Modelling impacts of tidal stream turbines on surface waves

doi:10.1016/j.renene.2018.05.098

Renewable Energy

Volume 130, January 2019, Pages 725-734

https://doi.org/10.1016/j.renene.2018.05.098 Get rights and content

Highlights

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Impact of tidal stream energy device on surface wave dynamics are studied.
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A 3D wave-current-sediment fully coupled large-scale numerical model is used.
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Impact of turbines on surface waves are incorporated in the large-scale model.
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Model prediction indicates a 3% turbine-caused drop in wave height.
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Impact of the wave height drop on bed stress in the immediate wake is small.

Abstract

A high resolution Computational Flow Dynamics (CFD) numerical model is built based on a laboratory experiment in this research to study impacts of tidal turbines on surface wave dynamics. A reduction of $\sim 3 %$ in wave height is observed under the influence of a standalone turbine located 0.4 m from the free surface. The artificial wave energy dissipation routine ‘OBSTACLE’ within FVCOM is shown to effectively capture the correct level of wave height reduction, reproducing the CFD results with significantly less computational effort.

The turbine simulation system is then applied to a series of test cases to investigate impact of a standalone turbine on bed shear stress. Results suggest an apparent increase in bed stress ( $\sim 7 %$ ) upstream of the turbine due to the inclusion of surface waves. However, in the immediate wake of the turbine, bed stress is dominated by the presence of the turbine itself, accounting for a $\sim 50 %$ increase, with waves having a seemingly negligible effect up to 9D (D is the turbine diameter) downstream of the turbine. Beyond this point, the effect of waves on bed shear stress become apparent again. The influence of OBSTACLE on bed stress is also noticeable in the far wake, showing a reduction of $\sim 2 %$ in wave height.

Introduction

As a very promising clean, non-carbon alternative to traditional fossil fuels, tidal stream energy has been gaining significant attention. However, despite the growing interest in this sector of renewable energy, our understanding of the impacts of tidal stream energy devices on the surrounding environment is still limited, largely due to the lack of data collected from on-site projects.

Alternatively, laboratory experiments and numerical simulations are widely adopted to investigate such impacts. For example, porous actuator disc simulators [[1], [2], [3]] and down-scaled turbine prototype models [4,5] have been used in laboratories to study turbine-induced impacts on passing flows and turbulence. Laboratory experiments are also carried out to study changes of wake recovery of a turbine subjected to opposing waves [6]. As a complement to laboratory experiments, Computational Flow Dynamics (CFD) modelling is also commonly applied. Similarly, works with turbines approximated as porous discs [[7], [8], [9]] and with realistic turbine geometry resolved in the computational mesh [[10], [11], [12]] have been published to reveal how flow patterns and turbulent mixing are changed by the turbine in near-field scale.

To study the far-field hydrodynamic changes caused by the operation of turbines and turbine arrays, numerical oceanographic models, such as Regional Ocean Modelling System (ROMS) [13] and The Unstructured Grid Finite Volume Community Ocean Model (FVCOM) [14], have also been used. Modifications have been made to such models in order to simulate the effect of tidal stream turbines on the flow motion. These modifications are mostly based on either the additional bottom friction approach [[15], [16], [17]] or the turbine-induced body force concept [[18], [19], [20], [21], [22], [23], [24]].

In an effort to account for turbine-induced impacts on turbulence in large scale oceanographic models [25], added three terms to the $k - ε$ closure within ROMS to model turbine related turbulence generation, dissipation and turbulence length-scale interference. These three terms were later adapted accordingly to accommodate the theory around which the MY-2.5 turbulence closure is based and applied in FVCOM by Ref. [26].

In terms of interactions between surface waves and tidal turbines, current research focus has been mainly put on the impact of waves on the performance of turbines due to its immediate industry relevance [[27], [28], [29], [30], [31], [32], [33]]. However, there is a lack of emphasis on the effects of turbines on surface waves in both physical experimental studies and numerical modelling. Because tidal turbines are normally expected to be installed in relatively shallow coastal waters due to difficulties in device installation and operation that would occur otherwise [2], they are likely to have a close proximity to the free surface and hence interfere with the propagation of surface waves. Also, the altered three-dimensional flow structure due to the presence of tidal turbines could also have influence on surface waves through wave-current interaction mechanisms. Surface waves, particularly in shallow coastal areas, can influence sediment transport dynamics significantly. For instance, vertical mixing in the water column due to wave activities can keep sediment in suspension for longer, inhibiting sediment deposition in the downstream areas of the turbine [34]. Also, wave actions can increase bottom shear stress, leading to enhanced sediment resuspension and erosion [35]. Further, through wave-current interactions, waves can drive longshore currents, contributing to long-term shoreline evolution [36,37]. Therefore, changes in wave dynamics caused by tidal turbines are of high importance in terms of fully understanding impact of tidal turbines on local and regional geomorphology.

Due to the aforementioned interactions, the primary objectives of the work documented in this paper are to first explore the potential impacts of tidal turbines on surface waves with the help of high resolution CFD simulations, and second, to develop a Horizontal Axis Tidal Turbine (HATT) simulation system that could implement the impacts of tidal stream turbines on surface waves with a realistic spatial scale.

This paper details one high resolution CFD model for tidal turbine impact assessment on surface waves. Understandings obtained from the CFD modelling then advise turbine parameterization in large scale oceanographic models. The high resolution modelling is based on a CFD solver — ANSYS FLUENT. The implementation of effects of turbine operation on surface waves is an extension of the turbine simulation platform reported in Ref. [26], which parameterized tidal turbines in the current and turbulence closure modules of FVCOM. Impacts of tidal turbines on surface waves are considered in this new model by modification of wave energy flux across the device. A thorough validation study is also presented in which the turbine representation and operation in the CFD models is validated against laboratory data collected from an experiment conducted at the University of Hull using their ‘Total Environment Simulator Laboratory Flume’ [61,62] and the FVCOM model is verified utilizing the CFD simulated results.

The structure of the paper is provided as follows for clarity. Firstly in Section 2 ANSYS FLUENT and the FVCOM model are introduced. The integration of turbine simulation within these two frameworks is also discussed in this section. Next, Section 3 introduces the exploratory CFD models which aim to reveal the impacts of turbines on surface waves. A set of experimental data was used for CFD model validation in this section. Section 4 details the verification study for the turbine implementation in FVCOM which considers surface waves. Note that as the experimental data available was considered insufficient for comprehensive validation, verification in this section is based on data generated via the CFD modelling detailed in Section 3. In Section 5, the turbine simulation system developed based on FVCOM is applied to test cases in order to reveal impacts of a standalone turbine on its surroundings which incorporate wave-current interaction processes. A set of discussion is presented in Section 6, followed by concluding remarks given in Section 7 to summarise important results from Sections 4 Verification of the FVCOM model, 5 Application —standalone turbine tests, along with suggestions for potential future developments.

Section snippets

ANSYS FLUENT — a CFD solver

FLUENT solves the three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations which can be written in tensor form as follows: $\frac{\partial ρ}{\partial t} + \frac{\partial ρ {\bar{u}}_{i}}{\partial x_{i}} = 0$ $\frac{\partial (ρ \bar{u_{i}})}{\partial t} + \frac{\partial (ρ \bar{u_{i}} \bar{u_{j}})}{\partial x_{j}} = - \frac{\partial \bar{P}}{\partial x_{i}} + \frac{\partial}{\partial x_{j}} [μ (\frac{\partial {\bar{u}}_{i}}{\partial x_{j}} + \frac{\partial {\bar{u}}_{j}}{\partial x_{i}}) - \frac{2}{3} μ \frac{\partial u_{j}}{\partial x_{i}} δ_{i j}] + \frac{\partial}{\partial x_{j}} (- ρ \bar{u_{i}^{'} u_{j}^{'}}) + F_{i}$ where ρ is the water density; t is time; μ is the molecular viscosity; $δ_{i j}$ is the Kronecker delta and $F_{i}$ are external body forces in the i directions $(x, y, z)$ . ${\bar{u}}_{i}$ $(\bar{u}, \bar{v}, \bar{w})$ and $u_{i}^{'}$ $(u^{'}, v^{'}, w^{'})$ are the time-averaged (mean) and fluctuating water velocities in the $x_{i}$

The CFD model

A CFD model is built in this research to study the impacts of tidal turbines on surface waves. It is based on an experiment carried out at the University of Hull using their ‘Total Environment Simulator Laboratory Flume’ [61,62]. The flume is 11 m in length, 1.6 m wide and 0.8 m deep. The water depth was 0.6 m throughout the experiment. The flow rate at the inlet was 0.3 m/s. A surface wave propagating in the direction of the flow was imposed upon the inlet. The wave height and wave period were

Verification of the FVCOM model

This section explores the possibility of using the OBSTACLE mentioned above to represent the observed rotor-caused wave height drop. Hence, a FVCOM based model was set up according to the above-mentioned experimental conditions. The mesh of the model has a uniform spatial resolution of 0.2 m (i.e. 1D) throughout the computational domain. Vertically, the water column is evenly divided into 50 sigma layers to accommodate the turbine representation in the current and turbulence modules recorded in

Application —standalone turbine tests

This section investigates the effects of the inclusion of waves and activation of OBSTACLE upon the bottom shear stress based on a series of tests carried out using a prototype 15 m diameter turbine model as the test bed [26]. Water depth of these cases is 45 m and the turbine hub is located at a depth of 22.5 m. The flow and wave conditions are set to reflect those of the Anglesey coast, North Wales, UK, which is identified as one of the potential locations for tidal energy exploitation [53].

Choice of turbine simulation method in FLUENT

Apart from VBM, there are a number of other methods that are widely used to model tidal turbines in CFD simulations, such as the Actuator Disc Method (ADM) which provides a momentum sink in the rotor disk fluid zone without the BEM [57], and the Moving Reference Frame (MRF) method which explicitly simulate the structure and the rotational motion of the turbine [58]. Compared to the fully resolved MRF, VBM has two well-documented limitations: 1) the mechanical turbulence caused by the turbine

Conclusions

The impact of turbines on surface waves is investigated in this study in light of the importance of surface waves on local/regional geomorphology and also as a response to the lack of attention on turbine-induced wave dynamic alternation in the literature. A CFD simulation with a turbine (blockage ratio 3.3% and TSR 5.5) located 0.4 m from the free surface revealed a $\sim 3 %$ reduction in wave height as well as a slight increase in wave length. To simulate the wave height drop in FVCOM, the OBSTACLE

Acknowledgement

X. Li would like to acknowledge support from the China Scholarship Council and the University of Liverpool. Dr. Sufian also provided the settings for VBM in ANSYS FLUENT. The authors are grateful to Brendan Murphy for his help setting-up and running the experiments. The authors would also like to acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC) to grant EP/J010359/1 (Interactions of flow, tidal stream turbines and local sediment bed under combined waves

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Modelling impacts of tidal stream turbines on surface waves

Highlights

Abstract

Introduction

Section snippets

ANSYS FLUENT — a CFD solver

The CFD model

Verification of the FVCOM model

Application —standalone turbine tests

Choice of turbine simulation method in FLUENT

Conclusions

Acknowledgement

Renew. Energy

Ocean Eng.

Renew. Energy

Renew. Energy

Renew. Energy

Renew. Energy

Renew. Energy

Ocean Model.

Renew. Energy

Appl. Ocean Res.

Renew. Energy

Renew. Energy

Renew. Energy

Renew. Energy

Renew. Energy

Renew. Energy

Renew. Energy

Renew. Energy

Renew. Energy

Energy Procedia

Int. J. Mar. Energy

Renew. Energy

Int. J. Mar. Energy

Coast. Eng.

Comput. Geosci.

J. Mar. Syst.

Renew. Energy

Appl. Energy

Coast. Eng.

Experimental characterisation of flow effects on marine current turbine behaviour and on its wake properties

IET Renew. Power Gener.

The impact of tidal stream turbines on 3D flow and bed shear stress measured with particle image velocimetry in a laboratory flume