Wave Tank Experiments Research Articles

Wave excitation force estimation of wave energy floats using extended Kalman filters

In many advanced control strategies the wave excitation force is key to determining the control input. However, it is often difficult to measure the excitation force on a Wave Energy Converter (WEC). The use of Kalman filters to estimate the wave excitation force based on readily available measurement data can potentially fill the gap between the development of WEC control strategies and the data that is available. Two different estimation methods using an nonlinear Extended Kalman Filter are tested on experimental wave tank data for a heaving semi-submerged float. The first method relies on directly including the excitation force as a state in the first order dynamics—which allows the “random walk” of the Kalman filter to identify an estimate of the excitation force. The second method of estimation involves modeling the wave excitation force as a harmonic oscillator comprised of sinusoidal components. Both methods are evaluated for a variety of incident waves and additional sensitivity analyses are performed to investigate the susceptibility of these estimation methods to changes in the model, measurement noise, and sampling rate.

On the transport and landfall of marine oil spills, laboratory and field observations

The dynamics of crude oil and different surface ocean drifters were compared to study the physical processes that govern the transport and landfall of marine oil spills. In a wave-tank experiment, drifters with drogue did not follow oil slicks. However, patches of undrogued drifters and thin bamboo plates did spread at the same rate and in the same direction as the crude oil slicks. Then, the trajectories of the Deepwater Horizon oil spill and 1300 drifters released near the spill source were investigated. Undrogued drifters were transported twice as fast as drogued drifters across the isobaths. 25% of the undrogued drifters landed, versus about 5% of the drogued ones, for the most part, on the same coastline locations where oil was found after Deepwater Horizon. Results highlight the importance of near surface gradients in controlling the cross-shelf transport and landing of surface material on the Gulf of Mexico's northern shores.

Rogue waves and analogies in optics and oceanography

We review the study of rogue waves and related instabilities in optical and oceanic environments, with particular focus on recent experimental developments. In optics, we emphasize results arising from the use of real-time measurement techniques, whilst in oceanography we consider insights obtained from analysis of real-world ocean wave data and controlled experiments in wave tanks. Although significant progress in understanding rogue waves has been made based on an analogy between wave dynamics in optics and hydrodynamics, these comparisons have predominantly focused on one-dimensional nonlinear propagation scenarios. As a result, there remains significant debate about the dominant physical mechanisms driving the generation of ocean rogue waves in the complex environment of the open sea. Here, we review state-of-the-art of rogue wave studies in optics and hydrodynamics, aiming to clearly identify similarities and differences between the results obtained in the two fields. In hydrodynamics, we take care to review results that support both nonlinear and linear interpretations of ocean rogue wave formation, and in optics, we also summarise results from an emerging area of research applying the measurement techniques developed for the study of rogue waves to dissipative soliton systems. We conclude with a discussion of important future research directions.

Nonlinear hydrodynamics analysis of a submerged spherical point absorber with asymmetric mass distribution

In previous work, a frequency-domain model was developed from linear potential theory to investigate the oscillation modes and efficiency of a single-tether 3 degree-of-freedom submerged spherical point absorber with asymmetric mass distribution (SPAMD). It was found that the trajectory of the device has a strong correlation with the performance of the wave energy converter. Specifically, the SPAMD can generate unique circular trajectories under long waves, producing up to 3 times power that of a generic single-tether point absorber (PA). However, this conclusion might not be valid for large buoy displacements due to increased nonlinear hydrodynamic effects (e.g. surface piercing, overtopping water, and vortex shedding). In this study, the trajectory of the SPAMD was analysed to determine the dominant nonlinear hydrodynamic effect that degrades the performance of a fully submerged system. The analysis was conducted in a numerical wave tank experiment (NWT), based on the Navier-Stokes equation and using the computational fluid dynamic toolbox OpenFOAM and the open-source library OLAFLOW for wave generation and absorption. The results obtained from NWT experiments show that surface piercing has the largest negative impact on the system’s performance, which compromises the efficiency of the SPAMD by modifying the trajectory and dissipating energy. As a result, the efficiency of the SPAMD significantly decreases for long waves when surface piercing is most likely to occur, which implies that submerged point absorbers are less efficient than the floating ones in this scenario. Furthermore, although the performance of the SPAMD were significantly compromised due to the effect of surface piercing, the resulting power improvement in comparison to the submerged generic point absorber was still considerable for some wave periods.

Experimental and numerical investigations of a two-body floating-point absorber wave energy converter in regular waves

This paper presents experimental and numerical studies on the hydrodynamics of a two-body floating-point absorber (FPA) wave energy converter (WEC) under both extreme and operational wave conditions. In this study, the responses of the WEC in heave, surge, and pitch were evaluated for various regular wave conditions. For extreme condition analysis, we assume the FPA system has a survival mode that locks the power-take-off (PTO) mechanism in extreme waves, and the WEC moves as a single body in this scenario. A series of Reynolds-averaged Navier–Stokes (RANS) simulations was performed for the survival condition analysis, and the results were validated with the measurements from experimental wave tank tests. For the FPA system in operational conditions, both a boundary element method (BEM) and the RANS simulations were used to analyze the motion response and power absorption performance. Additional viscous damping, primarily induced by flow separation and vortex shedding, is included in the BEM simulation as a quadratic drag force proportional to the square of body velocity. The inclusion of viscous drag improves the accuracy of BEM’s prediction of the heave response and the power absorption performance of the FPA system, which agrees well with experimental data and the RANS simulation results over a broad range of incident wave periods, except near resonance in large wave height scenarios. Overall, the experimental and numerical results suggest that nonlinear effects, caused by viscous damping and interaction between waves and the FPA, significantly influence the system response and power absorption performance. Nonlinear effects were found to be particularly significant when the wave height was large and the period was near or shorter than the resonant period of the FPA system. The study is expected to be helpful for understanding the nonlinear interaction between waves and the FPA system so that the structure of the FPA can be adequately designed.

Analytical and Experimental Investigation of Waves Propagating through Thin, Porous Walls for Coastal Protection Applications

Chen, Y.-H.; Chu, T., and Wang, K.-H., 2019. Analytical and experimental investigation of waves propagating through thin, porous walls for coastal protection applications. Journal of Coastal Research, 35(6), 1294–1306. Coconut Creek (Florida), ISSN 0749-0208.This paper presents a combined analytical and experimental investigation of the interaction between either a linear monochromatic or a nonlinear solitary wave and a thin, porous wall as a breakwater for coastal protection. This study will determine with validation a unique porous-wall property defined as a material constant in Darcy's law based on theoretical formulation of the porous flow condition using the laboratory data and will evaluate the effectiveness of the porous walls on the transmitted waves through the analytical solutions derived for both the monochromatic and solitary wave conditions. A series of wave tank experiments were performed to record wave elevations in front of and behind the porous walls. Monochromatic waves with various incident wave conditions were first generated to test eight thin, porous walls with varying pore sizes and porosities. The porous-wall properties determined and validated with monochromatic wave data are found to be dependent on the porosity and pore size. The transmitted wave height is reduced as the porosities of the walls decrease. When comparing two porous walls with the same porosity, less incident wave energy gets transmitted through the wall with a smaller pore diameter. Additionally, those porous walls were applied in laboratory tests with inputs of incident solitary waves of varying wave heights. The derived analytical solutions with the determined porous-wall material constants for the cases of solitary waves are verified with the results, similar to the monochromatic wave cases, showing good matches between the predicted and measured wave profiles. Overall, this study provides an important analytical approach with confirmed experimental verifications to be used for coastal engineering application analyzing waves interacting with a thin, porous wall.

Imbricated Coastal Boulder Deposits are Formed by Storm Waves, and Can Preserve a Long-Term Storminess Record

Coastal boulder deposits (CBD) are archives of extreme wave events. They are emplaced well above high tide, and may include megagravel clasts weighing tens or even hundreds of tonnes. But do they represent storms or tsunami? Many are interpreted as tsunami deposits based simply on clast size and inferences about transport, despite the fact that there are no direct observations documenting formation of these inbricated boulder clusters and ridges. In this study, we use force-balanced, dynamically scaled wave-tank experiments to model storm wave interactions with boulders, and show that storm waves can produce all the features of imbricated CBD. This means that CBD, even when containing megagravel, cannot be used as de facto tsunami indicators. On the contrary, CBD should be evaluated for inclusion in long-term storminess analysis.

Experimental Investigation of Ocean Wave Measurement Using Short-Range K-Band Radar: Dock-Based and Boat-Based Wind Wave Measurements

In this paper, an ocean wave measurement technique and a newly developed short-range K-band radar are tested. In previous work, the technique and its feasibility were studied based on numerical simulations and wave tank experiments, while its performance at sea was still unknown. Surface current, Stokes drift, and wave breaking can greatly complicate interpreting radar backscatters. The feasibility of the technique needed to be further investigated with sea experiments. Experiments were carried out at a stationary site and from a moving platform. The short-range K-band radar transmitted continuous wave and received backscatters at low-grazing angles. The Bragg-scattering from the radar’s effective footprint dominated the backscatters. The Doppler shift frequency of the Bragg-scattering was attributed to the phase velocity of Bragg waves and modulated by the surface motions induced by current, Stokes drift, platform, and gravity waves. These sources of the Doppler shift frequency were analyzed, and the components induced by wind waves were successfully retrieved and converted into wave spectra that were consistent with the measurements of wave rider buoy. The experimental investigation further validated the feasibility of using short-range K-band radar to measure ocean waves.

Numerical analysis and wave tank validation on the optimal design of a two-body wave energy converter

To improve the performance of a ‘point absorber’ type wave energy converter (WEC), an additional submerged body can be deployed. The submerged body can be used to increase the equivalent excitation force on the WEC, as well as provide resonance tuning. This paper presents numerical analysis and experimental validation of a two-body point absorber type WEC using a mechanical motion rectifier (MMR) based power takeoff. The two-body point absorber consists of a floating buoy connected to a neutrally buoyant submerged body via the power-takeoff. The mechanical motion rectifier and a ball screw translate the relative heave motion of the two bodies into unidirectional rotation, which in turn spins a generator. Frequency domain analysis suggests there is an optimal submerged body mass for maximum WEC power absorption. Regular wave simulations in the time domain are compared to the results obtained in the frequency domain. While the time domain and frequency domain results predict the same optimal mass ratio, time domain analysis provides a more complex and accurate power result. To validate the time domain model, experimental wave tank testing is conducted using a 1:30 scale model WEC. The experiment shows the two-body WEC can produce twice the amount of power as the single-body WEC with same floating buoy and can be further increased by PTO design and power electronics optimization. Wave tank testing also shows the two-body WEC has a capture width ratio up to 58% at 59 kW/m and 51% at 36 kW/m.

Wave-induced real-fluid effects in marine energy converters: Review and application to OWC devices

The performance assessment of industrial marine energy converters involves the integrated treatment of their hydrodynamic design and the optimization of their device hulls. Nowadays, such tasks require extensive experimental work and simulation plans, consuming considerable resources and time. In this comprehensive review of integrated approaches to numerical and experimental testing, the advantages and disadvantages of existing tools, from full-scale prototype and wave tank models to Computational Fluid Dynamics (CFD) and potential flow simulations, are all analysed. Likewise, current challenges such as experimental scale effects, numerical viscosity, and turbulence treatment are all studied. The novelty of this research is an integrated approach that employs experimental wave tank tests to validate a numerical wave tank model based on CFD that serves to calibrate a fast potential flow solver with Morison's correction terms. The model allows running, on tight resources, the necessary simulation for the design and optimisation of marine energy converters under multiple sea state conditions. Given the operating regimes of conventional marine energy converters, the results show that the influence of turbulence may be small, due to the unsteady nature of the oscillatory boundary layer flows.

Experiments on wave propagation in grease ice: combined wave gauges and particle image velocimetry measurements

Water wave attenuation by grease ice is a key mechanism for the polar regions, as waves in ice influence many phenomena such as ice drift, ice breaking and ice formation. However, the models presented so far in the literature are limited in a number of regards, and more insights are required from either laboratory experiments or fieldwork for these models to be validated and improved. Unfortunately, performing detailed measurements of wave propagation in grease ice, either in the field or in the laboratory, is challenging. As a consequence, laboratory data are relatively scarce, and often consist of only a couple of wave elevation measurements along the length of the wave tank. We present combined measurements of wave elevation using an array of ultrasonic probes, and water kinematics using particle image velocimetry (PIV), in a small-scale wave tank experiment. Experiments are performed over a wider frequency range than has been previously investigated. The wave elevation measurements are used to compute the wavenumber and exponential damping coefficient. In contrast to a previous study in grease ice, we find that the wavenumber is consistent with the mass loading model, i.e. it increases compared with the open water case. Wave attenuation is compared with a series of one-layer models, and we show that they satisfactorily describe the viscous damping occurring. PIV data are also consistent with exponential wave amplitude attenuation, and a proper orthogonal decomposition analysis reveals the existence of mean flows under the ice that are a consequence of the displacement and packing of the ice induced by the gradient in the wave-induced stress. Finally, we show that the dynamics of grease ice can generate eddy structures that inject eddy viscosity into the water under the grease ice, which would lead to enhanced mixing and participating in energy dissipation.

On the Assessment of Numerical Wave Makers in CFD Simulations

A fully non-linear numerical wave tank (NWT), based on Computational Fluid Dynamics (CFD), provides a useful tool for the analysis of coastal and offshore engineering problems. To generate and absorb free surface waves within a NWT, a variety of numerical wave maker (NWM) methodologies have been suggested in the literature. Therefore, when setting up a CFD-based NWT, the user is faced with the task of selecting the most appropriate NWM, which should be driven by a rigorous assessment of the available methods. To provide a consistent framework for the quantitative assessment of different NWMs, this paper presents a suite of metrics and methodologies, considering three key performance parameters: accuracy, computational requirements and available features. An illustrative example is presented to exemplify the proposed evaluation metrics, applied to the main NWMs available for the open source CFD software, OpenFOAM. The considered NWMs are found to reproduce waves with an accuracy comparable to real wave makers in physical wave tank experiments. However, the paper shows that significant differences are found between the various NWMs, and no single method performed best in all aspects of the assessment across the different test cases.

Elastic response of a light-weight floating support structure of FOWT with guywire supported tower

To obtain fundamental knowledge on the elastic response characteristics of a light-weight floating support structure of a FOWT (floating offshore wind turbine) with guywire supported tower, basic load transmission mechanism was investigated. Static analysis with elastic frame model, numerical analysis and wave tank experiment with elastically and dynamically similar segmented backbone model were conducted to clarify the dynamic elastic response characteristics of the structure. In the numerical analysis, analysis code of a rotor-floater-mooring-control coupled response NK-UTWind developed in University of Tokyo is used. It is clarified that when the rigidity of the frame structural part is low compared with guywire, the load is mainly borne by guywire under pitch motion. It was found that the tension fluctuation of guywire becomes large at wave period of 6 s when the inertial force due to pitch motion is large, and at wave period of 18–20 s when inclination of tower is larger the tension fluctuation also becomes large due to the overturning moment.

Stability of coupled human and stand-up paddle board

This study investigates the rider stability on a stand-up paddle board (SUP) based on board dimensions, rider body parameters, and skill level using a buoyant body dynamics approach. For this study, the system of a rider standing on an SUP is modeled as a planar motion of a rectangular cross-sectional buoyant body with an added mass attached at a fixed height. A stability map is developed to visualize stability regions for various system parameters. The result shows that eigenvalue contours are in good agreement with qualitative terms, such as beginner and professional. A new approach of describing rider skill level is proposed. Wave tank experiments are conducted to verify the analytic equations. Analytical results of the rectangular board show the same trend as the experimental results from a real SUP product.

Study on Bragg and Non-Bragg Scattering Mechanism and Frequency Shifts From Time-Varying Periodic Water Wave

In this paper, Bragg and non-Bragg scattering mechanisms and frequency shifts from time-varying periodic water wave are studied from three aspects: wave tank measurements with an ultrahigh frequency radar, numerical simulation using the method of moments, and theoretical derivation applying the small perturbation method. The scattering field, radar cross section (RCS), and frequency shifts are discussed in both horizontal and vertical polarizations. The wave tank observations show that backscattering enhancement occurs when water wavelength is an integer multiple of Bragg wavelength, and there are several Doppler harmonics with frequency shifts of water-wave frequency and its integer multiples. Numerical simulations indicate that these Doppler harmonics except the one associated with the water-wave phase velocity are caused by the water surface edge effect. Moreover, theoretical analyses, numerical simulations, and wave tank experiments all show a clear exponential relationship between backscattering RCS and wave height. In addition, we further analyze the bistatic scattering and find that the scattering field is composed of plane waves propagating in the directions determined by water wavelength and radio wavelength, the bistatic frequency shift is the harmonic frequency of water wave, and the bistatic RCS also has an exponential relation with water-wave height.

Experimental study on characteristics of air entrainment and distribution of void fraction in surf zone

The study of breaking wave-induced air bubble flows near the seashore is a great challenge. The process of air entrainment is highly complex and air bubbles evolution takes place over a wide range of spatial and temporal scales. Surf wave breaking can generate strong turbulence and entrain a large amount of air in the water and produce many air bubbles with different lifetimes. It is a typical two-phase flow involving water and air. The evolution and transport of wave-induced air bubbles can significantly affect coastal structures with respect to impact intensity, seawater-air exchange, sediment transport and wave energy dissipation. Studying the characteristics of wave-induced air bubbles in the nearshore wave breaking zone has important theoretical value and scientific significance. A better understanding of this process is also important to improving engineering practice and environmental protection in the nearshore region. Past studies have been focused on the transport law of the wave-induced air bubbles and the related statistics of air bubbles involved in the wave breaking process. Due to the complexity of water and gas mixing processes and the difficulty of the corresponding measurements, the prior research mainly focused on the formation and distribution of wave-induced air bubbles and their relationship with wave energy dissipation. However, quantitative research on how air bubble characteristics vary with different wave parameters still lacks in-depth discussion. This work presents a wave tank experiment to fully understand the void fraction distribution and its relationship with prominent wave parameters during the air entrainment process during surf wave breaking. Spatial and temporal variations of air entrainment and void fraction evolutions are recorded by an air bubble measuring system and high speed camera, respectively. The main factors affecting the distribution of void fraction are determined. The experimental data demonstrates that the distribution of void fraction is closely related to the water depth, prominent wave parameters and distance from the wave breaking location. We discuss the spatial and temporal evolution of the void fraction and derive an empirical formula to predict the void fraction by introducing two factors named C 0 and k 1. The factor k 1 is inversely proportional to wave height and C 0 varies with the wave steepness, relative water depth and dimensionless horizontal position. The empirical formula is used to analyze the relationship between each wave parameter and the distribution of void fraction. Experimental results show that the maximum void faction was close to 20% at the water surface and that the maximum penetration depth of air bubbles was close to 5 cm below the still water surface. As the water depth increased, void faction varied with e-index. Results of this work can be used to analyze and predict the spatiotemporal distribution of the void fraction in wave breaking zone and provide a good reference for further study of air bubble transport during surf wave breaking.

Peak forces on a point absorbing wave energy converter impacted by tsunami waves

Although a tsunami wave in deep sea can be simulated using linear shallow water theory, the wave dynamics of a tsunami running up a continental shelf is very complex, and different phenomena may occur, depending on the width and profile of the shelf, the topography of the coast, incident angle of the tsunami and other factors. How to simulate tsunami waves at an intermediate depth is studied in this paper by using three different simulation approaches for tsunamis, a soliton, a simulated high incident current and a dam-break approach. The surface wave profiles as well as the velocity- and pressure profiles for the undisturbed waves are compared. A regular Stokes 5th wave of the same amplitude is simulated for comparison. A wave energy converter model, previously validated with wave tank experiment, is then used to study the survivability of the Uppsala University wave energy device for the different waves. The force in the mooring line is studied together with the resulting force on a bottom mounted column, corresponding to the linear generator on the seabed.

Waves on a vortex: rays, rings and resonances

We study the scattering of surface water waves with irrotational draining vortices. At small depth, this system is a mathematical analogue of a rotating black hole and can be used to mimic some of its peculiar phenomena. Using ray-tracing methods, we exhibit the existence of unstable orbits around vortices at arbitrary depth. These orbits are the analogue of the light rings of a black hole. We show that these orbits come in pairs, one co-rotating and one counter-rotating, at an orbital radius that varies with the frequency. We derived an explicit formula for this radius in the deep-water regime. Our method is validated by comparison with recent experimental data from a wavetank experiment. We finally argue that these rings will generate a discrete set of damped resonances that we characterize and that could possibly be observed in future experiments.

Numerical and experimental study on a hemispheric point-absorber-type wave energy converter with a hydraulic power take-off system

This study examined the performance of a hemispherical point-absorber wave energy converter (WEC) with a hydraulic power take-off (PTO) system. The hydraulic PTO system for power generation was modeled as an approximate coulomb damping force, which was recently proposed to reduce numerical error. To examine the hydrodynamic performance of the WEC, a three-dimensional frequency-domain numerical wave tank technique was adopted, in which the wave radiation problem and diffraction problem for a hemispherical buoy were solved in succession to obtain the hydrodynamic coefficients. The Cummins equation was also adopted to simulate the buoy displacement and extracted wave power in the time domain. For comparison, a three-dimensional wave tank experiment was conducted. Various viscous damping and energy loss from WEC system and hydraulic cylinder pressure of the PTO system were measured from the experimental results, and these values were added to the governing equation of buoy motion. Therefore, the final numerical model of the WEC system contained the viscous damping and hydraulic PTO forces as well as the potential-flow-based hydrodynamic coefficients. Using the developed numerical model, the hemispheric buoy displacement and extracted wave power were calculated for various hydraulic pressures and input wave conditions to determine the optimal conditions for the maximum wave power.

Experimental and numerical study on a novel dual-resonance wave energy converter with a built-in power take-off system

A new concept of point-absorber wave energy converter (WEC) with a waterproof outer-floater and a built-in power take-off (BI-PTO) mechanism, named Dual-Resonance WEC (DR-WEC), is put forward and investigated by experiments and numerical simulations. The BI-PTO mechanism includes spring, sliding-mass and damping systems, where the spring system is the most complicated and should be designed specially. A 1:10 scale model is designed. The mechanical performance of the BI-PTO system is investigated by a bench test. The results have shown that the design is feasible, and the added inertia effect of the BI-PTO has a negative influence on the power output. The average mechanical efficiency of the BI-PTO is 65.8% with maximum up to 80.0%. The motion and power responses of the DR-WEC are studied by a wave tank experiment and a linear numerical model with corrected mechanical added mass and viscosity. The viscous added mass and damping correction coefficients are obtained by a free decay test. The good agreement between the experimental measurements and numerical simulations has indicated that the present numerical model with corrections is of enough accuracy and the effects of mooring system and other degree of freedoms on the heave motion and power responses can be ignored.