Applications of stochastic modeling in air traffic management:Methods, challenges and opportunities for solving air traffic problems under uncertainty

In this paper we provide a wide-ranging review of the literature on stochastic modeling applications within aviation, with a particular focus on problems involving demand and capacity management and the mitigation of air traffic congestion. From an operations research perspective, the main techniques of interest include analytical queueing theory, stochastic optimal control, robust optimization and stochastic integer programming. Applications of these techniques include the prediction of operational delays at airports, pre-tactical control of aircraft departure times, dynamic control and allocation of scarce airport resources and various others. We provide a critical review of recent developments in the literature and identify promising research opportunities for stochastic modelers within air traffic management

Optimal control of queueing systems with multiple heterogeneous facilities

This thesis discusses queueing systems in which decisions are made when customers arrive, either by individual customers themselves or by a central controller. Decisions are made concerning whether or not customers should be admitted to the system (admission control) and, if they are to be admitted, where they should go to receive service (routing control). An important objective is to compare the effects of "selfish" decision-making, in which customers make decisions aimed solely at optimising their own outcomes, with those of "socially optimal" control policies, which optimise the economic performance of the system as a whole. The problems considered are intended to be quite general in nature, and the resulting findings are therefore broad in scope. Initially, M/M/1 queueing systems are considered, and the results presented establish novel connections between two distinct areas of the literature. Subsequently, a more complicated problem is considered, involving routing control in a system which consists of heterogeneous, multiple-server facilities arranged in parallel. It is shown that the multiple-facility system can be formulated mathematically as a Markov Decision Process (MDP), and this enables a fundamental relationship to be proved between individually optimal and socially optimal policies which is of great theoretical and practical importance. Structural properties of socially optimal policies are analysed rigorously, and it is found that 'simple' characterisations of socially optimal policies are usually unattainable in systems with heterogeneous facilities. Finally, the feasibility of finding 'near-optimal' policies for large scale systems by using heuristics and simulation-based methods is considered

Azaadamantane Benzamide 5-HT4 Agonists: Gastrointestinal Prokinetic SC-54750

Azaadamantanone 1 was converted to a series of aminoazaadamantane benzamides 9a–d, which were profiled for serotonin receptor activity. Aminomethylazaadamantane SC-54750 is a potent 5-HT4 agonist and 5-HT3 antagonist with in vivo efficacy in gastroparesis models and also inhibits cisplatin-induced emesis

Stochastic Modelling of Aircraft Queues: A Review

In this paper we consider the modelling and optimal control of queues of aircraft waiting to use the runway(s) at airports, and present a review of the related literature. We discuss the formulation of aircraft queues as nonstationary queueing systems and examine the common assumptions made in the literature regarding the random distributions for inter-arrival and service times. These depend on various operational factors, including the expected level of precision in meeting pre-scheduled operation times and the inherent uncertainty in airport capacity due to weather and wind variations. We also discuss strategic and tactical methods for managing congestion at airports, including the use of slot controls, ground holding programs, runway configuration changes and aircraft sequencing policies

Resource allocation in congested queueing systems with time-varying demand:An application to airport operations

Motivated by the need to develop time-efficient methods for minimizing operational delays at severely congested airports, we consider a problem involving the distribution of a common resource between two sources of time-varying demand. We formulate this as a dynamic program in which the objective is based on second moments of stochastic queue lengths and show that, for sufficiently high volumes of demand, optimal values can be well-approximated by quadratic functions of the system state. We identify conditions which enable the strong performance of myopic policies and develop approaches to the design of heuristic policies by means of approximate dynamic programming (ADP) methods. Numerical experiments suggest that our ADP-based heuristics, which require very little computational effort, are able to improve substantially upon the performances of more naive decision-making policies, particularly if exogenous system parameters vary considerably as functions of time

Structure of an acetylating aldehyde dehydrogenase from the thermophilic ethanologen Geobacillus thermoglucosidasius

Acetylating aldehyde dehydrogenases (AcAldDH) catalyse the acetylation of Coenzyme‐A (CoA), or in reverse generate acetaldehyde from Acetyl‐CoA using NADH as a co‐factor. This article reports the expression, purification, enzyme assay, and X‐ray crystal structures of an AcAldDH from Geobacillus thermoglucosidasius (GtAcAldDH) to 2.1Å and in complex with CoA and NAD(+) to 4.0Å. In the structure, the AcAldDH forms a close‐knit dimer, similar to that seen in other Alcohol Dehydrogenase (ADH) structures. In GtAcAldDH, these dimers associate via their N‐termini to form weakly interacting tetramers. This mode of tetrameric association is also seen in an unpublished AcAldDH deposited in the PDB, but is in contrast to all other ADH structures, (including the one other published AcAldDH found in a bacterial microcompartment), in which the dimers bury a large surface area including the C‐termini. This novel mode of association sequesters the active sites and potentially reactive acyl‐enzyme intermediates in the center of the tetramer. In other respects, the structure is very similar to the other AcAldDH, binding the cofactors in a corresponding fashion. This similarity enabled the identification of a shortened substrate cavity in G. thermoglucosidasius AcAldDH, explaining the limitations on the length of substrate accepted by the enzyme

Galaxies at a redshift of similar to 0.5 around three closely spaced quasar sightlines

We examine the relationship between galaxies and the intergalactic medium at z < 1 using a group of three closely spaced background quasi-stellar objects (QSOs) with zem ≈ 1 observed with the Hubble Space Telescope. Using a new grouping algorithm, we identify groups of galaxies and absorbers across the three QSO sightlines that may be physically linked. There is an excess number of such groups compared to the number we expect from a random distribution of absorbers at a confidence level of 99.9 per cent. The same search is performed with mock spectra generated using a hydrodynamical simulation, and we find that the vast majority of such groups arise in dense regions of the simulation.We find that atz<0.5, groups in the simulation generally trace the large-scale filamentary structure as seen in the projected 2D distribution of the HI column density in an ∼30 h −1 Mpc region. We discover a probable sub-damped Lyman α system at z = 0.557 showing strong, low-ionization metal absorption lines. Previous analyses of absorption across the three sightlines attributed these metal lines to HI. We show that even when the new line identifications are taken into account, evidence remains for planar structures with scales of ∼1 Mpc absorbing across the three sightlines. We identify a galaxy at z = 0.2272 with associated metal absorption in two sightlines, each 200 kpc away. By constraining the star formation history of the galaxy, we show that the gas causing this metal absorption may have been enriched and ejected by the galaxy during a burst of star formation 2 Gyr ago