Merging Junction Research Articles

Vehicle Driving Safety of Underground Interchanges Using a Driving Simulator and Data Mining Analysis

In the process of driving in an underground interchange, drivers are faced with many challenges, such as being in a closed space, visual changes alternating between light and dark conditions, complex road conditions in the confluence section, and dense signage, which directly affect the safety and comfort of drivers in an underground interchange. Thus, driving simulation, building information modeling (BIM), and data mining were used to analyze the impact of underground interchange safety facilities on driving safety and comfort. Acceleration disturbance and steering wheel comfort loss values were used to assist the comfort analysis. The CART algorithm, classification decision trees, and neural networks were used for data mining, which uses a dichotomous recursive partitioning technique where multiple layers of neurons are superimposed to fit and replace very complex nonlinear mapping relationships. Ten different scenarios were designed for comparison. Multiple linear regression combined with ANOVA was used to calculate the significance of the control variables for each scenario on the evaluation index. The results show that appropriately reducing the length of the deceleration section can improve driving comfort, setting reasonable reminder signs at the merge junction can improve driving safety, and an appropriate wall color can reduce speed oscillation. This study indicates that the placement of traffic safety facilities significantly influences the safety and comfort of driving in underground interchanges. This study may provide support for the optimization of the design of underground interchange construction and internal traffic safety facilities.

Insights into the bubble formation dynamics in converging shape microchannels using CLSVOF method

Abstract Bubble formation in a square microchannel having a converging shape merging junction has been studied using the Coupled Level-Set and Volume-of-Fluid (CLSVOF) method. The influence of variations in merging junction angles, fluid properties, and operating conditions on the bubble length and pressure drop has been analyzed. The results show a direct relationship between surface tension, gas-liquid flow ratio, and the inverse relation of continuous phase viscosity with the bubble length. Moreover, opposite variations of these parameters are observed for pressure drop. This work reveals a discerning influence of the angle variations of merging junction on the interplay between inertial, viscous, and surface tension forces in the bubble formation mechanism. We envisage that this numerical work will be of significant interest for the process intensification in various industries that deal with gas-liquid microfluidic systems.

简单闭环路网交通流定常解

The steady-state solutions of traffic flow in a circled road network composed of 3 road sections and 2 junctions were studied under the assumption of the user equilibrium principle at the diverging junction. The results show that, the solution parameters and the dynamic behavior depend continuously on the total number of vehicles in the network. More precisely, the solution suggests a constant density in each road section when the total number of vehicles is not greater than the 1st critical density and not smaller than the 2nd critical density. When the total number of vehicles is between the 2 critical densities, the shock discontinuity or queuing appears upstream towards a bottleneck or a junction where the upstream capacity is greater than the downstream capacity. Complete analytical results were presented with the diverging and the merging junctions as bottlenecks, respectively.

Analysis of Traffic Evolution on Road Networks of a Roundabout

In this paper, the analyses of traffic evolution on the road network of a roundabout having three entrances and three exiting legs are conducted from macroscopic point of view. The road networks of roundabouts are modeled as a merging and diverging types 1×2 and 2×1 junctions. To study traffic evolution at junction, two cases have been considered, namely, demand and supply limited cases. In each case, detailed mathematical analysis and numerical tests have been presented. The analysis in the case of demand limited showed that rarefaction wave fills the portion of the road network in time. In the contrary, in supply limited case, traffic congestion occurs at merging junctions and shock wave propagating back results in reducing the performance of a roundabout to control traffic dynamics. Also, we illustrate density and flux profiles versus space discretization at different time steps via numerical simulation with the help of Godunov scheme.

On convexity of the robust freeway network control problem in the presence of prediction and model uncertainty

In the freeway network control (FNC) problem, the operation of a traffic network is optimized using only flow control. For special cases of the FNC problem, in particular the case when all merging junctions are controlled, there exist tight convex relaxations of the corresponding optimization problem. In practice, many parameters of this optimization problem are not known with certainty, in particular the fundamental diagram and predictions of future traffic demand. This uncertainty poses a challenge for control approaches that pursue a model- and optimization-based strategy. In this work, we propose a robust counterpart to the FNC problem, where we introduce uncertainty sets for both the fundamental diagram and future, external traffic demands and seek to optimize the system operation, minimizing the worst-case cost. For a network with controlled merging junctions, and assuming that certain technical conditions on the uncertainty sets are satisfied, we show that the robust counterpart of the FNC problem can be reduced to a convex, finite-dimensional and deterministic optimization problem, whose numerical solution is tractable.

Assessment of Hydraulic Network Models in Predicting Reverse Flows in OD Cooled Disc Type Transformer Windings

Predicting liquid flow distribution in the winding is crucial for transformer thermal design and hydraulic network models are usually used to guarantee reasonable liquid flow distribution. This paper is a continuation of the research effort in using computational fluid dynamics (CFD) to calibrate and improve the network modelling approach. The possibility is assessed of hydraulic network models in predicting the occurrence of reverse flows in pump-driven oil directed (OD) cooling modes. CFD modelling shows that the occurrence of reverse flows requires both high Reynolds number at the winding pass inlet and the “real” pass inlet velocity profile. To reflect the effect of pass inlet velocity profile on flow distribution, a winding pass is added beneath dividing and merging junctions in deriving minor pressure loss correlations. In addition, hybrid minor loss correlations are used to incorporate the effect of pass inlet velocity profile and to mitigate the inaccuracy of minor pressure loss segregation introduced by the inclusion of a winding pass beneath the junctions. Even with the above mentioned improvements, a conventional network model with one node for each junction still cannot predict reverse flows because of its architecture limit, hence a 3-node junction network model is then proposed. It is demonstrated that the newly developed 3-node junction hydraulic network model can predict the occurrence of reverse flows in OD cooled disc-type transformer windings.

An exact convex relaxation of the freeway network control problem with controlled merging junctions

We consider the freeway network control problem where the aim is to optimize the operation of traffic networks modeled by the cell transmission model via ramp metering and partial mainline demand control. Optimal control problems using the cell transmission model are usually non-convex, due to the nonlinear fundamental diagram, but a convex relaxation in which demand and supply constraints are relaxed is often used. Previous works have established conditions under which solutions of the relaxation can be made feasible with respect to the original constraints. In this work, we generalize these conditions and show that the control of flows into merging junctions is sufficient to do so if the objective is to minimize the total time spent in traffic. We derive this result by introducing an alternative system representation. In the new representation, the system dynamics are concave and state-monotone. We show that exactness of the convex relaxation of finite horizon optimal control problems follows from these properties. Deriving the main result via a characterization of the system dynamics allows one to treat arbitrary monotone, concave fundamental diagrams and several types of control for merging junctions in a uniform manner. The derivation also suggests a straightforward method to verify if the results continue to hold for extensions or modifications of the models studied in this work.

REDUCING A POSSIBILITY OF TRANSPORT CONGESTION ON FREEWAYS USING RAMP CONTROL MANAGEMENT

Merge junctions are the key elements in the freeway system, as they are likely to function as bottlenecks. Investigations into breakdown occurrence at ramp junctions have demonstrated that when the groups of several vehicles following each other enter the freeway from the ramp, they are expected to create ‘turbulence’ resulting from lane changes, decelerations of vehicles on the mainline and inevitably by the cars merging from the on-ramp. This turbulence can lead to breakdown when the level of mainline demand is adequately high. In other words, the impact of a ramp vehicle on capacity is higher than that of a mainline vehicle, which indicates that a part of vehicles will simultaneously occupy two lanes during the process of changing them thus momentarily decreasing the capacity of the link. This feature becomes particularly important near bottlenecks where it might reduce the already limited throughput. The article introduces the main approaches, methodology, principles and stages of transport demand management on freeways that are aimed at improving the operation quality of transport facilities, including road safety. The technique allows evaluating and optimizing a Ramp-Metering (RM) concept from the viewpoint of minimizing the length of queues on ramps and a possibility of transport congestion. The proposed algorithm estimates the probability of starting congestion formation on the ramp using objective information on traffic conditions in each segment of the highway, which is based on the criterion for vehicle density on the lane. The last chapter shows the examples of traffic flow optimization on Western bypass ramps in Vilnius comparing two strategies for access control management using one or several vehicles per lane. Conclusions, trends and work on future investigations are presented at the end of the article.

The Contribution of Ramp Demand in the Capacity of Merge Bottleneck Locations

Transportation engineers rely on the Highway Capacity Manual (HCM) for estimating capacity at freeway segments. According to the HCM 2010, the capacity of the basic freeway segments is a function of the free-flow speed and it ranges from 2,400 passenger cars per hour per lane (pc/h/ln) for FFS 70 or 75 mi/h, to 2,250 pc/h/ln for FFS 55 mi/h. The freeway merge segments methodology in the HCM 2010 Update uses these same capacity values in the analysis procedure, although research has shown that capacities at these bottleneck locations are considerably lower. In addition to that, researchers have also observed that capacity varies significantly from day to day and from one site to the other. Researchers acknowledge that driver behavior and frequent interactions between mainline and ramp vehicles at these junctions are the causal factor of these variations in capacity and the low capacity values; however, this has not been reflected in the updated version of the HCM 2010. Furthermore, the HCM 2010 Update does not account for the conflicting movements and the contribution of the ramp vehicles on the overall merge junction capacity. This paper investigates the relationship between freeway and ramp demand and capacity at merge junctions. For the purposes of this research, historic data at merge bottleneck locations across North America with different geometric and operational characteristics were analyzed. The results of the analysis show that, there is a clear correlation between ramp demand, freeway demand and freeway capacity. More specifically, higher demand on the on-ramps produces lower overall capacity values. In addition, this paper proposes new capacity values for merge junctions as a function of the freeway and ramp demand and number of lanes.

On the Consistency of Freeway Macroscopic Merging Models

Many first-order macroscopic models for freeway traffic have been developed since the seminal work on the well-known Lighthill–Whitham–Richards, or LWR, continuous model. The asymmetric cell transmission model (ACTM) is a widely accepted macroscopic model integrated into a variety of freeway traffic control frameworks. The model is based on the original cell transmission model; modified merge equations make it computationally tractable for optimization of ramp metering. However, when the common simplistic calibration of the model is used, an infeasible merging behavior is observed at uncontrolled merges, and the resulting queue lengths are not accurately estimated. This paper proposes an elegant analytical relationship, based on traffic flow theory, for calibrating the two parameters involved in the ACTM merge model. The method ensures the physical consistency of the dynamics involved in queue processes in the proximity of a noncontrolled merge junction. This method should be an essential feature of any freeway traffic model.

Computing Individual Path Marginal Cost in Networks with Queue Spillbacks

“Individual path marginal cost” (IPMC) is defined as the change in travel cost of one unit of flow on a time-dependent path caused by one unit of flow on another time-dependent path. Knowledge of IPMC is central to dynamic transportation modeling, for instance, to compute system-optimal network performance, to solve a dynamic origin–destination (O-D) estimation problem, and to analyze equity issues for travelers with different origins and destinations. This paper proposes a method of approximating IPMC for general networks, in which a cell transmission model–based kinematic wave model is used to model traffic dynamics. By tracing the changes in the cumulative flow curves of the bottleneck links on which queues form during dynamic network loading, an approximation method is developed to obtain the IPMC for the cases of merge junctions, diverge junctions, and general junctions. This method was applied to compute the total path marginal cost in a network. The results showed that vehicles at the beginning of the congestion duration had significantly larger marginal travel costs than other vehicles. The method was then applied to solve a dynamic O-D estimation problem with partial link-flow counts and historical O-D trip tables. With the incorporation of IPMC into the estimation procedure, both the O-D demands and the observed path travel times were successfully reproduced.

Enhancing Realism in Modeling Merge Junctions in Analytical Models for System-Optimal Dynamic Traffic Assignment

The existing analytical system-optimal dynamic traffic assignment (SO-DTA) model formulated with the linear programming (LP) approach usually assumes system control over vehicles in the entire network. This property would give rise to unreasonable priorities at merge junctions that are sometimes physically impossible to realize for the given roadway configuration. In this paper, we demonstrate that models with and without considering the merge-priority ratio would exhibit very different traffic patterns and route-choice behavior. To realistically model traffic flow on a transportation network, one should properly distinguish the level of control by drivers, roadway geometry, and system providers. This paper also attempts to develop an LP module that explicitly considers the merge-priority ratio of a merge junction and can potentially be incorporated into the existing LP formulation of the SO-DTA problem based on the cell-transmission model. By more realistically modelling the behavior of vehicles at merge junctions, the obtained solution can be used as a benchmark to compare control strategies developed without explicitly considering the merge-priority ratio at merge junctions or strategies developed with heuristic approaches.

Continuous kinematic wave models of merging traffic flow

Merging junctions are important network bottlenecks, and a better understanding of merging traffic dynamics has both theoretical and practical implications. In this paper, we present continuous kinematic wave models of merging traffic flow which are consistent with discrete Cell Transmission Models with various distribution schemes. In particular, we develop a systematic approach to constructing kinematic wave solutions to the Riemann problem of merging traffic flow in supply–demand space. In the new framework, Riemann solutions on a link consist of an interior state and a stationary state, subject to admissible conditions such that there are no positive and negative kinematic waves on the upstream and downstream links, respectively. In addition, various distribution schemes in Cell Transmission Models are considered entropy conditions. In the proposed analytical framework, we prove that the stationary states and boundary fluxes exist and are unique for the Riemann problem for both fair and constant distribution schemes. We also discuss two types of invariant merge models, in which local and discrete boundary fluxes are the same as global and continuous ones. With numerical examples, we demonstrate the validity of the analytical solutions of interior states, stationary states, and corresponding kinematic waves. Discussions and future studies are presented in the conclusion section.

Development and calibration of the Anisotropic Mesoscopic Simulation model for uninterrupted flow facilities

This paper presents the development, analysis, and calibration of the Anisotropic Mesoscopic Simulation (AMS) model for uninterrupted flow facilities, such as freeways. The proposed AMS model is a vehicle-based mesoscopic traffic simulation approach that explicitly considers the anisotropic property of traffic flow into the vehicle state update at each simulation step. The advantage of AMS is its ability to address a variety of uninterrupted flow conditions in a relatively simple, unified and computationally efficient manner. The discussions focus on the key modeling concepts, the analytical properties and numerical analysis, and the calibration process and results. The addressed analytical properties are the overtaking conditions, acceleration and deceleration rate bounds, and shockwaves. The numerical analysis includes both freeway segments as well as merging junctions. Considerable efforts were devoted to employ the Next-Generation Simulation (NGSIM) program datasets to calibrate the AMS model parameters. The high traffic fidelity and satisfactory computational efficiency make AMS a promising simulation approach for large-scale regional dynamic traffic simulation and assignment.

A Macro-kinetic Hybrid Model for Traffic Flow on Road Networks

Abstract We have developed a new hybrid model for an heterogeneous traffic flow, based on a coupling of the Lighthill — Whitham and Richards (LWR) macroscopic model and the kinetic model. On the highways of a road network, we consider the macroscopic description of the traffic flow and switch to the kinetic model to compute the mass flux through a junction. This new model reproduces the capacity drop phenomenon at a merge junction, for instance, without imposing any priority rule. We present some numerical simulations in which we compare the results of the hybrid model with those given by the fully macroscopic model. Furthermore, we illustrate the consequences of the velocity distribution on the flow through a merging junction.

Paramics Simulation of Periodic Oscillations Caused by Network Geometry

Traffic oscillations such as stop-and-go waves in a traffic system can occur for a variety of reasons, such as instabilities. This paper describes a study of a type of periodic oscillations caused by network geometry with a microscopic simulation model, Paramics. With careful tuning in Paramics, periodic oscillations in which traffic states change periodically between a diverging junction and a merging junction are successfully simulated. The formation process is discussed in detail. The study suggests that, given an appropriate network structure and traffic conditions, the local oscillations caused by randomness in car-following and lane-changing models in Paramics can be maintained and can become periodic and global. Finally, the consistencies between a macroscopic kinematic wave model and Paramics are discussed, and future research topics and the implications of the findings of the study are presented.

Paramics Simulation of Periodic Oscillations Caused by Network Geometry

Traffic oscillations such as stop-and-go waves in a traffic system can occur for a variety of reasons, such as instabilities. This paper describes a study of a type of periodic oscillations caused by network geometry with a microscopic simulation model, Paramics. With careful tuning in Paramics, periodic oscillations in which traffic states change periodically between a diverging junction and a merging junction are successfully simulated. The formation process is discussed in detail. The study suggests that, given an appropriate network structure and traffic conditions, the local oscillations caused by randomness in car-following and lanechanging models in Paramics can be maintained and can become periodic and global. Finally, the consistencies between a macroscopic kinematic wave model and Paramics are discussed, and future research topics and the implications of the findings of the study are presented.

Multi-step synthesis of nanoparticles performed on millisecond time scale in a microfluidic droplet-based system.

This paper reports a plug-based, microfluidic method for performing multi-step chemical reactions with millisecond time-control. It builds upon a previously reported method where aqueous reagents were injected into a flow of immiscible fluid (fluorocarbons)(H. Song et al., Angew. Chem. Int. Ed., 2003, 42, 768). The aqueous reagents formed plugs--droplets surrounded and transported by the immiscible fluid. Winding channels rapidly mixed the reagents in droplets. This paper shows that further stages of the reaction could be initiated by flowing additional reagent streams directly into the droplets of initial reaction mixture. The conditions necessary for an aqueous stream to merge with aqueous droplets were characterized. The Capillary number could be used to predict the behavior of the two-phase flow at the merging junction. By transporting solid reaction products in droplets, the products were kept from aggregating on the walls of the microchannels. To demonstrate the utility of this microfluidic method it was used to synthesize colloidal CdS and CdS/CdSe core-shell nanoparticles.

Numerical study of early stages of an impulsively started unsteady laminar flow past expanded trapezoidal cylinders

Characteristics of the development of an impulsively started flow around an expanded trapezoidal cylinder were studied numerically. A stream function‐vorticity formulation in a body coordinate system was used to describe the unsteady flow field. The inflow Reynolds number considered ranges from 25 to 1,000. Pressure contours, surface pressure coefficient and drag coefficient were studied through the streamline flow field. Main‐flow and sub‐flow regimes are identified through an analysis of the evolution of the flow characteristics. Typically, for a given expanded trapezoidal cylinder, it is noted that flow starts with minimum separation at the aft end. As time advances, symmetrical standing zone of recirculation develops aft of the cylinder. The rate of growth in width, length and structure of the aft end eddies depends on the Reynolds number. As time advances and at higher Reynolds numbers, separated flow from the leading edges of the trapezoidal cylinder develops along the upper and lower inclined surfaces of the trapezoidal cylinder. The separation bubbles on the upper and lower inclined surfaces of the cylinder grow towards the downstream regions with time and eventually merge with the swelling symmetrical eddies aft of the cylinder. This merging of the flows created a complex flow regime with a disturbed tertiary flow zone near the merging junction. For the flows considered here, eventually, depending on the Reynolds number and the expanded angle of the trapezoidal cylinder, the flow field develops into a specific category of symmetrical standing recirculatory flow with its own distinct characteristics.

Early stages of an impulsively started unsteady laminar flow past tapered trapezoidal cylinders

Characteristics of the developing recirculation region behind a tapered trapezoidal cylinder and its interaction with the separating shear layer from the leading edges were studied numerically for an impulsively started laminar flow. An unsteady stream function–vorticity formulation was used. The Reynolds numbers considered range from 25 to 1000. Pressure contours, surface pressure coefficient, wake length and drag coefficient were studied through the streamline flow field. Main flow and subflow regimes were identified by an analysis of the evolution of the flow characteristics. It was found that typically, for a given trapezoidal cylinder, flow starts with no separation. As time advances, the symmetrical standing zone of recirculation develops aft of the trapezoidal cylinder. The rate of growth in width, length and structure of the aft end eddies depends on the Reynolds number. In time, separated flow from the leading edges of the trapezoidal cylinder also develops and forms growing separation bubbles on the upper and lower inclined surfaces of the trapezoidal cylinder. As time advances, the separation bubbles on the upper and lower inclined surfaces of the cylinder grow towards the downstream regions and eventually merge with the swelling symmetrical eddies aft of the cylinder. This merging of the flows creates a complex flow regime with a disturbed tertiary flow zone near the merging junction. Eventually, depending on the Reynolds number and the tapered angle of the trapezoidal cylinder, the flow develops into a specific category of symmetrical standing recirculatory flow with its own distinct characteristics. Comparisons with the available results of other investigators showed very good agreement. © 1998 John Wiley & Sons, Ltd.