Hydrodynamic characteristics of slotted breakwaters under regular waves

The performance of a slotted breakwater consisting of one row of vertical slots was investigated theoretically and experimentally under normal regular waves. A simple theoretical model based on an eigenfunction was developed. The wave transmission, reflection, energy loss, and hydrodynamic force exerted on the breakwater were calculated for different values of the wave and structure parameters. The validity of the theoretical model was examined by comparing its results with theoretical and experimental results obtained from different studies. It was found that the transmission coefficient decreases with increasing dimensionless wavenumber (kh), increasing wave steepness (H _i/L), and decreasing breakwater porosity (ε). The reflection coefficient showed the opposite trend to the transmission coefficient. Also, about 20–50% of incident wave energy was lost due to the effect of the breakwater. In addition, the proposed theoretical model can be used for predicting the performance of slotted breakwaters and the hydrodynamic forces exerted on these structures using the friction coefficient f = 1.5.     

Maritime education factors and presenteeism: a comparative quantitative study

Presenteeism research has only recently extended from a construct capturing lost productivity due to attending work despite health issues to a construct representing students’ perceived academic performance. Aligning with presenteeism’s prevalent research paradigm, the pioneering studies used health-related issues to measure presenteeism. In contrast, this study used the Presenteeism and Perceived Academic Performance (PPAP) Scale, which the researcher developed for this study. The PPAP Scale comprises aspects of student behavior that support academic performance. This study filled gaps in the literature by investigating presenteeism as a concept associated with students’ perceived academic performance, measured with the PPAP Scale, and factors specific to maritime education. The factors investigated were self-identified by the study respondents, in sufficient numbers to support statistical analysis, as favorably (i.e., cruise and license/maritime instruction) or negatively (i.e., mandatory regimental activity, taps, morning or afternoon formations, and watch) impacting their academic performance. This study found no statistical evidence to suggest the level of presenteeism, measured with the PPAP Scale, among the study’s sample of license students is associated with factors perceived to favorably or negatively impact academic performance. Additional insight can be gained from license student participants’ responses to the study’s open-ended questions (e.g., a perceived imbalance between time available to allocate to their academics and time needed to fulfill regimental responsibilities).     

Incentivizing alternative fuel vehicles: the influence of transport policies,attitudes and perceptions

This paper aims to evaluate the influence of policies, attitudes and perceptions when incentivizing alternative fuel vehicles. The impact of possible policies such as gasoline taxes increases, purchase price subsidies, tax exemptions, and increases in fuel recharging station availability for alternative fuelled vehicles is evaluated using hybrid choice models. The models also allow assessing the sensitivity of latent variables (i.e., attitudes and perceptions) in the car purchase behaviour. The models are estimated using data from a stated choice survey collected in five Colombian cities. The latent variables are obtained from the rating of statements related to the transport system, environmental concern, vehicle preferences, and technology. The modelling approach includes regression between latent variables. Results show that environmental concern and the support for green transport policies have a positive influence on the intention to purchase alternative fuel vehicles. Meanwhile, people who reveal to be car-dependent prefer to buy standard fuelled vehicles. The analysis among cities shows similar trends in individual behaviour, although there are differences in attribute sensitivities. The policy scenario analysis revealed high sensitivity to capital cost and the need for extensive investments in refuelling stations for alternative fuel vehicles to become attractive. Nevertheless, all policies should not only be directed at infrastructure and vehicles but also be focused on user awareness and acceptance of the alternative fuel vehicles. The analysis suggests that in an environmentally conscious market, people prefer alternative fuels. However, if the transport policies support private transport, the market shares of alternative fuel vehicles will decrease.     

Indirect production function and the output effect of public transit subsidies

This paper uses an indirect production function to decompose the effects of subsidies on output into the lump-sum, cost and inefficiency effects. Using 2006 data for U.S. transit systems it estimates an indirect production function and uses the results to calculate these effects. It finds that the lump-sum effects exceed the other effects and that the average total effect of the subsidies is a 4.72% increase in output. The range of the output change shows that in many transit systems the output increases from the subsidies are quite large. The paper suggests that reductions in allocative inefficiencies from the subsidies would result in very large increases in output.     

On the numerical accuracy of the wave load distribution on a ship advancing in short and steep waves

Defining numerical uncertainty is an important part of the practical application of a numerical method. In the case of a ship advancing in short and steep waves, little knowledge exists on the solution behaviour as a function of discretisation resolution. This paper studies an interface-capturing (VOF) solution for a passenger ship advancing in steep (kA = 0.24) and short waves (L _w /L _pp = 0.16). The focus is to estimate quantitative uncertainties for the longitudinal distributions of the first–third harmonic wave loads in the ship bow area. These estimates are derived from the results of three systematically refined discretisation resolutions. The obtained uncertainty distributions reveal that even the uncertainty of the first harmonic wave load varies significantly along the ship bow area. It is shown that the largest local uncertainties of the first harmonic wave load relate to the differences in the local details of the propagating and deforming encountered waves along the hull. This paper also discusses the challenges that were encountered in the quantification of the uncertainties for this complex flow case.     

A behavioral choice model of the use of car-sharing and ride-sourcing services

There are a number of disruptive mobility services that are increasingly finding their way into the marketplace. Two key examples of such services are car-sharing services and ride-sourcing services. In an effort to better understand the influence of various exogenous socio-economic and demographic variables on the frequency of use of ride-sourcing and car-sharing services, this paper presents a bivariate ordered probit model estimated on a survey data set derived from the 2014–2015 Puget Sound Regional Travel Study. Model estimation results show that users of these services tend to be young, well-educated, higher-income, working individuals residing in higher-density areas. There are significant interaction effects reflecting the influence of children and the built environment on disruptive mobility service usage. The model developed in this paper provides key insights into factors affecting market penetration of these services, and can be integrated in larger travel forecasting model systems to better predict the adoption and use of mobility-on-demand services.     

Analyzing the dispersion of cargo vapors around a ship’s superstructure by means of wind tunnel experiments

In a previous study, it was found that cargo tank operations like cleaning and venting, lead to higher cargo vapor concentrations around the ship’s superstructure. Can wind tunnel experiments confirm these findings? Is there an improvement when using higher outlets at high velocities compared to lower outlets with a low outlet velocity? Is there a relation between relative wind speed and measured concentration? These questions were investigated in the Peutz wind tunnel. By using a tracer gas for the wind tunnel experiments, concentration coefficients have been calculated for various settings. The study shows that using high-velocity outlets is an efficient way to keep concentrations as low as possible. The only exception is for relative wind directions from the bow. In this last case using a manhole as ventilation outlet leads to lower concentrations. With increasing wind speeds the building downwash effect resulted in higher concentration coefficients near the main deck. This study confirms our on-board measurements and suggests the lowering of the ventilation inlet of the accommodation, so that the high-velocity outlet can be used safely at all times.     

Identification and quantification of link vulnerability in multi-level public transport networks: a passenger perspective

Robust public transport networks are important, since disruptions decrease the public transport accessibility of areas. Despite this importance, the full passenger impacts of public transport network vulnerability have not yet been considered in science and practice. We have developed a methodology to identify the most vulnerable links in the total, multi-level public transport network and to quantify the societal costs of link vulnerability for these identified links. Contrary to traditional single-level network approaches, we consider the integrated, total multi-level PT network in the identification and quantification of link vulnerability, including PT services on other network levels which remain available once a disturbance occurs. We also incorporate both exposure to large, non-recurrent disturbances and the impacts of these disturbances explicitly when identifying and quantifying link vulnerability. This results in complete and realistic insights into the negative accessibility impacts of disturbances. Our methodology is applied to a case study in the Netherlands, using a dataset containing 2.5 years of disturbance information. Our results show that especially crowded links of the light rail/metro network are vulnerable, due to the combination of relatively high disruption exposure and relatively high passenger flows. The proposed methodology allows quantification of robustness benefits of measures, in addition to the costs of these measures. Showing the value of robustness, our work can support and rationalize the decision-making process of public transport operators and authorities regarding the implementation of robustness measures.     

Port system evolution – the emergence of second-tier hubs

Some evidence has emerged of second-tier hubs inserting themselves between hubs and feeder ports, producing a new hierarchy of port networks. This article aims to establish the dynamics of this process based on illustrative cases in Asia, South America, and Europe. Findings reveal spatial factors to include a cluster of small ports with minimal sailing distance within a given range, suitable channel and berth depth, and ideally high capacity inland links. From the economic perspective, demand-side factors include a local captive market and aggregated demand to be captured from other ports, while supply-side factors include diseconomies of scale at traditional hubs, an increase in direct services, an increase in large feeder vessels calling from first-tier hubs which are then transhipped to smaller feeders for serving local ports, and an increase in overland servicing of local smaller ports. From a strategic perspective, vertical and horizontal integration in the shipping sector has produced extensive network economies, whereby shipping lines look to create group-specific port hierarchies, enhanced in the presence of aggressive management strategies and supportive policies. This finding suggests that proactive port stakeholders can in certain circumstances seize the opportunity to capture this role within their port range.