The paper examines the behavior of the supply chain through a system dynamic model. The theoretical part presents system dynamics, the Beer game and the simulation environment in which the model was created. In the practical part, the response of five surveyed decision-making order rules on the 4 patterns of demand is analyzed.

The framework of a stream ecosystem simulation model is described. Using this framework and data from two different geographical areas, a cold desert stream and a generalized mountain stream, exploratory sensitivity analysis was performed on the model. This was accomplished by qualitatively comparing outputs of a series of simulations in which a different level of a driving variable was used in each simulation. Based on these results, recommendations are made for improving the structure of the model.

In this study, biogeochemical regimes of Marguerite Bay and the adjacent part of the West Antarctic Peninsula (WAP) continental shelf were delineated through integration of nutrient, hydrographic, and biological measurements obtained during the LTER and SO GLOBEC studies during austral summer, autumn, and winter of 2001 and 2002. Marguerite Bay biogeochemical regime was found to differ from those of the adjacent WAP continental shelf. In terms of Treguer and Jacques (1992), Marguerite Bay is a combination of Coastal Continental Shelf Zone (CCSZ) and Seasonal Ice Zone (SIZ) distinguished by shallow mixing regime, high primary production and export production. At the end of the growing season (autumn) in both years, waters in Marguerite Bay were strongly depleted in nutrients (the deficits of total inorganic nitrogen (NO3-+NO2-+NH4+) and silica were >0.6 mol m-2 and >2.5 mol m-2, respectively). Observed ΔN/ΔP removal ratios of 10-12.5, lower than that of Redfield et al. (1963), and ΔSi/ΔN removal ratios as high as 4-5 indicated the dominance of diatoms. High autumnal ammonium stocks (>0.25 mol m-2) were observed in Marguerite Bay and were co-located with the areas of the highest nutrient deficits suggesting spatial coupling between primary and heterotrophic production during both years. Consistency of this feature was not disrupted by significant interannual variability of biological production in Marguerite Bay that resulted in ~30-50% reduction in nutrient deficits and ammonium stocks from the first year to the next.The other two biogeochemical regimes were at the central part of the continental shelf characterized by mixed phytoplankton community and at the outer shelf dominated by diatoms. Both regimes were characterized by considerably lower depletion of nutrients compare to those of the Marguerite Bay regime and were consistent between the two years. Interannual variability of biological production and possible sources of high ammonium stocks in Marguerite Bay were studied with a one-dimensional model, a modification of that of Walsh et al. (2001). The model attributed the decline in nutrient deficits to the difference in sea ice cover dynamics between two years. The greater sea ice presence led to the somewhat lower primary production during the second year compare to the fist one. Moreover, model's tight coupling between primary and bacterial production resulted in a decline of bacterial ammonification between the two years. Bacteria were found to be the primary source of ammonium in the Marguerite Bay model. Yet, 3-4-fold fluctuations in macro- and mesozooplankton biomass might have led to 15-25% variability in model's autumnal ammonium stocks.

Most of the early wind erosion research undertaken in Australia, concentrated on how wind erosion affects cultivated farm land. However, in the 1990's the focus of wind erosion research in Australia started to shift to include rangeland environments. Initially these rangeland experiments used experimental configurations that were developed for cultivated fields. This meant that in most cases a sampler was set up in the middle of a field and it was assumed that the data collected was representative of the field as a whole. It was also assumed that temporal changes in dust fluxes/concentration reflect overall changes in the land type erodibility and wind erosivity. However, recent experiments and field observations within the rangelands, of the Channel Country suggest that this assumption is not valid. These experiments and observations suggest that there are substantial spatial and temporal variations in erodibility within individual land types. Such variations complicate the interpretation of temporal and spatial erosion trends. In particular, this variability implies that it is difficult to compare sampler data between different wind erosion events. To begin quantifying and comparing sampler data between events within the rangeland environments, the Dust Source Interaction Simulation Model (DSism) was developed to simulate the effect that physical processes and spatial variations in erodibility have upon observed dust concentration pro- files. The modelling/simulation approach used is closely linked to experimental data via the extensive use of sensitivity testing. Another key feature of the DSism approach, is its flexibility in allowing different dust source areas to have particle emission characteristics. This combined sensitivity testing and simulation approach has provided new insights into the wind erosion processes. By using DSism, it has been possible to identify several key features of the wind erosion process within rangeland environments. The first observation is that spatial and temporal changes in erodibility produce distinct changes in both the vertical and crosswind dust concentration profiles. Further investigations, indicate that the dispersion processes in operation vary from event to event. In particular, the results presented here indicate that surface heating plays an important role in some wind erosion events. These results also suggest that even small variations in the vertical dust concentration profile can reflect temporal and spatial changes in processes and erodibility. Finally the simulation results show that the particle size distribution of a vertical dust concentration profile depends on (a) the processes in operation during a given event and (b) the spatial variation in the particle size emission characteristics of the various source areas. These findings have several important implications. In particular, they indicate that both the crosswind and vertical dust concentration profiles can be viewed as amalgamation of several distinct plumes from different dust source areas and that dust concentration profiles contain significant information about both the spatial distribution of sources and the processes in operation during any given event. Most field studies have used regression models to describe the variation in dust concentration with height. A problem with this approach is that it assumes that the variation in dust concentration with height, always has a given functional form (or shape) and that dust concentration always decreases with height. Field observations, indicate that this assumption is only valid for some events within rangeland environments and that dust concentration does not always decrease with height in these environments. In most cases, such variations from the regression fit have been assumed to be the result of experimental 'noise' (error) or spatial variations in erodibility. This thesis presents, modelling and field evidence, which suggests that such variations, are the result of a combination of spatial variations in erodibility and changes in thermal conditions.

Heat recovery steam generators (HRSG) play a very important role in combined cycle (CC) power plants, where steam is generated from a gas turbine exhaust and supplied at the appropriate pressure and temperature to steam turbines for further power generation. The power plants achieve an overall efficiency above 55% and are ideally suited for combined heat and power generation in utility systems. The performance of energy conversion is improved by reducing exergy losses which implies reducing the temperature difference between the combustion gas and the steam cycle. Thus recent HRSG designs include up to three pressure levels with reheat in the steam cycle for maximum energy recovery and the use of high pressure, high temperature superheater and reheater in CC plants. Super critical boilers are also conceivable. Since HRSG performance has a large impact on the overall efficiency of the CC power plant, an accurate simulation of the performance of the HRSG is necessary. We present a steady state HRSG model to support design and rating simulations of vertical units. The simulation model, called FELVAL, divides the boiler in its rows. The row model can also be divided several times following the tube length, to better estimate the fumes temperature distribution across the hot gas path. Another model, called SUFVAL, carries out the design as well as the automatic generation of the FELVAL units and all the needed connections. The log mean temperature difference (LMTD) method and the effectiveness-NTU (ε -NTU) method are alternatively used to compute the overall heat transferred in each part of the HRSG. The problem of convergence of boiler models with more than one row in parallel is discussed. Good initialisation of the different variables is crucial to obtain convergence. The models are tested on 2 references HRSG. The first one is an assisted circulation boiler that operates at 3 subcritical pressure levels. The second is a once through boiler able to operate above the critical pressure of water. These new models were introduced into a commercial software of data reconciliation (VALI of Belsim sa) already used by the engineering and design departments of a HRSG manufacturer. They thus have a general-purpose package enabling them to make design, data reconciliation and simulation with the same software. Moreover, the use of FELVAL model will enable them to simulate any type of boiler and to obtain informations on the change of the temperatures inside the heat exchangers. This information is crucial for well monitoring closely the operation of a boiler, and better understanding its behaviour. This knowledge improvement allows to limit the overdesign and the safety margins and to reduce the investment costs.

The need to develop sustainable resource management strategies for semi-arid and arid rangelands is acute as non-adapted grazing strategies lead to irreversible environmental problems such as desertification and associated loss of economic support to society. In such vulnerable ecosystems, successful implementation of sustainable management strategies depends on well-founded under-standing of processes at different scales that underlay the complex system dynamic. There is ample evidence that, in contrast to traditional sectoral approaches, only interdisciplinary research does work for resolving problems in conservation and natural resource management. In this thesis I combined a range of modeling approaches that integrate different disciplines and spatial scales in order to contribute to basic guidelines for sustainable management of semi-arid and arid range-lands. Since water availability and livestock management are seen as most potent determinants for the dynamics of semi-arid and arid ecosystems I focused on (i) the interaction of ecological and hydro-logical processes and (ii) the effect of farming strategies. First, I developed a grid-based and small-scaled model simulating vegetation dynamics and inter-linked hydrological processes. The simulation results suggest that ecohydrological interactions gain importance in rangelands with ascending slope where vegetation cover serves to obstruct run-off and decreases evaporation from the soil. Disturbances like overgrazing influence these positive feedback mechanisms by affecting vegetation cover and composition. In the second part, I present a modeling approach that has the power to transfer and integrate ecological information from the small scale vegetation model to the landscape scale, most relevant for the conservation of biodiversity and sustainable management of natural resources. I combined techniques of stochastic modeling with remotely sensed data and GIS to investigate to which ex-tent spatial interactions, like the movement of surface water by run-off in water limited environments, affect ecosystem functioning at the landscape scale. My simulation experiments show that overgrazing decreases the number of vegetation patches that act as hydrological sinks and run-off increases. The results of both simulation models implicate that different vegetation types should not only be regarded as provider of forage production but also as regulator of ecosystem functioning. Vegetation patches with good cover of perennial vegetation are capable to catch and conserve surface run-off from degraded surrounding areas. Therefore, downstream out of the simulated system is prevented and efficient use of water resources is guaranteed at all times. This consequence also applies to commercial rotational grazing strategies for semi-arid and arid rangelands with ascending slope where non-degraded paddocks act as hydrological sinks. Finally, by the help of an integrated ecological-economic modeling approach, I analyzed the relevance of farmers’ ecological knowledge for longterm functioning of semi-arid and arid grazing systems under current and future climatic conditions. The modeling approach consists of an ecological and an economic module and combines relevant processes on either level. Again, vegetation dynamics and forage productivity is derived by the small-scaled vegetation model. I showed that sustainable management of semi-arid and arid rangelands relies strongly on the farmers’ knowledge on how the ecosystem works. Furthermore, my simulation results indicate that the projected lower annual rainfall due to climate change in combination with non-adapted grazing strategies adds an additional layer of risk to these ecosystems that are already prone to land degradation. All simulation models focus on the most essential factors and ignore specific details. Therefore, even though all simulation models are parameterized for a specific dwarf shrub savanna in arid southern Namibia, the conclusions drawn are applicable for semi-arid and arid rangelands in general. / Nachhaltige Managementstrategien für semi-aride und aride Beweidungsgebiete sind äusserst bedeutend, da ein nicht nachhaltiges Management sehr schnell zu irreversiblen Umweltproblemen und damit verbundenem Verlust der ökonomischen Prosperität führt. Obwohl Wasserverfügbarkeit und Viehmanagement als die bedeutendsten Faktoren für die Dynamik semi-arider und arider Ökosysteme angesehen werden, ist deren Einfluss und Interaktion nicht genügend erforscht. Ziel der Dissertation war, das Wissen über diese Prozesse zu erweitern, um grundsätzliche Richtlinien für die nachhaltige Nutzung semi-arider und arider Beweidungsgebiete zu erstellen. Hierfür habe ich in dieser Arbeit, die aus drei aufeinander aufbauenden Teilen besteht, mehrere Modellierungstechniken kombiniert. Für den ersten Teil meiner Arbeit habe ich ein gitterbasiertes und kleinskaliges Modell entwickelt, welches die Vegetationsdynamik und damit verbundene hydrologische Prozesse wie Oberflächenabfluss und Evaporation simuliert. Da Entscheidungen zur nachhaltigen Nutzung von Resourcen auf der Landschaftsebene getroffen werden, stelle ich im zweiten Teil der Arbeit eine neue Methode vor, mit deren Hilfe man diese kleinskaligen ökologischen Informationen auf die Landschaftsebene übertragen kann. Um zu untersuchen wie Oberflächenabfluss das Funktionieren von Ökosystemen auf Landschaftsebene beeinflusst, habe ich Techniken der stochastischen Modellierung mit Techniken der Fernerkundung und GIS kombiniert.. Die Ergebnisse beider Simulationsmodelle implizieren, dass öko-hydrologische Interaktionen in Beweidungsgebieten mit ausgeprägter Topographie von Bedeutung sind. Verschiedene Vegetationstypen sollten nicht nur als Futterquelle für die Weidetiere betrachtet werden, sondern auch bezüglich ihrer Bedeutung als Regler der Ökosystemfunktion. Vegetationsbestände mit einem hohen Bedeckungsgrad an perennierender Vegetation können Oberflächenabfluss aus degradierten benachbarten Gebieten abfangen. Störungen wie Überweidung beeinflussen diesen positiven Rückkopplungsmechanismus negativ, indem sie Vegetationsbedeckung und -zusammensetzung verändern. Im letzten Teil der Arbeit habe ich mit Hilfe eines ökologisch-ökonomischen Simulationsmodells die Bedeutung des ökologischen Verständnisses der Farmer für ein langfristiges Funktionieren von semi-ariden und ariden Beweidungssystemen unter aktuellen und prognostizierten klimatischen Bedingungen untersucht. Auch hier wird die Vegetationsdynamik und – produktivität beider Module mit Hilfe des kleinskaligen Vegetationsmodells abgeleitet. Die Ergebnisse zeigen, dass ein nachhaltiges Management semi-arider und arider Savannen sehr stark vom Verständnis der Farmer für die Funktionsweise des Ökosystems abhängt. Des Weiteren weist das Modell darauf hin, dass ein durch den prognostizierten Klimawandel reduzierter Jahresniederschlag in Kombination mit nicht-angepassten Beweidungsstrategien ein hohes Risiko für diese Ökosysteme darstellt. Meine Arbeit trägt zu einem besseren Verständnis grundlegender Prozesse der Ökosystemdynamik einer ariden Zwergstrauchsavanne im südlichen Namibia bei. Da sich alle drei Simulationsmodelle auf grundlegende Faktoren konzentrieren und spezifische Details ignorieren, können die Schlussfolgerungen auch auf andere semi-aride und aride Beweidungsgebiete übertragen werden.

Efficient and reliable public transport systems are fundamental in promoting green growth developments in metropolitan areas. A large range of Advanced Public Transport Systems (APTS) facilitates the design of real-time operations and demand management. The analysis of transit performance requires a dynamic tool that will enable to emulate the dynamic loading of travelers and their interaction with the transit system. BusMezzo, a dynamic transit operations and assignment model was developed to enable the analysis and evaluation of transit performance and level of service under various system conditions and APTS. The model represents the interactions between traffic dynamics, transit operations and traveler decisions. The model was implemented within a mesoscopic traffic simulation model. The different sources of transit operations uncertainty including traffic conditions, vehicle capacities, dwell times, vehicle schedules and service disruptions are modeled explicitly. The dynamic path choice model in BusMezzo considers each traveler as an adaptive decision maker. Travelers’ progress in the transit system consists of successive decisions that are defined by the need to choose the next path element. The evaluations are based on the respective path alternatives and their anticipated downstream attributes. Travel decisions are modeled within the framework of discrete random utility models. A non-compensatory choice-set generation model and the path utility function were estimated based on a web-based survey. BusMezzo enables the analysis and evaluation of proactive control strategies and the impacts of real-time information provision. Several experiments were conducted to analyze transit performance from travelers, operator and drivers perspectives under various holding strategies. This analysis has facilitated the design of a field trial of the most promising strategy. Furthermore, a case study on real-time traveler information systems regarding the next vehicle arrival time investigated the impacts of various levels of coverage and comprehensiveness. As passengers are more informed, passenger loads are subject to more fluctuation due to the traveler adaptations. / QC 20111201

This is a Master’s Thesis that has been carried out at the Global Outbound Logistics department at Scania. Scania manufactures trucks, buses and engines. Some trucks and buses are delivered to markets where it, due to reduced customs duties and cheaper manpower, is more profitable to do the assembly locally at so called Regional Product Centres (RPCs). Since the components are produced far away from the RPC markets the lead times become long. In addition, the customers’ buying behaviour at the RPC markets is often not comparable to the European culture were a customer can accept to wait for weeks for a unit to be delivered. The long lead time in combination with the customer behaviour implies that the RPCs need to keep a certain selection of standard models of buses and trucks in stock. It has turned out to be difficult for the pipeline managers at the RPCs to place order volumes that correspond well to what will be delivered to the business units or distributors later on. The result of this is high stock levels at the RPCs, which leads to an important amount of tied up capital. Due to what is explained above, the purpose of this study is “to create a simulation model, based upon a process mapping, that visualises future volume levels in the pipeline due to different demand and ordering scenarios”. The short term target, which is also the target of this study, is to increase the RPCs understanding for how different demand and ordering scenarios influence the future volume levels in the pipeline. The long term target is to reduce tied up capital by adjusting buffer levels and lead times, while still ensuring a certain service level. The model should contribute to more accurate decision making with respect to the previous mentioned aspects. First, a high level process mapping was made in order to select which flows that were suitable for being subject for a detailed mapping. Second, a detailed mapping was made during which several RPC-, process- and function responsible were interviewed. After the detailed mapping, common denominators between the flows were identified and all activities were clustered into a solution that could be generalised and suitable for all flows. Factors such as lead times, deviation risks and capacity limitations were taken into account during the aggregation of activities. When a common view of the different RPC flows had been created, the mathematical relationships for how the goods can move throughout the process could be established. Then, the development and validation of the simulation model, which was an iterative process, could start. A directive was to build the simulation model in Microsoft Excel. Interviews were made with experienced model creators in order to find out how to create a user-friendly and robust model. The creation of the simulation model started with the development of a structure and then the content of each part was defined. A final validation, which consisted of sensitivity analysis and user trials, was finally done in order to ensure the simulation models functioning and accuracy. To conclude, a simulation model that will serve as a helpful tool for the RPCs when they are to decide which order volumes to place has been created. By clearly visualising the simulation results, the simulation model will hopefully increase the RPCs’ comprehension for how the pipeline works with respect to different ordering and demand scenarios. On top of this, the method used, the process mapping and the mathematical relationships that have been defined are important input for a possible future development of a more permanent and robust non-Microsoft Excel solution. This solution could probably be even more precise, automatically updated and have an even higher granularity.

This master thesis presents a novel methodology for the  development of simulation models  for hydraulic pumps and motors. In this work, a generic simulation model capable of representing multiple axial piston machines is presented, implemented and validated. Validation of the developed generic simulation model is done by comparing the results from the simulation model with experimental measurements. The development of the generic model is done using AMESim. Today simulation models are an integral part of any development process concerning hydraulic machines. An improved methodology for developing these simulation models will affect both the development cost and time in a positive manner. Traditionally, specific simulation models dedicated to a certain pump or motor are created. This implies that a complete rethinking of the model structure has to be done when modeling a new pump or motor. Therefore when dealing with a large number of pumps and motors, this traditional way of model development could lead to large development time and cost. This thesis work presents a unique way of simulation model development where a single model could represent multiple pumps and motors resulting in lower development time and cost. An automated routine for simulation model creation is developed and implemented. This routine uses the generic simulation model as a template to automatically create simulation models requested by the user. For this purpose a user interface has been created through the use of Visual Basic scripting. This interface communicates with the generic simulation model allowing the user to either change it parametrically or completely transform it into another pump or motor. To determine the level of accuracy offered by the generic simulation model, simulation results are compared with experimental data. Moreover, an optimization routine to automatically fine tune the simulation model is also presented.

Today, discrete event simulation is the only reliable tool for detailed analysis of complex behaviors of modern manufacturing systems. However, building high fidelity simulation models is expensive. Hence, it is important to improve the simulation modeling productivity. In this research, we explore two approaches for the improvement of simulation modeling productivity. One approach is the Virtual Factory Approach, using a general-purpose model for a system to achieve various simulation objectives with a single high fidelity model through abstraction. The other approach is the Reference Model Approach, which is to build fundamental building blocks for simulation models of any system in a domain with formal descriptions and domain knowledge. In the Virtual Factory Approach, the challenge is to show the validity of the methodology. We develop a formal framework for the relationships between higher fidelity and lower fidelity models, and provide justification that the models abstracted from a higher fidelity model are interchangeable with various abstract simulation models for a target system. For the Reference Model Approach, we attempt to overcome the weak points inherited from ad-hoc modeling and develop a formal reference model and a model generation procedure for discrete part manufacturing systems, which covers most modern manufacturing systems.