24 research outputs found
Implications of realistic non-steady environments for tidal turbine performance evaluation and mechanical load characterisation
Full text linkDecarbonising human life is core to sustainability, and many facets of our existence
will need to be carbon-neutral in the future. The energy sector, specifically
electricity generation, is at the forefront of this green transition globally. One of
the challenges in this context is a shift from on-demand generation provided by
burning hydrocarbons to the often intermittent nature of harnessing renewable
resources. This is especially significant when addressing the need for consistent,
reliable base loads. One answer to this challenge, and the focus of this thesis,
comes in the form of tidal energy.
Tidal stream turbine technology has advanced over the past few decades but is
still in its relative infancy. At this early stage in tidal turbine development, there
is a clear lack of in-service learning to draw from, so numerical and experimental
research is driving developments. Advancements in tidal turbine technology rely
on a better understanding of the interaction between turbine rotors and the environment.
The non-steady nature of tidal flows adds complexity to understanding
rotor loading. The inclusion of strongly sheared currents, turbulence and surface
waves all contribute to a unique operating environment.
The primary focus of this thesis is the mechanical performance of tidal turbine
rotors. Initially, research is undertaken to evaluate current methods for performance
benchmarking by assessing the classical performance indicators CP and
CT, or more specifically, scrutinising the normalising velocities used in constructing
these indicators. Two tank test data sets are analysed, one from IFREMER
and another from FloWave at the University of Edinburgh. On analysing onset
flow datasets from FloWave, it is shown that CP and CT can be biased by as
much as 22.44% and 15.58% respectively. The analysis of calibration data at the
IFREMER flume reveals that the method used to characterise a rotor plane area
(in the absence of a turbine) can introduce up to 12.04% change in CP and 7.88%
change in CT. These analyses highlight a significant challenge to benchmarking
rotors against each other. This leads to the recommendation at the FloWave
facility that performance benchmarking should be done using calibration flow
measurements rather than synchronous flow measurements. Additionally, these
calibration measurements should account for spatial variations in terms of transverse
shear in addition to the already specified vertical shear in IECTS62600-
200.
Next, considering dynamic mechanical performance, the effects of realistic,
complex ocean environments on rotor performance are studied. A novel numerical
model is developed on the OpenFOAM framework, which includes a twophase
fluid solver (water and air) that simulates surface waves, shear currents
and turbulence. An actuator line model is embedded within this to solve the
rotor loading. The model is scaled to a 1.5 MW machine, and by varying wave
configuration as well as rotor position in the water column, sensitivities in hub
loading are investigated in terms of torque and thrust, but also with regard to the
out-of-plane hub moment, which is hypothesised to be important when considering
turbine damage. A new metric, the maximum load span (MLS), is developed
in this thesis to describe the form of the out-of-plane hub moment. The MLS
was found to increase with both hub height and wave height. The MLS also
shows dependence on wave period, which changed the cyclic form of the out-ofplane
moment patterns, leading to what has been referred to as “wave-driven
moment-type dominance”. Longer wave periods resulted in moment loading
that was dominated by a yaw response, while shorter period waves resulted in a
predominantly pitching moment response.
The dynamic mechanical rotor loading characteristics uncovered in the numerical
simulations are verified experimentally. An experimental campaign was
undertaken as part of this research to further highlight the rotor loading in response
to surface waves. Complete validation of the out-of-plane load forms
was unachievable experimentally due to the challenge of maintaining a tightly
controlled constant speed of the University of Edinburgh turbine. However, applied
to tidal turbine rotors for the first time, a “load-loop” analysis approach
is used and verifies the changing form of the out-of-plane hub moment under
wave loading. The experimental campaign highlights a wave-driven moment-type
dominance dependent on both wave height and period, in slight contrast to the
numerical work, which only identifies wave period as the driving variable
Hydrodynamics of ebb-tidal jet and circulations within a tidal inlet and embayed beach system
No full textTidal inlets are integral components of many coastal systems worldwide, facilitating water, sediment, and ecological exchange between estuaries and the coastal ocean. Under specific geomorphological and hydrodynamic conditions, outflows of tidal inlets can form jet-like structures, known as ebb-tidal jets, that extend offshore and play a significant role in shaping coastal morphology and the circulation within and around the inlet system. Despite their importance, the structure, dynamics, and broader implications of ebb-tidal jets remain poorly understood globally.
At Montrose Bay in eastern Scotland, UK, a tidal inlet at the southern end is reported to generate a jet-like outflow in response to the flushing action during the ebb tide. This outflow is an example of an ebb-tidal jet. Montrose’s ebb-tidal jet develops within the proximity of beaches that have experienced a rapid shoreline recession, approximately 2 m/year. However, the formation and significance of Montrose’s ebb-tidal jet are still poorly understood, an issue that extends to similar coastal systems globally.
This thesis examines the hydrodynamics of ebb-tidal jets and their influence on the circulation within their coastal system, using Montrose Bay as a primary case study. A series of numerical modelling investigations were conducted using the Delft3D Flexible Mesh in both depth-averaged and three-dimensional modes. The study examined the structure, dynamics, and variability of the ebb-tidal jet under different forcing conditions, progressing from tidal-only simulations to wave-current interactions and density-driven flows. Insights from Montrose were then extended to three other estuarine systems in the UK through an idealised modelling framework.
At Montrose Bay, simulations validated against an ADCP measurement (RMSE 0.885) and satellite imagery revealed a highly asymmetrical ebb-tidal jet resulting from the interplay between the outflow jet, shoreline features, and ambient tidal current. Under tidal forcing alone, the jet exited the inlet at speeds of up to 2.2 m/s, extended over 2 km north-eastward offshore, and was accompanied by a counter-clockwise eddy with a flow speed of 0.5 m/s and a diameter larger than 1 km. This eddy formed a predominantly southward flow region for around 80% of the tidal cycle. During high-wave conditions, the jet’s interaction with wave-induced longshore currents formed a conduit mechanism for sediment export. Wave directions from the north-east impact a seaward push to the ebb-tidal jet, thereby promoting offshore transport. While the south-east waves induced a northward longshore current at the tip of the sheltering headland and deflected the ebb-tidal jet landward, then facilitated a northward sediment bypassing. Three-dimensional modelling validated with satellite images further identified that the ebb-tidal jet formed a buoyant surface plume of 0.5-1.0 m thickness capable of travelling up to 7 km offshore, driven by a combination of jet-induced launching, lateral spreading by the eddy, and advection and diffusion by the ambient current. Under a high discharge event, the plume could spread up to 17 km away from the inlet.
These findings were extended to three other estuarine systems using an idealised model. The results provided a first-order demonstration of the ebb-tidal jet structures, dynamics, and variability at those systems, that are in a good agreement against the turbid outflow observed in satellite imagery. Across all systems, high wave events activated similar sediment export conduit mechanisms that potentially transported sediment away from inlet-adjacent beaches. Their plumes behave similarly to those previously observed at Montrose.
This study offers the first comprehensive visualisation of the ebb-tidal jet at Montrose Bay and three other estuarine systems across the UK, demonstrating their temporal and spatial variability under different forcing conditions. The findings offer a new conceptual understanding of the sediment export mechanism induced by jet-wave interactions. It also demonstrates that the small-scale plume produced in such a system could travel a longer distance than earlier studies suggested. These insights are relevant for coastal management, as similar systems across the world are facing similar challenges, including simultaneous beach erosion, inlet sedimentation, and the development of a more efficient dredging-deposition strategy.
In conclusion, this thesis makes a significant contribution to the understanding of ebb-tidal jet hydrodynamics and their significance, with findings applicable not only to Montrose Bay but also to other similar systems across the UK and the world
Flow-induced vibration of an underwater lazy wave cable in unidirectional current
Full text linkThis paper describes measurements of the flow-induced vibration of an instrumented model cable in a lazy wave configuration immersed in unidirectional currents in the 2 m deep FloWave Fa- cility at the University of Edinburgh. The cable model, designed to represent a dynamic power cable used in offshore renewable energy structures for electricity transmission, has an external diameter (D) of 31 mm and a mass ratio of 1.22. The current speed was varied from 0.1 to 0.9 m/s and its direction was set at 0, 90, and 180 degrees relative to the initial longitudinal axis of the cable. An underwater Qualisys motion capture system measured the in-line (IL) and cross-flow (CF) displacement components at 36 locations along the length of the submerged cable. Local displacements, response frequencies, and travelling wave modes are determined for reduced velocity Ur ε (5.29, 47.69), and Reynolds number Re ε (103, 104). It is found that the root mean square (RMS) values of the displacement components exhibited an increasing trend with reduced velocity reaching 0.40D in the in-line direction and 0.45D in the cross-flow direction. For reduced velocity in the range from 5.29 to 10.58, the cable exhibited single frequency vibrations. For Ur > 10.58, the cable experienced broad-banded, multi-frequency responses. Along the cable, certain locations were found to execute distinct circular, elliptical, nearly linear, and figure-of-eight orbits at low Ur. A sudden phase shift was observed along the cable length, related to unsteady vortex-induced vibration (VIV), which effectively prevented lock-in occurring at high Ur
Application of high-order hydrodynamic models to floating offshore wind TLP: numerical and experimental analysis
No full textWith the large-scale development in the last decades of fixed offshore wind across Europe and the ever-more-present threat of climate change dominating national and global agenda, the exploitation of wind power in deep-water using floating wind turbines is gathering a significant amount of interest. Compared to other types of floating wind platforms, Tension-leg-platforms (TLPs) are less compliant systems resisting dynamic forces through their pre-tensioned tendons. Whilst this reduces the weight of the platform hull, understanding extreme loading cycles in the mooring system becomes an important design issue.
Recent research has revealed the importance of considering sum-frequency second-order and third-order loads to capture extreme events, such as slack-line events and ringing events. These events occur when high-frequency wave loads excite the resonant vertical modes of large and stiff floating systems. For the offshore wind industry, it is essential to ascertain whether such events can be stochastically predicted across numerous random sea-states, addressing both ultimate limit state and fatigue design scenarios.
This thesis presents a comprehensive review of existing numerical methods available to engineers for calculating second and third-order forces in aero-hydro-servo-elastic time domain solvers, commonly used in the offshore wind industry for assessing multiple design load cases. These models encompass potential (semi-analytical and BEM) and strip-theory approaches (Rainey and FNV). Subsequently, these approaches are applied to a complete academic floating offshore wind TLP platform, considering both fixed and fully dynamic conditions. The nonlinear hydrodynamic loading on the platform in the fixed condition is compared against high-fidelity simulations obtained using a Navier-Stokes CFD numerical wave tank. Furthermore, an experimental campaign is designed to investigate the application of these numerical models to dynamic conditions, encompassing wave and coupled wind-wave excitation.
Both the CFD and experimental results indicate that, while previous literature has primarily focused on inertial loads, viscous effects, particularly vertical drag, exert a more significant influence on the third-order response of the studied TLP system. This finding emphasises the necessity for further research into modelling viscous drag on complex floating structures. Additionally, the experimental campaign underscores the importance of characterizing both structural and viscous damping as significant parameters that strongly affect the resonant response of the floating offshore wind TLP system.
Finally, a time-frequency analysis of the response is undertaken which serves to identify the ringing events. This analysis shows that whilst numerical models fail to capture ringing events adequately, they can still predict their occurrence, albeit with lower amplitudes. This thesis thus presents a comparative review of the numerical wave loading approach available for the stochastic treatment of ringing and slack-line events in floating wind TLP systems, providing valuable insights for the industry
Characterisation of Turbulence at Sites with Coexisting Waves and Currents: An Analysis by Empirical Mode Decomposition
Full text linkThis paper presents a novel side information assisted Empirical Mode Decomposition (EMD) method for separating wave orbital velocity from a combined wave-tidal current velocity data, measured by three different Acoustic Doppler Current Profilers (ADCPs) deployed in the Pentland Firth, Orkney Waters, UK. The effectiveness of this technique is confirmed through two methods: (1) the spectra of the wave-removed velocity align with the Kolmogorov -5/3 power law, and (2) the extracted wave orbital velocities were used to derive significant wave heights, which were validated against wave heights measured by the same ADCPs and through numerical modelling carried out with a coupled wave-current tool (TOMAWAC-TELEMAC 3D). The decomposed data was further used to calculate three-dimensional turbulence intensities (TI) for combined wave-current, wave-only, and current-only conditions, across different flow speeds and wave heights. The results demonstrate the robustness of the decomposition method in various scenarios and quantify the individual TI contributions from waves, currents, and their combined effects. This work provides valuable insights into the dynamics of areas where waves and currents coexist
Wake Field Interaction in 3D Tidal Turbine Arrays: Numerical Analysis for the Pentland Firth
Full text linkThis study presents a methodology for applying the three-dimensional actuator disk-RANS technique in modeling tidal energy converters within a regional-scale simulation. Of particular interest are the robustness of the applied momentum source term and its effectiveness in modeling an array of full-sized tidal turbines under realistic hydrodynamic and operational conditions. The Inner Sound region, which is the site of commercial-scale deployment projects of the Pentland Firth in Scotland, is chosen as the study area. While the actuator disk approach had been used in past studies to parameterize the far-wake region of horizontal-axis tidal turbines, details of its three-dimensional implementation have not been thoroughly discussed. Criteria adopted in deciding the array location are presented in this study, along with the actuator disks’ detailed setup and constraints. The models are subjected to one operational characteristic that is similar to commercial devices in service to examine the accuracy of the imposed source term under complex flow conditions. The results demonstrate that the thickness of the disk imposed in the source term has a pronounced influence on the model outputs. In addition to accurately modeling flow propagation and wake interactions, the models are also able to replicate the observed asymmetrical tidal currents in the region. Because there is currently limited published material on the detailed application of the actuator disk approach in ocean-scale models, this study is hoped to fill the research gap and provide evidence, guidance, and examples of best practices for future studies
Influence of Ebb-Tidal Jet on Wave-Current Interaction at a Low Embayment Beach: A Numerical Modelling Approach
Full text linkThe coastline of Montrose Bay, in Scotland, UK is a 9 km sandy beach bounded by two rocky headlands that form a low embayment. The bay’s southern section, which has seen continuous erosion and sedimentation, is influenced by the combination of a highly asymmetrical ebb-tidal jet, alongshore tidal current, and oblique waves. This paper investigates the significance of the ebb-tidal jet in wave-current interaction at Montrose Bay using numerical modelling with Delft3D Flexible Mesh. The baseline model produces a good validation when compared against an ADCP measurement at the bay during July and December 2015. Idealized forcing scenarios reveal a complex interplay between the ebb-tidal jet and waves. The ebb-tidal jet is deflected seaward or landward by the wave-induced longshore currents, while simultaneously inducing wave refraction, resulting in a reduction in wave height, and a shifting in wave direction. The rip current and the bed shear stress at various conditions are further discussed. When high waves approach from the North-East, the combined effects of the ebb-tidal jet and deflection rip currents may create pathways for sediment transport to deeper, less dynamic environments. This study offers insights into the importance of ebb-tidal jets in bar-built tidal inlet – embayed beach systems and emphasizes their potential implications on erosion, sedimentation, and dredging-deposition activities
Thetis-SWAN: A Python-interfaced wave-current interactions coupled system
Full text linkWave–Current Interactions (WCI) emerge in nearshore coastal areas, prompting the development of coupled modelling systems to simulate these phenomena. We present a new multi-scale parallelised Python-interfaced WCI coupled system adopting a component-based approach enabling model-component integration without inhibiting their respective development. The underlying principles emphasise model equitability, flexibility and language interoperability. The hybrid model comprises the spectral wave model SWAN and the 2-D shallow-water equation model, Thetis. The coupling is performed through the Basic Model Interface. The coupled WCI model is the first to employ a Python interface, while maintaining the efficiency of different lower-level compiled programming languages, Fortran for SWAN and C for Thetis. We discuss the system implementation, architecture, and underlying physics considered. The coastal waters of Duck, NC, serve as a practical demonstration in simulating WCI. We then elaborate on the rationale for the coupled system design to inform the development of coupled modelling frameworks for environmental systems.</p
Wave-Induced Breaking and Wave Set-up Representation by Coupling Spectral Wave and Coastal Hydrodynamics Models
Full text linkA parallelised coupled two-dimensional model is developed to capture wave-current interactions at regional scales. The framework comprises of a spectral wave model, Simulating WAves Nearshore (SWAN), and a coastal hydrodynamics shallow-water equation model, Thetis. The two models are coupled through the Basic Model Interface (BMI) structure. They run iteratively and exchange information at prescribed time-intervals. SWAN provides the necessary parameters for the calculation of radiation stress, introduced in Thetis, upon solving the action density equation in a manner encompassing source terms accounting for deep- and shallow- water phenomena. In turn, Thetis returns water elevation and current velocity fields by considering the 2-D depth-averaged formulation of the shallow water equations. The coupled model's capability to account for depth- induced breaking, wave set-up and bed friction is tested using the physical modelling experiment of Boers (1997) on the behaviour of waves acting on a barred beach. The model’s results exhibit good correlation with experimental data, which consists of derived wave characteristics and water elevation measurements. Additionally, the model’s performance is compared against other coupled models with 3-D ocean model. The proposed model’s results showcase the same level of accuracy as other coupled models and could be extensible to 3-D modelling applications and complex geometrie
Missing health data pattern matching technique for continuous remote patient monitoring
Full text linkRemote patient monitoring (RPM) has been gaining popularity recently. However, health data acquisition is a significant challenge associated with patient monitoring. In continuous RPM, health data acquisition may miss health data during transmission. Missing data compromises the quality and reliability of patient risk assessment. Several studies suggested techniques for analyzing missing data; however, many are unsuitable for RPM. These techniques neglect the variability of missing data and provide biased results with imputation. Therefore, a holistic approach must consider the correlation and variability of the various vitals and avoid biased imputation. This paper proposes a coherent computation pattern-matching technique to identify and predict missing data patterns. The performance of the proposed approach is evaluated using data collected from a field trial. Results show that the technique can effectively identify and predict missing patterns. © 2023, The Author(s)
