292 research outputs found
A BEMT model for a high solidity, hubless and ducted tidal stream turbine
5th Oxford Tidal Energy Workshop (OTE 2016), 21-22 March 2016, Oxford, UKA Blade Element Momentum Theory (BEMT) model for ‘conventional’ 3 bladed designs of Tidal Stream Turbine (TST) is presented, with validations from scale model experiments carried out in a cavitation tunnel. Assumptions and limitations of the model are discussed in order to gauge potential use in assessing a high solidity, hubless and ducted TST design, which has been developed by OpenHydro. A number of adjustments to the model are considered, which are to be validated with fully blade resolved CFD studies and field data from a full scale device deployed at Paimpol-Bréhat, Brittany at the start of 2016 in collaboration with EDF.The Industrial Doctoral Centre for Offshore Renewable Energy (IDCORE) is funded by the Energy Technology partnership and the RCUK Energy Programme (Grant number EP/J500847/1)
Can one predict DNA Transcription Start Sites by studying bubbles?
It has been speculated that bubble formation of several base-pairs due to
thermal fluctuations is indicatory for biological active sites. Recent
evidence, based on experiments and molecular dynamics (MD) simulations using
the Peyrard-Bishop-Dauxois model, seems to point in this direction. However,
sufficiently large bubbles appear only seldom which makes an accurate
calculation difficult even for minimal models. In this letter, we introduce a
new method that is orders of magnitude faster than MD. Using this method we
show that the present evidence is unsubstantiated.Comment: 4 pages, 3 figures, accepted for publication in physical review
letter
Anharmonic stacking in supercoiled DNA
Multistep denaturation in a short circular DNA molecule is analyzed by a
mesoscopic Hamiltonian model which accounts for the helicoidal geometry.
Computation of melting profiles by the path integral method suggests that
stacking anharmonicity stabilizes the double helix against thermal disruption
of the hydrogen bonds. Twisting is essential in the model to capture the
importance of nonlinear effects on the thermodynamical properties. In a ladder
model with zero twist, anharmonic stacking scarcely affects the thermodynamics.
Moderately untwisted helices, with respect to the equilibrium conformation,
show an energetic advantage against the overtwisted ones. Accordingly
moderately untwisted helices better sustain local fluctuational openings and
make more unlikely the thermally driven complete strand separation.Comment: In pres
Adapting Conventional Tools to Analyse Ducted and Open Centre Tidal Stream Turbines
This is the author accepted manuscript. The final version is available from EWTEC via the link in this record.This paper details a hydrodynamic model based on Blade Element Momentum Theory (BEMT) developed to assess ’conventional’ 3-bladed tidal stream turbines (TSTs), adapted here to analyse an ’unconventional’ case of a ducted and open centre device. Validations against a more detailed coupled Reynolds averaged computational fluid dynamics (RANS-BEM) model shows excellent agreement, of within 2% up to the peak power condition, with associated computational times in the order of a few minutes on a single core. The paper demonstrates the application of hydrodynamic forces into a structural analysis tool, in order to assess blade stress distributions of a generic hubless turbine. Incorporation of parameters such as non-uniform inflows and blade weight forces are investigated, with their effects on stress profiles presented. Key findings include: i) the adapted BEMT model replicates the majority of turbine performance characteristics estimated through previous CFD assessments; ii) the proposed model reduces the computational effort by several orders of magnitude compared to the reference coupled CFD, making it suitable for engineering assessments iii) blade stress distribution profiles are quantified, detailing concentration zones and cyclic values for use in fatigue analyses. This work forms part of a greater project aimed to develop a suite of analytical tools to perform engineering assessments of bi-directional ducted TSTsThis research is carried out as part of the Industrial Doctoral centre for Offshore Renewable Energy (IDCORE), funded by the Energy Technology partnership and the RCUK Energy programme (Grant number EP/J500847/1), in collaboration with EDF R&D
Dynamical Superfluid-Insulator Transition in a Chain of Weakly Coupled Bose-Einstein Condensates
We predict a dynammical classical superfluid-insulator transition (CSIT) in a
Bose-Einstein condensate (BEC) trapped in an optical and a magnetic potential.
In the tight-binding limit, this system realizes an array of weakly-coupled
condensates driven by an external harmonic field. For small displacements of
the parabolic trap about the equilibrium position, the BEC center of mass
oscillates with the relative phases of neighbouring condensates locked at the
same (oscillating) value. For large displacements, the BEC remains localized on
the side of the harmonic trap. This is caused by a randomization of the
relative phases, while the coherence of each individual condensate in the array
is preserved. The CSIT is attributed to a discrete modulational instability,
occurring when the BEC center of mass velocity is larger than a critical value,
proportional to the tunneling rate between adjacent sites.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let
Dynamic mooring simulation with Code_Aster with application to a floating wind turbine
This is the final version of the article. Available from Elsevier via the DOI in this record.The design of reliable station-keeping systems for permanent floating structures such as offshore renewable energy devices is vital to their lifelong integrity. In highly dynamic and/or deep-water applications, including hydrodynamics and structural dynamics in the mooring analysis is paramount for the accurate prediction of the loading on the lines and hence their dimensioning. This article presents a new workflow based on EDF R&D's open-source, finite-element analysis tool Code_Aster, enabling the dynamic analysis of catenary mooring systems, with application to a floating wind turbine concept. The University of Maine DeepCwind-OC4 basin test campaign is used for validation, showing that Code_Aster can satisfactorily predict the fairlead tensions in both regular and irregular waves. In the latter case, all of the three main spectral components of tension observed in the experiments are found numerically. Also, the dynamic line tension is systematically compared with that provided by the classic quasi-static approach, thereby confirming its limitations. Robust dynamic simulation of catenary moorings is shown to be possible using this generalist finite-element software, provided that the inputs be organised consistently with the physics of offshore hydromechanics.IDCORE is funded by the ETI and the RCUK Energy programme, grant number EP/J500847/1. The authors are grateful for the funding provided by these institutions, and to EDF R&D for hosting and supervising the industrial doctorate which expressed the present work
Ratcheting Heat Flux against a Thermal Bias
Merely rocking the temperature in one heat bath can direct a steady heat flux
from cold to hot against a non-zero thermal bias in stylized nonlinear lattice
junctions that are sandwiched between two heat baths. Likewise, for an average
zero-temperature difference between the two contacts a net, ratchet-like heat
flux emerges. Computer simulations show that this very heat flux can be
controlled and reversed by suitably tailoring the frequency ( 100
MHz) of the alternating temperature field.Comment: 5 pages, 6 figure
Lengthscales and Cooperativity in DNA Bubble Formation
It appears that thermally activated DNA bubbles of different sizes play
central roles in important genetic processes. Here we show that the probability
for the formation of such bubbles is regulated by the number of soft AT pairs
in specific regions with lengths which at physiological temperatures are of the
order of (but not equal to) the size of the bubble. The analysis is based on
the Peyrard- Bishop-Dauxois model, whose equilibrium statistical properties
have been accurately calculated here with a transfer integral approach
Nonlinearity-induced conformational instability and dynamics of biopolymers
We propose a simple phenomenological model for describing the conformational
dynamics of biopolymers via the nonlinearity-induced buckling and collapse
(i.e. coiling up) instabilities. Taking into account the coupling between the
internal and mechanical degrees of freedom of a semiflexible biopolymer chain,
we show that self-trapped internal excitations (such as amide-I vibrations in
proteins, base-pair vibrations in DNA, or polarons in proteins) may produce the
buckling and collapse instabilities of an initially straight chain. These
instabilities remain latent in a straight infinitely long chain, because the
bending of such a chain would require an infinite energy. However, they
manifest themselves as soon as we consider more realistic cases and take into
account a finite length of the chain. In this case the nonlinear localized
modes may act as drivers giving impetus to the conformational dynamics of
biopolymers. The buckling instability is responsible, in particular, for the
large-amplitude localized bending waves which accompany the nonlinear modes
propagating along the chain. In the case of the collapse instability, the chain
folds into a compact three-dimensional coil. The viscous damping of the aqueous
environment only slows down the folding of the chain, but does not stop it even
for a large damping. We find that these effects are only weakly affected by the
peculiarities of the interaction potentials, and thus they should be generic
for different models of semiflexible chains carrying nonlinear localized
excitations.Comment: 4 pages (RevTeX) with 5 figures (EPS
Master equation approach to friction at the mesoscale
At the mesoscale friction occurs through the breaking and formation of local
contacts. This is often described by the earthquake-like model which requires
numerical studies. We show that this phenomenon can also be described by a
master equation, which can be solved analytically in some cases and provides an
efficient numerical solution for more general cases. We examine the effect of
temperature and aging of the contacts and discuss the statistical properties of
the contacts for different situations of friction and their implications,
particularly regarding the existence of stick-slip.Comment: To be published in Physical Review
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