Félicien Bonnefoy

Effect of non-ideal power take-off on the energy absorption of a reactively controlled one degree of freedom wave energy converter

In this paper, the effect of non-ideal actuators on the performance of reactive control for a heaving wave energy converter is studied. The concept of the control is to cancel all or part of the reactive terms in the equation of motion. The proposed control is causal, thus it may be applied in practice. Actuators efficiencies from 50 to 100% are considered. The methodology used in the study relies on mathematical and numerical modeling. Control performance is investigated in regular waves and in irregular waves, and also from the perspective of the annual mean absorbed power at a typical Western Atlantic site. Motion constraints are not taken into account in the analysis for sake of simplicity. As already shown in previous work, it is found that reactive control can increase the mean annual power absorption at the considered site by a factor 10 in case of ideal actuators. However, it is shown that actuators efficiency is critical to control performance, because of the large amount of reactive power involved in the control strategy. Thus, for low efficiencies actuators (<80%), control performance is a fraction of what it can be with ideal actuators (approximately 10%). Even with 90% efficiency, control performance is less than 30% of the ideal case. In the range 90–100%, every percent of increase in efficiency leads to significant increase in control performance.

Nonlinear Higher Order Spectral Solution for a Two-Dimensional Moving Load on Ice

We calculate the nonlinear response of an infinite ice sheet to a moving load in the time domain in two dimensions, using a higher-order spectral method. The nonlinearity is due to the moving boundary, as well as the nonlinear term in Bernoullis equation and the elastic plate equation. We compare the nonlinear solution with the linear solution and with the nonlinear solution found by Parau & Dias (J. Fluid Mech., vol. 460, 2002, pp. 281-305). We find good agreement with both solutions (with the correction of an error in the Parau & Dias 2002 results) in the appropriate regimes. We also derive a solitary wavelike expression for the linear solution - close to but below the critical speed at which the phase speed has a minimum. Our model is carefully validated and used to investigate nonlinear effects. We focus in detail on the solution at a critical speed at which the linear response is infinite, and we show that the nonlinear solution remains bounded. We also establish that the inclusion of nonlinearities leads to significant new behaviour, which is not observed in the linear solution.

Role of the basin boundary conditions in gravity wave turbulence

Gravity wave turbulence is studied experimentally in a large wave basin where irregular waves are generated unidirectionally. The role of the basin boundary conditions (absorbing or reflecting) and of the forcing properties are investigated. To that purpose, an absorbing sloping beach opposite to the wavemaker can be replaced by a reflecting vertical wall. We observe that the wave field properties depend strongly on these boundary conditions. Quasi-one dimensional field of nonlinear waves propagate before to be damped by the beach whereas a more multidirectional wave field is observed with the wall. In both cases, the wave spectrum scales as a frequency-power law with an exponent that increases continuously with the forcing amplitude up to a value close to -4, which is the value predicted by the weak turbulence theory. The physical mechanisms involved are probably different according to the boundary condition used, but cannot be easily discriminated with only temporal measurements. We have also studied freely decaying gravity wave turbulence in the closed basin. No self-similar decay of the spectrum is observed, whereas its Fourier modes decay first as a time power law due to nonlinear mechanisms, and then exponentially due to linear viscous damping. We estimate the linear, nonlinear and dissipative time scales to test the time scale separation that highlights the important role of a large scale Fourier mode. By estimation of the mean energy flux from the initial decay of wave energy, the Kolmogorov-Zakharov constant is evaluated and found to be compatible with a recent theoretical value.

Observation of resonant interactions among surface gravity waves

We experimentally study resonant interactions of oblique surface gravity waves in a large basin. Our results strongly extend previous experimental results performed mainly for perpendicular or collinear wave trains. We generate two oblique waves crossing at an acute angle, while we control their frequency ratio, steepnesses and directions. These mother waves mutually interact and give birth to a resonant wave whose properties (growth rate, resonant response curve and phase locking) are fully characterized. All our experimental results are found in good quantitative agreement with four-wave interaction theory with no fitting parameter. Off-resonance experiments are also reported and the relevant theoretical analysis is conducted and validated.

Comparison of Simulation and Tank Test Results of a Semi-submersible Floating Wind Turbine Under Wind and Wave Loads

A model of the Dutch Tri-floater semi-submersible platform equipped with the NREL 5MW wind turbine has been tested in the hydrodynamic and ocean engineering tank of Ecole Centrale Nantes under wind and wave loads. This paper aims at comparing the results obtained with numerical simulations with these experimental results. The numerical model is based on the FAST design code from NREL and a user defined platform load model for calculating hydrodynamic and mooring loads. This hydrodynamic model includes non linear hydrostatic and Froude-Krylov forces, diffraction/radiation forces obtained from linear potential theory and Morison forces to take into account viscous effects on the braces and heave plates.First the hydrodynamic model is calibrated against the results of free decay tests without wind. A good agreement is achieved by calibrating mooring properties and heave plates properties of the numerical model. Then a comparison of regular wave cases without wind is realised, and a fair agreement is observed for surge, heave and pitch motions of the floating system. Finally comparisons are realised for regular wave cases with a constant wind speed. A good agreement is observed for the steady state surge and pitch offset. Surge and heave motions also shown a good agreement, these degrees of freedom are not being strongly influenced by wind loading. For pitch motion, numerical simulations show differences around 0.4 rad/s wave frequency, for which model tests have shown a significant influence of wind loading on system motion.Copyright

Generation of Large Angle Bimodal Sea States Using One-Side Segmented Wavemaker

In the present work, we study the capabilities of a one-side segmented wavemaker to generate multidirectional sea states. We want to reproduce typical West Africa sea conditions in a rectangular basin, that is, two spectra representing swell and wind waves with ±45 deg main directions relative to the basin axis. For that purpose, we study two generation techniques proposed, respectively, by Dalrymple and Molin which rely on the control of sidewall reflections in order to enlarge the testing zone. Both numerical and experimental results show that the use of such methods in combination with a one-side wavemaker provides high quality wave fields as well as large extent testing area. The Molin method appears to be more efficient in terms ofwave field quality within the testing area although the tuning of its setting parameters requires more care than the Dalrymple method.

Deterministic Reconstruction and Prediction of a Non-Linear Wave Field Using Probe Data

The accurate simulation of non-linear sea states evolution over long time periods represents a great challenge, with number of applications in oceanography, marine engineering, security of people or marine transportation, etc... The aim of this study is to develop an efficient deterministic prediction model for irregular wave fields based on the exploitation of wave elevation time series given by one or more probes. We use the High-Order-Spectral model (HOS) to simulate numerically the wave field evolution, in order to take the non-linear effects up to a desired order into account. In this paper, we report on the development of an effective reconstruction scheme, for two dimensional wave fields and using one wave record, that allows us to get proper initial conditions for numerical simulations and nonlinear wave fields forecast.Copyright

In-vitro validation of 4D flow MRI measurements with an experimental pulsatile flow model

PURPOSE The purpose of this study was to assess the precision of four-dimensional (4D) phase-contrast magnetic resonance imaging (PCMRI) to measure mean flow and peak velocity (Vmax) in a pulsatile flow phantom and to test its sensitivity to spatial resolution and Venc. MATERIAL AND METHODS The pulsatile flow phantom consisted of a straight tube connected to the systemic circulation of an experimental mock circulatory system. Four-dimensional-PCMR images were acquired using different spatial resolutions (minimum pixel size: 1.5×1.5×1.5mm3) and velocity encoding sensitivities (up to three times Vmax). Mean flow and Vmax calculated from 4D-PCMRI were compared respectively to the reference phantom flow parameters and to Vmax obtained from two-dimensional (2D)-PCMRI. RESULTS 4D-PCI measured mean flow with a precision of -0.04% to+5.46%, but slightly underestimated Vmax when compared to 2D-PCMRI (differences ranging from -1.71% to -3.85%). 4D PCMRI mean flow measurement was influenced by spatial resolution (P<0.001) with better results obtained with smaller voxel size. There was no effect of Venc on mean flow measurement. Regarding Vmax, neither spatial resolution nor Venc did influence the precision of the measurement. CONCLUSION Using an experimental pulsatile flow model 4D-PCMRI is accurate to measure mean flow and Vmax with better results obtained with higher spatial resolution. We also show that Venc up to 3 times higher than Vmax may be used with no effect on these measurements.

Wave and Current Generation in Wave Flumes Using Axial-Flow Pumps

We investigate a new concept for wave and current generation. It consists of axial-flow pumps driven such as to generate an oscillatory flow through an orifice located at one end of the flume. Oscillations of the flow lead to the generation of water waves at the free surface. If the average of the flow is different from zero, a current is generated that superposes on the waves. In this study, we explored the technical capabilities of this concept and the influence of geometric parameters on wave and current generation. We used numerical and experimental modelling. Most noticeably, the numerical results indicate that this concept is well suited for the generation of long and high waves. An experimental setup has been designed and built. We used it to make an experimental proof of concept for the wave and current generation, including waves propagating against the current.

Non-Linear Initialization in Three-Dimensional High Order Spectra Deterministic Sea State Modeling

This study deals with the initialization of three-dimensional wave field computations. We carry out such simulations with an HOS model developed at LMF-ECN since 2002 and based on the work of West et al. (1987) and Dommermuth & Yue (1987). In such models, initial conditions for three-dimensional realistic sea state computation are obtained by linearly distributing energy density spectrum. This however implies a relaxation of the non-linear effects as proposed by Dommermuth (2000) for bi-dimensional monochromatic wave train and Tanaka (2001), over several tenths of wave periods. The present work tests those former initialization methods and exposes an alternative initialization based on a non-linear three-dimensional approach. Non-linear interaction processes are both accounted in the spectra of elevation and potential of velocity, in accordance with the formulation of Dalzell (1999) at second order in wave steepness. Non-linear energy calculation is then addressed and the efficiency of the methods as well as their possible impact on properties and statistics of the wave field are investigated.Copyright