Discourse analysis

Discourse analysis is an umbrella term for a range of methodological approaches that analyse the use and functions of talk and text within social interaction. These approaches are used across social science disciplines such as psychology, sociology, linguistics, anthropology and communication studies. Discourse analysis is interdisciplinary in nature, developed from work within speech act theory, ethnomethodology and semiology as well as post-structuralist theorists such as Michel Foucault and Jacques Derrida, and the later works of philosopher Ludwig Wittgenstein. Discourse analysis approaches are crucial for understanding human relationships because they focus primarily on interaction: how we talk to each other and the discursive practices (talking, writing) through which relationships develop, fall apart and so on. This entry covers central features of discourse analysis, methodological issues and some of the most commonly used versions of discourse analysis

Food abuse : Mealtimes, helplines and 'troubled' eating

Feeding children can be one of the most challenging and frustrating aspects of raising a family. This is often exacerbated by conflicting guidelines over what the ‘correct’ amount of food and ‘proper’ eating actually entails. The issue becomes muddier still when parents are accused of mistreating their children by not feeding them properly, or when eating becomes troubled in some way. Yet how are parents to ‘know’ how much food is enough and when their child is ‘full’? How is food negotiated on a daily level? In this chapter, we show how discursive psychology can provide a way of understanding these issues that goes beyond guidelines and measurements. It enables us to examine the practices within which food is negotiated and used to hold others accountable. Like the other chapters in this section of the book, eating practices can also be situations in which an asymmetry of competence is produced; where one party is treated as being a less-than-valid person (in the case of family practices, this is often the child). As we shall see later, the asymmetry can also be reversed, where one person (adult or child) can claim to have greater ‘access’ to concepts such as ‘appetite’ and ‘hunger’. Not only does this help us to understand the complexity of eating practices; it also highlights features of the parent/child relationshipi and the institutionality of families

Lagrangian transport and chaos in the near wake of the flow around an obstacle: a numerical implementation of lobe dynamics

In this paper we study Lagrangian transport in the near wake of the flow around an obstacle, which we take to be a cylinder. In this case, for the range of Reynolds numbers investigated, the flow is two-dimensional and time periodic. We use ideas and methods from transport theory in dynamical systems to describe and quantify transport in the near wake. We numerically solve the Navier-Stokes equations for the velocity field and apply these methods to the resulting numerical representation of the velocity field. We show that the method of lobe dynamics can be used in conjunction with computational fluid dynamics methods to give very detailed and quantitative information about Lagrangian transport. In particular, we show how the stable and unstable manifolds of certain saddle-type stagnation points on the cylinder, and one in the wake, can be used to divide the flow into three distinct regions, an upper wake, a lower wake, and a wake cavity. The significance of the division using stable and unstable manifolds lies in the fact that these invariant manifolds form a template on which the transport occurs. Using this, we compute fluxes from the upper and lower wakes into the wake cavity using the associated turnstile lobes. We also compute escape time distributions as well as compare transport properties for two different Reynolds numbers

Diffusion of a passive scalar from a no-slip boundary into a two-dimensional chaotic advection field

Using a time-periodic perturbation of a two-dimensional steady separation bubble on a plane no-slip boundary to generate chaotic particle trajectories in a localized region of an unbounded boundary layer flow, we study the impact of various geometrical structures that arise naturally in chaotic advection fields on the transport of a passive scalar from a local 'hot spot' on the no-slip boundary. We consider here the full advection-diffusion problem, though attention is restricted to the case of small scalar diffusion, or large Peclet number. In this regime, a certain one-dimensional unstable manifold is shown to be the dominant organizing structure in the distribution of the passive scalar. In general, it is found that the chaotic structures in the flow strongly influence the scalar distribution while, in contrast, the flux of passive scalar from the localized active no-slip surface is, to dominant order, independent of the overlying chaotic advection. Increasing the intensity of the chaotic advection by perturbing the velocity held further away from integrability results in more non-uniform scalar distributions, unlike the case in bounded flows where the chaotic advection leads to rapid homogenization of diffusive tracer. In the region of chaotic particle motion the scalar distribution attains an asymptotic state which is time-periodic, with the period the same as that of the time-dependent advection field. Some of these results are understood by using the shadowing property from dynamical systems theory. The shadowing property allows us to relate the advection-diffusion solution at large Peclet numbers to a fictitious zero-diffusivity or frozen-field solution, corresponding to infinitely large Peclet number. The zero-diffusivity solution is an unphysical quantity, but is found to be a powerful heuristic tool in understanding the role of small scalar diffusion. A novel feature in this problem is that the chaotic advection field is adjacent to a no-slip boundary. The interaction between the necessarily non-hyperbolic particle dynamics in a thin near-wall region and the strongly hyperbolic dynamics in the overlying chaotic advection field is found to have important consequences on the scalar distribution; that this is indeed the case is shown using shadowing. Comparisons are made throughout with the flux and the distributions of the passive scalar for the advection-diffusion problem corresponding to the steady, unperturbed, integrable advection field

Time-frequency analysis of chaotic systems

We describe a method for analyzing the phase space structures of Hamiltonian systems. This method is based on a time-frequency decomposition of a trajectory using wavelets. The ridges of the time-frequency landscape of a trajectory, also called instantaneous frequencies, enable us to analyze the phase space structures. In particular, this method detects resonance trappings and transitions and allows a characterization of the notion of weak and strong chaos. We illustrate the method with the trajectories of the standard map and the hydrogen atom in crossed magnetic and elliptically polarized microwave fields.Comment: 36 pages, 18 figure

Current-driven destabilization of both collinear configurations in asymmetric spin-valves

Spin transfer torque in spin valves usually destabilizes one of the collinear configurations (either parallel or antiparallel) and stabilizes the second one. Apart from this, balance of the spin-transfer and damping torques can lead to steady precessional modes. In this letter we show that in some asymmetric nanopillars spin current can destabilize both parallel and antiparallel configurations. As a result, stationary precessional modes can occur at zero magnetic field. The corresponding phase diagram as well as frequencies of the precessional modes have been calculated in the framework of macrospin model. The relevant spin transfer torque has been calculated in terms of the macroscopic model based on spin diffusion equations.Comment: 4 pages, 4 figure

Index k saddles and dividing surfaces in phase space, with applications to isomerization dynamics

In this paper we continue our studies of the phase space geometry and dynamics associated with index k saddles (k > 1) of the potential energy surface. Using normal form theory, we give an explicit formula for a "dividing surface" in phase space, i.e. a co-dimension one surface (within the energy shell) through which all trajectories that "cross" the region of the index k saddle must pass. With a generic non-resonance assumption, the normal form provides k (approximate) integrals that describe the saddle dynamics in a neighborhood of the index k saddle. These integrals provide a symbolic description of all trajectories that pass through a neighborhood of the saddle. We give a parametrization of the dividing surface which is used as the basis for a numerical method to sample the dividing surface. Our techniques are applied to isomerization dynamics on a potential energy surface having 4 minima; two symmetry related pairs of minima are connected by low energy index one saddles, with the pairs themselves connected via higher energy index one saddles and an index two saddle at the origin. We compute and sample the dividing surface and show that our approach enables us to distinguish between concerted crossing ("hilltop crossing") isomerizing trajectories and those trajectories that are not concerted crossing (potentially sequentially isomerizing trajectories). We then consider the effect of additional "bath modes" on the dynamics, which is a four degree-of-freedom system. For this system we show that the normal form and dividing surface can be realized and sampled and that, using the approximate integrals of motion and our symbolic description of trajectories, we are able to choose initial conditions corresponding to concerted crossing isomerizing trajectories and (potentially) sequentially isomerizing trajectories.Comment: 49 pages, 12 figure