Disk heating by more than one spiral density wave

We consider a differentially rotating, 2D stellar disk perturbed by two steady state spiral density waves moving at different patterns speeds. Our investigation is based on direct numerical integration of initially circular test-particle orbits. We examine a range of spiral strengths and spiral speeds and show that stars in this time dependent gravitational field can be heated (their random motions increased).This is particularly noticeable in the simultaneous propagation of a 2-armed spiral density wave near the corotation resonance (CR), and a weak 4-armed one near the inner and outer 4:1 Lindblad resonances. In simulations with 2 spiral waves moving at different pattern speeds we find: (1) the variance of the radial velocity, sigma_R^2, exceeds the sum of the variances measured from simulations with each individual pattern; (2) sigma_R^2 can grow with time throughout the entire simulation; (3) sigma_R^2 is increased over a wider range of radii compared to that seen with one spiral pattern; (4) particles diffuse radially in real space whereas they don't when only one spiral density wave is present. Near the CR with the stronger, 2-armed pattern, test particles are observed to migrate radially. These effects take place at or near resonances of both spirals so we interpret them as the result of stochastic motions. This provides a possible new mechanism for increasing the stellar velocity dispersion in galactic disks. If multiple spiral patterns are present in the Galaxy we predict that there should be large variations in the stellar velocity dispersion as a function of radius.Comment: 20 pages, 13 figures. Submitted to MNRA

The Place of Widening Participation

This chapter reports on a small-scale study to explore the learning careers of mature, non-traditionally qualified students. It reveals how they (re)engage with learning, and their transition from studying on a bespoke access course delivered in a university to study at undergraduate level. The study highlights the barriers and challenges students overcome and identifies their motivations and dispositions to learning. Whilst the majority of access courses are delivered in further education colleges, a small number of universities deliver bespoke access courses. These courses, which are usually designed to facilitate progression within the institution, enable students to experience teaching and learning in a university setting, facilities and support services. The research adopted a qualitative approach, using interviews as the main method of data collection. Emerging findings reveal that (re)engaging with education and the transition from ‘access student’ to undergraduate student is not seamless and without challenges. Such students ‘often undergo a unique and profound experience’ (Burnell, 2015:6) as they enter this new space which provides an opportunity to create new social and class-related identities. The transition to undergraduate study is easier when learners are familiar with the routines and rhythms of higher education in general and the facilities and services of a particular institution

Field Theory of Propagating Reaction-Diffusion Fronts

The problem of velocity selection of reaction-diffusion fronts has been widely investigated. While the mean field limit results are well known theoretically, there is a lack of analytic progress in those cases in which fluctuations are to be taken into account. Here, we construct an analytic theory connecting the first principles of the reaction-diffusion process to an effective equation of motion via field-theoretic arguments, and we arrive at the results already confirmed by numerical simulations

Ionising radiation exposure from medical imaging – A review of Patient's (un) awareness

Introduction: Medical imaging is the main source of artificial radiation exposure. Evidence, however, suggests that patients are poorly informed about radiation exposure when attending diagnostic scans. This review provides an overview of published literature with a focus on nuclear medicine patients on the level of awareness of radiation exposure from diagnostic imaging. Methods: A review of available literature on awareness, knowledge and perception of ionising radiation in medical imaging was conducted. Articles that met the inclusion criteria were subjected to critical appraisal using the Mixed Methods Appraisal Tool. Results: 140 articles identified and screened for eligibility, 24 critically assessed and 4 studies included in synthesis. All studies demonstrated that patients were generally lacking awareness about radiation exposure and highlighted a lack of communication between healthcare professionals and patients with respect to radiation exposure. Conclusion: Studies demonstrate a need to better inform patients about their radiation exposure, and further studies focusing on nuclear medicine patients are particularly warranted. Implications for practice: Adequate and accurate information is crucial to ensure the principle of informed consent is present

Hopping Conduction and Bacteria: Transport in Disordered Reaction-Diffusion Systems

We report some basic results regarding transport in disordered reaction-diffusion systems with birth (A->2A), death (A->0), and binary competition (2A->A) processes. We consider a model in which the growth process is only allowed to take place in certain areas--"oases"--while the rest of space--the "desert"--is hostile to growth. In the limit of low oasis density, transport is mediated through rare "hopping" events, necessitating the inclusion of discreteness effects in the model. By first considering transport between two oases, we are able to derive an approximate expression for the average time taken for a population to traverse a disordered medium.Comment: 4 pages, 2 figure

A tensile strength apparatus with the facility to monitor negative pore-water pressure

This paper presents a new testing method for investigating the behaviour of clayey geomaterials subjected to a tensile (negative) total stress. The method includes the use of high capacity tensiometers to measure the pore-water pressure of the test specimen, an aspect which has not been demonstrated in any other direct tensile testing method. This addition allows interpretation of failure data in terms of effective stress rather than total stress, which is the approach that should be pursued in the saturated range. The test specimen shape and loading method have been modified from those commonly seen in existing literature to ensure that the direction of the major principal stress in the failure zone coincides with the direction of the externally applied tensile force, allowing for a more accurate analysis of tensile failure. Results are shown for saturated specimens and compared to results obtained for the same soil in uniaxial compression, using a modified version of the presented uniaxial tensile method, and a triaxial compression test. It is demonstrated that crack initiation occurs by shear failure if the data is interpreted in terms of effective stress rather than total stress, and that the failure mechanisms under tension do not differ from compression