2,867 research outputs found

    An hierarchical approach to hull form design

    Get PDF
    As ship design tools become more integrated and more advanced analysis tools are introduced, the ability to rapidly develop and modify hull forms becomes essential. Modern hull design applications give an experienced user the ability to create almost any shape of hull. However, the direct manipulation of hull surface representations is laborious and may limit the exploration of design concept to the fullest extent. Transformation of parent forms and parametric hull generation tools can provide a quick solution, but neither method is conducive for innovative design. A hull design tool is required that can integrate the separate techniques creating a fair hull form surface that can be modified easily throughout the design process. This paper explores the concept of separating the hull surface into global and local features by establishing a hierarchical definition structure and introduces some of the benefits of this approach

    Phase diagram of congested traffic flow: an empirical study

    Full text link
    We analyze traffic data from a highway section containing one effective on-ramp. Based on two criteria, local velocity variation patterns and expansion (or nonexpansion) of congested regions, three distinct congested traffic states are identified. These states appear at different levels of the upstream flux and the on-ramp flux, thereby generating a phase diagram of the congested traffic flow. Compared to our earliear reports (including cond-mat/9905292) based on 14 day traffic data, the present paper uses a much larger data set (107 days) and the analysis is carried in a more systematic way, which leads to the modification of a part of interpretation in the earlier reports. Observed traffic states are compared with recent theoretical analyses and both agreeing and disagreeing features are found.Comment: More extensive and systematic version of earlier reports (including cond-mat/9905292). A part of interpretation in earlier reports is modified. 6 two-column pages. To appear in Phys. Rev. E (tentatively scheduled for Oct. 1 issue

    Automatic collision avoidance of ships

    Get PDF
    One of the key elements in automatic simulation of ship manoeuvring in confined waterways is route finding and collision avoidance. This paper presents a new practical method of automatic trajectory planning and collision avoidance based on an artificial potential field and speed vector. Collision prevention regulations and international navigational rules have been incorporated into the algorithm. The algorithm is fairly straightforward and simple to implement, but has been shown to be effective in finding safe paths for all ships concerned in complex situations. The method has been applied to some typical test cases and the results are very encouraging

    SPIDA: Abstracting and generalizing layout design cases

    Get PDF
    Abstraction and generalization of layout design cases generate new knowledge that is more widely applicable to use than specific design cases. The abstraction and generalization of design cases into hierarchical levels of abstractions provide the designer with the flexibility to apply any level of abstract and generalized knowledge for a new layout design problem. Existing case-based layout learning (CBLL) systems abstract and generalize cases into single levels of abstractions, but not into a hierarchy. In this paper, we propose a new approach, termed customized viewpoint - spatial (CV-S), which supports the generalization and abstraction of spatial layouts into hierarchies along with a supporting system, SPIDA (SPatial Intelligent Design Assistant)

    Steady state solutions of hydrodynamic traffic models

    Full text link
    We investigate steady state solutions of hydrodynamic traffic models in the absence of any intrinsic inhomogeneity on roads such as on-ramps. It is shown that typical hydrodynamic models possess seven different types of inhomogeneous steady state solutions. The seven solutions include those that have been reported previously only for microscopic models. The characteristic properties of wide jam such as moving velocity of its spatiotemporal pattern and/or out-flux from wide jam are shown to be uniquely determined and thus independent of initial conditions of dynamic evolution. Topological considerations suggest that all of the solutions should be common to a wide class of traffic models. The results are discussed in connection with the universality conjecture for traffic models. Also the prevalence of the limit-cycle solution in a recent study of a microscopic model is explained in this approach.Comment: 9 pages, 6 figure

    Interpreting the Wide Scattering of Synchronized Traffic Data by Time Gap Statistics

    Full text link
    Based on the statistical evaluation of experimental single-vehicle data, we propose a quantitative interpretation of the erratic scattering of flow-density data in synchronized traffic flows. A correlation analysis suggests that the dynamical flow-density data are well compatible with the so-called jam line characterizing fully developed traffic jams, if one takes into account the variation of their propagation speed due to the large variation of the netto time gaps (the inhomogeneity of traffic flow). The form of the time gap distribution depends not only on the density, but also on the measurement cross section: The most probable netto time gap in congested traffic flow upstream of a bottleneck is significantly increased compared to uncongested freeway sections. Moreover, we identify different power-law scaling laws for the relative variance of netto time gaps as a function of the sampling size. While the exponent is -1 in free traffic corresponding to statistically independent time gaps, the exponent is about -2/3 in congested traffic flow because of correlations between queued vehicles.Comment: For related publications see http://www.helbing.or

    Effects of Nitrogen Limitation on Hydrological Processes in CLM4-CN

    Get PDF
    http://globalchange.mit.edu/research/publications/2253The role of nitrogen limitation on photosynthesis downregulation and stomatal conductance has a significant influence on evapotranspiration and runoff. In the current Community Land Model with coupled Carbon and Nitrogen cycles (CLM4-CN), however, the carbon and water coupling in stomata is not linked to nitrogen limitation. We modify the incomplete linkages between carbon, nitrogen, and water, and examine how nitrogen limitation affects hydrological processes in CLM4-CN. In addition, we evaluate if the modification can improve the simulation of carbon and water fluxes. Applying the effects of nitrogen limitation on stomatal conductance significantly decreases leaf photosynthesis. It leads to a reduction in canopy transpiration, thereby increasing total runoff, mainly due to increasing subsurface runoff. More available soil water for vegetation from the reduced transpiration helps increase gross primary productivity (GPP) in the relatively moisture-limited regions of grassland/steppe and savanna. But, in the tropics and boreal forest regions, changes in soil water by nitrogen limitation are insignificant, and GPP decreases directly by down-regulated leaf photosynthesis. Decreasing canopy transpiration and increasing runoff from nitrogen limitation improve simulating latent heat flux and runoff by reducing high biases for latent heat flux in the tropics and low biases for runoff in the tropics and northern high-latitudes. In addition, the CLM4-CN with leaf-level nitrogen limitation reduces high model biases in tropical GPP. Thus, nitrogen limitation on the leaf-level significantly affects hydrological processes in CLM4-CN and improves the simulation of carbon and water fluxes.The study was supported by the joint project between Lehigh and MIT Joint Program on the Science and Policy of Global Change through a consortium of industrial sponsors and Federal grants

    Human behavior as origin of traffic phases

    Full text link
    It is shown that the desire for smooth and comfortable driving is directly responsible for the occurrence of complex spatio-temporal structures (``synchronized traffic'') in highway traffic. This desire goes beyond the avoidance of accidents which so far has been the main focus of microscopic modeling and which is mainly responsible for the other two phases observed empirically, free flow and wide moving jams. These features have been incorporated into a microscopic model based on stochastic cellular automata and the results of computer simulations are compared with empirical data. The simple structure of the model allows for very fast implementations of realistic networks. The level of agreement with the empirical findings opens new perspectives for reliable traffic forecasts.Comment: 4 pages, 4 figures, colour figures with reduced resolutio

    Enhancing and visualising data on soils, land use and the environment.

    Get PDF
    End of Project ReportA computer based system was developed to produce new information, charts and map data on soils, environment and land use for environmental decision support. The process involved manipulation of data in tabular and electronic map form by combining features from digitised maps and tables to develop an information system of linked and harmonised data. This report reviews and illustrates the findings with outputs in the form of maps and tables. Maps presenting sheep census information for the years 1970- 1991 show greatly increased livestock densities, in the latter years. These changes mainly occurred in the (i) traditional drystock areas of the midlands (ii) south east arable areas and (iii) south east Connacht dry stock/sheep areas and reflect economic and structural changes arising out of Government policy and membership of the EU. Maps depicting the temporal changes in dairy livestock densities were not so dramatic; however the structural change is particularly reflected in the reduction of dairying in the Connacht/Midlands regions. The introduction of milk quotas in the 1980's accelerated the changes which had been taking place since the early 1970's. In comparison to dairy cows, the increased concentration of drystock seems to be more widely spread. High concentrations of drystock occurred in the east midlands in 1970. By 1980, the high concentration of dry cattle had spread to North and South Midlands. The density of cattle increased further in Leinster and Munster areas by 1991. In Connaught, where sheep numbers had increased markedly between 1970 and 1991, cattle numbers remained relatively static. The digitisation of detailed soil survey maps was undertaken in this project and a new soil survey map of the Lough Derg Catchment was produced using Great Soil Groups as the mapping unit. This map is included in the report.European Union Structural Funds (EAGGF

    Superconductivity in CoO2_2 Layers and the Resonating Valence Bond Mean Field Theory of the Triangular Lattice t-J model

    Get PDF
    Motivated by the recent discovery of superconductivity in two dimensional CoO2_2 layers, we present some possibly useful results of the RVB mean field theory applied to the triangular lattice. Away from half filling, the order parameter is found to be complex, and yields a fully gapped quasiparticle spectrum. The sign of the hopping plays a crucial role in the analysis, and we find that superconductivity is as fragile for one sign as it is robust for the other. Nax_xCoO2y_2\cdot yH2_2O is argued to belong to the robust case, by comparing the LDA Fermi surface with an effective tight binding model. The high frequency Hall constant in this system is potentially interesting, since it is pointed out to increase linearly with temperature without saturation for T >> Tdegeneracy_{degeneracy}.Comment: Published in Physical Review B, total 1 tex + 9 eps files. Erratum added as separate tex file on November 7, 2003, a numerical factor corrected in the erratum on Dec 3, 200
    corecore