Molecular Dynamics Studies on Nanoscale Gas Transport

Three-dimensional molecular dynamics (MD) simulations of nanoscale gas flows are studied to reveal surface effects. A smart wall model that drastically reduces the memory requirements of MD simulations for gas flows is introduced. The smart wall molecular dynamics (SWMD) represents three-dimensional FCC walls using only 74 wall Molecules. This structure is kept in the memory and utilized for each gas molecule surface collision. Using SWMD, fluid behavior within nano-scale confinements is studied for argon in dilute gas, dense gas, and liquid states. Equilibrium MD method is employed to resolve the density and stress variations within the static fluid. Normal stress calculations are based on the Irving-Kirkwood method, which divides the stress tensor into its kinetic and virial parts. The kinetic component recovers pressure based on the ideal gas law. The particle-particle virial increases with increased density, while the surface-particle virial develops due to the surface force field effects. Normal stresses within nano-scale confinements show anisotropy induced primarily by the surface force-field and local variations in the fluid density near the surfaces. For dilute and dense gas cases, surface-force field that extends typically 1nm from each wall induces anisotropic normal stress. For liquid case, this effect is further amplified by the density fluctuations that extend beyond the three field penetration region. Outside the wall force-field penetration and density fluctuation regions the normal stress becomes isotropic and recovers the thermodynamic pressure, provided that sufficiently large force cut-off distances are utilized in the computations. Next, non-equilibrium SWMD is utilized to investigate the surface-gas interaction effects on nanoscale shear-driven gas flows in the transition and free molecular flow regimes. For the specified surface properties and gas-surface pair interactions, density and stress profiles exhibit a universal behavior inside the wall force penetration region at different flow conditions. Shear stress results are utilized to calculate the tangential momentum accommodation coefficient (TMAC) between argon gas and FCC walls. The TMAC value is shown to he independent of the now properties and Knudsen number in all simulations. Velocity profiles show distinct deviations from the kinetic theory based solutions inside the wall force penetration depth, while they match the linearized Boltzmann equation solution outside these zones. Afterwards, surface effects are studied as a function of the surface-gas potential strength ratio (ϵ wf/ϵff) for the shear driven argon gas flows in the early transition and tree molecular flow regimes. Results show that increased ϵwf/ϵ ff results in increased gas density, leading towards monolayer adsorption on surfaces. The near wall velocity profile shows reduced gas slip, and eventually velocity stick with increased ϵwf/ϵ ff. Similarly, using MD predicted shear stress values and kinetic theory, TMAC are calculated as a function of ϵwf/ϵ ff and TMAC values are shown to be independent of the Knudsen number. Results indicate emergence of the wall force field penetration depth as an additional length scale for gas flows in nano-channels, breaking the dynamic similarity between rarefied and nano-scale gas flows solely based on the Knudsen and Mach numbers

Fluid velocity slip and temperature jump at a solid surface

A comprehensive review of current analytical models, experimental techniques, and influencing factors is carried out to highlight the current challenges in this area. The study of fluid-solid boundary conditions has been ongoing for more than a century, starting from gas-solid interfaces and progressing to that of the more complex liquid-solid case. Breakthroughs have been made on the theoretical and experimental fronts but the mechanism behind the phenomena remains a puzzle. This paper provides a review of the theoretical models, and numerical and experimental investigations that have been carried out till date. Probable mechanisms and factors that affect the interfacial discontinuity are also documented

ASA 2021 Statistics and Information Systems for Policy Evaluation

This book includes 25 peer-reviewed short papers submitted to the Scientific Opening Conference titled “Statistics and Information Systems for Policy Evaluation”, aimed at promoting new statistical methods and applications for the evaluation of policies and organized by the Association for Applied Statistics (ASA) and the Department of Statistics, Computer Science, Applications DiSIA “G. Parenti” of the University of Florence, jointly with the partners AICQ (Italian Association for Quality Culture), AICQ-CN (Italian Association for Quality Culture North and Centre of Italy), AISS (Italian Academy for Six Sigma), ASSIRM (Italian Association for Marketing, Social and Opinion Research), Comune di Firenze, the SIS – Italian Statistical Society, Regione Toscana and Valmon – Evaluation & Monitoring

Exploring a capability-demand interaction model for inclusive design evaluation

Designers are required to evaluate their designs against the needs and capabilities of their target user groups in order to achieve successful, inclusive products. This dissertation presents exploratory research into the specific problem of supporting analytical design evaluation for Inclusive Design. The analytical evaluation process involves evaluating products with user data rather than testing with actual users. The work focuses on the exploration of a capability-demand model of product interaction as the basis for analytical inclusive evaluation. This model suggests that by comparing the measured sensory, cognitive and motor capabilities of a user population to the corresponding product demands, the degree of fit between users and products can be assessed. The research problem was addressed by firstly examining theories of human function and performance together with existing sources of user capability data. It was found that user capability data was fragmented and lacking in terms of predicting design exclusion and difficulty at the population level. More fundamentally, however, it was found that the relationships between measured capability in populations with low functional capacity and real world task performance with products (such as errors, times and difficulty) were not well understood. Given that an understanding of these relationships are necessary to guide capability data collection and to drive valid and robust analytical evaluation methods, the research effort focused on exploring these relationships via empirical and analytical studies. The research process culminated in an experimental study with nineteen users of various functional capability profiles performing tasks with four consumer products (a clock radio, a mobile phone, a blender and a vacuum cleaner). Measures of user capability were related to corresponding product demands (on those capabilities) and task outcome measures. A complex picture emerged, where linear relationships did not generally account for significant variance in task outcome measures. Further, it appeared that multiple capabilities were possibly interacting in unknown ways to support real world interaction. These indicative results point to the further investigation of multivariate and non-linear models for describing capability-demand relationships, and also the replication of similar studies with larger sample sizes to confirm the relationships observed. The resulting overall recommendation, therefore, is that there is a need to direct research efforts in this critical but largely unexplored area of capability-demand model building for Inclusive Design evaluation

Measuring and Modeling Viscoelastic Relaxation of the Lithosphere with Application to the Northern Volcanic Zone, Iceland

Viscoelastic relaxation of the stress perturbation caused by an earthquake or diking event can produce measurable ground deformation over 100 km away from the source. We consider the role of viscoelastic relaxation in two different contexts. First, we explore the role that post-seismic relaxation may play in loading a fault over the entire seismic cycle. Viscous relaxation recycles the stress that is shed by the co-seismic fault, acting to reload the fault with stresses in a non-linear fashion. Under conditions of rapid post-seismic relaxation and slow tectonic loading, stress recycling via viscoelastic relaxation can lead to clustering of earthquakes in time. The second context in which we consider viscoelastic relaxation involves the lithospheric response to a mid-ocean ridge rifting episode in Northern Iceland. The diking and subsequent relaxation act as a natural rock mechanics experiment, and in measuring and modeling the post-rifting response we aim to constrain the rheological properties of the Icelandic lithosphere. In order to use post-seismic or post-rifting relaxation to probe properties of the lithosphere, we must be able to precisely measure surface deformation. To that end, we have developed a couple of new interferometric synthetic aperture radar (InSAR) processing approaches: (1) Automatically producing multiple interferograms in a common coordinate system and (2) removing displacements caused by ocean tidal loading from InSAR observations. Both of these developments are essential as we begin to consider the systematic use of tens to hundreds of interferograms

Lay perceptions of health, housing and community on the Kent Coast, England

Lay perceptions of health inequalities are becoming increasingly important in developing local housing strategies and many coastal areas have attracted recent attention because of high levels of deprivation. This paper draws from the findings of 14 socioeconomically and geographically representative focus groups as part of the wider French British Interreg IIIA project examining health inequalities and health behaviours in South East England and Northern France. Kent coastal areas were identified as being of particular and unique interest, leading to a wider literature review of socio-economic and health inequalities more generally in coastal towns and the effect of geography on health. Participants in the focus groups particularly suggested that the loss of traditional industries – notably the holiday trade (tourism), but also other local employment – had led to new low-income, deprived communities, including immigrant communities, whose needs often went unmet. Participants identified the changing nature of coastal or seaside housing from guest house to residential living accommodation and the relationship to the benefit system as being of particular concern, affecting both physical and mental health and the wider environment. However, participants also described successful local community-led regeneration solutions which could run alongside new local authority responsibilities to tackle health inequalities. The focus group findings suggest that lay perceptions are in many ways close to recent governmental research findings which identify the coastal regions as unique environments, some with similar levels of deprivation to inner urban and rural areas and lacking sufficient public investment. The results of this study suggest that the public have additional concerns around housing allocation policies creating marginal coastal communities and how these needs might be addressed. New strategies need to involve the communities affected. Although this can prove challenging, there is a new range of legislative provisions to tackle complex and multifaceted housing, social, economic and environmental conditions faced by those suffering some of the most acute health inequalities

The critical slip distance for seismic and aseismic fault zones of finite width

We present a conceptual model for the effective critical friction distance for fault zones of finite width. A numerical model with 1D elasticity is used to investigate implications of the model for shear traction evolution during dynamic and quasi-static slip. The model includes elastofrictional interaction of multiple, parallel slip surfaces, which obey rate and state friction laws with either Ruina (slip) or Dieterich (time) state evolution. A range of slip acceleration histories is investigated by imposing perturbations in slip velocity at the fault zone boundary and using radiation damping to solve the equations of motion. The model extends concepts developed for friction of bare surfaces, including the critical friction distance L, to fault zones of finite width containing wear and gouge materials. We distinguish between parameters that apply to a single frictional surface, including L and the dynamic slip weakening distance do, and those that represent slip for the entire fault zone, which include the effective critical friction distance, Dcb, and the effective dynamic slip weakening distance Do. A scaling law for Dcb is proposed in terms of L and the fault zone width. Earthquake source parameters depend on net slip across a fault zone and thus scale with Dcb, Do, and the slip at yield strength Da. We find that Da decreases with increasing velocity jump size for friction evolution via the Ruina law, whereas it is independent of slip acceleration rate for the Dieterich law. For both laws, Da scales with fault zone width and shear traction exhibits prolonged hardening before reaching a yield strength. The parameters Dcb and Do increase roughly linearly with fault zone thickness. This chapter and a companion chapter in the volume discuss the problem of reconciling laboratory measurements of the critical friction distance with theoretical and field-based estimates of the effective dynamic slip weakening distance

Reliability prediction in early design stages

In the past, reliability is usually quantified with sufficient information available. This is not only time-consuming and cost-expensive, but also too late for occurred failures and losses. For solving this problem, the objective of this dissertation is to predict product reliability in early design stages with limited information. The current research of early reliability prediction is far from mature. Inspired by methodologies for the detail design stage, this research uses statistics-based and physics-based methodologies by providing general models with quantitative results, which could help design for reliability and decision making during the early design stage. New methodologies which accommodate component dependence, time dependence, and limited information are developed in this research to help early accurate reliability assessment. The component dependence is considered implicitly and automatically without knowing component design details by constructing a strength-stress interference model. The time-dependent reliability analysis is converted into its time-independent counterpart with the use of the extreme value of the system load by simulation. The effect of dependent interval distribution parameters estimated from limited point and interval samples are also considered to obtain more accurate system reliability. Optimization is used to obtain narrower system reliability bounds compared to those from the traditional method with independent component assumption or independent distribution parameter assumption. With new methodologies, it is possible to obtain narrower time-dependent system reliability bounds with limited information during early design stages by considering component dependence and distribution parameter dependence. Examples are provided to demonstrate the proposed methodologies --Abstract, page iv