Roger Brooks

Simplifying Sirius: sensitivity analysis and development of a meta-model for wheat yield prediction

A sensitivity analysis and analysis of the structure of the Sirius wheat model has resulted in the development of a simpler meta-model, which produced very similar yield predictions to Sirius of potential and water-limited yields at two locations in the UK, Rothamsted and Edinburgh. This greatly increases the understanding of the nature and consequences of the relationships implicit within Sirius. The analysis showed that the response of wheat crops to climate could be explained using a few simple relationships. The meta-model aggregates the three main Sirius components, the calculation of leaf area index, the soil water balance model and the evapotranspiration calculations, into simpler equations. This results in a requirement for calibration of fewer model parameters and means that weather variables can be provided on a monthly rather than a daily time-step, because the meta-model can use cumulative values of weather variables. Consequently the meta-model is a valuable tool for regional impact assessments when detailed input data are usually not available. Because the meta-model was developed from the analysis of Sirius, rather than from statistical fitting of yield to weather data, it should perform well for other locations in Great Britain and with different management scenarios.

Simplification in the simulation of manufacturing systems

Given the widespread acceptance of the importance of simplicity in management science models, the scarcity of research into simplification is perhaps surprising. In the simulation of manufacturing systems, simplification is often not attempted and, on the (misguided) assumption that more detailed models are necessarily more accurate and therefore better, common practice is to build and use the most complex model that can be built in the time available. However, for cases where the only results required are averages, such as long term throughput rates, it will often be possible to reduce the model to such a simple version that an analytical solution becomes feasible and the simulation redundant. An eight stage procedure is proposed for doing the reductions and two manufacturing case studies are described.

Conceptual Modeling for Discrete-Event Simulation

Bringing together an international group of researchers involved in military, business, and health modeling and simulation, Conceptual Modeling for Discrete-Event Simulation presents a comprehensive view of the current state of the art in the field. The book addresses a host of issues, including: What is a conceptual model? How is conceptual modeling performed in general and in specific modeling domains? What is the role of established approaches in conceptual modeling? Each of the books six parts focuses on a different aspect of conceptual modeling for simulation. The first section discusses the purpose and requirements of a conceptual model. The next set of chapters provides frameworks and tools for conceptual modeling. The book then describes the use of soft systems methodology for model structuring as well as the application of software engineering methods and tools for model specification. After illustrating how conceptual modeling is adopted in the military and semiconductor manufacturing, the book concludes with a discussion on future research directions. This volume offers a broad, multifaceted account of the field by presenting diverse perspectives on what conceptual modeling entails. It also provides a basis upon which these perspectives can be compared.

Component-based simulation on the Web?

Various forms of distributed simulation are possible over the world-wide web, including simple multiple replications of the same model, client-server architectures for one or more simultaneously running models and the distributed operation of one or more linked models. Like all web-based operations, these simulations are slow due to current bandwidth limitations, but that could change in the next few years. Languages such as Java make this distributed work possible within standard web-browsers such as Internet Explorer and Netscape, though security considerations mean that this is not always straightforward. Component-based simulation stems from the ideas of object-orientation, which enable libraries of simulation based components to be developed for re-use. The development of the world-wide-web means that distributed component, discrete simulation libraries in Java are now feasible. This paper reviews some of these developments and considers requirements for such distributed libraries, drawing on our experience at Lancaster.

Empirical investigations of conceptual modeling and the modeling process

Conceptual modeling, deciding what to include in the model, is a very important task in the modeling process. However, it has so far received relatively little attention in the literature and there is a lack of empirical data. This paper describes three empirical studies on conceptual modeling and the modeling process. In the first one, data on the time spent on different topics during real simulation projects by an expert and by groups of novice modelers was collected and analyzed. The second study was a questionnaire survey to obtain data on conceptual models and modeling processes for projects carried out by experienced modelers. The third study was an experiment to investigate the effect of model size on the ease of understanding of the model.

Critical issues in the development of component-based discrete simulation

The development of computer simulation methods has proceeded in parallel with those in other areas of computing. Thus, interest in component-based approaches is evident both in general software engineering and in computer simulation, with advocates arguing that component-based approaches are far superior to more conventional ways of developing software and applications. Potential benefits are reduced cost and development time arising from reuse of components, as well as easier model adaptation due to the features of extensibility and evolvability. If these are to be realised, then organisations must develop ways to ensure that costs and benefits are shared and should ensure that suitable libraries and component distribution frameworks are available. In a simulation context it is also crucial to ensure that the issue of model credibility and validity is addressed even if the components are thought valid.

Improving the understanding of conceptual modelling

Conceptual modelling, determining what to include in the model and at what level of detail, affects model performance directly. However, it has so far received little attention in the literature mainly because conceptual modelling is viewed as more of an art than science. This paper describes a study on conceptual modelling in which data on the time spent on the different topics during real simulation projects by an expert and by novice modellers was collected and analysed. There were several differences between the data for the expert and the data for the novices, possibly indicating a difference in the modelling style.

UNDERSTANDING THE DYNAMICS OF INDUSTRIAL NETWORKS USING KAUFFMAN BOOLEAN NETWORKS

Industrial networks offer a prime example of the tension between system stability and change. Change is necessary as a response to environmental variation, whereas stability provides the underpinning for long-term investment and the exploitation of efficiencies. Whilst one of the key themes in industrial network research has been the dynamics of change, relatively little work, empirical or theoretical, has been devoted to the dynamics of stability. This paper presents a new approach to this problem by using Boolean networks, which were originally devised by Stuart Kauffman as an abstract model of genetic networks. The elements in the model are connected by rules of Boolean logic, and a novel aspect of this research is that the elements represent the industrial network exchanges rather than stock entities (the organizations). The model structure consists of interactions between the exchanges, and the results represent the pattern of exchange episodes. A total of 42 networks were modeled and the dynamics analyzed in detail, and five of these cases are presented in this paper. The models produced realistic behavior and provided some insights into the reasons for stability in industrial networks.

Experimental investigation of the impacts of virtual reality on discrete-event simulation

This paper presents the results of experimental studies that were undertaken to test the impacts of virtual reality (VR) on discrete event simulation (DES). The experiments focused on spotting errors in the DES model. The models were developed in 2D and 3D/VR displays using WITNESS. The 2D display used icons and other visualization techniques that confine its scope to essentially flat 2D surface. On the other hand, the 3D display was represented by means of a three-axis spatial position (XYZ) plots, but appeared on a two-dimensionally mappings, otherwise known as 2.5D. The experiments involved paid participants who were recruited from amongst the staff and students of Lancaster University, UK. The results showed that it is easier, and faster to spot errors in 3D/VR model than in 2D. The findings also indicated that users can easily understand the modeled operation of 3D/VR display compared to 2D, irrespective of background or technical ability.

The population genetics of the self-incompatibility polymorphism in Papaver rhoeas . XI. The effects of limited pollen and seed dispersal, overlapping generations and variation in plant size on the variance of S-allele frequencies in populations at equilibrium

The effects of limited pollen and seed dispersal, of overlap between generations and of variation in plant size on the steady-state variance of S-allele frequencies have been investigated in a simulated population of size 3840 containing 16 S-alleles whose initial frequencies were exactly equal. Simulations were run with each of the 16 possible combinations of these four factors to investigate their effects on the time for the population to reach steady state and on the average variance of S-allele frequencies in steady state. The time to steady state appeared to be relatively unaffected by any of the factors and was about 50 generations. However, the steady-state variance was markedly affected, with variation in plant size increasing this variance by an average of 228 per cent and overlapping generations decreasing the variance by an average of 30 per cent. The effects of limited pollen and seed dispersal were individually small, although their combined effect was to increase the steady-state variance by an average of 12 per cent. Limited seed and pollen dispersal, when combined with variation in plant size, caused the alleles to cluster. The four factors together caused a large increase in the average steady-state variance. Furthermore, even when a population is in steady state, the variance for a particular generation can be considerably greater than this average value. Consequently the frequencies of the S-alleles of a population in steady state can be very different. It is possible, therefore, that the large variation in S-allele frequencies found in samples taken from Papaver rhoeas populations is consistent with their being in steady state.