Large-scale Simulation Models Research Articles

Reynolds-number dependency in homogeneous, stationary two-dimensional turbulence

Turbulent solutions of the two-dimensional Navier–Stokes equations are a paradigm for the chaotic space–time patterns and equilibrium distributions of turbulent geophysical and astrophysical ‘thin’ flows on large horizontal scales. Here we investigate how homogeneous, stationary two-dimensional turbulence varies with the Reynolds number (Re) in stationary solutions with large-scale, random forcing and viscous diffusion, also including hypoviscous diffusion to limit the inverse energy cascade. This survey is made over the computationally feasible range in Re ≫ 1, approximately between 1.5 × 103 and 5.6 × 106. For increasing Re, we witness the emergence of vorticity fine structure within the filaments and vortex cores. The energy spectrum shape approaches the forward-enstrophy inertial-range form k−3 at large Re, and the velocity structure function is independent of Re. All other statistical measures investigated in this study exhibit power-law scaling with Re, including energy, enstrophy, dissipation rates and the vorticity structure function. The scaling exponents depend on the forcing properties through their influences on large-scale coherent structures, whose particular distributions are non-universal. A striking result is the Re independence of the intermittency measures of the flow, in contrast with the known behaviour for three-dimensional homogeneous turbulence of asymptotically increasing intermittency. This is a consequence of the control of the tails of the distribution functions by large-scale coherent vortices. Our analysis allows extrapolation towards the asymptotic limit of Re → ∞, fundamental to geophysical and astrophysical regimes and their large-scale simulation models where turbulent transport and dissipation must be parameterized.

Three-dimensional electromagnetic model of the pulsed-powerZ-pinch accelerator

A three-dimensional, fully electromagnetic model of the principal pulsed-power components of the 26-MA ZR accelerator [D. H. McDaniel et al., in Proceedings of the 5th International Conference on Dense $Z$-Pinches (AIP, New York, 2002), p. 23] has been developed. This large-scale simulation model tracks the evolution of electromagnetic waves through the accelerator's intermediate-storage capacitors, laser-triggered gas switches, pulse-forming lines, water switches, triplate transmission lines, and water convolute to the vacuum insulator stack. The insulator-stack electrodes are coupled to a transmission-line circuit model of the four-level magnetically insulated vacuum-transmission-line section and double-post-hole convolute. The vacuum-section circuit model is terminated by a one-dimensional self-consistent dynamic model of an imploding $z$-pinch load. The simulation results are compared with electrical measurements made throughout the ZR accelerator, and are in good agreement with the data, especially for times until peak load power. This modeling effort demonstrates that 3D electromagnetic models of large-scale, multiple-module, pulsed-power accelerators are now computationally tractable. This, in turn, presents new opportunities for simulating the operation of existing pulsed-power systems used in a variety of high-energy-density-physics and radiographic applications, as well as even higher-power next-generation accelerators before they are constructed.

Evaluation of water quality in an urban park for environmental sensitization: A large scale simulation model

Urban parks are important components of the urban environment since they are critical to improving livelihoods and the environment in densely populated cities, offering also shelter to several wildlife species and opportunities for outdoor educational activities. The park of Environmental Information and Sensitization „Antonis Tritsis” is the largest park of the Athens metropolitan area (∼1.15 km2) including an artificial system of six large reservoirs referred as lakes of total surface of approximately 6 hectares (5–6% of its total surface) constituting the largest surface of fresh water within the city structure. The aforementioned system is a „controlled” one since it is fed by two distinctly different, monitored water sources, namely the pumped local water and the occasional runoff from rains. The bottom of the „lakes” is covered by geomembrane allowing the system to be considered as a very large scale simulation experiment. The results obtained provide insight into the biochemical functioning of the water system which constitutes a useful tool as a model. Two seasonal samplings were carried out, the first during the dry summer period when water supply for the lakes is provided exclusively from a local drilling and the second in autumn of 2007 directly after an intense rainfall. The physicochemical parameters measured were temperature, conductivity, suspended particulate matter, dissolved oxygen and pH and those directly related to eutrophication were ammonium, nitrites, nitrates, phosphates, chlorophyll a and dissolved organic carbon. The microbial charge was also assessed through analyses of Enterococci, Coliforms and Escherichia coli. From the results obtained it became clear that water deriving from the drilling has high concentrations of nitrates and the microbiological parameters studied. Runoff, which washes out the nearby urban area, introduces significant quantities of nutrients, mostly phosphates, to the lake system. In the wetland in the northern part of the park, where lakeside vegetation is abundant, concentrations of both nitrates and phosphates are significantly reduced. Measurements of dissolved organic carbon and chlorophyll a indicate increased algal presence in the central lakes, linked to a certain extent to aesthetic deterioration.

Large-scale simulation modeling system for industrial construction

Industrial construction is a complex production system involving multiple supply chains. Engineers need a tool that enables them to plan and improve the system by testing different theories. This research developed a special modeling system tailored for building large-scale simulation models for industrial construction. The modeling system can be used to model an industrial construction project as a whole supply chain system, covering spool drafting, material supply, spool fabrication, module assembly, and site installation, complete with details of the production level. The entire production system can be conceived in terms of not only activities, but also product flows and information flows by the developed modeling system. The developed model was utilized to execute several simulation experimentations to test the various theories. This testing ultimately facilitates the improvement of the performance of the whole industrial construction project system.

Physical Activity into the Meal Glucose—Insulin Model of Type 1 Diabetes: In Silico Studies

A simulation model of a glucose-insulin system accounting for physical activity is needed to reliably simulate normal life conditions, thus accelerating the development of an artificial pancreas. In fact, exercise causes a transient increase of insulin action and may lead to hypoglycemia. However, physical activity is difficult to model. In the past, it was described indirectly as a rise in insulin. Recently, a new parsimonious model of exercise effect on glucose homeostasis has been proposed that links the change in insulin action and glucose effectiveness to heart rate (HR). The aim of this study was to plug this exercise model into our recently proposed large-scale simulation model of glucose metabolism in type 1 diabetes to better describe normal life conditions. The exercise model describes changes in glucose-insulin dynamics in two phases: a rapid on-and-off change in insulin-independent glucose clearance and a rapid-on/slow-off change in insulin sensitivity. Three candidate models of glucose effectiveness and insulin sensitivity as a function of HR have been considered, both during exercise and recovery after exercise. By incorporating these three models into the type 1 diabetes model, we simulated different levels (from mild to moderate) and duration of exercise (15 and 30 minutes), both in steady-state (e.g., during euglycemic-hyperinsulinemic clamp) and in nonsteady state (e.g., after a meal) conditions. One candidate exercise model was selected as the most reliable. A type 1 diabetes model also describing physical activity is proposed. The model represents a step forward to accurately describe glucose homeostasis in normal life conditions; however, further studies are needed to validate it against data.

Configuration Management for Large-Scale Scientific Computing at the UK Met Office

The UK Met Office's flexible configuration management (FCM) system uses existing open source tools, adapted for use with high-performance scientific Fortran code, to help manage evolving code in its large-scale climate simulation and weather forecasting models. FCM has simplified the development process, improved team coordination, and reduced release cycles.

A large-scale simulation model of pandemic influenza outbreaks for development of dynamic mitigation strategies

Limited stockpiles of vaccine and antiviral drugs and other resources pose a formidable healthcare delivery challenge for an impending human-to-human transmittable influenza pandemic. The existing preparedness plans by the Center for Disease Control and Health and Human Services strongly underscore the need for efficient mitigation strategies. Such a strategy entails decisions for early response, vaccination, prophylaxis, hospitalization and quarantine enforcement. This paper presents a large-scale simulation model that mimics stochastic propagation of an influenza pandemic controlled by mitigation strategies. The impact of a pandemic is assessed via measures including total numbers of infected, dead, denied hospital admission and denied vaccine/antiviral drugs, and also through an aggregate cost measure incorporating healthcare cost and lost wages. The model considers numerous demographic and community features, daily human activities, vaccination, prophylaxis, hospitalization, social distancing, and hourly accounting of infection spread. The simulation model can serve as the foundation for developing dynamic mitigation strategies. The simulation model is tested on a hypothetical community with over 1100 000 people. A designed experiment is conducted to examine the statistical significance of a number of model parameters. The experimental outcomes can be used in developing guidelines for strategic use of limited resources by healthcare decision makers. Finally, a Markov decision process model and its simulation-based reinforcement learning framework for developing mitigation strategies are presented. The simulation-based framework is quite comprehensive and general, and can be particularized to other types of infectious disease outbreaks.

Application of cellular automata modeling to seismic elastodynamics

This article details application of a physics-based cellular automata (CA) computational approach to model seismic events in an idealized linear-elastic medium. Application of rectangular-celled CA to the seismic problem is shown to yield discrete equations equivalent to the centered-difference finite difference (FD) approach. However, it is emphasized that the discrete equations are arrived at from the ‘bottom up’ using local rules vice ‘top-down’ discretization of global partial differential equations. A further distinction between the two methods concerns the location of stresses and its impact on boundary conditions: the CA approach assigns stresses to the cell faces while the FD approach assigns stress collocated with displacement components at a single node. These differences may provide important perspective on modeling arbitrary geometry with a finite difference-like approach based on cell assembly, similar to finite element analysis. Implementation of the CA paradigm using autonomous, local cells fits naturally with object-oriented programming practices and lends itself readily to distributed computing. Results are provided for an example ground-shock simulation in which a differentiated Gaussian pulse acts on the surface of a linear-elastic half-space. The CA perspective suggests a simple treatment for the free-surface boundary condition. Comparison of the computed pressure, shear, and surface waves to those computed using a staggered-grid finite difference approach demonstrates very good agreement. In addition, the simulation results suggest that the CA approach may exhibit less ‘ringing’ as waves pass, and more symmetry in left-ward and right-ward moving waves. Future directions exploiting attractive attributes of the CA approach are suggested, to include large-scale simulation, multi-resolution analysis, and coupled-field modeling.

Evaluating Regional Contra-Flow and Phased Evacuation Strategies for Texas Using a Large-Scale Dynamic Traffic Simulation and Assignment Approach

After Hurricane Rita, the need for systematic and quantitative assessments of evacuation planning and operations strategies was recognized. Both contra-flow operation and phased evacuation were put into rigorous analysis through a large-scale regional traffic simulation modeling approach. After extensive data preparation and model developments, the Central Texas Evacuation network (CTE) in Dynamic Urban Systems in Transportation (DynusT) was created. The analysis results are presented and elaborated through relevant measures of effectiveness as well as innovative visualization techniques in this paper. The results show that the contra-flow strategy yields considerable improvements in all evacuation corridors in spite of several hot spots requiring further mitigation. The phased evacuation strategy in conjunction with the contra-flow strategy brings forth further improvements, particularly for the coastal high risk zones.

Dropping Out of Social Security

The liability facing a pay-as-you-go social security system can be calculated in several ways. The exact liability measure chosen can significantly affect the conversion of a public pay-as-you-go system to a system based on individually funded accounts. Most conversions, including that which took place in Chile, as well as in many plans to convert the US system, assume the largest measure, known as the That measure pays many workers who have contributed to the public system more money than the public system is actually worth to them, thereby placing a larger burden on future generations. Other liability measures, though, are hard to implement due to an information asymmetry between the government and individuals about an individual's skill level. This paper demonstrates that a very simple reform plan - simply letting people drop out of social security - generates a truthful revelation equilibrium in which agents reveal private information about their skill level. The new assumed liability measure can be as little as half of the shutdown liability as the new measure more accurately assigns a liability for each individual based on their true value of remaining in social security. A smaller liability, therefore, is passed to future generations which also generates quicker transition paths. Moreover, interestingly, the drop out method also does a better job of protecting the welfare of the initial elderly when general revenue is used to pay for the transition. Simulation evidence is provided using a large-scale lifecycle simulation model that allows for heterogeneous skill levels. The evidence demonstrates the importance of the dropping out approach relative to the traditional conversion method that assumes the shutdown liability.

Heat Flux Distribution And Flame Shapes On The Inert Facade

Fire spread in high rise buildings from floor to floor occurs if flames emerge and extend on the façade of the building to cause ignition in the floor above the floor where flashover has developed. Even though considerable effort has been exerted to address this issue, proposed relations for heat fluxes on façade are incomplete and contradictory because the relevant physics have been poorly clarified. By systematically performing a series small scale experiments having various enclosure geometries, door-like openings and fire locations, the physics and new relations are underpinned for the emerging flames on inert facades in ventilation controlled (under-ventilated) fires at the floor of fire origin. To limit the variables and uncertainties, propane and methane gas burners create a controlled (theoretical) heat release rate at the source. Gas temperatures inside the enclosure and at the opening, heat fluxes on the façade wall, flame contours (by a Charge Coupled Device camera-CCD camera) and heat release rates (by oxygen calorimetry) inside and outside the enclosure have been measured. The gas temperatures inside the enclosure were uniform for aspect ratio (length to width) of the enclosure from one to one to three to one. Previous relations for the air inflow and heat release rate inside the enclosure were verified. The flames are highly radiative because soot can be formed at high temperatures inside the enclosure before the combustion gases and the unburned fuel exit the enclosure. The heat fluxes on an inert façade, both at the centreline and off-center above the opening, have been well correlated by identifying two length scales. One related to the effective area of the outflow ( ??1 ), and the other represented the length after which the flames turn from horizontal to vertical ( ?? 3 ). Finally, Correlation of the maximum flame width which is valid for the case having aspect ratio of the opening (width to height) between 0.375 and 2 was proposed in this paper. The results can be used for engineering calculations for real fires and for validation of new large eddy scale simulation models.

Microsimulation Calibration Using Speed-Flow Relationships

A microsimulation calibration methodology based on matching speed-flow graphs from field and simulation is presented. Evaluation and automation of matching speed-flow graphs are based on methods from pattern recognition. The methodology was applied to the US-101 freeway network in San Francisco, California, by using an evolutionary algorithm. The methodology is compared to traditional methods of calibration based on capacity and is shown to perform better. In addition, a small-scale test-network simulation model was developed to assist in calibration of large-scale simulation models. The performance of the test-network-based calibration is comparable to the US-101 simulation model.

Controlling endemic cholera with oral vaccines.

BackgroundAlthough advances in rehydration therapy have made cholera a treatable disease with low case-fatality in settings with appropriate medical care, cholera continues to impose considerable mortality in the world's most impoverished populations. Internationally licensed, killed whole-cell based oral cholera vaccines (OCVs) have been available for over a decade, but have not been used for the control of cholera. Recently, these vaccines were shown to confer significant levels of herd protection, suggesting that the protective potential of these vaccines has been underestimated and that these vaccines may be highly effective in cholera control when deployed in mass immunization programs. We used a large-scale stochastic simulation model to investigate the possibility of controlling endemic cholera with OCVs.Methods and FindingsWe construct a large-scale, stochastic cholera transmission model of Matlab, Bangladesh. We find that cholera transmission could be controlled in endemic areas with 50% coverage with OCVs. At this level of coverage, the model predicts that there would be an 89% (95% confidence interval [CI] 72%–98%) reduction in cholera cases among the unvaccinated, and a 93% (95% CI 82%–99%) reduction overall in the entire population. Even a more modest coverage of 30% would result in a 76% (95% CI 44%–95%) reduction in cholera incidence for the population area covered. For populations that have less natural immunity than the population of Matlab, 70% coverage would probably be necessary for cholera control, i.e., an annual incidence rate of ≤ 1 case per 1,000 people in the population.ConclusionsEndemic cholera could be reduced to an annual incidence rate of ≤ 1 case per 1,000 people in endemic areas with biennial vaccination with OCVs if coverage could reach 50%–70% depending on the level of prior immunity in the population. These vaccination efforts could be targeted with careful use of ecological data.

Conceptual modelling for designing large-scale simulations

Large-scale complex simulations take many years to develop and cost millions of dollars. Development of such simulations requires many areas of expertise and poses significant technical and managerial challenges. Some of the challenges can be met with the development and use of a conceptual model (CM). This paper describes a life cycle for the development of a simulation CM. The role of the CM in large-scale complex simulation model design is explained. The objectives under which a simulation CM should be created and used are enunciated. The life cycle presented provides a structured blueprint of simulation CM development and specifies the work products to be created under the designated processes together with the integrated verification and validation activities.

RESERVOIR SIMULATION—UPSCALING, STREAMLINES AND PARALLEL COMPUTING

Simulation of petroleum reservoirs is becoming more and more complex due to increasing necessity to model heterogeneity of reservoirs for accurate reservoir performance prediction. With high oil prices and less easy oil, accurate reservoir management tools such as simulation models are in more demand than ever before. The aim is to capture and preserve reservoir heterogeneity when changing over from a detailed geocellular model to a flow simulation model, minimising errors when upscaling and preventing excessive numerical dispersion by employing variable and innovative grids, as well as improved computational algorithms.For accurate and efficient simulation of large-scale models there are essentially three choices: upscaling, which involves averaging of parameters for several blocks, resulting in a coarser model that executes faster; the use of streamline simulation, which uses a more optimal grid, combined with a different computational algorithm for increased efficiency; and, the use of parallel computing techniques, which use superior hardware configurations for efficiency gains. With uncertainty screening of various multiple geostatistical realisations and investigation of alternative development scenarios— now commonplace for determining reservoir performance—computational efficiency and accuracy in modelling are paramount. This paper summarises the main techniques and methodologies involved in considering geocellular models for flow simulation of reservoirs, commenting on advantages and disadvantages among the various possibilities. Starting with some historic comments, the three modes of simulation are reviewed and examples are given for illustrative purposes, including a case history for the Bayu-Undan Field, Timor Sea.

Linking Electronic Medical Records To Large-Scale Simulation Models: Can We Put Rapid Learning On Turbo?

One method for rapid learning is to use data from electronic medical records (EMRs) to help build and validate large-scale, physiology-based simulation models. These models can than be used to help answer questions that cannot be addressed directly from the EMR data. Their potential uses include analyses of physiological pathways; simulation and design of clinical trials; and analyses of clinical management tools such as guidelines, performance measures, priority setting, and cost-effectiveness. Linking the models to EMR data also facilitates tailoring analyses to specific populations. The models' power and accuracy can be improved by linkage to comprehensive, person-specific, longitudinal data from EMRs.

Heat fluxes and flame heights in façades from fires in enclosures of varying geometry

Fire spread in high rise buildings from floor to floor occurs if flames emerge and extend on the façade of the building to cause ignition in floors above the floor of fire origin. Even though considerable effort has been exerted to address this issue, proposed relations for flame heights and heat fluxes are incomplete and contradictory because the relevant physics have been poorly clarified. By performing numerous experiments in small scale enclosures having various door-like openings and fire locations, the physics and new relations are underpinned for flames on façade emerging from (under-ventilated) ventilation controlled fires at the floor of fire origin. To limit the variables and uncertainties, propane and methane gas burners created controlled (theoretical) heat release rates at the source. Gas temperatures inside the enclosure and at the opening, heat fluxes on the façade wall, flame contours (by a CCD camera) and heat release rates (by oxygen calorimetry) inside and outside the enclosure have been measured. The gas temperatures inside the enclosure were uniform for aspect ratio (length to width) of the enclosure varying from one to three to one. Previous relations for the air inflow and heat release rate inside the enclosure were verified. These flames are highly radiative because soot can be formed at high temperatures inside the enclosure before the combustion gases and the unburned fuel exit the enclosure. Remarkably the efficiency of combustion is one for well over-ventilated and very under-ventilated fires by it dropped to 80% for burning conditions around stoichiometric. The flame height and heat fluxes have been well correlated by identifying new length scales related to the effective area of the outflow and the length after which the flow turns from horizontal to vertical due to buoyancy. The results can be used for engineering calculations for real fires and for validation of new large eddy scale simulation models.

Development of Jeju–Haenam HVDC system model for dynamic performance study

This paper deals with the development of the simulation models of the Jeju–Haenam HVDC system and its dynamic performance study in order to verify the control characteristics of the HVDC system. Also, the model development requirement and criteria are discussed. Guidelines for developing large-scale simulation models for detailed electromagnetic studies are presented according to the results of the modeling project.

Parametric Model Reduction for Fast Simulation of Cyclic Voltammograms

Model order reduction is a well-established technique for fast simulation of large-scale models based on ordinary differential equations, especially those in the field of integrated circuits and microelectro-mechanical systems. In this paper, we propose the use of parametric model reduction for fast simulation of a cyclic voltammogram. Instead of being considered as a time varying system, the model for a cyclic voltammogram is treated as a system with a parameter (applied voltage) which is to be preserved during model reduction. Because voltage is preserved in the symbolic form during model reduction, we can simulate the cyclic voltammogram with a reduced system and therefore invest much less time and memory as compared with direct simulation based on the original large-scale model. We present our approach for a case study based on scanning electrochemical microscopy.

Mitigation strategies for pandemic influenza in the United States

Recent human deaths due to infection by highly pathogenic (H5N1) avian influenza A virus have raised the specter of a devastating pandemic like that of 1917-1918, should this avian virus evolve to become readily transmissible among humans. We introduce and use a large-scale stochastic simulation model to investigate the spread of a pandemic strain of influenza virus through the U.S. population of 281 million individuals for R(0) (the basic reproductive number) from 1.6 to 2.4. We model the impact that a variety of levels and combinations of influenza antiviral agents, vaccines, and modified social mobility (including school closure and travel restrictions) have on the timing and magnitude of this spread. Our simulations demonstrate that, in a highly mobile population, restricting travel after an outbreak is detected is likely to delay slightly the time course of the outbreak without impacting the eventual number ill. For R(0) < 1.9, our model suggests that the rapid production and distribution of vaccines, even if poorly matched to circulating strains, could significantly slow disease spread and limit the number ill to <10% of the population, particularly if children are preferentially vaccinated. Alternatively, the aggressive deployment of several million courses of influenza antiviral agents in a targeted prophylaxis strategy may contain a nascent outbreak with low R(0), provided adequate contact tracing and distribution capacities exist. For higher R(0), we predict that multiple strategies in combination (involving both social and medical interventions) will be required to achieve similar limits on illness rates.