“限时服务规则”下的复线船闸多目标调度优化

针对现有船舶过闸排队规则的欠缺,基于“限时服务规则”,构建复线船闸多目标双层优化调度模型:上层模型用于获得两个闸室安全区域的船舶排布可行方案;下层模型用于获得不同船舶排布可行方案的优化闸次数。下层模型分两个阶段完成:对符合“限时服务规则”的船舶,构建以闸次最少为目标的0-1规划模型,获得此类船舶安排的闸次;对其余船舶按照“先到先服务规则”,构建以闸次最少、闸室利用率最大为目标的多目标决策模型,获得不同船舶排布可行方案应该安排的频次。以位于江苏省干线航道上的某复线船闸某日24小时内过闸船舶的数据为例,计算结果表明:采用本文优化模型获得的优化方案与“经验编排方式”相比,两座船闸各节约2个闸次,两个船闸的平均闸室利用率分别提高了3.66和4.72个百分点。     

A key-lock-pair oriented access control scheme for the growth of users and files

A new access control scheme for the growth of users and files in file protection systems is proposed. Our scheme associates each user with a user key and each file with a file key. For each key, there are some corresponding locks, that can be extracted from a nonsingular matrix. Through simple operations on keys and locks, privacy decisions of the protection system can easily be revealed. Furthermore, by employing our method, whenever a new user or file is joined, the corresponding key values and lock values will be determined immediately without changing any previously defined keys and locks.     

Solutions for Initial-Boundary-Value Problems Representing Gravity Currents Arising from Variable Inflow

In this article, we report on theoretical and numerical studies of models for suddenly initiated variable inflow gravity currents in rectangular geometry. These gravity currents enter a lighter, deep ambient fluid at rest at a time‐dependent rate from behind a partially opened lock gate and their subsequent dynamics is modeled in the buoyancy‐inertia regime using ½‐layer shallow water theory. The resistance to flow that is exerted by the ambient fluid on the gravity current is accounted for by a front condition which involves a non‐dimensional parameter that can be chosen in accordance with experimental observations. Flow filament theory is used to arrive at expressions for the variable inflow velocity under the assumptions of an inviscid and incompressible fluid moving through an opening of fixed area which is suddenly opened under a lock gate at one end of a large rectangular tank. The fluid in the lock is subjected to a (possibly) time varying pressure applied uniformly over its surface and the finite movement of the free surface is accounted for. Finding this time‐dependent inflow velocity, which will then serve as a boundary condition for the solution of the shallow‐water equations, involves solving forced non‐linear ordinary differential equations and the form of this velocity equation and its attendant solutions will, in general, rule out finding self‐similar solutions for the shallow‐water equations. The existence of self‐similar solutions requires that the gravity currents have volumes proportional to t ^α , where α≥ 0 and t is the time elapsed from initiation of the flow. This condition requires a point source of fluid with very special properties for which both the area of the gap and the inflow velocity must vary in a related and prescribed time‐dependent manner in order to preserve self‐similarity. These specialized self‐similar solutions are employed here as a check on our numerical approach. In the more natural cases that are treated here in which fluids flow through an opening of fixed dimensions in a container an extra dimensional parameter is introduced thereby ruling out self‐similarity of the solutions for the shallow‐water equations so that the previous analytical approaches to the variable inflow problem, involving the use of phase‐plane analysis, will be inapplicable. The models developed and analyzed here are expected to provide a first step in the study of situations in which a storage container is suddenly ruptured allowing a heavy fluid to debouch at a variable rate through a fixed opening over level terrain. They also can be adapted to the study of other situations where variable inflow gravity currents arise such as, for example, flows of fresh water from spring run‐off into lakes and fjords, flows from volcanoes and magma chambers, discharges from locks and flash floods.     

A proactive approach for simultaneous berth and quay crane scheduling problem with stochastic arrival and handling time

For a container terminal system, efficient berth and quay crane (QC) schedules have great impact on the improvement of both operation efficiency and customer satisfaction. In this paper we address berth and quay crane scheduling problems in a simultaneous way, with uncertainties of vessel arrival time and container handling time. The berths are of discrete type and vessels arrive dynamically with different service priorities. QCs are allowed to move to other berths before finishing processing on currently assigned vessels, adding more flexibility to the terminal system. A mixed integer programming model is proposed, and a simulation based Genetic Algorithm (GA) search procedure is applied to generate robust berth and QC schedule proactively. Computational experiment shows the satisfied performance of our developed algorithm under uncertainty.     

An algorithm for early unlocking of entities in database transactions

Assume that a DBA is presented with (1) a database management system which controls concurrency by locking and (2) a set of transactions in which the programmer has specified lock steps and action steps. The objective is to insert unlocks to create a safe transaction system (i.e., all its legal schedules are guaranteed to be serializable) in which the sum of time units entities are locked is minimized. Although this is shown to be an NP-hard problem, an improvement can be made over two phase locking. In this paper we present an efficient algorithm that uses the database structure imposed by the way the transactions visit the entities to allow entities to be unlocked before all entities have been locked.     

A multiscale time model with piecewise constant argument for a boundedly rational monopolist

We present a dynamic model for a boundedly rational monopolist who, in a partially known environment, follows a rule-of-thumb learning process. We assume that the production activity is continuously carried out and that the costly learning activity only occurs periodically at discrete time periods, so that the resulting dynamical model consists of a piecewise constant argument differential equation. Considering general demand, cost and agent’s reactivity functions, we show that the behavior of the differential model is governed by a nonlinear discrete difference equation. Differently from the classical model with smooth argument, unstable, complex dynamics can arise. The main novelty consists in showing that the occurrence of such dynamics is caused by the presence of multiple (discrete and continuous) time scales and depends on size of the time interval between two consecutive learning processes, in addition to the agent’s reactivity and the sensitivity of the marginal profit.     

Developing an implementation capacity: justifications from prior research

Implementing innovative operational research solutions into organizations can be messy. Pragmatic inquiry suggests the first step in dissolving a mess is to determine the mindset, or set of concepts, that will be used by decision makers to inform their day to day choice of activities. Van de Ven and Poole reviewed much of the organizational change literature and identified four reasons why change occurs. They labelled them, life cycle, evolution, teleology and dialectic. It would seem logical to suggest that any attempt to make sense of the mess of implementation needs to span these four reasons. However, these reasons need to be operationalized into a mindset for implementers. To do this, the management implementation literature was reviewed under each of these reasons. The result is a justification of four concepts that can create a mindset likely to improve the implementation capacity of organizations. This mindset is that organizations make greater use of ‘champions’ ‘continuous improvement’ ‘job rotation’ and ‘debate’ over alternative activities. Exactly how so, is for individual organizations to interpret.     

The virtues of locking by symbolic names

We propose an algorithm for improving the concurrency of two phase locked transaction systems, which use symbolic-name locking. The algorithm determines by preanalysis which entities can be unlocked before all locks have been obtained, without comprising serializability. This extends the work we published (J. Algorithms 7 (1986) 146–156), in three ways. First, the transactions are not restricted to exclusive locks and may use shared locks as well. Second, a method is proposed to prevent the potential problem of cascading restarts, which results from unlocking of entities before commitment. Third, the transactions may be designed for a distributed database.     

Optimal sequencing in the presence of setup times for tow/barge traffic through a river lock

Queues of tow/barges form when a river lock is rendered inoperable due to lock malfunction, a tow/barge accident or adverse lock operating conditions. In this paper, we develop model formulations that allow the queue to be cleared using a number of differing objectives. Of particular interest is the presence of different setup times between successive passages of tow/barges through the lock. Dependent on the objective chosen, we are able to show that certain ordering protocols may be used to markedly reduce the sequencing search space for N tow/barges from the order of N! to 2^N. We present accompanying linear and nonlinear integer programming formulations and carry out computational experiments on a representative set of problems.     

Preservice Elementary Teachers' Conceptions of What Causes Night and Day

Two different procedures were utilized to study 50 preservice elementary teachers' conceptions of what causes night and day on earth. The numbers of responses representing alternative conceptions for the models with written and verbal explanation procedures were 27 and 14, respectively. The alternative conception most frequently expressed for both procedures was that night and day are caused by the earth revolving around the sun. The frequency with which alternative conceptions were expressed by future teachers is a problem which should be addressed with instruction using models.     

Exact and heuristic methods for placing ships in locks

The ship placement problem constitutes a daily challenge for planners in tide river harbours. In essence, it entails positioning a set of ships into as few lock chambers as possible while satisfying a number of general and specific placement constraints. These constraints make the ship placement problem different from traditional 2D bin packing. A mathematical formulation for the problem is presented. In addition, a decomposition model is developed which allows for computing optimal solutions in a reasonable time. A multi-order best fit heuristic for the ship placement problem is introduced, and its performance is compared with that of the left-right-left-back heuristic. Experiments on simulated and real-life instances show that the multi-order best fit heuristic beats the other heuristics by a landslide, while maintaining comparable calculation times. Finally, the new heuristic’s optimality gap is small, while it clearly outperforms the exact approach with respect to calculation time.     

Production scheduling with alternative process plans

This paper deals with a scheduling problem with alternative process plans that was motivated by a production of wire harnesses where certain parts can be processed manually or automatically by different types of machines. Only a subset of all the given activities will form the solution, so the decision whether the activity will appear in the final schedule has to be made during the scheduling process. The problem considered is an extension of the resource constrained project scheduling problem (RCPSP) with unary resources, positive and negative time-lags and sequence dependent setup times. We extend classic RCPSP problem by a definition of alternative branchings, represented by the Petri nets formalism allowing one to define alternatives and parallelism within one data structure. For this representation of the problem, an integer linear programming model is formulated and the reduction of the problem, using time symmetry mapping, is shown. Finally, a heuristic algorithm based on priority schedule construction with an unscheduling step is proposed for the nested version of the problem and it is used to solve the case study of the wire harnesses production.     

Water transport on infinite graphs

If the nodes of a graph are considered to be identical barrels – featuring different water levels – and the edges to be (locked) water‐filled pipes in between the barrels, consider the optimization problem of how much the water level in a fixed barrel can be raised with no pumps available, that is, by opening and closing the locks in an elaborate succession. This model is related to an opinion formation process, the so‐called Deffuant model. We consider the initial water profile to be given by i.i.d. unif(0,1) random variables, investigate the supremum of achievable water levels at a given node – or to be more precise, the support of its distribution – and ask in which settings it becomes degenerate, that is, reduces to a single value. This turns out to be the case for all infinite connected quasi‐transitive graphs, with exactly one exception: the two‐sided infinite path.     

Maximizing the net present value of a project under uncertainty

We address the maximization of a project’s expected net present value when the activity durations and cash flows are described by a discrete set of alternative scenarios with associated occurrence probabilities. In this setting, the choice of scenario-independent activity start times frequently leads to infeasible schedules or severe losses in revenues. We suggest to determine an optimal target processing time policy for the project activities instead. Such a policy prescribes an activity to be started as early as possible in the realized scenario, but never before its (scenario-independent) target processing time. We formulate the resulting model as a global optimization problem and present a branch-and-bound algorithm for its solution. Extensive numerical results illustrate the suitability of the proposed policy class and the runtime behavior of the algorithm.     

Application of hybrid RANS-LES models to the prediction of flow behaviour in an industrial crystalliser

Knowledge of the residence time and shear rates in industrial crystallisers is critical for any assessment of the performance of these vessels from a chemical engineering perspective. It is unlikely that the range of expected residence time behaviours, or the shear rates can be predicted accurately with a RANS model. In the current study, a wide range of hybrid RANS-LES models, including Stress-Blended Eddy Simulation (SBES), were used to predict the flow field in a laboratory-scale alumina precipitator with the objectives of both quantifying the accuracy of the models and assessing if the hybrid models provide significant improvement over a previous RANS modelling study. Predicted mean and fluctuating velocities have been compared with Laser Doppler Velocimetry (LDV) data from a laboratory-scale alumina precipitator. The results achieved show that hybrid RANS-LES models can accurately predict both the mean and fluctuating velocities in the precipitator vessel. Importantly, as the mesh is refined, agreement with experimental data improves and differences between model predictions reduce, showing that sensitivity to the sub-grid scale model reduces if all the relevant large-scale turbulence structures are explicitly resolved. Prediction of fluctuating velocities is found to be more accurate than that achieved in a previous RANS modelling study. The SBES results are found to be mesh-independent, and to give closer agreement with experimental data, on a coarser mesh than both the SST-DDES and SST-SAS approaches as the model formulation allows rapid transition to an explicit LES model immediately outside the wall boundary layer. This is an important result for industrial simulation due to the significant reduction in simulation times needed.     

PIV observation of instantaneous velocity structure of lock release gravity currents in the slumping phase

The experiments of lock release gravity currents were performed in a rectangular channel with salt water and fresh water. The spreading law of the salt water is validated by using a digital video to record the progress of the current. Detailed instantaneous velocity structure of lock release gravity currents in the slumping phase was studied experimentally with particle image velocimetry (PIV). The time variation of the spatial distribution of velocity and vorticity is obtained, which shows some qualitative characters of the current as well as the effects of the bottom boundary layer and upper mixing layer.     

Storage space allocation models for inbound containers in an automatic container terminal

This paper studies the problem of improving the operations efficiency for retrieving inbound containers in a modern automatic container terminal. In the terminal, when an external truck arrives to collect a container stored in a specific container block, it waits at one end of the block where an automatic stack crane will retrieve the container and deliver it to the truck. With the aim of reducing the expected external truck waiting time which is determined by how the containers are stored in a block, we propose two correlated approaches for the operations efficiency improvement, (1) by designing an optimized block space allocation to store the inbound containers after they are discharged from vessels, and (2) by conducting overnight re-marshaling processes to re-organize the block space allocation after some containers are retrieved. For the block space allocation problem, we consider three optimization models under different strategies of storing containers, namely, a non-segregation model, a single-period segregation model, and a multiple-period segregation model. Optimal solution methods are proposed for all three models. For the re-marshaling problem with a given time limit, we find that the problem is NP-hard and develop a heuristic algorithm to solve the problem. We then use simulation to validate our models and solution approaches. Simulation results reveal important managerial insights such as the advantage of the multiple-period segregation over the myopic single-period segregation, the possibility of overflow of the segregation model, and the benefit of re-marshaling.     

Strategies simulation in an aggregate bank model

A simple balance sheet simulation model was built up to help the top management to evaluate policies which directly affect the whole bank profit.We focused on three main alternative strategies, i.e.: (1) a growth in deposits as a measure of total activity expansion; (2) a change in the interest rates margin; (3) different provision policies for bad debts.According to the chosen strategy, the model allows for a quantitative response in the P and L account and in self-financing, showing the consequent flow of funds.The model was thought to be adaptable and responsive in a rapidly-changing environment. A first scheme refers to the whole bank activity; the same scheme with some adjustments refers to the foreign branches activity. Residually, it is possible to examine the bank's domestic activity.     

Branch and bound based solution algorithms for the budget constrained discrete time/cost trade-off problem

The time/cost trade-off models in project management aim to reduce the project completion time by putting extra resources on activity durations. The budget problem in discrete time/cost trade-off scheduling selects a time/cost mode for each activity so as to minimize the project completion time without exceeding the available budget. There may be alternative modes that solve the budget problem optimally and each solution may have a different total cost value. In this study we consider the budget problem and aim to find the minimum cost solution among the minimum project completion time solutions. We analyse the structure of the problem together with its linear programming relaxation and derive some mechanisms for reducing the problem size. We solve the reduced problem by branch and bound based optimization and heuristic algorithms. We find that our branch and bound algorithm finds optimal solutions for medium-sized problem instances in reasonable times and the heuristic algorithms produce high quality solutions very quickly.     

The coastal seaspace patrol sector design and allocation problem

In this paper, we model and solve the problem of designing and allocating coastal seaspace sectors for steady-state patrolling operations by the vessels of a maritime protection agency. The problem addressed involves optimizing a multi-criteria objective function that minimizes a weighted combination of proportional measures of the vessels’ distances between home ports and patrol sectors, the sector workload, and the sector span. We initially assure contiguity of each patrol sector in our mixed-integer programming formulation via an exponential number of subtour elimination constraints, and then propose three alternative solution methods, two of which are based on reformulations that suitably replace the original contiguity representation with a polynomial number of constraints, and a third approach that employs an iterative cut generation procedure based on identifying violated subtour elimination constraints. We further enhance these reformulations with symmetry defeating constraints, either in isolation or in combination with a suitable perturbation of the objective function using weighted functions based on such constraints. Computational comparisons are provided for solving the problem using the original formulation versus either of our three alternative solution approaches for a representative instance. Overall, a model formulation based on Steiner tree problem (STP) constructs and enhanced by the reformulation-linearization technique (RLT) yielded the best performance.