A survey of simulation techniques in commerce and defence

Despite the developments in Modelling and Simulation (M&S) tools and techniques over the past years, there has been a gap in the M&S research and practice in healthcare on developing a toolkit to assist the modellers and simulation practitioners with selecting an appropriate set of techniques. This study is a preliminary step towards this goal. This paper presents some results from a systematic literature survey on applications of M&S in the commerce and defence domains that could inspire some improvements in the healthcare. Interim results show that in the commercial sector Discrete-Event Simulation (DES) has been the most widely used technique with System Dynamics (SD) in second place. However in the defence sector, SD has gained relatively more attention. SD has been found quite useful for qualitative and soft factors analysis. From both the surveys it becomes clear that there is a growing trend towards using hybrid M&S approaches

Selective COX-2 inhibitors and risk of myocardial infarction

Selective inhibitors of cyclooxygenase- 2 ( COX- 2, ` coxibs') are highly effective anti-inflammatory and analgesic drugs that exert their action by preventing the formation of prostanoids. Recently some coxibs, which were designed to exploit the advantageous effects of non- steroidal anti-inflammatory drugs while evading their side effects, have been reported to increase the risk of myocardial infarction and atherothrombotic events. This has led to the withdrawal of rofecoxib from global markets, and warnings have been issued by drug authorities about similar events during the use of celecoxib or valdecoxib/ parecoxib, bringing about questions of an inherent atherothrombotic risk of all coxibs and consequences that should be drawn by health care professionals. These questions need to be addressed in light of the known effects of selective inhibition of COX- 2 on the cardiovascular system. Although COX- 2, in contrast to the cyclooxygenase-1 ( COX- 1) isoform, is regarded as an inducible enzyme that only has a role in pathophysiological processes like pain and inflammation, experimental and clinical studies have shown that COX- 2 is constitutively expressed in tissues like the kidney or vascular endothelium, where it executes important physiological functions. COX- 2- dependent formation of prostanoids not only results in the mediation of pain or inflammatory signals but also in the maintenance of vascular integrity. Especially prostacyclin ( PGI(2)), which exerts vasodilatory and antiplatelet properties, is formed to a significant extent by COX- 2, and its levels are reduced to less than half of normal when COX- 2 is inhibited. This review outlines the rationale for the development of selective COX- 2 inhibitors and the pathophysiological consequences of selective inhibition of COX- 2 with special regard to vasoactive prostaglandins. It describes coxibs that are currently available, evaluates the current knowledge on the risk of atherothrombotic events associated with their intake and critically discusses the consequences that should be drawn from these insights. Copyright (C) 2005 S. Karger AG, Basel

Meeting the four-hour deadline in an A&E department

This is the print version of the Article. The official published version can be obtained from the link below - Copyright @ 2011 EmeraldPurpose – Accident and emergency (A&E) departments experience a secondary peak in patient length of stay (LoS) at around four hours, caused by the coping strategies used to meet the operational standards imposed by government. The aim of this paper is to build a discrete-event simulation model that captures the coping strategies and more accurately reflects the processes that occur within an A&E department. Design/methodology/approach – A discrete-event simulation (DES) model was used to capture the A&E process at a UK hospital and record the LoS for each patient. Input data on 4,150 arrivals over three one-week periods and staffing levels was obtained from hospital records, while output data were compared with the corresponding records. Expert opinion was used to generate the pathways and model the decision-making processes. Findings – The authors were able to replicate accurately the LoS distribution for the hospital. The model was then applied to a second configuration that had been trialled there; again, the results also reflected the experiences of the hospital. Practical implications – This demonstrates that the coping strategies, such as re-prioritising patients based on current length of time in the department, employed in A&E departments have an impact on LoS of patients and therefore need to be considered when building predictive models if confidence in the results is to be justified. Originality/value – As far as the authors are aware this is the first time that these coping strategies have been included within a simulation model, and therefore the first time that the peak around the four hours has been analysed so accurately using a model

Mean shear flows generated by nonlinear resonant Alfven waves

In the context of resonant absorption, nonlinearity has two different manifestations. The first is the reduction in amplitude of perturbations around the resonant point (wave energy absorption). The second is the generation of mean shear flows outside the dissipative layer surrounding the resonant point. Ruderman et al. [Phys. Plasmas 4, 75 (1997)] studied both these effects at the slow resonance in isotropic plasmas. Clack et al. [Astron. Astrophys. 494}, 317 (2009)] investigated nonlinearity at the Alfven resonance, however, they did not include the generation of mean shear flow. In this present paper, we investigate the mean shear flow, analytically, and study its properties. We find that the flow generated is parallel to the magnetic surfaces and has a characteristic velocity proportional to $\epsilon^{1/2}$, where $\epsilon$ is the dimensionless amplitude of perturbations far away from the resonance. This is, qualitatively, similar to the flow generated at the slow resonance. The jumps in the derivatives of the parallel and perpendicular components of mean shear flow across the dissipative layer are derived. We estimate the generated mean shear flow to be of the order of $10{\rm kms}^{-1}$ in both the solar upper chromosphere and solar corona, however, this value strongly depends on the choice of boundary conditions. It is proposed that the generated mean shear flow can produce a Kelvin--Helmholtz instability at the dissipative layer which can create turbulent motions. This instability would be an additional effect, as a Kelvin--Helmholtz instability may already exist due to the velocity field of the resonant Alfven waves. This flow can also be superimposed onto existing large scale motions in the solar upper atmosphere.Comment: 11 page

Guidance and control system research for improved terminal area operations

Several guidance and control system research and development activities aimed at improving the operational capabilities of commercial aircraft in the terminal area are described. The guidance and control systems were designed to improve the capacity and efficiency of terminal area operations, enhance the approach and landing capability of aircraft in adverse weather conditions, and reduce the impact of aircraft noise perceived on the ground. Specific performance features include the ability to capture and track steep glideslopes, use short final approaches, perform flares with reduced longitudinal touchdown dispersion and execute high speed runway rollout and turnoff. Results obtained from simulation studies or flight tests are shown for each of the algorithms