73 research outputs found
Derogation on the EU Nitrates Directive: does it make a difference?
No full textThe paper discusses an ex ante evaluation of the derogation on the Nitrates Directive for Flanders, Belgium, which is a case of intensive but highly productive livestock areas. The aim is to develop an accurate simulation model to detect small differences in manure surpluses caused by changes of manure production and/or utilization. The system of models consists of various modules to fine tune the calculations of manure production, fertilizing behaviour and manure allocation and disposal on and off farm. The results show that derogation may cause the existing manure surpluses to expire, if only nitrogen limits are considered and no transactions costs are taken into account. When also phosphate fertilization limits are considered, the increase in manuring possibilities is much lower than expected. Ongoing research focuses on the marginal shifts in manure surplus at farm level and possible effects of transactions costs.Nitrates Directive, derogation, modelling, Environmental Economics and Policy, Livestock Production/Industries,
VLM Optimization: Increasing efficiency through heating chamber optimization
No full textIn the space industry miniaturization has been a trend for decades, with satellite masses ranging from 0.1-10kg for pico- and nano-satellites. Propulsion subsystems for these satellites have not kept up with this trend performance-wise. They are often limited by safety regulations requiring inert propellants andpower budget constraints of the satellite.Vaporizing Liquid Microthruster (VLM) are a type of resistojet considered for pico- and nano-satellites and are currently in development at Space Engineering (SE) department of the Technical University of Delft (TUD). VLM expel an electrically heated (inert) propellant, which is stored in the liquid phase.Currently, VLM are inefficient and design tools are needed to find more optimal designs. In this research such a design tool is developed, and applied to thrusters with water as a propellant. A distinction with earlier research on VLM is that specific attention was paid to the field of microchannel flow boiling, so that knowledge from this discipline could be applied in the heat transfer model of the VLM heating channels. The design tool was successfully developed, and is extendible with novel heat transfer and pressure drop relations when required for future research. When applied to 15 mN thrust range, the outcome is that the chamber temperature can be increased without increasing the total power consumption significantly. Heating chamber design also tends to be more optimal if more and smaller channels are used than in earlier published VLM designs.The optimal wall temperatures were about 40 K higher than the chamber temperature. The optimal channel width and wall thickness were constrained by structural limitations, at 20 and 50 micron, respectively. The optimal number of channels increased with thrust from 3-4 channels at 1 mN to 8-9 at 5 mN.Aerospace Engineerin
Study on Remote Control System Using VLM 5020
No full textpublisherThe author, has made an experimental remote control System Using VLM 5020. In this paper, the author constructs system’s block diagram, and describes the behavior of each block. On a trial production, the system has proved to be practicable.departmental bulletin pape
Improving demand forecasting in the air cargo handling industry: A case study
No full textAir transportation plays a crucial role in the agile and dynamic environment of contemporary supply chains. This industry is characterized by high air cargo demand uncertainty, making forecasting extremely challenging. An in-depth case-study has been undertaken in order to explore and untangle the factors influencing demand forecasting and consequently to improve the operational performance of an Air Cargo Handling Company. It has been identified that in practice, the demand forecasting process does not provide the necessary level of accuracy, to effectively cope with the high demand uncertainty. This has a negative impact on a whole range of air cargo operations, but especially on the management of the workforce, which is the most expensive resource in the air cargo handling industry. Besides forecast inaccuracy, a range of additional hidden factors that affect operations management have been identified. A number of recommendations have been made to improve demand forecasting and workforce management
IOBPCS based models and the customer order decoupling point
No full textThe inventory and order based production control system (IOBPCS) is mainly a model of a forecast driven
production system where the production decision is based on the forecast in combination with the deviation
between target inventory and actual inventory. The model has been extended in various directions by including
e.g. WIP feedback but also by interpreting the inventory as an order book and hence representing a customer order
driven system. In practice a system usually consists of one forecast driven subsystem in tandem with a customer
order driven subsystem and the interface between the two subsystems is represented by information flows and a
stock point referred to as the customer order decoupling point (CODP). The CODP may be positioned late, as in
make to stock systems, or early, as in make to order systems, but in any case the model should be able to capture
the properties of both subsystems in combination. A challenge in separating forecast driven from customer order
driven is that neither one of inventory or order book should be allowed to take on negative values, and hence nonlinearities
are introduced making the model more difficult to solve analytically unless the model is first linearized.
In summary the model presented here is based on two derivatives of IOBPCS that are in tandem and interfaces
related to where the demand information flow is decoupled and the positioning of the CODP
On the analysis of lead-time disturbances in production and inventory control models
No full textChanges in the lead-time can lead to supply chain inefficiencies and risks. In this paper, we investigate the
effects of lead-time disturbances on the system’s output responses of a production and inventory control model. In the adaption process of the control system for lead-time disturbance analysis, the resulting model becomes nonlinear. Hence nonlinear control theory in combination with simulation is used to analyse the impact of leadtime changes on the transient and steady state responses of order rate, inventory and work in process. Assuming constant customer demand, small perturbation theory is applied to linearise the model and to find the transfer functions relating the system’s outputs to the lead-time input. We find that the order rate, inventory and work in process transfer functions are input-dependent. In order words, the output responses depend on the input type, amplitude and direction of changes in the lead-time. When leadtime increases, the system has a relatively slow transient response and, as expected, work in process inventory levels increase and order rates are higher. However, step decreases in the lead-time can cause significant underdamped dynamics in the system. This work demonstrates that, although lead-time reduction is associated with service level improvement, increased flexibility and cost reductions, its implementation has to be carefully planned since a quick time compression may lead to undesirable oscillations in the supply chain system. In contrast, increased lead-times, associated say with a disturbance, yield slow recovery requiring adjustment of control parameters to increase resilience
The impact of freight transport capacity limitations on supply chain dynamics
No full textWe investigate how capacity limitations in the transportation system affect the dynamic behaviour of supply chains. We are interested in the more recently defined, 'backlash' effect. Using a system dynamics simulation approach, we replicate the well-known Beer Game supply chain for different transport capacity management scenarios. The results indicate that transport capacity limitations negatively impact on inventory and backlog costs, although there is a positive impact on the 'backlash' effect. We show that it is possible for both backlog and inventory to simultaneous occur, a situation which does not arise with the uncapacitated scenario. A vertical collaborative approach to transport provision is able to overcome such a trade-off. © 2013 Taylor & Francis
Fertilization: trade-offs between manure abatement and plant productivity
No full textIn 2005, 30% of the Flemish farms faced a manure excess, while at aggregated level still 9.7% of the emission rights were unused. This means that, despite the various possibilities, Flemish farmers do not succeed in an effective exchange of manure between farms. In current paper is shown how inorganic fertilizer use influences the use and exchange of organic nitrogen. Because of the mutual interdependency between organic and inorganic nitrogen emission rights (or quota), inorganic nitrogen use limits the emission rights for organic nitrogen. Utilisation of these emission rights are analysed as a trade-offs choice between plant productivity (use of inorganic nitrogen) and manure disposal, as the major abatement alternative of manure production. Farmers still prefer inorganic fertilizers because of their effect on plant productivity and income. However, by changing the quota rent of organic nitrogen, the fertilization behaviour can be influenced. A higher quota rent of organic nitrogen would increase the use of manure. This trade-off behaviour seriously influences effectiveness of policies. When the objective is to lower the total nitrogen use, a mere reduction of organic quota can partially be counteracted by a higher inorganic nitrogen use. When the objective is to better spread the manure, increasing the quota rent for deficit farms will increase their acceptance of manure.manure abatement, nutrient emission rights, Tobit model, Crop Production/Industries,
Monitoring Megathrust-Earthquake-Cycle-Induced Relative Sea-Level Changes near Phuket, South Thailand, Using (Space) Geodetic Techniques
No full textTemporal changes in vertical land motion (VLM) in and around Phuket Island in southern Thailand following the great 2004 Sumatra–Andaman megathrust earthquake have impacted the relative sea-level change estimates based on tide-gauge (TG) records. To better monitor the VLM, two new continuous global navigation satellite system (GNSS) stations have been installed in the past 5 years, situated on bedrock both near and at the Koh Taphao Noi Island TG in Phuket, which together with older global positioning system (GPS) data provides a clear insight in the VLM of Phuket Island from 1994 onward. In addition, satellite altimetry (SALT) data has been analyzed since 1992. The VLM (GPS) position and relative (TG) and absolute (SALT) sea-level change time series were successfully combined in pairs to validate each independent result (e.g., SALT − GNSS = TG): prior to the 2004 earthquake, the relative sea-level rise in Phuket was 1.0 ± 0.7 mm/yr, lower by 2.4 ± 0.2 mm/yr than the absolute sea-level rise caused by VLM. After the earthquake, nonlinear post-seismic subsidence has caused the VLM to drop by 10 cm in the past 17 years, resulting, by the end of 2020, in a relative sea-level rise by up to 16 cm. During the same period, other TG stations in south Thailand recorded similar sea-level increases. Combination with SALT further suggests that, prior to 2005, uplift (5.3 ± 1.4 mm/yr) of the coastal region of Ranong (north of Phuket) resulted in a relative sea-level fall, but since then, post-seismic-induced negative VLM may have significantly increased coastal erosion along the entire Andaman Sea coastline
The impact of nonlinear dynamics on the resilience of a grocery supply chain
No full textPurpose of this paper: In an effort to improve operational and logistical efficiencies, UK grocery retailers combined primary and secondary distribution increasing the importance of designing resilient replenishment systems in the distribution centre. This paper has the purpose to analyse the resilience performance of the distribution centre stock ordering system within a grocery retailer. Design/methodology/approach: A system dynamics approach is used for framing and building a credible representation of the real system. Mathematical analysis of the nonlinear model based on nonlinear control engineering techniques in combination with system dynamics simulation have been used to understand the behaviour of stock and shipment output responses in the distribution centre given step and periodic demand signals. Findings: Preliminary mathematical analysis through nonlinear control theory techniques has been undertaken in order to gain initial insights in the understanding of the replenishment control model. This practice allowed the researcher to identify specific behaviour change in the DC stock and shipment responses, which are key indicators for assessing supply chain resilience, without going through a time-consuming simulation process. Transfer function analysis and describing function serve as a guideline for undertaking system dynamics simulation. Value: This paper aims to fill the gap in the literature of supply chain resilience by using quantitative system dynamics methods to assess the resilience performance of a grocery retailer. In this way, we also supplement the literature with empirical data. Moreover, we explore different analytical methods since simulation is the predominant method for quantitative analysis of system dynamics. Research limitations/implications (if applicable): This research is limited to the dynamics of single-echelon supply chain systems. Although the EPOS sales data and the store replenishment system have been considered in the validation process, this study has focused on analysing the resilience performance of the DC replenishment system only. Considering the multi-echelon supply chain is intended for further research activities. Practical implications (if applicable): The findings suggest that the distribution centre replenishment system can be re-designed in order to improve the supply chain resilience performance. The ‘As Is’ scenario produces slow response of stock levels and inventory targets are never recovered due to a permanent offset
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