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Item type:Publication, Access status: without files , Forces on a submerged sub sea tidal kite in surface proximitySub-sea tidal kites, while still at an early stage of development, might be an efficient and cost effective way of extracting energy from marine currents [9]. During normal operating conditions the kite is positioned deep in the water column and would ideally be built neutrally buoyant. For operation and maintenance (O&M) situations, or if a fault occurs, it is important to surface the kite in a controlled manner. While the behaviour of wing like profiles in currents is well understood, the assessment of the behaviour in surface proximity and under wave action is not trivial [1]. We employ an efficient boundary element code called panMARE [2] to simulate the effect of surface proximity and wave current interaction on a sub-sea kite. Comparison with experimental data from [1] demonstrates the suitability of the method to simulate forces on a submerged foil for varying immersion depths and angles of attack. Simulations are then performed to investigate the combined effect of waves and current to inform on the most suitable met-ocean conditions for kite retrieval.Publicationtype: Conference Paper100 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Designing short-term wave traces to assess wave power devices(2018-03-01); ; ; It is generally accepted that an estimate of mean power capture for a wave energy converter (WEC) in a given sea state can only be obtained over many hundreds (or thousands) of wave cycles. The difficulty stems from the fact that WECs typically exhibit significant nonlinearities in their responses. A reduction in the number of wave cycles needed to obtain accurate results would allow the use of numerical tools for design optimization tasks that are currently too computationally demanding. In this paper, experimental time traces are analyzed to provide reasonable estimates of relative variations in device performance using short-duration sea states. We examine the suitability of various metrics of surface elevation time traces by comparing corresponding WEC data of interest. The results show that carefully selected wave traces can be used to reliably assess variations in power output due to changes in hydrodynamic design or wave climate. It is also demonstrated how confidence levels increase with running time, so in the future simulations could be run until sufficient accuracy is achieved to choose the best design.Publicationtype: Journal Article128 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Optimising power take-off of an oscillating wave surge converter using high fidelity numerical simulationsOscillating wave surge converters are a promising technology to harvest ocean wave energy in the near shore region. Although research has been going on for many years, the characteristics of the wave action on the structure and especially the phase relation between the driving force and wave quantities like velocity or surface elevation have not been investigated in detail. The main reason for this is the lack of suitable methods. Experimental investigations using tank tests do not give direct access to overall hydrodynamic loads, only damping torque of a power take off system can be measured directly. Non-linear computational fluid dynamics methods have only recently been applied in the research of this type of devices. This paper presents a new metric named wave torque, which is the total hydrodynamic torque minus the still water pitch stiffness at any given angle of rotation. Changes in characteristics of that metric over a wave cycle and for different power take off settings are investigated using computational fluid dynamics methods. Firstly, it is shown that linearised methods cannot predict optimum damping in typical operating states of OWSCs. We then present phase relationships between main kinetic parameters for different damping levels. Although the flap seems to operate close to resonance, as predicted by linear theory, no obvious condition defining optimum damping is found.Publicationtype: Journal ArticleScopus© Citations 26 254 459 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Designing short term wave traces to assess wave power devices(International Society of Offshore and Polar Engineers (ISOPE), 2016-06); ; ; In recent years modern numerical methods have been employed in the design of Wave Energy Converters (WECs), however the high computational costs associated with their use makes it prohibitive to undertake simulations involving statistically relevant numbers of wave cycles. Experimental tests in wave tanks could also be performed more efficiently and economically if short time traces, consisting of only a few wave cycles, could be used to evaluate the hydrodynamic characteristics of a particular device or design modification. Ideally, accurate estimations of device performance could be made utilizing results obtained from investigations with a relatively small number of wave cycles. However the difficulty here is that many WECs, such as the Oscillating Wave Surge Converter (OWSC), exhibit significant non-linearity in their response. Thus it is challenging to make accurate predictions of annual energy yield for a given spectral sea state using short duration realisations of that sea. This is because the non-linear device response to particular phase couplings of sinusoidal components within those time traces might influence the estimate of mean power capture obtained. As a result it is generally accepted that the most appropriate estimate of mean power capture for a sea state be obtained over many hundreds (or thousands) of wave cycles. This ensures that the potential influence of phase locking is negligible in comparison to the predictions made. In this paper, potential methods of providing reasonable estimates of relative variations in device performance using short duration sea states are introduced. The aim of the work is to establish the shortness of sea state required to provide statistically significant estimations of the mean power capture of a particular type of Wave Energy Converter. The results show that carefully selected wave traces can be used to reliably assess variations in power output due to changes in the hydrodynamic design or wave climate.Publicationtype: Conference Paper48 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Development and validation of a procedure for numerical vibration analysis of an oscillating wave surge converterDuring extreme sea states so called impact events can be observed on the wave energy converter Oyster. In small scale experimental tests these impact events cause high frequency signals in the measured load which decrease confidence in the data obtained. These loads depend on the structural dynamics of the model. Amplification of the loads can occur and is transferred through the structure from the point of impact to the load cell located in the foundation. Since the determination of design data and load cases for Wave Energy Converters originate from scale experiments, this lack of confidence has a direct effect on the development. Numerical vibration analysis is a valuable tool in the research of the structural load response of Oyster to impact events, but must take into account the effect of the surrounding water. This can be done efficiently by adding an added mass distribution, computed with a linearised potential boundary element method. This paper presents the development and validation of a numerical procedure, which couples the OpenSource boundary element code NEMOH with the Finite Element Analysis tool CodeAster. Numerical results of the natural frequencies and mode shapes of the structure under the influence of added mass due to specific structural modes are compared with experimental results.Publicationtype: Journal ArticleScopus© Citations 13 246 361 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Steps towards fully nonlinear simulations of arrays of OWSC(2016); OWSCs in waves reflect, refract and radiate waves in different directions depending on geometrical,structural and dynamic properties of the flap. Array interaction describes the changes induced on theexcitation of flaps in an array compared to the excitation of a single flap.Renzi and Dias, 2013, Renzi et al., 2014, investigated array effects of OWSCs using a linearised semi-analytical and a linearised FEM method. However, research suggests that the applicability of linear meth-ods for the simulation of OWSCs is limited to very small flap angles (Crooks et al., 2014, Crooks et al., 2016).Linear inviscid assumptions seem to break down in typical operating conditions (Folley et al., 2004,Asmuth et al., 2014). It can therefore be assumed that the accuracy of linear methods in predicting the characteristic wave pat-tern around a flap and the interaction between multiple such devices is limited when applied to realisticoperating conditions with typical pitch motion amplitudes.Although RANS CFD tools have been shown to reproduce the motion of single flaps in waves within thelevels of experimental accuracy and can provide detailed data of all field variables like surface elevation,pressure or velocity (Schmitt and Els ¨asser, 2015a), the simulation of arrays of WECs remains an openchallenge. Due to numerical dissipation water waves simulated using volume of fluid methods tend todiminish in height and require careful spatial and temporal discretisation.The simulation of multiple moving bodies requires adaptation of the mesh and constitutes a considerablecomputational effort. As with physical test facilities numerical wave tanks require non-reflecting bound-ary conditions and wave makers.Publicationtype: Conference Paper54 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Determination of non-linear damping coefficients of bottom-hinged oscillating wave surge converters using numerical free decay testsLinear wave theory models are commonly applied to predict the performance of bottom-hinged Oscillating Wave Surge Converters (OWSC) in operational sea states. To account for non-linear effects, the additional input of coefficients not included in the model itself becomes necessary. In ocean engineering it is common practice to obtain damping coefficients of floating structures from free decay tests. This paper presents results obtained from experimental tank tests and numerical computational fluid dynamics simulations of OWSC’s. Agreement between numerical and experimental methods is found to be very good, with CFD providing more data points at small amplitude rotations. Analysis of the obtained data reveals that linear quadratic-damping, as commonly used in time domain models, is not able to accurately model the occurring damping over the whole regime of rotation amplitudes. The authors conclude that a hyperbolic function is most suitable to express the instantaneous damping ratio over the rotation amplitude and would be the best choice to be used in coefficient based time domain models.Publicationtype: Conference Paper36