Nonlinear Breit-Wheeler pair creation with bremsstrahlung γ\gamma rays

Electron-positron pairs are produced through the Breit-Wheeler process when energetic photons traverse electromagnetic fields of sufficient strength. Here we consider a possible experimental geometry for observation of pair creation in the highly nonlinear regime, in which bremsstrahlung of an ultrarelativistic electron beam in a high-$Z$ target is used to produce $\gamma$ rays that collide with a counterpropagating laser pulse. We show how the target thickness may be chosen to optimize the yield of Breit-Wheeler positrons, and verify our analytical predictions with simulations of the cascade in the material and in the laser pulse. The electron beam energy and laser intensity required are well within the capability of today's high-intensity laser facilities.Comment: 12 pages, 5 figure

Solar influences on dynamical coupling between the stratosphere and troposphere

We use a simplified atmospheric general circulation model (AGCM) to investigate the response of the lower atmosphere to thermal perturbations in the lower stratosphere. The results show that generic heating of the lower stratosphere tends to weaken the sub-tropical jets and the tropospheric mean meridional circulations. The positions of the jets, and the extent of the Hadley cells, respond to the distribution of the stratospheric heating, with low latitude heating displacing them poleward, and uniform heating displacing them equatorward. The patterns of response to the low latitude heating are similar to those found to be associated with solar variability in previous observational data analysis, and to the effects of varying solar UV radiation in sophisticated AGCMs. In order to investigate the chain of causality involved in converting the stratospheric thermal forcing to a tropospheric climate signal we conduct an experiment which uses an ensemble of model spin-ups to analyse the time development of the response to an applied stratospheric perturbation. We find that the initial effect of the change in static stability at the tropopause is to reduce the eddy momentum flux convergence in this region. This is followed by a vertical transfer of the momentum forcing anomaly by an anomalous mean circulation to the surface, where it is partly balanced by surface stress anomalies. The unbalanced part drives the evolution of the vertically integrated zonal flow. We conclude that solar heating of the stratosphere may produce changes in the circulation of the troposphere even without any direct forcing below the tropopause. We suggest that the impact of the stratospheric changes on wave propagation is key to the mechanisms involved

The educational scorecard: The Start of our Journey

Against a backdrop of public sector cuts, increasing university fees and high youth unemployment, we are facing challenges in Higher Education to demonstrate the value of our courses. Assessing the value of learning, however, is not straight forward. This paper reports on a study of evaluation processes on a post-graduate, professionally accredited diploma delivered at a selection of post 1992 universities. The driver for the study was a concern that current evaluation processes do not fully demonstrate the value of the course nor take into account the needs of multiple stakeholders. The project included benchmarking University evaluation processes and conducting a dialogue with stakeholders. The study adopted a qualitative management research approach, involving: a review of current practice, comparison with a sample of equivalent courses, and consultation (in the form of focus groups and semi-structured interviews) with a sample of students and employers. The paper discusses findings and proposes recommendations for future evaluation procedures in the design of an ‘educational scorecard’ for the course. Kaplan and Norton’s ‘balanced scorecard (1996) concept was adapted to provide a mechanism to represent and balance the needs of different stakeholder groups in the education process. We argue that we cannot truly assess accountability and comparability without engaging a range of stakeholders, not only in soliciting their views on the outcome of the learning but also in the design and implementation of evaluation processes. The ‘educational scorecard’ presented in this paper is developed specifically for the post-graduate diploma in Human Resource Management (PDHRM) at Leeds Metropolitan University and the report concludes with some initial reflections on the benefits of adopting the scorecard methodology The proposed model is flexible and may be adapted for other HE institutions and courses

Convective instability and transient growth in flow over a backward-facing step

Transient energy growths of two- and three-dimensional optimal linear perturbations to two-dimensional flow in a rectangular backward-facing-step geometry with expansion ratio two are presented. Reynolds numbers based on the step height and peak inflow speed are considered in the range 0–500, which is below the value for the onset of three-dimensional asymptotic instability. As is well known, the flow has a strong local convective instability, and the maximum linear transient energy growth values computed here are of order 80×103 at Re = 500. The critical Reynolds number below which there is no growth over any time interval is determined to be Re = 57.7 in the two-dimensional case. The centroidal location of the energy distribution for maximum transient growth is typically downstream of all the stagnation/reattachment points of the steady base flow. Sub-optimal transient modes are also computed and discussed. A direct study of weakly nonlinear effects demonstrates that nonlinearity is stablizing at Re = 500. The optimal three-dimensional disturbances have spanwise wavelength of order ten step heights. Though they have slightly larger growths than two-dimensional cases, they are broadly similar in character. When the inflow of the full nonlinear system is perturbed with white noise, narrowband random velocity perturbations are observed in the downstream channel at locations corresponding to maximum linear transient growth. The centre frequency of this response matches that computed from the streamwise wavelength and mean advection speed of the predicted optimal disturbance. Linkage between the response of the driven flow and the optimal disturbance is further demonstrated by a partition of response energy into velocity components

Convective instability and transient growth in steady and pulsatile stenotic flows

We show that suitable initial disturbances to steady or long-period pulsatile flows in a straight tube with an axisymmetric 75%-occlusion stenosis can produce very large transient energy growths. The global optimal disturbances to an initially axisymmetric state found by linear analyses are three-dimensional wave packets that produce localized sinuous convective instability in extended shear layers. In pulsatile flow, initial conditions that trigger the largest disturbances are either initiated at, or advect to, the separating shear layer at the stenosis in phase with peak systolic flow. Movies are available with the online version of the paper

Estimation of unsteady aerodynamic forces using pointwise velocity data

A novel method to estimate unsteady aerodynamic force coefficients from pointwise velocity measurements is presented. The methodology is based on a resolvent-based reduced-order model which requires the mean flow to obtain physical flow structures and pointwise measurement to calibrate their amplitudes. A computationally-affordable time-stepping methodology to obtain resolvent modes in non-trivial flow domains is introduced and compared to previous existing matrix-free and matrix-forming strategies. The technique is applied to the unsteady flow around an inclined square cylinder at low Reynolds number. The potential of the methodology is demonstrated through good agreement between the fluctuating pressure distribution on the cylinder and the temporal evolution of the unsteady lift and drag coefficients predicted by the model and those computed by direct numerical simulation.Comment: In revie

Stress corrosion cracking of titanium alloys: SCC velocity: concentration of TiCl3

Stress corrosion cracking of titanium alloys, velocity of cracking in aqueous and methanol solutions and halogenated organic solvents, concentration of TiCl3 in crack

Dimensions of Grace: Factor Analysis of Three Grace Scales

Measuring grace is challenging. Prior research found the Grace Scale (GS), Richmont Grace Scale (RGS), and The Amazing Grace Scale (TAGS) to be reliable, have promising convergent and divergent validity, and to inter-correlate strongly. However, they may tap different constructs, or grace may be multidimensional (Bufford, Blackburn, Sisemore, & Bassett, 2015). Here two exploratory factor analyses of the combined items showed five factors: experiencing God’s grace, costly grace, grace to self, grace from others, and grace to others, partially paralleling Watson, Chen and Sisemore (2011). Items from all three scales loaded on Factor 1, only items from the RGS loaded on Factor 2. The remaining factors were mostly GS items and a few RGS items. The three scales measure somewhat different constructs. Preliminary validity for the five factors is promising. Regressions showed that combinations of the other four proposed scales accounted for at most about one third of the variance on any given grace factor. The five factors showed different patterns of relationships to criterion variables. We propose a 36 item Dimensions of Grace Scale combining items from all three scales for further exploration