Frequency, Stochastic and Time Domain Models for an Articulated Wave Power Device

2008 ◽  
Author(s):  
Jose´ J. Caˆndido ◽  
Paulo A. P. Justino
Keyword(s):  
Stochastic Model ◽  
Time Domain ◽  
Domain Model ◽  
Irregular Waves ◽  
Power Device ◽  
Wave Power ◽  
Regular Waves ◽  
Sea State ◽  
Frequency Model ◽  
Non Linear

A frequency domain model to describe the behaviour of an articulated system in regular waves is developed. A stochastic model is found for the articulated wave power device. Probability density functions for relevant parameters that characterize the wave power system behaviour are found for different sea state conditions. Average values for useful power and capture length are also obtained for these sea state conditions. A time domain model is also developed for the articulated wave power device. Results for a linear power take-off configuration and regular waves are obtained and compared to the results obtained from the frequency model. A non-linear power take-off configuration is found. Results in regular wave conditions are obtained for this configuration, set in terms of different characteristic parameters. Results for this non-linear configuration for the power take-off and irregular waves are also computed and presented. Finally, these results are compared to the results obtained from the stochastic model.

Frequency and Time-Domain Models for a Two-Body Wave Power Device That Extracts Energy From Sea Waves by Relative Pitch Motion

2008 ◽  
Author(s):  
Paulo Justino ◽  
Jose´ C. Amador ◽  
Tiago Morais ◽  
Marc Hadden ◽  
Nuno Ferreira
Keyword(s):  
Time Domain ◽  
Body Wave ◽  
Domain Analysis ◽  
Hydrodynamic Performance ◽  
Irregular Waves ◽  
Power Device ◽  
Wave Power ◽  
Sea Waves ◽  
Pitch Motion ◽  
Relative Pitch

In this paper analytical and numerical tools are used to describe the behaviour of a new wave power device. This device extracts energy from sea waves by relative pitch motion between their two independent parts. Frequency-domain analysis is carried out assuming that its hydrodynamic performance and its power take-off equipment behaviour may be considered linear. For regular waves optimal mechanical damping coefficients are computed assuming restrictions to hold for relative pitch motion between the two device parts. Useful power, capture length as well as rotations and displacements are computed. Time-domain analysis is necessary if one needs to compute the device’s performance under more realistic operational conditions. In order to get device’s variables trajectories for irregular sea conditions, a time-domain approach is adopted since the hydraulic power take-off equipment exhibits a non-linear behaviour. Results are obtained for irregular waves, i.e. for different sea states, and thus trajectories for several device variables are presented. Power available to the hydraulic machine is computed and presented.

A Nonlinear Numerical Algorithm for Time-Domain Hydrodynamic Simulations of Vessel Motions in the Presence of Waves

2012 ◽  
Author(s):  
Eduardo Pasquetti ◽  
Luiz Cristóvão G. Coelho ◽  
Marcelo A. S. Neves ◽  
Mauro C. Oliveira ◽  
Paulo T. T. Esperança ◽  
...  
Keyword(s):  
Free Surface ◽  
Time Domain ◽  
Relative Position ◽  
Geometric Modeling ◽  
Domain Model ◽  
Irregular Waves ◽  
Wave Forces ◽  
Hydrodynamic Simulations ◽  
Non Linear ◽  
Hull Forms

Linear approaches have been traditionally employed to simulate the dynamic behavior of floating vessels and its interaction with regular or irregular waves. Some difficulties arise when large waves and vessel motions occur. Under these circumstances, the linear assumptions to compute the restoring and wave forces, which are computed on the mean position of body and water surface, are not capable of accurately representing the physics of the interactions between waves and vessels. Hydrostatic analysis of generic hull forms has already been implemented with a geometrical face representation of the hull and also internal ballast and oil tanks [1]. With the goal of improving the modeling the non-linear computation of hydrostatic in waves (at the instantaneous free surface) is implemented, thus using a generic geometric modeling of the hull to perform hydrodynamic simulations of vessel motions in the presence of waves. Additionally, for the computation of the instantaneous non-linear Froude-Krylov force (6 DOF time-domain model) the contribution of each geometrical face to the global Froude-Krylov force is calculated at the exact relative position of the vessel and the incident waves. After computing the relative position of each face, possibly being cut at the free surface, the pressure at the wetted face centers determines the contribution to the integral calculation. The paper presents the main aspects of the proposed methodology and highlights its capabilities and differences with respect to the linear approach. Complementarily, comparisons with model experiments are discussed.

scholarly journals Prediction of deepwater riser VIV with an improved time domain model including non-linear structural behavior

2021 ◽  
Vol 236 ◽  
pp. 109508
Author(s):  
Sang Woo Kim ◽  
Svein Sævik ◽  
Jie Wu ◽  
Bernt Johan Leira
Keyword(s):  
Time Domain ◽  
Domain Model ◽  
Structural Behavior ◽  
Non Linear ◽  
Deepwater Riser

Experimental Study on Hydrodynamics of Hybrid Deep-V Monohull With Different Built-Up Appendages

2018 ◽  
Author(s):  
Shuzheng Sun ◽  
Hui Li ◽  
Muk Chen Ong
Keyword(s):  
Model Test ◽  
Performance Model ◽  
Hydrodynamic Performance ◽  
Irregular Waves ◽  
Hydrodynamic Characteristics ◽  
Vertical Acceleration ◽  
Test Results ◽  
Regular Waves ◽  
Sea State ◽  
Seakeeping Performance

The hydrodynamic characteristics of a hybrid deep-V monohull with different built-up appendages are investigated experimentally in order to improve the resistance and seakeeping performance. Model tests have been carried out to study the hydrodynamic performance between a bare deep-V vessel and a deep-V monohull with different built-up appendage configurations (i.e. a hybrid deep-V monohull). From the model test results, it is found that the existence of the appendages will reduce the amplitude of pitching angle and bow vertical acceleration compared to that of the bare deep-V vessel in heading regular waves. However, the resistances for the hybrid deep-V monohull with built-up appendages are increased 15.6% for Fn = 0.264, and 0.1% for Fn = 0.441 compared to the resistance of the bare deep-V vessel. The model test results of seakeeping performance in irregular waves show that the hybrid deep-V monohull gives a better seakeeping performance than the deep-V vessel. The pitching angle and bow vertical acceleration of the hybrid deep-V monohull containing a built-up appendage are reduced 15.3% and 20.6% compared to the deep-V monohull in irregular waves at Fn = 0.441 in 6th class sea state (H1/3 = 6m).

Time Domain Simulations of Ship Maneuvering and Roll Motion in Regular Waves Based on a Hybrid Method

2019 ◽  
Author(s):  
Chengqian Ma ◽  
Ning Ma ◽  
Xiechong Gu
Keyword(s):  
Time Domain ◽  
Hybrid Approach ◽  
Wave Force ◽  
Domain Model ◽  
Viscous Force ◽  
Scale Model ◽  
Regular Waves ◽  
Ship Maneuvering ◽  
Frequency Wave ◽  
Hydrodynamic Derivatives

Abstract Ship maneuvering performance and rolling in waves under complicated environmental conditions are of significant importance for safety and economic reasons. The existing methods for predicting the maneuvering in adverse sea conditions can be categorized into unified two-time scale model, hybrid approach and CFD method. However, traditional potential methods rely tightly on ship viscous force data from test results, and CFD methods of free running ship require large computational resources consumption. In this paper, a 4-DOF (surge, sway, yaw and roll) model based on MMG method considering the wave effect is established to predict the trajectory and rolling motion with better time efficiency. The 1st order wave force and mean 2nd order drift force in this time-domain model are calculated by the 3D panel method and Cummins impose response function. Instead of model experiments, the hydrodynamic derivatives in the maneuvering model can be calculated by RANS-based numerical simulations of the Planar Motion Mechanism (PMM) test in calm water. Verification for grid convergence is also conducted according to state-of-the-art study. The predicted turning trajectory and rolling angle of the S175 containership in regular waves using CFD results show better agreement with experiment data than empirical formula results. Furthermore, it has been demonstrated that this model is also capable of predicting the ship motion in regular waves with practical accuracy. And the effects of the wave frequency, wave height are investigated consequently base on numerical simulation results.

Vortex Induced Vibrations of Pipelines With Non-Linear Seabed Contact Properties

2016 ◽  
Author(s):  
Jan V. Ulveseter ◽  
Svein Sævik ◽  
Carl M. Larsen
Keyword(s):  
Frequency Domain ◽  
Linear Model ◽  
Time Domain ◽  
Structural Model ◽  
Life Time ◽  
Domain Model ◽  
Linear Interaction ◽  
Vortex Induced Vibrations ◽  
Non Linear ◽  
Soil Damping

A promising time domain model for calculation of cross-flow vortex induced vibrations (VIV) is under development at the Norwegian University of Science and Technology. Time domain, as oppose to frequency domain, makes it possible to include non-linearities in the structural model. Pipelines that rest on an irregular seabed will experience free spans. In these areas VIV is a concern with respect to the fatigue life. In this paper, a time domain model for calculation of VIV on free spanning pipelines is proposed. The model has non-linear interaction properties consisting of discrete soil dampers and soil springs turning on or off depending on the pipeline response. The non-linear model is compared to two linear models with linear stiffness and damping properties. One linear model is based on the promising time domain VIV model, while the other one is based on RIFLEX and VIVANA, which calculates VIV in frequency domain. Through four case studies the effect of seabed geometry, current velocity and varying soil damping and soil stiffness is investigated for a specific pipeline. The results show that there is good agreement between the results produced by VIVANA and the linear model. The non-linear model predicts smaller stresses at the pipe shoulders, which is positive for the life time estimations. Soil damping does not influence the response significantly.

Current spectrum estimation using Prony's estimator and coherent resampling

2014 ◽  
Vol 33 (3) ◽  
pp. 989-997 ◽  
Author(s):  
Michał Lewandowski ◽  
Janusz Walczak
Keyword(s):  
Fourier Transform ◽  
Frequency Domain ◽  
Time Domain ◽  
Domain Model ◽  
Spectrum Estimation ◽  
Nonlinear Load ◽  
Frequency Model ◽  
Content Type ◽  
Base Frequency ◽  
Current Spectrum

Purpose – A highly accurate method of current spectrum estimation of a nonlinear load is presented in this paper. Using the method makes it possible to evaluate the current injection frequency domain model of a nonlinear load from previously recorded time domain voltage and current waveforms. The paper aims to discuss these issues. Design/methodology/approach – The method incorporates the idea of coherent resampling (resampling synchronously with the base frequency of the signal) followed by the discrete Fourier transform (DFT) to obtain the frequency spectrum. When DFT is applied to a synchronously resampled signal, the spectrum is free of negative DFT effects (the spectrum leakage, for example). However, to resample the signal correctly it is necessary to know its base frequency with high accuracy. To estimate the base frequency, the first-order Prony's frequency estimator was used. Findings – It has been shown that the presented method may lead to superior results in comparison with window interpolated Fourier transform and time-domain quasi-synchronous sampling algorithms. Research limitations/implications – The method was designed for steady-state analysis in the frequency domain. The voltage and current waveforms across load terminals should be recorded simultaneously to allow correct voltage/current phase shift estimation. Practical implications – The proposed method can be used in case when the frequency domain model of a nonlinear load is desired and the voltage and current waveforms recorded across load terminals are available. The method leads to correct results even when the voltage/current sampling frequency has not been synchronized with the base frequency of the signal. It can be used for off-line frequency model estimation as well as in real-time DSP systems to restore coherent sampling of the analysed signals. Originality/value – The method proposed in the paper allows to estimate a nonlinear load frequency domain model from current and voltage waveforms with higher accuracy than other competitive methods, while at the same time its simplicity and computational efficiency is retained.

Frequency and Time Domain Analysis of Vortex Induced Vibrations for Free Span Pipelines

2002 ◽  
Author(s):  
Carl M. Larsen ◽  
Kamran Koushan ◽  
Elizabeth Passano
Keyword(s):  
Time Domain ◽  
Structural Model ◽  
Linear Time ◽  
Time Domain Analysis ◽  
Linear Response Theory ◽  
Domain Model ◽  
Vortex Induced Vibrations ◽  
Non Linear ◽  
New Strategy ◽  
Stress Prediction

The present paper will discuss various models for calculation of vortex induced vibrations (VIV) of free span pipelines, and present a new strategy for such analyses. Applications of traditional models are presented and their limitations discussed. The new approach is based on the combination of an empirical linear frequency domain model, and a non-linear time domain structural model. The first step is to carry out the VIV analysis according to linear response theory, and next introduce the calculated hydrodynamic forces to the non-linear structural model. The benefit from using the non-linear model is to describe stresses at the shoulders more accurately, which is important since fatigue damage in many cases will be largest in this area. The conclusion is that the interaction between pipe and seafloor is crucial for accurate stress prediction, and that a non-linear time domain model will give the most accurate result.

A Comparison of Biradial and Wells Air Turbines on the Mutriku Breakwater OWC Wave Power Plant

2017 ◽  
Author(s):  
J. C. C. Henriques ◽  
W. Sheng ◽  
A. F. O. Falcão ◽  
L. M. C. Gato
Keyword(s):  
Power Plant ◽  
Wave Energy ◽  
Time Domain ◽  
Guide Vane ◽  
Basque Country ◽  
Domain Model ◽  
Wave Power ◽  
Wells Turbine ◽  
Sharp Drop ◽  
Demonstration Site

The Mutriku breakwater wave power plant is located in the Bay of Biscay, in Basque Country, Spain. The plant is based on the oscillating water column (OWC) principle and comprises 16 air chambers, each of them equipped with a Wells turbine coupled to an electrical generator with a rated power of 18.5 kW. The IDMEC/IST Wave Energy Group is developing a novel self-rectifying biradial turbine that aims to overcome several limitations of the Wells turbine, namely the sharp drop in efficiency above a critical flow rate. The new turbine is symmetrical with respect to a mid-plane perpendicular to the axis of rotation. The rotor is surrounded by a pair of radial-flow guide vane rows. Each guide vane row is connected to the rotor by an axisymmetric duct whose walls are flat discs. In the framework of the “OPERA” European H2020 Project, the new biradial turbine will be tested at Mutriku and later will be installed and tested on a floating OWC wave energy converter — the OCEANTEC Marmok-5’s — to be deployed at BiMEP demonstration site in September of 2017. The aim of the present paper is to perform critical comparisons of the performance of the new biradial and the Wells turbine that is presently installed at Mutriku. This is based on results from a time-domain numerical model. For the purpose, a new hydrodynamic frequency domain model of the power plant was developed using the well know WAMIT software package. This was used to build a time-domain model based on the Cummins approach.

scholarly journals Comparative Framework Linking Some Capabilities of Small Array Dispositions of Generic Points Absorbers

2017 ◽  
Author(s):  
Lorenzo Baños Hernandez
Keyword(s):  
Degrees Of Freedom ◽  
Radiation Force ◽  
Domain Model ◽  
Irregular Waves ◽  
Wave Direction ◽  
Regular Waves ◽  
Wave Regime ◽  
Variable Distance ◽  
Motion Characteristics ◽  
Generic Points

This thesis completion works it out to deepen into diverse modeling techniques for Point Absorbers. A combined frequency-time domain model is conceived, designed and developed in Matlab with Fortran as a base, leading to obtain physical variables of primary importance, namely position, velocity and power to energy net balance relationships of absorption. Integration of different degrees of freedom with heave as main executable leads in turn to a single buoy motion focus. Acquisition of the needed hydrodynamic coefficients is provided through application of NEMOH \& BEMIO solvers due to the Boundary Element Methodology. Initially, this Wave-to-motion model is validated by comparison with previous experimental results for a floating cone cylinder shape (Buldra-FO3). A single, generic, vertical floating cylinder is contemplated then, that responds to the action of the passing regular waves excitation. Later, two equally sized vertical floating cylinders aligned with the incident wave direction are modeled for a variable distance between the bodies. For both unidirectional regular and irregular waves as an input in deep water, we approximate the convolutive radiation force function term through the Prony method. By changing the spatial disposition of the axisymmetric buoys, using for instance triangular or rectangular shaped arrays of three and four bodies respectively, the study delves into motion characteristics for regular waves. The results highlight efficient layouts for maximizing the energy production whilst providing important insights into their performance, revealing displacement amplification- and capture width-ratios, while deriving in possible interpretations of scenarios related to the the known park effect. These terms are encompassed by the novelty of a new conceptual Post-Processing methodology in the field, which leads to obtain an optimal distance for the separated bodies with effective energy absorption in a regular wave regime. In conclusion, this computational excursion envisions and proposes potential fields of study, which will surely enhance new connections and link this renewable energy form.

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