavitation erosion area in unsteady cavitating flows using a modified approach

Estimation of cavitation erosion area remains a challenging issue. Only an accurate estimation of the erosion-sensitive area can effectively reveal the various mechanisms of cavitation erosion. In this study, large eddy simulation is applied to simulate the unsteady cavitating flows, and an energy approach is developed to estimate cavitation erosion area. This approach is modified by taking into account hydrodynamic efficiency, which is deduced from the perspective of shock wave decays. The numerical estimation is then validated based on an axisymmetric nozzle cavitating flow. The numerical simulation results indicate that the modified approach can reasonably estimate the cavitation erosion area. In addition, various hydrodynamic cavitation erosion mechanisms are numerically analyzed based on the cavitating flow around a NACA0015 hydrofoil. The numerical simulation is able to reproduce the experimentally observed U-shaped cavitation structure. The interaction between cavitation and vortex has an important influence on the distribution of cavitation erosion. The areas in which the U-shaped cavitation pass may be in high erosion risk.

Study of the Influence of Aspect Ratios on Hydrodynamic Performance of a Symmetrical Elliptic Otter Board

The otter board, which is designed to maintain the horizontal opening of trawl nets, is a vital component of a trawl system. It requires a high lift-to-drag ratio, which is directly related to the trawling efficiency and economic effectiveness of the single trawler. To improve the hydrodynamic efficiency of a symmetrical elliptic otter board, four model otter boards, i.e., aspect ratio (AR) = 0.507, 0.640, 0.766, and 0.895, were designed in the present work and the effects of aspect ratios on the hydrodynamic performance of the otter board were investigated by flume tank experiments. Further, the k-ε EARSM turbulence model was adopted to analyze the hydrodynamic coefficients and the flow distribution around the otter board using the computational fluid dynamics (CFD) method. The optimal aspect ratio was obtained based on the analysis of experimental data, wherein the lift coefficient, the drag coefficient, and the lift-to-drag ratio at different angles of attack (AOA) were measured. The results show that the symmetrical elliptic otter board model is within the critical Reynolds number region when the Reynolds number is larger than 1.682 × 105, and its hydrodynamic coefficient is consistent with the real otter board. When the AR was 0.766, the elliptic otter board had the best hydrodynamic performance, of which the lift coefficient and the lift-to-drag ratio were 1.05 and 1.14 fold that of the initial otter board (AR = 0.640), and the volume of the wing-tip vortex reaches a maximum. The results show the hydrodynamic performance of the symmetrical elliptic otter board, and parameter optimization of the otter board has also been provided for reference.

Tip Clearance Effect on The Tip Leakage Vortex Evolution and Wake Instability of a Ducted Propeller

The occurrence of a tip leakage vortex (TLV) is a special phenomenon of ducted propellers, which has a significant influence on the propeller’s hydrodynamic performance and efficiency. The inception, evolution, and instability of the TLV under different tip clearance sizes have a direct impact on the cavitation and acoustic characteristics. A simulation was set up to calculate the open-water performance of a standard ducted propeller. The open-water characteristics (OWCs) were compared with the experimental data to verify the feasibility of the method. Furthermore, to capture the influence of tip clearance size on the vortex structure evolution and wake dynamics, the improved delayed detached eddy simulation (IDDES) method was adopted to simulate four groups of ducted propellers with different tip clearances. The results showed that with the increase in the gap-to-span ratio (GSR), KTD and η0 gradually decreased, while KQ and KTB increased, but a peak point existed. Moreover, the TLV became thicker, indicating damage to the energy recycling process. The fast Fourier transform (FFT) of several wake points showed pressure pulsations of the wake ranging from the blade-passing frequency to the shaft frequency, and the evolution process accelerated with the increase in the GSR. The power spectral density (PSD) analysis showed that the energy of the wake enhanced with the increase in the GSR. In particular, the vortex interactions could cause pulses in low-GSR conditions, which could intensify the excitation force of the propeller and also have a certain impact on the noise level.

A novel Geno-fuzzy based model for hydrodynamic efficiency prediction of a land-fixed oscillating water column for various front wall openings, power take-off dampings and incident wave steepnesses

Accurate efficiency estimation of a wave energy converter (WEC) is a key concept in the design stage. Oscillating water column (OWC) is a promising type of WEC due to its advantages such as proved concept, operational simplicity, accessibility, reliability etc. In this study, for accurate efficiency estimation of an OWC, a novel Geno-fuzzy based model (GENOFIS) was developed, firstly, by improving Adaptive Neuro-Fuzzy inference system (ANFIS) and secondly, incorporating the Genetic algorithms (GAs). Data for training and testing phases of the models were obtained from an extensive wave flume experiments for various OWC underwater opening heights and applied PTO dampings under different incident waves. Both models performed remarkably with a slightly better performance of GENOFIS. The superiority of the GENOFIS model stemmed from that its high performance was attained with substantially low fuzzy rules (only three) where ANFIS required incomparably large fuzzy rules (twenty-seven) and yet achieved a slightly lower performance. Accordingly, very few numbers of fuzzy rules enable the construction of GENOFIS model structure with low complexity, which, in turn, immensely reduce the computational time required. Developed less complicated GENOFIS model is parsimonious, unlikely to suffer from overfitting and has high interpretability and practicality.