Mark Atherton

Non-Newtonian and flow pulsatility effects in simulation models of a stented intracranial aneurysm

Three models of different stent designs implanted in a cerebral aneurysm, originating from the Virtual Intracranial Stenting Challenge’07, are meshed and the flow characteristics simulated using commercial computational fluid dynamics (CFD) software in order to investigate the effects of non-Newtonian viscosity and pulsatile flow. Conventional mass inflow and wall shear stress (WSS) output are used as a means of comparing the CFD simulations. In addition, a WSS distribution is presented, which clearly discriminates in favour of the stent design identified by other groups. It is concluded that non-Newtonian and pulsatile effects are important to include in order to avoid underestimating WSS, to understand dynamic flow effects, and to discriminate more effectively between stent designs.

Robust optimization of cardiovascular stents: a comparison of methods

Modern engineering design contains both creative and analytic components. This paper discusses the design process and illustrates links between design optimization and conceptual design through the re-design of a cardiovascular stent. A comparison is presented of two methods for design improvement: genetic algorithms (GA) and model-based robust engineering design (RED). Computational fluid dynamics (CFD) models are used to generate measurements of the quality of competing designs based on the concept of dissipated power. Alternative performance measures are also discussed. Environmental noise is introduced into the analysis and consideration is given to the treatment of discrete and continuous design parameters. Improved designs are identified using both methods and verified with further CFD analyses, and the benefits of each method are discussed.

A finite-element-based formulation for sensitivity studies of piezoelectric systems

Sensitivity analysis is a branch of numerical analysis which aims to quantify the effects that variability in the parameters of a numerical model have on the model output. A finite-element-based sensitivity analysis formulation for piezoelectric media is developed here and implemented to simulate the operational and sensitivity characteristics of a piezoelectric-based distributed mode actuator (DMA). The work acts as a starting point for robustness analysis in the DMA technology.

Multi-scale interaction of particulate flow and the artery wall

We discuss, from the perspective of basic science, the physical and biological processes which underlie atherosclerotic (plaque) initiation at the vascular endothelium, identifying the widely separated spatial and temporal scales which participate. We draw on current, related models of vessel wall evolution, paying particular attention to the role of particulate flow (blood is not a continuum fluid), and proceed to propose, then validate all the key components in a multiply-coupled, multi-scale modeling strategy (in qualitative terms only, note). Eventually, this strategy should lead to a quantitative, patient-specific understanding of the coupling between particulate flow and the endothelial state.

Effect of surface roughness on friction behaviour of steel under boundary lubrication

The friction behaviour of grinded and polished surfaces was evaluated by using a reciprocal sliding tester under lubrication with PAO, PAO + ZnDTP and PAO + ZnDTP + MoDTC. Friction coefficients on the smooth surfaces showed higher values compared to those on the rough surfaces. For lubrication incorporating PAO and PAO + ZnDTP + MoDTC, friction coefficients on both the smoothest and the roughest surfaces decreased with sliding time. On the other hand, friction coefficients between these extremes decreased with sliding time. In this paper, the effects of surface roughness on friction behaviour are discussed.

Some fundamental aspects of self-levitating sliding contact bearings and their practical implementations:

In this study, fundamental aspects and mechanisms of acoustic levitation together with governing equations are presented first. Then, the acoustic levitation phenomenon is considered as a new way to design air suspension systems capable of self-levitation. A particular emphasis is laid on journal bearings and their specific geometrical configuration. A practical feasibility of using acoustic levitation to separate contacting surfaces is supported and illustrated by results of experimental testing of a number of prototype devices.

COMMUNITY STRUCTURE DETECTION IN THE EVOLUTION OF THE UNITED STATES AIRPORT NETWORK

This paper investigates community structure in the US Airport Network as it evolved from 1990 to 2010 by looking at six bi-monthly intervals in 1990, 2000 and 2010, using data obtained from the Bureau of Transportation Statistics of the US Department of Transport. The data contained monthly records of origin–destination pairs of domestic airports and the number of passengers carried. The topological properties and the volume of people traveling are both studied in detail, revealing high heterogeneity in space and time. A recently developed community structure detection method, accounting for the spatial nature of these networks, is applied and reveals a picture of the communities within. The patterns of communities plotted for each bi-monthly interval reveal some interesting seasonal variations of passenger flows and airport clusters that do not occupy a single US region. The long-term evolution of the network between those years is explored and found to have consistently improved its stability. The more recent structure of the network (2010) is compared with migration patterns among the four US macro-regions (West, Midwest, Northeast and South) in order to identify possible relationships and the results highlight a clear overlap between US domestic air travel and migration.

Bond Graph Based Sensitivity and Uncertainty Analysis Modelling for Micro-Scale Multiphysics Robust Engineering Design

Components within micro-scale engineering systems are often at the limits of commercial miniaturization and this can cause unexpected behavior and variation in performance. As such, modelling and analysis of system robustness plays an important role in product development. Here schematic bond graphs are used as a front end in a sensitivity analysis based strategy for modelling robustness in multiphysics micro-scale engineering systems. As an example, the analysis is applied to a behind-the-ear (BTE) hearing aid. By using bond graphs to model power flow through components within dierent physical domains of the hearing aid, a set of dierential equations to describe the system dynamics is collated. Based on these equations, sensitivity analysis calculations are used to approximately model the nature and the sources of output uncertainty during system operation. These calculations represent a robustness evaluation of the current hearing aid design and oer a means of identifying potential for improved designs of multiphysics systems by way of key parameter identification.

Bond graph analysis in robust engineering design

Within engineering design, optimization often involves building models of working systems to improve design objectives such as performance, reliability and cost. Bond graph models express systems in terms of energy flow and can be used to identify key factors that influence system behaviour. Robust Engineering Design (RED) is a strategy for the optimization of systems through experimentation and empirical modelling; however, experiments can often be prohibitively expensive for large or complex systems. By using bond graphs as a front-end to RED, experiments on systems could be designed more efficiently, reducing the number of experiments required for accurate empirical modelling. Two case study examples are given which show that bond graphs can be used to good effect in the empirical analysis of engineering systems.

NFAL Prototype Design and Feasibility Analysis for Self-Levitated Conveying

ABSTRACT In order to avoid friction and scratching when conveying object, an acoustic levitation prototype was designed to verify the feasibility. The modal shapes and the forced harmonic shapes of the prototype are obtained by an ANSYS coupled field computation with a one-quarter symmetry model and the levitation capacity was assessed by the use of groups of simulation and physical testing. The simulation results showed that the pure flexural and mixed flexural wave shapes with different wave numbers existed at some specific frequency. The amplitude in the central point of an aluminum plate having four piezo-electric discs glued to the bottom surface was simulated for a frequency spectrum. The experimental results confirmed the theoretical results and the feasibility of the prototype and confirm that objects can be floated at several resonant frequencies under forced vibrating condition. The system can provide largest bearing capacity when both the piezoelectric disc and the plate resonances coincide.