Minimum Time Lags Research Articles

A comparison of column-generation approaches to the Synchronized Pickup and Delivery Problem

In the Synchronized Pickup and Delivery Problem (SPDP), user-specified transportation requests from origin to destination points have to be serviced by a fleet of homogeneous vehicles. The task is to find a set of minimum-cost routes satisfying pairing and precedence, capacities, and time windows. Additionally, temporal synchronization constraints couple the service times at the pickup and delivery locations of the customer requests in the following way: a request has to be delivered within prespecified minimum and maximum time lags (called ride times) after it has been picked up. The presence of these ride-time constraints severely complicates the subproblem of the natural column-generation formulation of the SPDP so that it is not clear if their integration into the subproblem pays off in an integer column-generation approach. Therefore, we develop four branch-and-cut-and-price algorithms for the SPDP based on column-generation formulations that use different subproblems. Two of these subproblems are considered for the first time in this paper have not been studied before. We derive new dominance rules and labeling algorithms for their effective solution. Extensive computational results indicate that integrating either both types of ride-time constraints or only the maximum ride-time constraints into the subproblem results in the strongest overall approach.

Simplifying activity networks under generalized precedence relations to extended CPM networks

AbstractThe critical path method (CPM) is a network planning technology with a simple and intuitive representation. However, it can only represent a special precedence relation—the finish‐to‐start minimum time lag. Currently, generalized precedence relations (GPRs) are represented by complex activity networks. We simplify complex activity networks under GPRs by representing the GPRs in a new way. This new extended CPM network is similar to the standard CPM network with the exception that arcs may have negative lengths and cycles. The extended CPM network facilitates problems with GPRs. Although many current models and algorithms are advanced and efficient for the CPM network problem, they are difficult or impossible to improve for GPRs applications. The extended CPM network, however, combats these problems and serves as an illustration of resource leveling with activity splitting. The current 0‐1 binary model formulation for the resource leveling with just finish‐to‐start minimum time lags is also applied to the problem with GPRs using the extended CPM network.

Robust execution strategies for project scheduling with unreliable resources and stochastic durations

The resource-constrained project scheduling problem with minimum and maximum time lags (RCPSP/max) is a general model for resource scheduling in many real-world problems (such as manufacturing and construction engineering). We consider RCPSP/max problems where the durations of activities are stochastic and resources can have unforeseen breakdowns. Given a level of allowable risk, $$\alpha $$?, our mechanisms aim to compute the minimum robust makespan execution strategy. Robust makespan for an execution strategy is any makespan value that has a risk less than $$\alpha $$?. The risk for a makespan value, $$M$$M given an execution strategy, is the probability that a schedule instantiated from the execution strategy will not finish before $$M$$M given the uncertainty over durations and resources. We make three key contributions: (a) firstly, we provide an analytical evaluation of resource breakdowns and repairs on executions of activities; (b) we then incorporate such information into a local search framework and generate execution strategies that can absorb resource and durational uncertainties; and (c) finally, to improve robustness of resulting strategies, we propose resource breakdown aware chaining procedure with three different metrics. This chaining procedure computes resource allocations by predicting the effect of breakdowns on robustness of generated strategies. Experiments show effectiveness of our proposed methods in providing more robust execution strategies under uncertainty.

A branch and bound algorithm to minimise the total tardiness in the two-machine permutation flowshop scheduling problem with minimal time lags

In this paper, we consider the two-machine permutation flowshop scheduling problem with minimal time lags and the objective of minimising total tardiness. Dominance criteria are developed to establish precedence constraints between jobs in an optimal schedule and a lower bound on the total tardiness of the problem is derived. A branch and bound algorithm for this problem is developed to reach an optimal solution using lower bound and dominance criteria. Also, computationally efficient heuristics are proposed. Results of computational experiments on randomly generated problems are reported.

Animal Models—Decoding the Molecular Biology of Oral Cancer

Animal models have long been used to understand the initiation and progression of carcinogenesis, including that of oral mucosa.(1) One of the earliest models used was the chemical-induced oral cancer model, among which the Syrian Hamster check pouch was preferred for its ideal anatomical location and physiological features.(2) Salley et al(3) demonstrated that the cheek pouch mucosa underwent gradual changes from hyperplasia, carcinoma in situ to squamous cell carcinoma when exposed to polycyclic hydrocarbon 9, 10 dimethyl-1,2, benzanthracene (DMBA). Morris(4) standardized the dosage of carcinogen to 0.5% solution of DMBA in acetone and established that 5-week old animals were ideal to induce tumor with minimum time lag and maximum yield. Lin et al(5) demonstrated the synergistic effect of arecaidine with DMBA.

Minimizing total tardiness in the permutation flowshop scheduling problem with minimal and maximal time lags

In this research, we study permutation flowshop scheduling problem with minimal and maximal time lags while minimizing the total tardiness. The time lags are defined between couples of successive operations of jobs. Each time lag is greater than or equal to a prescribed value called minimal time lag and smaller than or equal to a prescribed value called maximal time lag. A new mathematical formulation is proposed. Upper bounds are provided by applying heuristic procedures based on known and new rules. Then, new lower bounds are derived by applying different methods where the main one is the Lagrangian relaxation. In order to make the last technique a viable approach to the considered problem, an auxiliary formulation is adopted and the Lagrangian multipliers are updated using the subgradient algorithm. Then, results of computational experiments are reported.

Characterization of Ethiopian mega hydrogeological regimes using GRACE, TRMM and GLDAS datasets

Understanding the spatio-temporal characteristics of water storage changes is crucial for Ethiopia, a country that is facing a range of challenges in water management caused by anthropogenic impacts as well as climate variability. In addition to this, the scarcity of in situ measurements of soil moisture and groundwater, combined with intrinsic “scale limitations” of traditional methods used in hydrological characterization are further limiting the ability to assess water resource distribution in the region. The primary objective of this study is therefore to apply remotely sensed and model data over Ethiopia in order to (i) test the performance of models and remotely sensed data in modeling water resources distribution in un-gauged arid regions of Ethiopia, (ii) analyze the inter-annual and seasonal variability as well as changes in total water storage (TWS) over Ethiopia, (iii) understand the relationship between TWS changes, rainfall, and soil moisture anomalies over the study region, and (iv) identify the relationship between the characteristics of aquifers and TWS changes. The data used in this study includes; monthly gravity field data from the Gravity Recovery And Climate Experiment (GRACE) mission, rainfall data from the Tropical Rainfall Measuring Mission (TRMM), and soil moisture from the Global Land Data Assimilation System (GLDAS) model. Our investigation covers a period of 8years from 2003 to 2011. The results of the study show that the western part and the north-eastern lowlands of Ethiopia experienced decrease in TWS water between 2003–2011, whereas all the other regions gained water during the study period. The impact of rainfall seasonality was also seen in the TWS changes. Applying the statistical method of Principal Component Analysis (PCA) to TWS, soil moisture and rainfall variations indentified the dominant annual water variability in the western, north-western, northern, and central regions, and the dominant seasonal variability in the western, north-western, and the eastern regions. A correlation analysis between TWS and rainfall indicated a minimum time lag of zero to a maximum of six months, whereas no lag is noticeable between soil moisture anomalies and TWS changes. The delay response and correlation coefficient between rainfall and TWS appears to be related to recharge mechanisms, revealing that most regions of Ethiopia receive indirect recharge. Our results also show that the magnitude of TWS changes is higher in the western region and lower in the north-eastern region, and that the elevation influences soil moisture as well as TWS.

Upper and lower bounds for the permutation flowshop scheduling problem with minimal time lags

In this paper, we consider the problem of scheduling \(n\) jobs in an \(m\)-machine permutation flowshop with minimal time lags between consecutive operations of each job. The processing order of jobs is to be the same for each machine. The time lag is defined as the waiting time between two consecutive operations of each job. Upper bounds for the problem are provided by applying heuristic procedures based on known and new rules. Lower bounds based on Moore’s algorithm and logic-based Benders decomposition are developed. For the last one, we define a long time horizon on the last machine divided into many segments of time. We combine a mixed integer linear programming to allocate jobs to time segments and scheduled using the constraint programming. Then, computational results are reported.

A multi-objective approach for a project scheduling problem with due dates and temporal constraints infeasibilities

In this paper, we study a multi-mode resource-constrained project scheduling problem (RCPSP) which considers time and work generalised precedence relationships with minimal and maximal time lags and due dates where each activity requires only one unit of resource (e.g. a worker, a machine, etc.). To find a feasible solution for this problem is NP-hard and therefore for instances where a feasible solution has not been found, an appropriate real-life approach would consist of providing the decision-maker with a collection of quality solutions with a trade-off between due dates and temporal constraints violations. We propose a multi-objective evolutionary algorithm for the generation of an approximation to the optimal Pareto front with the objectives of minimising the project tardiness, and the time and work precedence relationships infeasibilities. The algorithm is basically a genetic algorithm with a multi-objective management of the evolution and is complemented by several local searches. Local Searches are based on justification of strong components. In the literature, this methodology has been previously used successfully with activities, but never with strong components. Computational experiments have been carried out to test the relative efficiency of different versions of the algorithm.

Converging to periodic schedules for cyclic scheduling problems with resources and deadlines

Cyclic scheduling has been widely studied because of the importance of applications in manufacturing systems and in computer science. For this class of problems, a finite set of tasks with precedence relations and resource constraints must be executed repetitively while maximizing the throughput. Many applications also require that execution schedules be periodic i.e. the execution of each task is repeated with a fixed global period w.The present paper develops a new method to build periodic schedules with cumulative resource constraints, periodic release dates and deadlines. The main idea is to fix the period w, to unwind the cyclic scheduling problem for some number of iterations, and to add precedence relations so that the minimum time lag between two successive executions of any task equals w. Then, using any usual (not cyclic) scheduling algorithm to compute task starting times for the unwound problem, we prove that either the method converges to a periodic schedule of period w or it fails to compute a schedule. A non-polynomial upper bound on the number of iterations to unwind in order to guarantee that cyclic precedence relations and resource constraints are fulfilled is also provided. This method is successfully applied to a real-life problem, namely the software pipelining of inner loops on an embedded VLIW processor core by using a Graham list scheduling algorithm.

On the Babylonian Sighting-Criterion for the Lunar Crescent and its Implications for Egyptian Lunar Data

The aim of the present paper is to establish a lower boundary condition of a possible successful sighting of the lunar crescent based on Babylonian predictions and observations. This means that all Egyptian chronological options showing deviations in positive direction with visibilities below this boundary condition will be dismissed as impossible. Although a theoretical limit of 10° for the visibility criterion, i.e. a minimum lagtime between sunset and moonset of 40 minutes, is mentioned in text TU 11, the Babylonian Astronomical Normal Star Almanacs contain several predictions for first or last visibility with values smaller than 10° and the Astronomical Diaries a few such observations. Thus, a value of 10° for the lagtime is an inappropriate lower boundary condition which cannot be used as knock-out criterion for Egyptian chronological options based on lunar data.

Thermal, economical and environmental analysis of insulated building walls in a cold climate

In this study, thermal, economical and environmental effects in insulated building walls are numerically investigated under dynamic thermal conditions for two different wall structures and two different insulation materials. The investigation is carried out for different wall orientations during the winter period in Kars city which is one of the coldest cities of Turkey.For this purpose, a computer model based on an implicit finite difference procedure, which has been previously validated, is used under steady periodic conditions. Firstly, thermal characteristics such as yearly heating transmission load, yearly averaged time lag and decrement factor are calculated for heating season. The results show that maximum time lag, minimum decrement factor and lowest heating load are obtained in the brick wall with XPS (Extruded polystyrene) while minimum time lag, maximum decrement factor and highest heating load are obtained in the concrete wall with EPS (Expanded polystyrene).Secondly, yearly heating loads obtained under dynamic conditions are used as inputs to an economic model for the determination of the optimum insulation thickness. The optimum insulation thicknesses, energy savings and payback periods are calculated by using life-cycle cost analysis over lifetime of 20years of the building. For heating season, it is seen that the lowest value of heating load, optimum insulation thickness and energy saving is obtained for the south-facing wall while highest value of them is obtained for the north-facing wall. The results show that for heating season, the most economical orientation is south-facing wall with an optimum insulation thickness of 9.2cm at brick wall with XPS.Lastly, fuel consumption and emissions of CO2 and SO2 are calculated by taking into consideration wall orientations for different structure and insulation materials. It is seen that as the insulation thickness increases, the yearly heating transmission load and consequently, fuel consumption and emissions decrease. The results show that for 9cm insulation thickness, this decrement is 85% for all oriented walls.

Analysis of a high‐throughput cone‐and‐plate apparatus for the application of defined spatiotemporal flow to cultured cells

The shear stresses derived from blood flow regulate many aspects of vascular and immunobiology. In vitro studies on the shear stress-mediated mechanobiology of endothelial cells have been carried out using systems analogous to the cone-and-plate viscometer in which a rotating, low-angle cone applies fluid shear stress to cells grown on an underlying, flat culture surface. We recently developed a device that could perform high-throughput studies on shear-mediated mechanobiology through the rotation of cone-tipped shafts in a standard 96-well culture plate. Here, we present a model of the three-dimensional flow within the culture wells with a rotating, cone-tipped shaft. Using this model we examined the effects of modifying the design parameters of the system to allow the device to create a variety of flow profiles. We first examined the case of steady-state flow with the shaft rotating at constant angular velocity. By varying the angular velocity and distance of the cone from the underlying plate we were able to create flow profiles with controlled shear stress gradients in the radial direction within the plate. These findings indicate that both linear and non-linear spatial distributions in shear stress can be created across the bottom of the culture plate. In the transition and "parallel shaft" regions of the system, the angular velocities needed to provide high levels of physiological shear stress (5 Pa) created intermediate Reynolds number Taylor-Couette flow. In some cases, this led to the development of a flow regime in which stable helical vortices were created within the well. We also examined the system under oscillatory and pulsatile motion of the shaft and demonstrated minimal time lag between the rotation of the cone and the shear stress on the cell culture surface.

Looking for the best modes helps solving the MRCPSP/max

The multi-mode resource-constrained project scheduling problem with minimum and maximum time lags MRCPSP/max is a very general project scheduling problem with multiple execution modes per activity, renewable and non-renewable resources and minimum and maximum time lags between activities. In this paper, we describe SA-EVA, an algorithm for the problem. SA-EVA first searches for the best mode for each activity, without considering renewable resources. In this phase a simulated annealing is applied. Once a mode vector has been chosen, the problem reduces to the RCPSP/max, which SA-EVA solves with EVA, an algorithm designed in Ballestín et al. [2009. An evolutionary algorithm for the resource-constrained project scheduling problem with minimum and maximum time-lags. Journal of Scheduling, 14 (4), online]. Computational results show that SA-EVA outperforms the state-of-the-art algorithms in medium and large instances.

Lexicographic optimization of a permutation flow shop scheduling problem with time lag constraints

AbstractThis paper considers the permutation flow shop scheduling problem with minimal and maximal time lags. Time lags are defined as intervals of time that must exist between every pair of consecutive operations of a job. The objective is to hierarchically minimize two criteria, the primary criterion is the minimization of the number of tardy jobs and the secondary one minimizes the makespan. We propose a mixed integer mathematical programming formulation which can be solved with the subroutine CPLEX. We also propose several versions of simulated annealing algorithm to heuristically solve the problem. Computational experiments to compare the proposed procedures are presented and discussed.

The Dynamics of the Skin Temperature of the Dead Sea

We explored the dynamics of the temperature of the skin layer of the Dead Sea surface by means of in situ meteorological and hydrographic measurements from a buoy located near the center of the lake. The skin temperature is most highly correlated to air temperature (0.93–0.98) in all seasons. The skin temperature is much less correlated to the bulk surface water temperature in the summer (0.80), when the lake is thermally stratified, and uncorrelated in the winter, when the Dead Sea is vertically mixed. Low correlations were found between the skin temperature and the solar radiation and wind speed in all seasons. The skin, with its low thermal inertia, responds immediately to the atmospheric forcing. Heat fluxes across the sea surface are also presented. The high correlation of skin temperature to air temperature with minimal time lag is a result of the nearly immediate response of the thin skin layer to the surface heat fluxes, primarily the sensible heat flux.

Effects of multiple matings on reproductive fitness of male and female Diaeretiella rapae

AbstractMating frequency and the amount of sperm transferred during mating have important consequences on progeny sex ratio and fitness of haplodiploid insects. Production of female offspring may be limited by the availability of sperm for fertilizing eggs. This study examined multiple mating and its effect on fitness of the cabbage aphid parasitoid Diaeretiella rapae McIntosh (Hymenoptera: Aphidiidae). Female D. rapae mated once, whereas males mated with on average more than three females in a single day. The minimum time lag between two consecutive matings by a male was 3 min, and the maximum number of matings a male achieved in a day was eight. Sperm depletion occurred as a consequence of multiple mating in D. rapae. The number of daughters produced by females that mated with multiple‐mated males was negatively correlated with the number of matings achieved by these males. Similarly, the proportion of female progeny decreased in females that mated with males that had already mated three times. Although the proportion of female progeny resulting from multiple mating decreased, the decrease was quicker when the mating occurred on the same day than when the matings occurred once per day over several days. Mating success of males initially increased after the first mating, but then males became ‘exhausted’ in later matings; their mating success decreased with the number of prior matings. The fertility of females was affected by mating with multiple‐mated males. The study suggests that male mating history affects the fitness of male and female D. rapae.

Navigators for motion detection during real-time MRI-guided radiotherapy

An MRI-linac system provides direct MRI feedback and with that the possibility of adapting radiation treatments to the actual tumour position. This paper addresses the use of fast 1D MRI, pencil-beam navigators, for this feedback. The accuracy of using navigators was determined on a moving phantom. The possibility of organ tracking and breath-hold monitoring based on navigator guidance was shown for the kidney. Navigators are accurate within 0.5 mm and the analysis has a minimal time lag smaller than 30 ms as shown for the phantom measurements. The correlation of 2D kidney images and navigators shows the possibility of complete organ tracking. Furthermore the breath-hold monitoring of the kidney is accurate within 1.5 mm, allowing gated radiotherapy based on navigator feedback. Navigators are a fast and precise method for monitoring and real-time tracking of anatomical landmarks. As such, they provide direct MRI feedback on anatomical changes for more precise radiation delivery.

Arsenic contamination in central-east India: new lessons for environmental health

This paper reports high levels and variability in the arsenic (As) levels at locations identified as one of the highest As-contaminated locations in the world. The high levels and variability in As levels were established by a strict monitoring protocol and the minimum possible time lag between the sampling and analysis. Special sampling arrangements were created at the central-east Indian location to monitor regularly the various water quality parameters at four specific locations. Out of the four monitored locations the locations 1–3 were found contaminated on more than 90% and the public water supply source (location 4) was found contaminated on more than 38% sampling episodes as per Indian standards. If the WHO standards for As (max. 0.01 mg/L) are considered then all the inhabitants were supplied with unacceptable drinking water on ∼93% of the sampled days during the sampling period. The paper identifies many complexities in the management of As-contamination and the perils of low level contamination in the As-endemic areas.

Robust Local Search for Solving RCPSP/max with Durational Uncertainty

Scheduling problems in manufacturing, logistics and project management have frequently been modeled using the framework of Resource Constrained Project Scheduling Problems with minimum and maximum time lags (RCPSP/max). Due to the importance of these problems, providing scalable solution schedules for RCPSP/max problems is a topic of extensive research. However, all existing methods for solving RCPSP/max assume that durations of activities are known with certainty, an assumption that does not hold in real world scheduling problems where unexpected external events such as manpower availability, weather changes, etc. lead to delays or advances in completion of activities. Thus, in this paper, our focus is on providing a scalable method for solving RCPSP/max problems with durational uncertainty. To that end, we introduce the robust local search method consisting of three key ideas: (a) Introducing and studying the properties of two decision rule approximations used to compute start times of activities with respect to dynamic realizations of the durational uncertainty; (b) Deriving the expression for robust makespan of an execution strategy based on decision rule approximations; and (c) A robust local search mechanism to efficiently compute activity execution strategies that are robust against durational uncertainty. Furthermore, we also provide enhancements to local search that exploit temporal dependencies between activities. Our experimental results illustrate that robust local search is able to provide robust execution strategies efficiently.