Towards a unified approach for the hydrodynamic modelling of WECs: Effective linkage of non-linearity and viscous damping in potential flow models

Lead Research Organisation: Imperial College London
Department Name: Civil & Environmental Engineering


This work is undertaken in the context of the UK-China "Marine Development Feasibility Studies" call. This call was developed through a joint workshop between the UK and China research councils. As part of this workshop, four key areas were identified as critical to the development of wave and tidal energy in China. The research proposed here spans across two of these areas: "Increasing Survivability" and "Holistic Integrated Design Tools".

More specifically, the aims and objectives of the present work address important questions related to "Understanding the system dynamics to increase reliability and survivability". In the context of holistic design tools, the research addresses improvements in "CFD techniques (including potential flow models) to assess fluid structure interactions".

These two key elements of the present work are tightly linked, with one building upon the other. Whilst enhanced CFD techniques act as important design tools for wave energy conversion (WEC) developers, they are also urgently required to enable and understanding of WEC loading regimes. A firm understanding of loading regimes, in turn, will lead to improved design for survivability. These novel techniques are also crucial in addressing issues related to device reliability and fatigue loading. In addressing these challenges, the understanding of viscous effects and nonlinearities are central.

The enhanced physical understanding of both viscous effects and nonlinearity in wave structure interactions is believed to stimulate the development of wave energy extraction in both China and the UK. In the case of China, this is hoped to reduce the high dependence on fossil resources currently imported from abroad. For the UK, the development of wave energy is an important element in meeting the long-term CO2 emission targets.

The new unified model to be developed in this project will lead to a more efficient and accurate coupling method for the prediction of hydrodynamic characteristics, dynamic loadings and fatigue effects. Taken as a whole, this provides the base for the improved design of floating WECs, their survivability, and practical maintenance.

Planned Impact

The project goals are tightly linked to two areas identified as critical to the development of wave energy in China. Having arisen as a result of a scoping meeting, the importance of the work is clearly recognised by both UK and China research councils (EPSRC and MOST).

Current industry practice in hydrodynamic modelling of WECs is based on methodologies developed during the 1980s (linear potential flow). There are clear deficiencies in these methodologies, most importantly the omission of nonlinear effects and viscous effects. The proposed work programme seeks to address theses deficiencies, and primarily targets the WEC industry, providing a state-of-the art methodology for load calculations.

This includes device developers, engineering consultancies, and certification agencies. The description of the competing influence of nonlinearity and viscous damping will also make a significant impact on the wider academic community in offshore engineering. More specifically, the increased confidence in the prediction of hydrodynamic device damping and oscillating water column performance will greatly assist in estimating potential power capture for new and existing devices.

Dr Spinneken is research active in the UK marine energy community, and regularly attends UKCMER management meetings as well as the annual assembly. Through this avenue, Dr Spinneken will ensure that the findings of the present work are communicated to the UK academic community and other UKCMER stakeholders. Similarly, Dr Lu is the PI of two NSF funded projects that address physical processes of hydrodynamic damping with high relevance to wave energy extraction. Dr Lu will ensure that the present findings are suitably communicated to the Chinese academic community.

The work is part of the first EPSRC-China call of the recently established Newton fund. The principle target of the fund is to enable international collaborative research between the UK and rapidly developing partners. The investigators at ICL and DUT will strive to promote the targets of the fund, and assist in establishing its international recognition.


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Description The present research concerns the hydrodynamics and loading of marine renewable energy systems. From a loading perspective, two characteristics of such systems are particularly challenging. First, extreme wave events are associated with significant nonlinearity, which makes it difficult to predict accurate loads based on simplified engineering models. Second, under large motion excursions, the structure immersing into the fluid often causes vortices to be shed, which may significantly damp the overall motion and generally affect the dynamic response. Vortex generation is also a nonlinear phenomenon, which can often be neglected for small motions (low velocities) but is key to large amplitude motions or high velocities.

Historically, these two areas (nonlinear wave mechanics and vortex generation) have been considered separately, and are now established reasonably well if considered in isolation. However, to date, very few researchers have considered the joint description of these two problems, particularly in relation to free surface flows. Exploring the interactions between these two load drivers formed the primary objective of this grant.

Through the funded research we have demonstrated, for the first time, how nonlinear wave mechanics and vortex generation interplay in the context of a highly dynamic structure. A recent publication associated with this grant investigates the interaction of steep waves with both fixed and floating structures. The paper provides a novel view on the so-called wave excitation and wave radiation problems, leading to significant improvements in our understanding of the dynamics of floating structures as a whole. For example, the physical interpretation of observed nonlinear wave-structure interactions establishes that these may indeed be predicted using simple engineering approximations, reducing the complexity of design calculations significantly. The recent publication focuses on an explanation of the underpinning physics in a two-dimensional formulation subjected to relatively simple wave cases. A follow up paper, extending this work to realistic sea conditions, has also been submitted. Two on-going PhD projects, one funded by EPSRC and one funded through an Imperial College scholarship, support the extension of the concepts developed towards realistic geometries.
Exploitation Route The extreme loading of floating structures addressing nonlinear / viscous effects can be taken forward in at least two ways:

(1) Continue work on the interaction of nonlinear wave loading in the presence of viscous effects, where the focus lies on a detailed assessment of nonlinear wave mechanics, and the vortex generation is considered in an engineering-type approach, for example through the addition of a drag term.

(2) Investigate various coupling approaches between nonlinear potential flow solutions and viscous models. The EPSRC UK-China grant has laid the foundations for this work. This type of approach can also be applied to the moonpool problem or the resonant fluid gap problem, with applications to a multitude of areas including vessel behaviour and shipping.
Sectors Aerospace, Defence and Marine,Energy

Description The main output of the work is that it can lead to simplified engineering solutions. At present, the relative importance of wave-induced nonlinearity and vortex-induced damping is not well understood. The publications associated with this grant outline new engineering-type methodologies, for example the application of a quadratic Morrison-type drag term or a two-parameter Weibull fit. The grant PI also continues to engage with standards organisations. He presently acts as the UK principle expert on the development of IEC TC114 PT 62600-103 "Guidelines for the early stage development of wave energy converters: best practice and recommended procedures for the testing of pre-prototype devices". The work funded as part of this grant directly contributes to standard development. The grant PI has also recently joined the BSI on their wider committee to support the development of TC114 (standardisation of wave and tidal energy) as a whole.
First Year Of Impact 2015
Sector Aerospace, Defence and Marine,Energy
Impact Types Economic

Description Collaboration with Dalian University of Technology 
Organisation Dalian University of Technology
Country China 
Sector Academic/University 
PI Contribution Our contributions mainly lie in the development of a numerical approach and a physical interpretation to underpin laboratory observations made at Dalian University of Technology (DUT). The DUT laboratory investigation concerns the resonant gap problem. The UK research team visited the Chinese partner and gave two seminars in China. Joint publications are currently in preparation.
Collaborator Contribution The project partner, Dalian University of Technology, provided a substantial experimental dataset recorded prior to this funding. The dataset provided both the motivation for the work and also enabled a structured development of a corresponding numerical approach. The Chinese research team visited the UK and gave a seminar at Imperial College London. Joint publications are currently in preparation.
Impact Joint publications are currently in preparation.
Start Year 2014