Efficient maritime navigation through dynamic obstructions at close range is still a serious issue faced by mariners. There
have been studies focusing on collision risk assessment in the past, but the majority were based on the first person perspective,
with area-based ship domain concepts that are defined around either the ownship or the obstacle. Such methods are acceptable
for encounters where the ownship is required to manoeuvre according to the collision regulations (COLREGs), but they will
not work correctly if the ownship is the stay-on party. This article presents an alternative method of assessing the collision
risk for surface ships in close-range encounters that is compliant with the COLREGs as well as other ships from different
perspectives. 相似文献
Constructive interference between tidal stream turbines in multi-rotor fence configurations arrayed normally to the flow has been shown analytically, computationally, and experimentally to enhance turbine performance. The increased resistance to bypass flow due to the presence of neighbouring turbines allows a static pressure difference to develop in the channel and entrains a greater flow rate through the rotor swept area. Exploiting the potential improvement in turbine performance requires that turbines either be operated at higher tip speed ratios or that turbines are redesigned in order to increase thrust. Recent studies have demonstrated that multi-scale flow dynamics, in which a distinction is made between device-scale and fence-scale flow events, have an important role in the physics of flow past tidal turbine fences partially spanning larger channels. Although the reduction in flow rate through the fence as the turbine thrust level increases has been previously demonstrated, the within-fence variation in turbine performance, and the consequences for overall farm performance, is less well understood. The impact of turbine design and operating conditions, on the performance of a multi-rotor tidal fence is investigated using Reynolds-Averaged Navier-Stokes embedded blade element actuator disk simulations. Fences consisting of four, six, and eight turbines are simulated, and it is demonstrated that the combination of device- and fence-scale flow effects gives rise to cross-fence thrust and power variation. These cross-fence variations are also a function of turbine thrust, and hence design conditions, although it is shown simple turbine control strategies can be adopted in order to reduce the cross-fence variations and improve overall fence performance. As the number of turbines in the fence, and hence fence length, increases, it is shown that the turbines may be designed or operated to achieve higher thrust levels than if the turbines were not deployed in a fence configuration.
Mackie et al. (Values of travel time savings in the UK. Report to Department for Transport. Institute for Transport Studies, University of Leeds & John Bates Services, Leeds and Abingdon, 2003) proposed an identity relating the value of time (VoT) for commute and leisure travel to income and travel cost, reporting the prevalence of ‘cost damping’ (i.e. the phenomenon where VoT increases as travel cost increases). This identity (or a variant thereof) has been adopted within official methods for estimating VoT in the UK, Switzerland and The Netherlands. The present paper shows that Mackie et al.’s identity: (i) implies linear preferences, not strictly convex preferences as reported by Mackie et al.; (ii) complies with homogeneity and symmetry by construction; (iii) complies with adding-up if and only if VoT is unit elastic with respect to income; (iv) complies with negativity if VoT is unit elastic or greater with respect to income; (v) violates both adding-up and negativity in the case of the 2003 UK national VoT study. We propose alternative identities which comply with adding-up and homogeneity by construction, and offer comparable fit to Mackie et al.’s identity on the UK VoT dataset. We also find that the imposition of adding-up and negativity on Mackie et al.’s identity, through appropriate constraint on model estimation, leads to an increase of around 20% in valuations from the 2003 UK dataset. 相似文献
Although there is widespread recognition of the potential of ports as logistics centres, widely accepted performance measurements for such centres have yet to be developed. The essence of logistics and supply chain management is an integrative approach to the interaction of different processes and functions within a firm extended to a network of organizations for the purpose of cost reduction and customer satisfaction [1]. The logistics approach often adopts a cost trade-off analysis between functions, processes and even supply chains [2]. This approach could be beneficial to port efficiency by directing port strategy towards relevant value-added logistics activities. This paper seeks to show that through conceptualizing ports from a logistics and supply chain management approach, it is possible to suggest a relevant framework of port performance. A proposed framework is tested in a survey of port managers and other international experts. 相似文献
The paper offers a historical overview of the container revolution. It starts from the origin more than 50 years ago, presenting
how the pioneer Malcom McLean came to this idea that has changed the nature of Shipping. It then presents how and why container
shipping was gradually accepted and what has been the main trends during the last 50 years: the increase in vessels’ and containers’
size and in vessels’ speed. Finally, the paper investigates what the future will bring and notably sheds light on the increase
in containers’ size and on the improvements in terminal and rail services. 相似文献
In this paper, a new framework for idle speed control is presented [The idle control methodology described paper is protected by international patent law]. A simple new discrete event based engine model evolving in the crank domain is given and is parameterized using a combination of dynamic identification and steady state calibration. The majority of the non-linearities can then be inverted and a second synthetic output is introduced. This second output is the torque reserve of the engine and enables control of the tradeoff between disturbance rejection capabilities and engine efficiency. A linear controller is designed using the H 8 loopshaping technique and results are presented from dynamometer based engine tests. 相似文献