| 1. |
- Bagheri, Shervin, et al.
(författare)
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Transition delay using control theory
- 2011
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Ingår i: Philosophical Transactions. Series A. - : The Royal Society. - 1364-503X .- 1471-2962. ; 369:1940, s. 1365-1381
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Tidskriftsartikel (refereegranskat)abstract
- This review gives an account of recent research efforts to use feedback control for the delay of laminar-turbulent transition in wall-bounded shear flows. The emphasis is on reducing the growth of small-amplitude disturbances in the boundary layer using numerical simulations and a linear control approach. Starting with the application of classical control theory to two-dimensional perturbations developing in spatially invariant flows, flow control based on control theory has progressed towards more realistic three-dimensional, spatially inhomogeneous flow configurations with localized sensing/actuation. The development of low-dimensional models of the Navier-Stokes equations has played a key role in this progress. Moreover, shortcomings and future challenges, as well as recent experimental advances in this multi-disciplinary field, are discussed.
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| 2. |
- Bahaj, AbuBakr S.
(författare)
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Marine current energy conversion: the dawn of a new era in electricity production
- 2013
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Ingår i: Philosophical Transactions. Series A. - : The Royal Society. - 1364-503X .- 1471-2962.
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Tidskriftsartikel (refereegranskat)abstract
- Marine currents can carry large amounts of energy, largely driven by the tides, which are a consequence of the gravitational effects of the planetary motion of the Earth, the Moon and the Sun. Augmented flow velocities can be found where the underwater topography (bathymetry) in straits between islands and the mainland or in shallows around headlands plays a major role in enhancing the flow velocities, resulting in appreciable kinetic energy. At some of these sites where practical flows are more than 1 m s−1, marine current energy conversion is considered to be economically viable. This study describes the salient issues related to the exploitation of marine currents for electricity production, resource assessment, the conversion technologies and the status of leading projects in the field. This study also summarizes important issues related to site development and some of the approaches currently being undertaken to inform device and array development. This study concludes that, given the highlighted commitments to establish favourable regulatory and incentive regimes as well as the aspiration for energy independence and combating climate change, the progress to multi-megawatt arrays will be much faster than that achieved for wind energy development.
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| 3. |
- Batten, W.M.J., et al.
(författare)
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The accuracy of the actuator disc-RANS approach for predicting the performance and far wake of a horizontal axis tidal stream turbine
- 2013
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Ingår i: Philosophical Transactions. Series A. - : The Royal Society. - 1364-503X .- 1471-2962.
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Tidskriftsartikel (refereegranskat)abstract
- The actuator disc-RANS model has been widely used in wind and tidal energy to predict the wake of a horizontal axis turbine. The model is appropriate where large scale effects of the turbine on a flow are of interest, for example, when considering environmental impacts, or arrays of devices. The accuracy of the model for modelling the wake of tidal stream turbines has not been demonstrated, and flow predictions presented in the literature for similar modelled scenarios vary significantly. This paper compares the results of the actuator disc-RANS model, where the turbine forces have been derived using a blade element approach, to experimental data measured in the wake of a scaled turbine. It also compares the results to those of a simpler uniform actuator disc model. The comparisons show that the model is accurate and can predict up to 94\% of the variation in the experimental data measured on the centreline of the wake. The study demonstrates that the actuator-disc RANS model is an accurate approach for modelling a turbine wake, and conservative approach to predict performance and loads. It can therfore applied to similar scenarios with confidence.
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| 4. |
- Berggren, Karl-Fredrik, et al.
(författare)
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Electrons in one dimension
- 2010
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Ingår i: Philosophical Transactions. Series A. - : The Royal Society. - 1364-503X .- 1471-2962. ; 368, s. 1141-1162
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Forskningsöversikt (refereegranskat)abstract
- In this article, we present a summary of the current status of the study of the transport of electrons confined to one dimension in very low disorder GaAs–AlGaAs heterostructures. By means of suitably located gates and application of a voltage to ‘electrostatically squeeze’ the electronic wave functions, it is possible to produce a controllable size quantization and a transition from two-dimensional transport. If the length of the electron channel is sufficiently short, then transport is ballistic and the quantized subbands each have a conductance equal to the fundamental quantum value 2e2/h, where the factor of 2 arises from the spin degeneracy. This mode of conduction is discussed, and it is shown that a number of many-body effects can be observed. These effects are discussed as in the spin-incoherent regime, which is entered when the separation of the electrons is increased and the exchange energy is less than kT. Finally, results are presented in the regime where the confinement potential is decreased and the electron configuration relaxes to minimize the electron–electron repulsion to move towards a two-dimensional array. It is shown that the ground state is no longer a line determined by the size quantization alone, but becomes two distinct rows arising from minimization of the electrostatic energy and is the precursor of a two-dimensional Wigner lattice.
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| 5. |
- Black, John H, 1949
(författare)
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H3+ at the interface between astrochemistry and astroparticle physics
- 2012
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Ingår i: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. - : The Royal Society. - 1364-503X .- 1471-2962. ; 370:1978, s. 5130-5141
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Tidskriftsartikel (refereegranskat)abstract
- The H3+ molecular ion has been used by Oka and collaborators to trace the rate of ionization by cosmic rays in the interstellar medium. More energetic cosmic rays also produce diffuse γ-radiation. Now that several supernova remnants (SNRs) have been identified as γ-ray sources, it is possible to use spectroscopy of molecular ions to search for enhanced ionization rates that would pinpoint the SNRs as the accelerators of cosmic rays. It is proposed that the warm, dilute molecular gas revealed by Graphic absorption in the central molecular zone of the Galaxy can also be investigated via radio recombination lines of atoms and possibly triatomic hydrogen.
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| 6. |
- Borgdorff, J., et al.
(författare)
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Performance of distributed multiscale simulations
- 2014
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Ingår i: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. - : The Royal Society. - 1364-503X .- 1471-2962. ; 372:2021
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Tidskriftsartikel (refereegranskat)abstract
- Multiscale simulations model phenomena across natural scales using monolithic or component-based code, running on local or distributed resources. In this work, we investigate the performance of distributed multiscale computing of component-based models, guided by six multiscale applications with different characteristics and from several disciplines. Three modes of distributed multiscale computing are identified: supplementing local dependencies with large-scale resources, load distribution over multiple resources, and load balancing of small- and large-scale resources. We find that the first mode has the apparent benefit of increasing simulation speed, and the second mode can increase simulation speed if local resources are limited. Depending on resource reservation and model coupling topology, the third mode may result in a reduction of resource consumption.
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| 7. |
- Church, Ross, et al.
(författare)
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Properties of long gamma-ray bursts from massive compact binaries.
- 2013
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Ingår i: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Science. - : The Royal Society. - 1364-503X .- 1471-2962. ; 371:1992
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Tidskriftsartikel (refereegranskat)abstract
- We consider the implications of a model for long-duration gamma-ray bursts in which the progenitor is spun up in a close binary by tidal interactions with a massive black-hole companion. We investigate a sample of such binaries produced by a binary population synthesis, and show that the model predicts several common features in the accretion on to the newly formed black hole. In all cases, the accretion rate declines as approximately t(-5/3) until a break at a time of order 10(4) s. The accretion rate declines steeply thereafter. Subsequently, there is flaring activity, with the flare peaking between 10(4) and 10(5) s, the peak time being correlated with the flare energy. We show that these times are set by the semi-major axis of the binary, and hence the process of tidal spin-up; furthermore, they are consistent with flares seen in the X-ray light curves of some long gamma-ray bursts.
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| 8. |
- Daly, T., et al.
(författare)
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Modelling of the flow field surrounding tidal turbine arrays for varying positions in a channel
- 2013
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Ingår i: Philosophical Transactions. Series A. - : The Royal Society. - 1364-503X .- 1471-2962.
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Tidskriftsartikel (refereegranskat)abstract
- The modelling of tidal turbines and the hydrodynamic effects of tidal power extraction represents a relatively new challenge in the field of computational fluid dynamics. Many different methods of defining flow and boundary conditions have been postulated and examined to determine how accurately they replicate the many parameters associated with tidal power extraction. This paper outlines the results of numerical modelling analysis carried out to investigate different methods of defining the inflow velocity boundary condition. This work is part of a wider research programme investigating flow effects in tidal turbine arrays. Results of this numerical analysis were benchmarked against previous experimental work conducted at the University of Southampton Chilworth hydraulics laboratory. Results show significant differences between certain methods of defining inflow velocities. However, certain methods do show good correlation with experimental results. This correlation would appear to justify the use of these velocity inflow definition methods in future numerical modelling of the far-field flow effects of tidal turbine arrays.
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| 9. |
- Dutta, Prabal, et al.
(författare)
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Operating systems and network protocols for wireless sensor networks
- 2012. - 7
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Ingår i: Philosophical Transactions of the Royal Society A: Mathematical Physical & Engineering Sciences. - : The Royal Society. - 1364-503X .- 1471-2962. ; 370
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Tidskriftsartikel (refereegranskat)abstract
- Sensor network protocols exist to satisfy the communication needs of diverse applications, including data collection, event detection, target tracking and control. Network protocols to enable these services are constrained by the extreme resource scarcity of sensor nodes—including energy, computing, communications and storage—which must be carefully managed and multiplexed by the operating system. These challenges have led to new protocols and operating systems that are efficient in their energy consumption, careful in their computational needs and miserly in their memory footprints, all while discovering neighbours, forming networks, delivering data and correcting failures.
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| 10. |
- Ehrenfreund, Pascale, et al.
(författare)
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The evolution of organic matter in space
- 2011
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Ingår i: Philosophical Transactions. Series A. - London : The Royal Society. - 1364-503X .- 1471-2962. ; 369:1936, s. 538-554
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Tidskriftsartikel (refereegranskat)abstract
- Carbon, and molecules made from it, have already been observed in the early Universe. During cosmic time, many galaxies undergo intense periods of star formation, during which heavy elements like carbon, oxygen, nitrogen, silicon and iron are produced. Also, many complex molecules, from carbon monoxide to polycyclic aromatic hydrocarbons, are detected in these systems, like they are for our own Galaxy. Interstellar molecular clouds and circumstellar envelopes are factories of complex molecular synthesis. A surprisingly high number of molecules that are used in contemporary biochemistry on the Earth are found in the interstellar medium, planetary atmospheres and surfaces, comets, asteroids and meteorites and interplanetary dust particles. Large quantities of extra-terrestrial material were delivered via comets and asteroids to young planetary surfaces during the heavy bombardment phase. Monitoring the formation and evolution of organic matter in space is crucial in order to determine the prebiotic reservoirs available to the early Earth. It is equally important to reveal abiotic routes to prebiotic molecules in the Earth environments. Materials from both carbon sources (extra-terrestrial and endogenous) may have contributed to biochemical pathways on the Earth leading to life’s origin. The research avenues discussed also guide us to extend our knowledge to other habitable worlds.
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