ORCID Profile
0000-0001-7556-1193
Current Organisation
University of Oxford
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Publisher: American Geophysical Union (AGU)
Date: 04-2019
DOI: 10.1029/2018JC014471
Publisher: Cambridge University Press (CUP)
Date: 30-09-2021
DOI: 10.1017/JFM.2021.766
Abstract: The limit of power extraction by a device which makes use of constructive interference, i.e. local blockage, is investigated theoretically. The device is modelled using actuator disc theory in which we allow the device to be split into arrays and these then into sub-arrays an arbitrary number of times so as to construct an $n$ -level multi-scale device in which the original device undergoes $n-1$ sub- isions. The alternative physical interpretation of the problem is a planar system of arrayed turbines in which groups of turbines are homogeneously arrayed at the smallest $n\\mathrm {th}$ scale, and then these groups are homogeneously spaced relative to each other at the next smallest $n-1\\mathrm {th}$ scale, with this pattern repeating at all subsequent larger scales. The scale-separation idea of Nishino & Willden ( J. Fluid. Mech. , vol. 708, 2012 b , pp. 596–606) is employed, which assumes mixing within a sub-array occurs faster than mixing of the by-pass flow around that sub-array, so that in the $n$ -scale device mixing occurs from the inner scale to the outermost scale in that order. We investigate the behaviour of an arbitrary level multi-scale device, and determine the arrangement of actuator discs ( $n\\mathrm {th}$ level devices) which maximises the power coefficient (ratio of power extracted to undisturbed kinetic energy flux through the net disc frontal area). We find that this optimal arrangement is close to fractal, and fractal arrangements give similar results. With the device placed in an infinitely wide channel, i.e. zero global blockage, we find that the optimum power coefficient tends to unity as the number of device scales tends to infinity, a 27/16 increase over the Lanchester–Betz limit of $0.593$ . For devices in finite width channels, i.e. non-zero global blockage, similar observations can be made with further uplift in the maximum power coefficient. We discuss the fluid mechanics of this energy extraction process and examine the scale distribution of thrust and wake velocity coefficients. Numerical demonstration of performance uplift due to multi-scale dynamics is also provided. We demonstrate that bypass flow remixing and ensuing energy losses increase the device power coefficient above the limits for single devices, so that although the power coefficient can be made to increase, this is at the expense of the overall efficiency of energy extraction which decreases as wake-scale remixing losses necessarily rise. For multi-scale devices in finite overall blockage two effects act to increase extractable power an overall streamwise pressure gradient associated with finite blockage, and wake pressure recoveries associated with bypass-scale remixing.
Publisher: The Royal Society
Date: 12-2015
Abstract: This paper investigates the size and structure of large waves on the open ocean. We investigate how nonlinear physics modifies waves relative to those predicted by a linear model. We run linear random simulations and extract extreme waves and the surrounding sea-state. For each extreme event, we propagate the waves back in time under linear evolution before propagating the wave-field forward using a nonlinear model. The differences between large linear and nonlinear wave-groups are then examined. The general trends are that under nonlinear evolution, relative to linear evolution, there is, on average, little or no extra litude in the nonlinear simulations that there is an increase in the width of the crest of the wave-group and a contraction of large wave-groups in the mean wave direction that large waves tend to move to the front of a wave-packet meaning that the locally largest wave is relatively bigger than the wave preceding it and that nonlinearity can increase the duration of extreme wave events. In all these trends, there is considerable scatter, although the effects observed are clear. Our simulations show that nonlinearity does play an important part in the formation of extreme waves on deep water.
Publisher: The Royal Society
Date: 2020
DOI: 10.1098/RSOS.191127
Abstract: Uncertainty affects estimates of the power potential of tidal currents, resulting in large ranges in values reported for sites such as the Pentland Firth, UK. Kreitmair et al. (2019, R. Soc. open sci. 6 , 180941. ( doi:10.1098/rsos.191127 )) have examined the effect of uncertainty in bottom friction on tidal power estimates by considering idealized theoretical models. The present paper considers the role of bottom friction uncertainty in a realistic numerical model of the Pentland Firth spanned by different fence configurations. We find that uncertainty in removable power estimates resulting from bed roughness uncertainty depends on the case considered, with relative uncertainty between 2% (for a fully spanned channel with small values of mean roughness and input uncertainty) and 44% (for an asymmetrically confined channel with high values of bed roughness and input uncertainty). Relative uncertainty in power estimates is generally smaller than (input) relative uncertainty in bottom friction by a factor of between 0.2 and 0.7, except for low turbine deployments and very high mean values of friction. This paper makes a start at quantifying uncertainty in tidal stream power estimates, and motivates further work for proper characterization of the resource, accounting for uncertainty inherent in resource modelling.
Publisher: Elsevier BV
Date: 06-2014
Publisher: American Society of Mechanical Engineers
Date: 08-06-2014
Abstract: There are various candidate sites for tidal stream energy extraction in the English Channel. In this paper we examine the tidal stream resource at Portland Bill and the south coast of the Isle of Wight. A depth-averaged numerical model is developed and compared to field measurements. The presence of rows of tidal turbines is simulated using a line-discontinuity to represent the head loss across the turbines. The head loss is given by linear momentum actuator disc theory. At each site the length of the turbine rows, the local blockage ratio, and the location of the turbines are varied. For Portland Bill the presence of an array with multiple rows of turbines is also considered. We find that it is likely that (based purely on the hydrodynamics) power could viably be extracted at each site, with the mean power produced by each site being in the order of 10s MW.
Publisher: Cambridge University Press (CUP)
Date: 28-04-2016
DOI: 10.1017/JFM.2016.247
Abstract: Although wind and tidal turbines operate in turbulent shear flow, most theoretical results concerning turbine performance, such as the well-known Betz limit, assume the upstream velocity profile is uniform. To improve on these existing results we extend the classical actuator disc model in this paper to investigate the performance of an ideal turbine in steady, inviscid shear flow. The model is developed on the assumption that there is negligible lateral interaction in the flow passing through the disc and that the actuator applies a uniform resistance across its area. With these assumptions, solution of the model leads to two key results. First, for laterally unbounded shear flow, it is shown that the normalised power extracted is the same as that for an ideal turbine in uniform flow, if the average of the cube of the upstream velocity of the fluid passing through the turbine is used in the normalisation. Second, for a laterally bounded shear flow, it is shown that the same normalisation can be applied, but allowance must also be made for the fact that non-uniform flow bypassing the turbine alters the background pressure gradient and, in turn, the turbines ‘effective blockage’ (so that it may be greater or less than the geometric blockage, defined as the ratio of turbine disc area to cross-sectional area of the flow). Predictions based on the extended model agree well with numerical simulations approximating the incompressible Euler equations. The model may be used to improve interpretation of model-scale results for wind and tidal turbines in tunnels/flumes, to investigate the variation in force across a turbine and to update existing theoretical models of arrays of tidal turbines.
Publisher: Elsevier BV
Date: 12-2013
Publisher: The Royal Society
Date: 08-01-2014
Abstract: Several locations in the Pentland Firth, UK, have been earmarked for the deployment of separate farms of tidal turbines. However, recent numerical modelling suggests that these farms will be inter-dependent and that they must work together to optimize their collective performance. To explain this inter-dependence, in this paper we develop an electrical circuit analogy to describe flow through the Pentland Firth, in which parallel connections in the circuit represent different sub-channels formed by the islands of Swona, Stroma and the Pentland Skerries. The analogy is introduced in stages, beginning with turbines placed in a single channel, then turbines placed in a sub-channel connected in parallel to another sub-channel, and finally more complicated arrangements, in which turbines are installed both in parallel and in series within a multiply connected channel. The analogy leads to a general formula to predict the tidal power potential of turbines placed in a sub-channel connected in parallel to another sub-channel, and a predictive model for more complicated multiply connected channel arrangements. Power estimates made using the formula and predictive model (which may be applied using only measurements of the undisturbed natural tidal hydrodynamics) are shown to agree well with numerical model predictions for the Pentland Firth, providing useful insight into how to best develop the resource.
Publisher: SAGE Publications
Date: 13-02-2015
Abstract: Extracting power from the tide is a potential avenue for renewable energy production but is also a significant engineering challenge. This challenge has many different aspects but the basic problem is the hydrodynamic problem of converting the movement of the ocean into mechanical power. This paper presents a review of some of the hydrodynamic modelling techniques which can be used to model tidal barrages and tidal turbines. The analysis of these is broken down into different length scales, ranging from a single device, to an array of devices, and up to regional scales. As well as discussing modelling techniques some of the hydrodynamic problems, such as resource assessment and efficiency of power generation, are discussed.
Publisher: AIP Publishing
Date: 10-2016
DOI: 10.1063/1.4963777
Abstract: In the open ocean, the formation of large waves is non-linear. This non-linearity modifies the shape and structure of steep waves relative to that expected in a linear model. Work by Adcock et al. [“Nonlinear dynamics of wave-groups in random seas: Unexpected walls of water in the open ocean,” Proc. R. Soc. A 471(2184), 20150660 (2015)] used a numerical model to show that, for a wave spectrum representative of that of a storm in the North Sea, non-linear dynamics resulted in the following changes to the shape of extreme wave groups relative to a linear model: expansion of the wave-crest laterally, contraction of the wave-group in the mean wave direction, and a movement of the largest wave to the front of the wave-group. They found only moderate elevation above that expected by the linear model. This paper extends this work to explore the influence of spectral bandwidth and directional spreading on these results. We find that sea-states with low directional spreads are more likely to see significant non-linear changes to extreme wave-groups. However, rather surprisingly, extreme waves formed in seas with broader spectra generally, although not always, see greater non-linear changes for the range of spectra studied here. We also observe that the lateral expansion to the wave-group occurs even for relatively modest sea-states and is a predictable feature. By contrast, the contraction of the wave-group in the mean wave direction only occurs occasionally in the steepest sea-states—although when this is triggered the change can be very dramatic.
Publisher: SAGE Publications
Date: 24-07-2014
Abstract: The Pentland Firth, Scotland, is one of the World’s prime locations for the eventual installation of large farms of tidal stream turbines. This paper seeks to improve the upper bound estimate of available power output obtained by Adcock et al . (2013) who used a depth-integrated numerical model of the region containing the Pentland Firth with the outer boundary forced solely by M 2 and S 2 tidal constituents. Herein, the analysis is extended to include six additional tidal constituents and the model run for 11.5 years, more than half of the 18.6-year lunar nodal cycle, to allow variations over this to be analysed. The consequent increase in available power is estimated, and the variation in power output over an 11-year period is examined. Although further power could theoretically be extracted from the additional six tidal constituents, this would require the tidal turbine farm to have such a low capacity factor that it would probably be economically unfeasible.
Publisher: Society for Underwater Technology
Date: 11-2015
DOI: 10.3723/UT.33.075
Publisher: Elsevier BV
Date: 03-2014
Publisher: Elsevier BV
Date: 03-2014
Publisher: The Royal Society
Date: 08-09-2013
Abstract: This paper assesses an upper bound for the tidal stream power resource of the Pentland Firth. A depth-averaged numerical model of the tidal dynamics in the region is set-up and validated against field measurements. Actuator disc theory is used to model the effect of turbines on the flow, and to estimate the power available for generation after accounting for losses owing to mixing downstream of the turbines. It is found that three rows of turbines extending across the entire width of the Pentland Firth and blocking a large fraction of the channel can theoretically generate 1.9 GW, averaged over the spring–neap cycle. However, generation of significantly more power than this is unlikely to be feasible as the available power per additional swept area of turbine is too small to be viable. Our results differ from those obtained using simplified tidal channel models of the type used commonly in the literature. We also use our numerical model to investigate the available power from rows of turbines placed across various subchannels within the Pentland Firth, together with practical considerations such as the variation in power over the spring–neap tidal cycle and the changes to natural tidal flows which result from power extraction.
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Thomas Adcock.