4,314 research outputs found
Energy potential of a tidal fence deployed near a coastal headland
Enhanced tidal streams close to coastal headlands appear to present ideal locations for the deployment of tidal energy devices. In this paper, the power potential of tidal streams near an idealized coastal headland with a sloping seabed is investigated using a near-field approximation to represent a tidal fence, i.e. a row of tidal devices, in a two-dimensional depth-averaged numerical model. Simulations indicate that the power extracted by the tidal fence is limited because the flow will bypass the fence, predominantly on the ocean side, as the thrust applied by the devices increases. For the dynamic conditions, fence placements and headland aspect ratios considered, the maximum power extracted at the fence is not related in any obvious way to the local undisturbed kinetic flux or the natural rate of energy dissipation due to bed friction (although both of these have been used in the past to predict the amount of power that may be extracted). The available power (equal to the extracted power net of vertical mixing losses in the immediate wake of devices) is optimized for devices with large area and small centre-to-centre spacing within the fence. The influence of energy extraction on the natural flow field is assessed relative to changes in the M2 component of elevation and velocity, and residual bed shear stress and tidal dispersion
Modelling tidal energy extraction in a depth-averaged coastal domain
An extension of actuator disc theory is used to describe the properties of a tidal energy device, or row of tidal energy devices, within a depth-averaged numerical model. This approach allows a direct link to be made between an actual tidal device and its equivalent momentum sink in a depth-averaged domain. Extended actuator disc theory also leads to a measure of efficiency for an energy device in a tidal stream of finite Froude number, where efficiency is defined as the ratio of power extracted by one or more tidal devices to the total power removed from the tidal stream. To demonstrate the use of actuator disc theory in a depth-averaged model, tidal flow in a simple channel is approximated using the shallow water equations and the results are compared with the published analytical solutions. © 2010 © The Institution of Engineering and Technology
Decuplet Baryon Structure from Lattice QCD
The electromagnetic properties of the SU(3)-flavor baryon decuplet are
examined within a lattice simulation of quenched QCD. Electric charge radii,
magnetic moments, and magnetic radii are extracted from the E0 and M1 form
factors. Preliminary results for the E2 and M3 moments are presented giving the
first model independent insight to the shape of the quark distribution in the
baryon ground state. As in our octet baryon analysis, the lattice results give
evidence of spin-dependent forces and mass effects in the electromagnetic
properties. The quark charge distribution radii indicate these effects act in
opposing directions. Some baryon dependence of the effective quark magnetic
moments is seen. However, this dependence in decuplet baryons is more subtle
than that for octet baryons. Of particular interest are the lattice predictions
for the magnetic moments of and for which new recent
experimental measurements are available. The lattice prediction of the
ratio appears larger than the experimental ratio, while the
lattice prediction for the magnetic moment ratio is in good
agreement with the experimental ratio.Comment: RevTeX manuscript, 34 pages plus 21 figures (available upon request
Glueball matrix elements on anisotropic lattices
We describe a lattice calculation of the matrix elements relevant for
glueball production in radiative decays. The techniques for such a
calculation on anisotropic lattices with an improved action are outlined. We
present preliminary results showing the efficacy of the computational method.Comment: 3 pages (LaTeX), 3 figures (PostScript), Presented at Lattice '9
Simulating chemistry efficiently on fault-tolerant quantum computers
Quantum computers can in principle simulate quantum physics exponentially
faster than their classical counterparts, but some technical hurdles remain.
Here we consider methods to make proposed chemical simulation algorithms
computationally fast on fault-tolerant quantum computers in the circuit model.
Fault tolerance constrains the choice of available gates, so that arbitrary
gates required for a simulation algorithm must be constructed from sequences of
fundamental operations. We examine techniques for constructing arbitrary gates
which perform substantially faster than circuits based on the conventional
Solovay-Kitaev algorithm [C.M. Dawson and M.A. Nielsen, \emph{Quantum Inf.
Comput.}, \textbf{6}:81, 2006]. For a given approximation error ,
arbitrary single-qubit gates can be produced fault-tolerantly and using a
limited set of gates in time which is or ; with sufficient parallel preparation of ancillas, constant average
depth is possible using a method we call programmable ancilla rotations.
Moreover, we construct and analyze efficient implementations of first- and
second-quantized simulation algorithms using the fault-tolerant arbitrary gates
and other techniques, such as implementing various subroutines in constant
time. A specific example we analyze is the ground-state energy calculation for
Lithium hydride.Comment: 33 pages, 18 figure
On the Effect of Quantum Interaction Distance on Quantum Addition Circuits
We investigate the theoretical limits of the effect of the quantum
interaction distance on the speed of exact quantum addition circuits. For this
study, we exploit graph embedding for quantum circuit analysis. We study a
logical mapping of qubits and gates of any -depth quantum adder
circuit for two -qubit registers onto a practical architecture, which limits
interaction distance to the nearest neighbors only and supports only one- and
two-qubit logical gates. Unfortunately, on the chosen -dimensional practical
architecture, we prove that the depth lower bound of any exact quantum addition
circuits is no longer , but . This
result, the first application of graph embedding to quantum circuits and
devices, provides a new tool for compiler development, emphasizes the impact of
quantum computer architecture on performance, and acts as a cautionary note
when evaluating the time performance of quantum algorithms.Comment: accepted for ACM Journal on Emerging Technologies in Computing
System
Nucleon Axial Form Factor from Lattice QCD
Results for the isovector axial form factors of the proton from a lattice QCD
calculation are presented for both point-split and local currents. They are
obtained on a quenched lattice at with Wilson
fermions for a range of quark masses from strange to charm. We determine the
finite lattice renormalization for both the local and point-split currents of
heavy quarks. Results extrapolated to the chiral limit show that the
dependence of the axial form factor agrees reasonably well with experiment. The
axial coupling constant calculated for the local and the point-split
currents is about 6\% and 12\% smaller than the experimental value
respectively.Comment: 8 pages, 5 figures (included in part 2), UK/93-0
Effects of imperfections for Shor's factorization algorithm
We study effects of imperfections induced by residual couplings between
qubits on the accuracy of Shor's algorithm using numerical simulations of
realistic quantum computations with up to 30 qubits. The factoring of numbers
up to N=943 show that the width of peaks, which frequencies allow to determine
the factors, grow exponentially with the number of qubits. However, the
algorithm remains operational up to a critical coupling strength
which drops only polynomially with . The numerical dependence of
on is explained by analytical estimates that allows to
obtain the scaling for functionality of Shor's algorithm on realistic quantum
computers with a large number of qubits.Comment: 10 pages, 10 figures, 1 table. Added references and new data. Erratum
added as appendix. 1 Figure and 1 Table added. Research is available at
http://www.quantware.ups-tlse.fr
Qualitative evaluation of a physical activity-based chronic disease prevention program in a low-income, rural South African setting
Introduction: Chronic diseases, an increasing global concern, are prevalent in the low-income communities of South Africa, where rural health systems bear the double burden of infectious and chronic diseases. The Discovery Healthy Lifestyle Programme (DHLP) is a physical activity-based chronic disease prevention program that has been implemented in a low-income, rural setting in South Africa. The DHLP consists of both school- and primary healthcare clinic-based interventions for learners (Healthnutz) and adults (Live it Up), facilitated by teachers, nurses and community volunteers. The aim of this evaluation was to qualitatively assess the process by which the DHLP was implemented, identifying enabling factors and barriers.
Methods: Data were collected in target communities at schools and clinics from semi-structured focus groups of program leaders and members, teachers and community members (n=45), situational analyses of the school physical activity environment, informal community observations and informal interviews with program coordinators.
Results: The target communities faced socioeconomic and health inequalities and remained under-resourced and under-served. In spite of these and other challenges, the DHLP was well received by community members and stakeholders. It was valued by respondents for its health and psychosocial outcomes, evidenced by increased knowledge and awareness of the importance of physical activity and healthy lifestyles, and positively altered perceptions of physical activity. Program implementers believed the Live it Up component was growing, and this suggested the sustainability of the program. There were, however, some concerns about the fidelity of the Healthnutz intervention, due to timetabling difficulties. Despite this, teachers were positive about the
program and its value for their learners, staff and school. The community characteristics of being under-resourced and underserved appeared to positively influence DHLP implementation. Local government involvement in the DHLP resulted in greater ownership of the program, which enabled successful implementation.
Conclusions: This study presents a unique opportunity to assess the implementation and sustainability requirements of programs in environments of limited resources, considerable burden of infectious and chronic diseases and extensive socioeconomic challenges. The findings suggest that through enhancement of knowledge, transfer of appropriate skills and the provision of an enabling environment, participation in physical activity can be effectively promoted in a low-income, rural setting. Physical activity interventions that promote the participation and empowerment of rural communities can be feasible and accessible, thereby assisting in addressing the growing burden of chronic diseases in low-income
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