Departure Period Research Articles

Optimizing the loaded train combination problem at a heavy-haul marshalling station

In this paper, we address a loaded train combination problem at a heavy-haul marshalling station. This problem lies on assigning/combining loaded inbound trains to/into heavier outbound trains, and determining the actual departure time of outbound trains. By using a time-discretized modelling technique, we formulate the studied problem as a mixed integer linear programming model, to minimize the weighted sum of the total railcar dwell time and the total railcar extra transfer time. Several variable fixing rules and valid inequalities are designed to strengthen the proposed model. An iterated search algorithm, in which an intensification search and a neighborhood search are executed iteratively, is developed to solve large-scale problems efficiently. In the intensification search, the proposed model is reduced into a simpler assignment model to compute an incumbent solution, by fixing the departure time of outbound trains. In the neighborhood search, a rolling horizon heuristic is designed to improve the incumbent solution, by dividing the departure period of outbound trains into multiple overlapped shorter stages, and re-optimizing the solution in each stage while maintaining that in other stages unchanged. Different scales of instances, randomly generated from a real-world marshalling station in one of the busiest Chinese heavy-haul railways, are used to test the proposed approaches. Computational results demonstrate that our algorithm computes (near-)optimal solutions (with a maximum relative gap of 1.73% only) for all the tested instances within a maximum computation time shorter than 13 min. Our approach can also find better solutions (with an average improvement rate of 3.27%) in shorter computation time, when compared to the empirical method used in practice.

2-D numerical study of ferrofluid droplet formation from microfluidic T-junction using VOSET method

This article conducts a two-dimensional numerical model to simulate the ferrofluid droplet formation from microfluidic T-junction under inhomogeneous magnetic fields with diverse strengths. This external magnetic field is produced by two electric straight wires in a finite computational domain. A coupled volume-of-fluid and level-set interface tracking method (VOSET) is adopted to capture the evolution of two-phase interface. Meanwhile, a two-region computational domain method is designed for situations that the droplets are in close contact with the solid boundaries for the fluid flow. All 2-D numerical simulations are implemented by a self-developed CFD code, named as MHT (Multi-concept Heat Transfer). The numerical results show a significant inhibition effect in droplet formation at the presence of external magnetic field. With the increase of the current intensity, the magnetic force of the ferrofluid droplet increases and decreases periodically, especially when the electric current intensity is less than 60 A. The increasing current intensity enlarges the departure diameter and prolongs the departure period of ferrofluid droplet, especially when the current intensity in the range 12 A∼54A. In the cases of electric current within [12A, 54 A], the departure diameter growths monotonically and nearly in a quadratic manner with the increase of the current intensity. However, when the current intensity exceeds 60 A, the departure characteristic of ferrofluid will be changed due to ferrofluid droplet absorbed on the upper wall of the main channel.

Max-Plus Linear Equation System and Its Application on Railway Network System

Max-Plus algebra is the set of R U {-~} with R is the set of all real numbersthat are equipped with maximum operation and addition. Max-Plus algebra is able tomodel several types of Discrete Event System (DES) which are nonlinear in conventional algebra to be linear in Max-Plus algebra, so we can do further analysis of the system. Types of DES will be linear in the form of Max-Plus algebra which only synchronizes without any concurrency such as railway network systems, production systems, traffic lights, etc. This research discusses the application of the linear Max-Plus equation in the train schedules and involves synchronization between trains. The result of this study are obtained DAOP VI Yogyakarta rail network system model in the form of x(k + 1) = A otimes x (k) which is then used to determine the departure period and the time of initial train departure. The departure period is obtained from the eigenvalue (lambda) from the A matrix and the initial departure is obtained from the eigenvector corresponding to lambda. The calculation shows that the train departure period is T = 588 minutes.

A mechanistic model for nucleate boiling heat transfer performance with lubricant-refrigerant mixture

The present study proposes a rationally based model to investigate the influence of lubricant on the nucleate boiling characteristics of refrigerant-lubricant mixtures. This model is developed based on site-activation boiling mechanism along with detailed physical parameters in association with thermal and hydrodynamic processes, including waiting period for bubble incipience, bubble growth and departure period. Yet, the model also encompasses the polymer adsorption theory and energy gap concept to address the effect of lubricant with different chemical structure and physical property on the heat transfer phenomenon during each period of nucleate boiling individually. In addition, by using the partition function with Boltzmann energy distribution in different energy state, bubble density can be expressed in an analytic form to facilitate the calculation of heat transfer coefficient. Based on the proposed model, the presence of lubricant appreciably changes interfacial energy upon metal-liquid and liquid-bubble interfaces. The lubricant prefers lying on metal surface, thereby influencing surface coverage concentration when bubble is initiating. Such lubricant-rich layer near metal surface significantly alters the waiting period of nucleate boiling process, bubble size, growth and departure time, bubble density and superheat on heating surface. In essence, the presence of lubricant dramatically influences heat transfer performance. The proposed model is validated against some recent test data for R-134a/POE, R-1234ze/POE and R-134a/PVE refrigerant/lubricants mixtures.

Technology of operational management in the activities of port elevators

The article deals with the main problems of port elevators, which lead to incomplete use of their capacities. They include: the problem of reducing the capacity factor of the elevator capacity, the problem of lack of rhythm of grain picking operations, the problem of inconsistency with railway transport, the problem of storage of different crops and crops of different quality, the problem of loss of time during the shipment of grain to the ship. The necessity of detailed planning of port elevators' operations by the determination of flows of work passing through the system, as well as the control of their implementation, is proved. The graphical model of implementation of the operational function of the port elevator is suggested, which will allow to coordinate its operations and to check the possibility of their implementation during the departure period of the ship. This model establishes the order of operations, as well as the main planning documents that regulate them. The use of a technological map of the implementation of the operational function for describing the list and sequence of basic operations for the formation of a ship batch and its shipment, conditions and requirements for their implementation, as well as information about the equipment used, is suggested. The graphical model and the technological map of the implementation of the port elevator's operational function provide information on the rational sequence and coherence of the operations' planning for the formation of the ship party and its shipment, which reduces the time loss, provides the opportunity to coordinate the work of the elevator with its transportation system.

Optimizing the Number of Express Freight Trains on a High-speed Railway Corridor by the Departure Period

In China, the transportation capacity of high-speed railways is gradually sufficient to provide services for high-value express freight besides meeting passenger demands. However, similar as the passengers, express freight demands fluctuate and show clear peak and trough periods daily. Therefore, optimizing running numbers of express freight trains on high-speed railway by periods is quite necessary to guarantee the revenues of railway industry and to meet the various requirements of all consignees simultaneously. First, a space-period-pattern three-dimensional network with virtual arcs is built to describe the departure period selection and the stopping or skip-stopping operations. Second, by constructing the arcs' impedance function, the user equilibrium principle is introduced to optimize the express freight flow distribution for each pattern in each period. To elaborate the comprehensive goal of balancing the relationship between profits and the flow distribution, a bi-level programming model is established. The upper model addresses the railway industry's maximum profits, and the lower model addresses the minimum and similar impedance values of the final express freight flow distribution. Finally, through the use of a hybrid algorithm that combines the heuristic genetic algorithm with the Frank-Wolfe algorithm, an experimental case of 10 stations on the Beijing-Xi'an high-speed railway corridor is used to validate the study. The results show that the express freight volume is reasonably distributed to each kind of virtual arc, the impedance value of each pattern is minimized and almost equalized, and the profits of railway industry are maximized by optimizing the number and departure time of trains by each pattern in each period on the basis of meeting all the express freight demands.

Lattice Boltzmann simulation of seawater boiling in the presence of non-condensable gas

In this study, three-dimensional (3D) boiling of seawater containing non-condensable gas (NCG) was simulated using the modified pseudopotential lattice Boltzmann model for multiphase multicomponent flow, and the effects of wettability and the NCG concentration on the heat flux between the heat source and its adjacent fluids, bubble departure period and diameter were analyzed. The simulation results show that, with an increase in the NCG concentration, the heat flux, bubble departure period and diameter decreased, while the effect of NCG on the bubble departure diameter and period plateaued as contact angle proceeded. Additionally, the boiling mechanisms of seawater containing NCG in a horizontal fissure were also analyzed. With an increase in the NCG concentration, the bubble departure diameter decreased while the bubble departure period increased. Furthermore, the influence was weakened with an increase in Re number.

PEMODELAN WAKTU TUNGGU PENUMPANG PADA JALUR ANGKUTAN DALAM KOTA PALU MENGGUNAKAN ALJABAR MAX-PLUS

Passenger waiting time is the time required by passengers starting from the stops until getting transport. The purpose of this research is to get the model of waiting time passengers on the freight line in the Palu city. The first step is the preparation of directed graphs based on the existring routes, then calculate the mileage and travel time using synchronization rules and power algorithm with initial vector 𝑥 = 0 obtained the value 𝑝 = 2, 𝑞 = 1 and 𝑐 = 102by using the appplication rock Scilab 5.5.2 and Map-Plus Toolbox obtained eigenvalues as the departure period of 102 and eigenvektor as the initial departure time.Keywords : Eigenvalues, Eigenvectors, Max-Plus Algebra, Waiting Time.

Simulation of three-dimensional bubble formation and interaction using the high-density-ratio lattice Boltzmann method

The growth and departure of bubbles from an orifice and their interactions is a common phenomenon. In this paper, a multiphase flow model based on the lattice Boltzmann method is constructed to study this process, with an improved interface capture method being adopted to deal with the high density ratio between liquid and gas. A virtual layer is added outside the computational domain to handle the boundary condition at the orifice. The processes of bubble formation and interaction are well simulated by the model, and the results agree well with those of theoretical calculations for single-bubble formation and departure diameter and period. Bubble formation is controlled by the inertial force of the gas, buoyancy, surface tension, and the viscous force of the liquid. The inertial force of the gas and buoyancy promote bubble growth and departure, respectively, while the surface tension controls bubble deformation and the viscous force affects the necking process. Increases in both the orifice size and the gas outflow velocity can enlarge the departure diameter of the bubble, but the orifice size plays a more important role here. Both of these two parameters have obvious effects on the departure period. For the interaction of bubbles growing from orifices with the same or different sizes, different coalescence categories are distinguished according to the stage of coalescence, and the characteristics of the coalescence process are considered in terms of their dependence on surface tension, viscous force, and buoyancy. Finally, the departure diameter and departure period of coalesced bubbles are studied under the different coalescence conditions.

Performance of Kenyan Listed Companies Pre and Post Chief Executive Officer Dismissal, Retirement and Resignation Events

In the decade between years 2001 – 2011 Kenya experienced change of CEOs of listed firms through dismissal, resignation and retirement which created a need to establish whether CEO departure events had a significant influence on stock returns and profitability of the affected firms. In this research 9 CEO departure events 3 dismissal, 3 retirement and 3 resignation events were analyzed. Abnormal stock returns were computed by employing the market model and profitability of the affected listed firms was measured by employing return on assets (ROA). Paired samples t-test was employed to examine whether there was a significant difference in the abnormal returns and profitability (ROA) during pre and post CEO departure period. The findings indicated that there was no significant difference in abnormal stock returns and profitability in the pre and post CEO departure events. The findings implied that NSE investors were not excited by change of CEOs and that the NSE is semi strong form efficient.

Lattice Boltzmann simulation of nucleate boiling in micro-pillar structured surface

A two-particle distribution functions lattice Boltzmann method is used to simulate periodic bubble nucleation, growth and departure from a micro-pillar structured surface. Bubble growth process and the effects of micro-pillar character scales on bubble departure period are investigated in this paper. Fluid hydrodynamic and thermodynamic are characterized by the density distribution function and the temperature distribution function respectively. Pseudo-potential model and Peng-Robinson equations of state are selected in our simulations. Constant temperature boundary condition is applied to the bottom solid. It is found that bubble generated in the vicinity of the micro-pillars, but in the same superheat, the smooth surface has no phase change phenomenon. This result indicates that micro-structure can contribute to the nucleation of the fluid phase change. Geometrical dimensions of micro-pillars have obvious effects on the growth of the bubble. Specifically, we investigated the influence of the distance between two pillars and the height of the pillars on the bubble growth. Results show that, to some extent, the distance of two micro-pillars “n” and the height of micro-pillar “h” have obvious effects on the bubble growth. For the height “h”, the smaller of “h”, the bigger of heat flux and the smaller of bubble departure periodic. For the distance “n”, it has no obvious effect on surface heat flux and bubble departure periodic but has an important effect on deciding whether it can form an activated nucleate site. Only proper “n” and “h” can be chosen to ensure an activated nucleation site according to our simulation results.

Pseudo-potential MRT - thermal LB simulation of flow boiling in vertical tubes

The flow boiling in vertical tubes is simulated by the MRT pseudo-potential - thermal LB model to study the effect of contact angle. No empirical correlations are given in this simulation. To validate the model, the relationship between the acceleration of gravity and the departure velocity of bubble in the departure period is compared with the empirical formula. The effect of dynamic contact angle on boiling is investigated. The simulation results of the dynamic contact angle and the static contact angle show that the dynamic contact angle has great influence on the bubble behavior. The key features of dynamic contact angle, i.e. the advancing contact angle, receding contact angle and contact angle hysteresis, are investigated. The mechanisms of differences of entire bubble period and bubble departure diameter under various contact angles are discussed. The heat transfer coefficient, the critical heat flux and the flow pattern of different surfaces are investigated. The results suggest that the critical heat flux of hydrophilic surface are higher than that of hydrophobic surface.

Grunt to go—Vocal coordination of group movements in redfronted lemurs

AbstractTo remain cohesive as a group, individuals must coordinate their movements between resources. In many species, vocalisations are used in this context. While some species have specific movement calls, others use calls which are also employed in different contexts. The use of such multicontextual calls has rarely been studied quantitatively, especially during both the pre‐departure and departure period associated with collective decisions. We thus investigated the use of close calls (“grunts”) for the coordination of collective movements in four groups of wild redfronted lemurs (Eulemur rufifrons) in Kirindy Forest, Western Madagascar. Group movements are started by an initiator, who moves away from the group and is joined by followers setting out in the same direction. We observed collective movements and recorded vocalisations from 18 focal individuals (54 movements recorded for followers, 21 for initiators). The grunt rate of both initiators and followers was higher in the pre‐departure period than in a control context (i.e., during foraging). Initiators of collective movements grunted more often than followers in the pre‐departure period as well as at individual departure. The latter difference was due to the initiators’ grunt rates increasing earlier than the followers’ and remaining at an elevated level for longer. These observations suggest that grunts serve to coordinate the departure by indicating the individual's readiness to move. The pre‐departure period, in which both initiators and followers showed an elevated grunt rate, may provide the basis for a shared decision on departure time. The difference in initiator and follower call rates suggests that grunts may have a recruitment function, but playback experiments are required to test this potential function. Overall, our study describes how multicontextual close calls can function as movement calls, with changes in call rate providing a potential feedback mechanism for the timing of group movements. This study thus contributes to a more detailed understanding of the mechanisms of group coordination and collective decision‐making.

Single bubble dynamics during subcooled nucleate boiling on a vertical heater surface: An experimental analysis of the effects of surface characteristics

In the present experimental study we investigated the effects of surface characteristics, such as wettability and roughness, on nucleate boiling in de-ionized water at a vertical heater. In the experiments, bubbles were generated from an artificial nucleation site on a stainless steel heater surface. High-resolution optical imaging has been used to capture the bubble life cycle, that is, departure, sliding, and lift-off. We found, that the lower wettability leads to larger departure diameter, longer sliding and larger lift-off diameter of bubbles. Also surface roughness effects have been analyzed and it was found that bubble departure and lift-off diameters are smaller and departure period is longer for a smooth surface. Bubble sliding velocity was found faster for a rough surface compared to a smooth surface. It was also found that the roughness is very influential to bubble growth and departure, which can be explained by considering its interaction with the microlayer underneath the bubble. An “optimal roughness”, which accelerates the bubble growth, was found. The knowledge gained from this study shall be particularly useful to improve nucleate boiling models for numerical simulations.

Visualization study on bubble dynamical behavior in subcooled flow boiling under various subcooling degree and flowrates

An experimental study was conducted to investigate the characteristics of bubble dynamical behavior in subcooled flow nucleate boiling in an upward annular channel. The bubble dynamical behavior under wide ranges of subcooling degrees (ΔTsub=5–50K) and flowrates (vm=0.1–0.8m/s) were observed and recorded with high spatial and temporal resolutions by using a high speed video camera and a Cassergrain microscopic lens. The bubble behaviors at different subcooling degrees and flowrates were found to be quite different. At a medium subcooling degree, the bubble deformation at upstream-side interface during its growing and departure period was observed and was attributed to the condensation caused by the cold liquid coming from the subcooled bulk flow. At high subcooling degrees and low flowrates, contracting deformation was found in both upstream and downstream bottom of the bubble, the phenomenon of two successive bubbles was also observed. At high subcooling degrees but high flowrates, bubble was strongly pushed by the incoming liquid flow and concave deformation in the middle of bubble upstream-side interface was observed. At lower subcooling degrees, bubble behaviors at different flowrates are quite similar. The bubble shape was very close to a sphere, only very weak deforming phenomena was observed.

Direct numerical simulation of bubble dynamics in subcooled and near-saturated convective nucleate boiling

With the long-term objective of Critical Heat Flux (CHF) prediction, bubble dynamics in convective nucleate boiling flows has been studied using a Direct Numerical Simulation (DNS). A sharp-interface phase change model which was originally developed for pool boiling flows is extended to convective boiling flows. For physical scales smaller than the smallest flow scales (smaller than the grid size), a micro-scale model was used. After a grid dependency study and a parametric study for the contact angle, four cases of simulation were carried out with different wall superheat and degree of subcooling. The flow structures around the growing bubble were investigated together with the accompanying physics. The relation between the heat flux evolution and the bubble growth was studied, along with investigations of bubble diameter and bubble base diameter evolutions across the four cases. As a validation, the evolutions of bubble diameter and bubble base diameter were compared to experimental observations. The bubble departure period and the bubble shapes show good agreement between the experiment and the simulation, although the Reynolds number of the simulation cases is relatively low.

Investigation of nanofluid bubble characteristics under non-equilibrium conditions

We report experimental and theoretical investigations of the bubble characteristics during the oscillatory growth period for several nanofluids. The nanoparticles were found to affect liquid–gas and solid surface tensions, which modulated the bubble contact angle, radius of triple line, bubble volume and the dynamics of bubble growth. To increase the accuracy of the Young–Laplace equation predictions during the bubble growth in the oscillatory period, a new method multi-section bubble (MSB) approach was developed. In this method, the bubble was divided into n sections (i.e., n=1:N) and the Young–Laplace equation was solved for each section individually. As N increases, within each section the effects of inertia force and viscosity become reduced comparing to that of the liquid-gas surface tension. Unlike the conventional Young–Laplace approach (i.e., N=1), the new approach is able to predict the bubble characteristics reliably in the following cases: (a) the oscillatory period when bubble is fluctuating; (b) the departure period when bubble is stretched upward, right before departure; and (c) the high shear stress condition when gas velocity is relatively high.

A sharp-interface phase change model for a mass-conservative interface tracking method

A new phase-change model has been developed for a mass-conservative interface tracking method. The mass transfer rate is directly calculated from the heat flux at the liquid–vapor interface, and the phase change takes place only in the cells which include this interface. As a consequence of the sharpness of the mass transfer rate distribution, the velocity jump across the interface can be captured, and high accuracy can be maintained. The method has been implemented in an incompressible Navier–Stokes equations solver employing a projection method based on a staggered finite-volume algorithm on Cartesian grids. The model has been verified for one-dimensional phase-change problems and a three-dimensional simulation of a growing vapor bubble in a superheated liquid under zero gravity condition. The computed results agree with theoretical solutions, and the accuracy of the model is confirmed to be of second-order in space using a grid refinement study. A three-dimensional simulation of a rising vapor bubble in a superheated liquid under gravity has been performed as a validation case, and good agreement with experimental data is obtained for the bubble growth rate. As a demonstration of the applicability of the method to engineering problems, a nucleate boiling simulation is presented with a comparison to experimental data. Good agreement is obtained for the bubble shapes and the bubble departure period. In all the simulation cases, strict mass conservation is satisfied.

Cave visitation by temperate zone bats: effects of climatic factors

We investigated the effects of climatic variables on the flight activity of bats at the entrance of a hibernaculum (Kateřinská cave, Moravian Karst, CZ). Activity was recorded automatically using a double infrared-light logging system. Climatic factors influenced not only seasonal but also night-to-night and overnight patterns in cave visitation. The effect of individual variables and their contribution to variability in activity levels changed during the year. (1) Flight activity during late hibernation (5 March–14 April) was positively affected by the mean ambient temperature (Tavg) and negatively affected by previous day minimal temperature. (2) During the departure period (15 April–4 June), nightly activity correlated with Tavg and Pavg (mean barometric pressure). Previous day rainfall caused a decline in the activity levels. (3) Summer activity (5 June–26 July) increased as the range of daily temperature (Tdif max−min) increased and was suppressed by previous day rainfall. In contrast, a higher amount of rainfall (>10 mm) in the study day caused an increase in activity, likely due to bats sheltering. (4) During swarming (5 September–14 November), activity was positively related to Tavg, Pavg and the amount of rainfall. (5) During hibernation (15 November–4 March), temperature (Tavg and Tdif max−min) was the best predictor of the activity level. The percentage of nights on which activity occurred increased with increasing temperature during hibernation and late hibernation. Activity occurred even at temperatures<0 °C (Tmin=−13.2 °C). The recordings were all positive at Tmax≥6.2 °C. The activity within corresponding temperature groups was significantly lower during hibernation than during late hibernation. We review possible explanations for the patterns observed.

BUBBLE DEPARTURE FROM METAL-GRAPHITE COMPOSITE SURFACES AND ITS EFFECTS ON POOL BOILING HEAT TRANSFER

The formation and growth processes of a bubble in the vicinity of graphite micro-fiber tips on metal-graphite composite boiling surfaces and their effects on boiling behavior are investigated. It is discovered that a large number of micro bubbles are formed first at the micro scratches and cavities on the metal matrix in pool boiling. By virtue of the non-wetting property of graphite, once the growing micro bubbles touch the graphite tips, the micro bubbles are sucked by the tips and merged into larger micro bubbles sitting on the end of the tips. The micro bubbles grow rapidly and coalesce to form macro bubbles, each spanning several tips. The necking process of a detaching macro bubble is analyzed. It is revealed that a liquid jet is produced by sudden break-off of the bubble throat. The composite surfaces not only have higher temperatures in micro- and macrolayers but also make higher frequency of the bubble departure, which increase the average heat fluxes in both the bubble growth stage and in the bubble departure period. Based on these analyses, the enhancement mechanism of pool boiling heat transfer on composite surfaces is clearly revealed.