Inertial clustering of particles in high-Reynolds-number turbulence

We report experimental evidence of spatial clustering of dense particles in homogenous, isotropic turbulence at high Reynolds numbers. The dissipation-scale clustering becomes stronger as the Stokes number increases and is found to exhibit similarity with respect to the droplet Stokes number over a range of experimental conditions (particle diameter and turbulent energy dissipation rate). These findings are in qualitative agreement with recent theoretical and computational studies of inertial particle clustering in turbulence. Because of the large Reynolds numbers a broad scaling range of particle clustering due to turbulent mixing is present, and the inertial clustering can clearly be distinguished from that due to mixing of fluid particles.     

Cyclical adjustment of point-in-time PD

Banking regulation stipulates that to calculate minimum capital requirements a long-term average of annual default probability (PD) should be used. Typically, logistic regression is applied with a 12-month sample period to obtain retail PD estimates. Thus the output will reflect the default rate in the sample, and not the long-term average. The ensuing calibration problem is addressed in the paper by a ‘variable scalar methodology’, based on an actual application in a commercial bank. Using quarterly intra-bank loss data over 15 years, a state-space model of the credit cycle is estimated by a Kalman filter, resulting in a structural decomposition of the credit cycle. This yields an adjustment factor for each point in the cycle for each of two client segments. The regulatory compliance aspects of such a framework, as well as some practical issues are presented and discussed.     

Simulation of single start station for Edmonton EMS

The City of Edmonton's Emergency Medical Services (EMS) department proposed to move to a ‘single start station system’ (SS system) in which all ambulances would begin and end their shifts at the same location. We developed a discrete event simulation model to estimate the impact of this change and subsequently used this model to explore other changes to Edmonton EMS operations, including the addition of stations, the addition of ambulances, different shifts, and a different redeployment system. We found that a SS system increased average unit availability and the fraction of calls reached within the department's response time standard, particularly during the current shift changeover periods. The paper describes the development and validation of the simulation model and summarizes the results of its application.     

Maximizing throughput in finite-source parallel queue systems

Motivated by the dispatching of trucks to shovels in surface mines, we study optimal routing in a Markovian finite-source, multi-server queueing system with heterogeneous servers, each with a separate queue. We formulate the problem of routing customers to servers to maximize the system throughput as a Markov Decision Process. When the servers are homogeneous, we demonstrate that the Shortest Queue policy is optimal, and when the servers are heterogeneous, we partially characterize the optimal policy and present a near-optimal and simple-to-implement policy. We use the model to illustrate the substantial benefits of pooling, by comparing it to the permanent assignment of customers to servers.     

Transport risk models for hazardous materials: revisited

Several proposed path evaluation functions for hazardous materials transport use an approximation. Modified functions that avoid the approximation violate two reasonable axioms. We present new models by redefining the decision problem as one of satisfying demand at the destination. The new models satisfy the axioms and are relatively tractable.     

A linear model for surface mining haul truck allocation incorporating shovel idle probabilities

We present models of trucks and shovels in oil sand surface mines. The models are formulated to minimize the number of trucks for a given set of shovels, subject to throughput and ore grade constraints. We quantify and validate the nonlinear relation between a shovel’s idle probability (which determines the shovel’s productivity) and the number of trucks assigned to the shovel via a simple approximation, based on the theory of finite source queues. We use linearization to incorporate this expression into linear integer programs. We assume in our integer programs that each shovel is assigned a single truck size but we outline how one could account for multiple truck sizes per shovel in an approximate fashion. The linearization of shovel idle probabilities allows us to formulate more accurate truck allocation models that are easily solvable for realistic-sized problems.     

Electron attachment time-of-flight mass spectrometry reveals fcc bulk-like packing in CO2 aggregates

Electron attachment time-of-flight mass spectrometry reveals geometrical shell closings in (CO₂)_70N350 van der Waals aggregates. Quantitative structural parameters obtained from the observed geometrical shell closing pattern are compared to theory. These parameters are consistent with bulk-like distorted face-centered-cubic (fcc) packing of CO₂ aggregates in this size range. A magic number is seen in the mass spectrum at N=116 which was previously shown to result from a nearly spherical object with bulk-like fcc packing. These results confirm that the critical value for the cluster-to-bulk packing transition lies below N100, in qualitative agreement with prior experimental and theoretical work.     

Effective quenching of fragment formation in negative ion oligonucleotide matrix-assisted laser desorption/ionization mass spectrometry through sodium adduct formation

Metastable decay of the sodium-free and the sodium adducts of the negatively charged hexameric nucleotides 5'-d(TTXYTT)-3' (X and Y are dC, dG or dA) in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF-MS) is reported. We show that the sodium adduct formation may be used to quantitatively quench dissociation channels in oligonucleotide MALDI-MS, and we argue, that by means of effective steps in the sample preparation that lead to per-sodiation of the sample, the sample preparation and the mass resolution of oligonucleotide mass spectra in MALDI may be improved substantially. The dependency of the fragment formation on the degree of sodiation is also discussed in context with the underlying fragmentation mechanisms.     

Dissociative electron attachment to gas phase valine: a combined experimental and theoretical study

Using a crossed electron/molecule beam technique the dissociative electron attachment (DEA) to gas phase L-valine, (CH(3))(2)CHCH(NH(2))COOH, is studied by means of mass spectrometric detection of the product anions. Additionally, ab initio calculations of the structures and energies of the anions and neutral fragments have been carried out at G2MP2 and B3LYP levels. Valine and the previously studied aliphatic amino acids glycine and alanine exhibit several common features due to the fact that at low electron energies the formation of the precursor ion can be characterized by occupation of the pi* orbital of the carboxyl group. The dominant negative ion (M-H)(-) (m/Z=116) is observed at electron energies of 1.12 eV. This ion is the dominant reaction product at electron energies below 5 eV. Additional fragment ions with m/Z=100, 72, 56, 45, 26, and 17 are observed both through the low lying pi* and through higher lying resonances at about 5.5 and 8.0-9.0 eV, which are characterized as core excited resonances. According to the threshold energies calculated here, rearrangements play a significant role in the formation of DEA fragments observed from valine at subexcitation energies.     

Scheduling ambulance crews for maximum coverage

This paper addresses the problem of scheduling ambulance crews in order to maximize the coverage throughout a planning horizon. The problem includes the subproblem of locating ambulances to maximize expected coverage with probabilistic response times, for which a tabu search algorithm is developed. The proposed tabu search algorithm is empirically shown to outperform previous approaches for this subproblem. Two integer programming models that use the output of the tabu search algorithm are constructed for the main problem. Computational experiments with real data are conducted. A comparison of the results of the models is presented.