Characterisation of materials with hyperelastic microstructures through computational homogenisation and optimisation methods.

The constitutive modelling of microheterogeneous materials is a subject of considerable practical and theoretical interest. Among many approaches computational homogenisation is particularly powerful and versatile. This is based on the numerical estimation of the mechanical response of a volume element representing the material's microstructure. This thesis is concerned with computational homogenisation and its particular use in characterising materials with hyperelastic microstructures through an optimisation based methodology. Details of a finite element implementation of the computational homogenisation procedure are presented. These are derived from a variational treatment of the homogenisation problem. Examples of the application of the method to hyperelastic microstructures are reported. Next a procedure to provide a convenient characterisation of the behaviour of composite material is considered. This consists of adopting a conventional explicit model to approximate the macroscopic mechanical behaviour. Parameters of the model are chosen by established optimisation methods so that the macro model best fits the calculated homogenised response of a model of the microstructure. The optimisation based methodology is applied to the problem of modelling the constitutive behaviour of artery walls

Electrostatic electron cyclotron instabilities near the upper hybrid layer due to electron ring distributions

A theoretical study is presented of the electrostatic electron cyclotron instability involving Bernstein modes in a magnetized plasma. The presence of a tenuous thermal ring distribution in a Maxwellian plasma decreases the frequency of the upper hybrid branch of the electron Bernstein mode until it merges with the nearest lower branch with a resulting instability. The instability occurs when the upper hybrid frequency is somewhat above the third, fourth, and higher electron cyclotron harmonics, and gives rise to a narrow spectrum of waves around the electron cyclotron harmonic nearest to the upper hybrid frequency. For a tenuous cold ring distribution together with a Maxwellian distribution an instability can take place also near the second electron cyclotron harmonic. Noise-free Vlasov simulations are used to assess the theoretical linear growth-rates and frequency spectra, and to study the nonlinear evolution of the instability. The relevance of the results to laboratory and ionospheric heating experiments is discussed

Foehn winds in the McMurdo Dry Valleys, Antarctic: The origin of extreme warming events

Foehn winds resulting from topographic modification of airflow in the lee of mountain barriers are frequently experienced in the McMurdo Dry Valleys (MDVs) of Antarctica. Strong foehn winds in the MDVs cause dramatic warming at onset and have significant effects on landscape forming processes; however, no detailed scientific investigation of foehn in the MDVs has been conducted. As a result, they are often misinterpreted as adiabatically warmed katabatic winds draining from the polar plateau. Herein observations from surface weather stations and numerical model output from the Antarctic Mesoscale Prediction System (AMPS) during foehn events in the MDVs are presented. Results show that foehn winds in the MDVs are caused by topographic modification of south-southwesterly airflow, which is channeled into the valleys from higher levels. Modeling of a winter foehn event identifies mountain wave activity similar to that associated with midlatitude foehn winds. These events are found to be caused by strong pressure gradients over the mountain ranges of the MDVs related to synoptic-scale cyclones positioned off the coast of Marie Byrd Land. Analysis of meteorological records for 2006 and 2007 finds an increase of 10% in the frequency of foehn events in 2007 compared to 2006, which corresponds to stronger pressure gradients in the Ross Sea region. It is postulated that the intra- and interannual frequency and intensity of foehn events in the MDVs may therefore vary in response to the position and frequency of cyclones in the Ross Sea region

Numerical simulation of unconstrained cyclotron resonant maser emission

When a mainly rectilinear electron beam is subject to significant magnetic compression, conservation of magnetic moment results in the formation of a horseshoe shaped velocity distribution. It has been shown that such a distribution is unstable to cyclotron emission and may be responsible for the generation of Auroral Kilometric Radiation (AKR) an intense rf emission sourced at high altitudes in the terrestrial auroral magnetosphere. PiC code simulations have been undertaken to investigate the dynamics of the cyclotron emission process in the absence of cavity boundaries with particular consideration of the spatial growth rate, spectral output and rf conversion efficiency. Computations reveal that a well-defined cyclotron emission process occurs albeit with a low spatial growth rate compared to waveguide bounded simulations. The rf output is near perpendicular to the electron beam with a slight backward-wave character reflected in the spectral output with a well defined peak at 2.68GHz, just below the relativistic electron cyclotron frequency. The corresponding rf conversion efficiency of 1.1% is comparable to waveguide bounded simulations and consistent with the predictions of kinetic theory that suggest efficient, spectrally well defined radiation emission can be obtained from an electron horseshoe distribution in the absence of radiation boundaries.Publisher PD

High-coherence electron and ion bunches from laser-cooled atoms

XXVIII ICPEACCold atom electron and ion sources produce electron bunches and ion beams by photoionisation of laser cooled atoms. They offer high coherence and the potential for high brightness, with applications including ultrafast electron diffractive imaging of dynamic processes at the nanoscale. Here we present our cold atom electron/ion source, with an electron temperature of less than 10 K and a transverse coherence length of 10 nm. We also discuss experiments investigating space-charge effects with ions and the production of ultra-fast electron bunches using a femto-second laser. In the latter experiment we show that it is possible to produce both cold and fast electron bunches with our source.A. J. McCulloch, D. V. Sheludko, C. T. Putkunz, S. D. Saliba, D. J. Thompson, R. W. Speirs, D. Murphy, J. Torrance, B. M. Sparkes, and R. E. Scholte

Numerical simulations of unbounded cyclotron-maser emissions

Numerical simulations have been conducted to study the spatial growth rate and emission topology of the cyclotron-maser instability responsible for stellar/planetary auroral magnetospheric radio emission and intense non-thermal radio emission in other astrophysical contexts. These simulations were carried out in an unconstrained geometry, so that the conditions existing within the source region of some natural electron cyclotron masers could be more closely modelled. The results have significant bearing on the radiation propagation and coupling characteristics within the source region of such non-thermal radio emissions

Numerical simulation of astrophysical cyclotron-maser emission

Numerical simulations have been conducted at the University of Strathclyde to study the spatial growth rate and emission topology of the cyclotron maser instability responsible for auroral magnetospheric radio emission from stars and planets and intense non-thermal radio emission in other astrophysical contexts. The results have significant bearing on the radiation propagation characteristics and highly debated question of escape from the source region

Nonequilibrium phase transition in the sedimentation of reproducing particles

We study numerically and analytically the dynamics of a sedimenting suspension of active, reproducing particles, such as growing bacteria in a gravitational field. In steady state we find a non-equilibrium phase transition between a `sedimentation' regime, analogous to the sedimentation equilibrium of passive colloids, and a `uniform' regime, in which the particle density is constant in all but the top and bottom of the sample. We discuss the importance of fluctuations in particle density in locating the phase transition point, and report the kinetics of sedimentation at early times.Comment: 4 pages, 4 figure

Direct activation of oestrogen receptor- α by interleukin-6 in primary cultures of breast cancer epithelial cells

Interleukin 6 (IL-6) is secreted by breast tumours and shows synergistic activity with 17β-oestradiol (E2), leading to increases in reductive 17β-hydroxysteroid dehydrogenase activity in breast cancer epithelial cells. However, the mechanisms involved are poorly understood. Using short-term epithelial cultures established from primary breast tumours, we have examined whether IL-6 could directly affect transcriptional activity of oestrogen reception α (ERα). Tumour epithelial cultures were established from 15 breast tumours, grown to 70% confluence and transiently transfected with a plasmid reporter containing the vitellogenin oestrogen response element and the luciferase coding sequence (ERE-TK-LUC). Following transfection, cells were incubated with E2, IL-6, the pure anti-oestrogen ZM 182780 or combinations of these substances for 48 h. Luciferase activity was then measured in cell lysates. E2 caused a dose-dependent increase in luciferase expression, causing a maximum threefold stimulation at 100 p M. In the presence of IL-6, transcriptional activity was increased by up to 2.5-fold in ERα+cultures (11/15). In combination with E2, synergistic effects were observed with increases in luciferase activity of up to sixfold over controls. This effect could be blocked by treatment with ZM 182780. Pre-incubation of cells with an antibody directed against the signalling component of IL-6, gp130, was ineffective in blocking the E2 response. This antibody reduced, but did not completely block the effect of IL-6 either alone or in combination with E2, suggesting cross-talk between the two signalling pathways. In conclusion, these results provide evidence for direct transcriptional activation of ERα by IL-6. © 2000 Cancer Research Campaig