329 research outputs found
Are ghost surfaces quadratic-flux-minimizing?
Two candidates for "almost-invariant" toroidal surfaces passing through
magnetic islands, namely quadratic-flux-minimizing (QFMin) surfaces and ghost
surfaces, use families of periodic pseudo-orbits (i.e. paths for which the
action is not exactly extremal). QFMin pseudo-orbits, which are
coordinate-dependent, are field lines obtained from a modified magnetic field,
and ghost-surface pseudo-orbits are obtained by displacing closed field lines
in the direction of steepest descent of magnetic action, . A generalized Hamiltonian definition of ghost
surfaces is given and specialized to the usual Lagrangian definition. A
modified Hamilton's Principle is introduced that allows the use of Lagrangian
integration for calculation of the QFMin pseudo-orbits. Numerical calculations
show QFMin and Lagrangian ghost surfaces give very similar results for a
chaotic magnetic field perturbed from an integrable case, and this is explained
using a perturbative construction of an auxiliary poloidal angle for which
QFMin and Lagrangian ghost surfaces are the same up to second order. While
presented in the context of 3-dimensional magnetic field line systems, the
concepts are applicable to defining almost-invariant tori in other
degree-of-freedom nonintegrable Lagrangian/Hamiltonian systems.Comment: 8 pages, 3 figures. Revised version includes post-publication
corrections in text, as described in Appendix C Erratu
An analysis of a swimmer’s passive wave resistance using experimental data and CFD simulations
The passive resistance of a swimmer on the free surface has previously been researched experimentally. The contribution of wave resistance to total drag for a swimmer with a velocity around 2.0 m.s-1 was found to vary from 5% for Vorontsov and Rumyantsev (2000), to 21 % for Toussaint et al. (2002) and up to 60% according to Vennell et al. (2006). The exact resistance breakdown of a swimmer remains unknown due to difficulties in the direct measurement of wave resistance. As noted by Sato and Hino (2010), this lack of experimental data makes it difficult to validate numerical simulations of swimmers on the free surface.This study is therefore aimed at presenting direct measurements of a swimmer’s total drag and wave resistance, along with the longitudinal wave cuts which may be used to validate numerical simulations. In this paper, experimental data of a swimmer’s resistance are presented at two different velocities (case 1 = 1.7 m.s-1 and case 2 = 2.1 m.s-1). Total drag was measured using force block dynamometers mounted on a custom-built tow rig (Webb et al., 2011). Moreover, a longitudinal wave cut method was used to directly evaluate wave resistance (Eggers, 1955).The two conditions tested were simulated using the open-source Computational Fluid Dynamics (CFD) code OpenFOAM (OpenFOAM® (2013)). The body geometry is a generic human form, morphed into the correct attitude and depth using the above- and under-water video footage recorded during the experiment. 3D Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations were performed using the Volume of Fluid (VOF) method to solve the air-water interface. A similar numerical technique was used by Banks (2013a) to assess the passive resistance of a swimmer. Two cases were simulated and the error in total drag compared to the experimental data was found to be 1 % and 22 % respectively. In this paper, the resistance components over a swimmer’s typical range of speeds are investigated and compared with the experimental dat
The effect of swimsuit resistance on freestyle swimming race time.
It is known that swimming equipment (suit, cap and goggles) can affect the total resistance of a swimmer, and therefore impact the resulting swimming speed and race time. After the 2009 swimming world championships (WC) the international swimming federation (FINA) banned a specific type of full body suit, which resulted in an increase in race times for subsequent WC events. This study proposes that the 2009 suits provided a reduction in swimming resistance and aims to quantify this resistance reduction for male and female freestyle events. Due to the practical difficulties of testing a large sample of swimmers a simulation approach is adopted. To quantify the race time improvement that the 2009 suits provided, an equivalent 2009 “no-suit” dataset is created, incorporating the general trend of improving swimming performance over time, and compared to the actual 2009 times. A full race simulation is developed where the start, turn, underwater and surface swimming phases are captured. Independent resistance models are used for surface and underwater swimming; coupled with a leg propulsion model for underwater undulatory swimming and freestyle flutter kick, and a single element arm model to simulate freestyle arm propulsion. A validation is performed to ensure the simulation captures the change in swimming speed with changes to resistance and is found to be within 5% of reality. Race times for an equivalent “no-suit” 2009 situation are simulated and the total resistance reduced to achieve the actual 2009 race times. An average resistance reduction of 4.8% provided by the 2009 suits is identified. A factor of 0.47 ± 10%, to convert resistance changes to freestyle race time changes is determine
Action-gradient-minimizing pseudo-orbits and almost-invariant tori
Transport in near-integrable, but partially chaotic,
degree-of-freedom Hamiltonian systems is blocked by invariant tori and is
reduced at \emph{almost}-invariant tori, both associated with the invariant
tori of a neighboring integrable system. "Almost invariant" tori with rational
rotation number can be defined using continuous families of periodic
\emph{pseudo-orbits} to foliate the surfaces, while irrational-rotation-number
tori can be defined by nesting with sequences of such rational tori. Three
definitions of "pseudo-orbit," \emph{action-gradient--minimizing} (AGMin),
\emph{quadratic-flux-minimizing} (QFMin) and \emph{ghost} orbits, based on
variants of Hamilton's Principle, use different strategies to extremize the
action as closely as possible. Equivalent Lagrangian (configuration-space
action) and Hamiltonian (phase-space action) formulations, and a new approach
to visualizing action-minimizing and minimax orbits based on AGMin
pseudo-orbits, are presented.Comment: Accepted for publication in a special issue of Communications in
Nonlinear Science and Numerical Simulation (CNSNS) entitled "The mathematical
structure of fluids and plasmas : a volume dedicated to the 60th birthday of
Phil Morrison
Microbial oxidation of arsenite in a subarctic environment: diversity of arsenite oxidase genes and identification of a psychrotolerant arsenite oxidiser
Background: Arsenic is toxic to most living cells. The two soluble inorganic forms of arsenic are arsenite (+3) and arsenate (+5), with arsenite the more toxic. Prokaryotic metabolism of arsenic has been reported in both thermal and moderate environments and has been shown to be involved in the redox cycling of arsenic. No arsenic metabolism (either dissimilatory arsenate reduction or arsenite oxidation) has ever been reported in cold environments (i.e. < 10°C).
Results: Our study site is located 512 kilometres south of the Arctic Circle in the Northwest Territories, Canada in an inactive gold mine which contains mine waste water in excess of 50 mM arsenic. Several thousand tonnes of arsenic trioxide dust are stored in underground chambers and microbial biofilms grow on the chamber walls below seepage points rich in arsenite-containing solutions. We compared the arsenite oxidisers in two subsamples (which differed in arsenite concentration) collected from one biofilm. 'Species' (sequence) richness did not differ between subsamples, but the relative importance of the three identifiable clades did. An arsenite-oxidising bacterium (designated GM1) was isolated, and was shown to oxidise arsenite in the early exponential growth phase and to grow at a broad range of temperatures (4-25°C). Its arsenite oxidase was constitutively expressed and functioned over a broad temperature range.
Conclusions: The diversity of arsenite oxidisers does not significantly differ from two subsamples of a microbial biofilm that vary in arsenite concentrations. GM1 is the first psychrotolerant arsenite oxidiser to be isolated with the ability to grow below 10°C. This ability to grow at low temperatures could be harnessed for arsenic bioremediation in moderate to cold climates
Interdisciplinary research collegium in advanced maritime systems design
The education of naval architects, marine engineers and others who are the active contributors to the ship design processes is heavily focussed on engineering fundamentals, often aligned with traditional university course constraints. The concept of a research collegium is described whose aim is to provide an environment where young people in their formative postgraduate years can learn and work in a small, mixed discipline group drawn from the worldwide maritime community to develop their skills whilst completing a project in advanced ship design. The brief that initiates each project sets challenging user requirements which encourage each team to develop an imaginative solution, using their individual knowledge and experience, together with learning derived from teaching which form a common element of the early part of the collegiu
Relaxed MHD states of a multiple region plasma
We calculate the stability of a multiple relaxation region MHD (MRXMHD)
plasma, or stepped-Beltrami plasma, using both variational and tearing mode
treatments. The configuration studied is a periodic cylinder. In the
variational treatment, the problem reduces to an eigenvalue problem for the
interface displacements. For the tearing mode treatment, analytic expressions
for the tearing mode stability parameter , being the jump in the
logarithm in the helical flux across the resonant surface, are found. The
stability of these treatments is compared for displacements of an
illustrative RFP-like configuration, comprising two distinct plasma regions.
For pressure-less configurations, we find the marginal stability conclusions of
each treatment to be identical, confirming analytic results in the literature.
The tearing mode treatment also resolves ideal MHD unstable solutions for which
: these correspond to displacement of a resonant interface.
Wall stabilisation scans resolve the internal and external ideal kink. Scans
with increasing pressure are also performed: these indicate that both
variational and tearing mode treatments have the same stability trends with
, and show pressure stabilisation in configurations with increasing edge
pressure. Combined, our results suggest that MRXMHD configurations which are
stable to ideal perturbations plus tearing modes are automatically in a stable
state. Such configurations, and their stability properties, are of emerging
importance in the quest to find mathematically rigorous solutions of ideal MHD
force balance in 3D geometry.Comment: 11 pages, 3 figures, 22nd IAEA Fusion Energy Conference, Geneva,
Switzerland. Submitted to Nuclear Fusio
Svestka's Research: Then and Now
Zdenek Svestka's research work influenced many fields of solar physics,
especially in the area of flare research. In this article I take five of the
areas that particularly interested him and assess them in a "then and now"
style. His insights in each case were quite sound, although of course in the
modern era we have learned things that he could not readily have envisioned.
His own views about his research life have been published recently in this
journal, to which he contributed so much, and his memoir contains much
additional scientific and personal information (Svestka, 2010).Comment: Invited review for "Solar and Stellar Flares," a conference in honour
of Prof. Zden\v{e}k \v{S}vestka, Prague, June 23-27, 2014. This is a
contribution to a Topical Issue in Solar Physics, based on the presentations
at this meeting (Editors Lyndsay Fletcher and Petr Heinzel
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