Turbulence Closure with a Topography-parameter-free Single Equation Model

A new topography-parameter-free turbulence closure, based on a transport equation for the pseudo eddy viscosity, is described. The model is tested against experimental data of several flow cases across the Mach number range and compared to Menter's single equation model and to the Spalart-Allmaras model. The main conclusion is that the new closure outperforms the other two models in both low and high speed flows.     

Routing in Queues with Delayed Information

We compare two routing-control strategies in a high-speed communication network with c parallel channels (routes), where information on service completions in down-stream servers is randomly delayed. The controller can either hold arriving messages in a common buffer, dispatching them to servers only when the delayed information becomes available (Wait option), or route jobs to the various channels, in a round-robin fashion, immediately upon their arrival. Interpreting the delays as servers's vacations and considering overall queue sizes as a measure of performance, we show that the Wait strategy is superior as long as the mean information delay is below a threshold. We calculate threshold values for various combinations of load and c and show that, for a given load, the threshold increases with c and, for fixed c, the threshold decreases with an increasing load. If information is delayed on arrival instants, rather than on service completions, we show that the system can be viewed as a tandem queue and derive a generalization of a queue-decomposition result obtained by Altman, Kofman and Yechiali.     

Statistical mechanics analysis of the “twins paradox”

The aging of the two brothers in the twins paradox is analyzed through the space-time evolution of the densities that correspond to their internal complex structure. Taking into account their relative motion, it is shown that the traveling brother evolves over a shorter interval of time than his twin, which makes him younger than his brother.     

Lighting-up the dynamics of telomerization and DNA replication by CdSe-ZnS quantum dots

CdSe-ZnS core-shell quantum dots (QDs) act as photochemical centers for lighting-up the dynamics of telomerization or DNA replication.     

Site-directed deep electronic tunneling through a molecular network

Electronic tunneling in a complex molecular network of N(>2) donor/acceptor sites, connected by molecular bridges, is analyzed. The "deep" tunneling dynamics is formulated using a recursive perturbation expansion, yielding a McConnell-type reduced N-level model Hamiltonian. Applications to models of molecular junctions demonstrate that the donor-bridge contact parameters can be tuned in order to control the tunneling dynamics and particularly to direct the tunneling pathway to either one of the various acceptors.     

Cyclic Bernoulli polling

We introduce, analyse and optimize the class of Bernoulli random polling systems. The server movescyclically among N channels (queues), butChange-over times between stations are composed ofwalking times required to move from one channel to another andswitch-in times that are incurredonly when the server actually enters a station to render service. The server uses aBernoulli random mechanism to decide whether to serve a queue or not: upon arrival to channeli, it switches in with probabilityp _i , or moves on to the next queue (w.p. 1 —p _i ) without serving any customer (e.g. packet or job). The Cyclic Bernoulli Polling (CBP) scheme is independent of the service regime in any particular station, and may be applied to any service discipline. In this paper we analyse three different service disciplines under the CBP scheme: Gated, Partially Exhaustive and Fully Exhaustive. For each regime we derive expressions for (i) the generating functions and moments of the number of customers (jobs) at the various queues at polling instants, (ii) the expected number of jobs that an arbitrary departing job leaves behind it, and (iii) the LST and expectation of the waiting time of a cutomer at any given queue. The fact that these measures of performance can be explicitly obtained under the CBP is an advantage over all parameterized cyclic polling schemes (such as the k-limited discipline) that have been studied in the literature, and for which explicit measures of performance are hard to obtain. The choice of thep _i 's in the CBP allows for fine tuning and optimization of performance measures, as well as prioritization between stations (this being achieved at a low computational cost). For this purpose, we develop a Pseudo-conservation law for amixed system comprised of channels from all three service disciplines, and define a Mathematical Program to find the optimal values of the probabilities {p _i } _i ^N =1 so as to minimize the expected amount of unfinished work in the system. Any CBP scheme for which the optimalp _i 's are not all equal to one, yields asmaller amount of the expected unfinished work in the system than that in the standard cyclic polling procedure with equivalent parameters. We conclude by showing that even in the case of a single queue, it is not always true thatp ₁=1 is the best strategy, and derive conditions under which it is optimal to havep ₁ < 1.Supported by a Grant from the France-Israel Scientific Cooperation (in Computer Science and Engineering) between the French Ministry of Research and Technology and the Israeli Ministry of Science and Technology, Grant Number 3321190.     

Could ionic gamma-elimination be concerted: clocking the internal displacement across a cyclobutane ring

Carbanion 1, obtained by a nucleophilic attack of PhSe- on 3-chlorobicyclobutane-carbonitrile in DME undergoes both protonation and elimination as shown in eq 1. Alcohols of increasing acidity in the following order: t-BuOH, i-PrOH, MeOH, trifluoroethanol (TFE), and hexafluoro2-propanol (HFIP) were used as proton donors. An Eigen-type plot of the log of the product ratio (protonation/elimination) vs the pK(a) of the alcohols, levels off for the two most acidic alcohols, TFE and HFIP which react at a diffusion-controlled rate. The partitioning of the products between protonation and elimination enables, therefore, the determination of the rate constant for the internal elimination as approximately 3 x 10(10) s(-1). Ab initio calculations at the B3LYP/6-31G level show that the elimination from a model carbanion (4, eq 4) occurs in a barrierless process. Simulation of the experimental reaction by including solvation effects using the Onsager model, shows that using the dielectric constant of DME (7.2) stabilizes, as expected, the carbanion and prevents a spontaneous elimination. In the absence of solvation effects, using Me- as a base, a complete elimination of HCl (proton removal and leaving-group expulsion) took place from 3-chlorocyclobutanecarbonitrile in a barrierless process without the formation of any discrete intermediate.     

Enol-enamine tautomerism in crystals of 1,3-bis(pyridin-2-yl) propan-2-one: a combined crystallographic and quantum-chemical investigation of the effect of packing on tautomerization processes

The enolpyridine, OH-ketoenamime, NH equilibrium in crystals of 1,3-bis(pyridin-2-yl)propan-2-one was studied using temperature-dependent single-crystal X-ray diffraction. The relative population of the different tautomers was found to be sensitive to the temperature in the range of 100-300 K, illustrating the small thermodynamic difference between these two tautomers. This energy resemblance is partially attributed to the molecular packing in the crystal, where the molecules are arranged in the form of dimers. Ab initio electronic energy calculations (HF/6-31G** and MP2/6-31G**) reveal the effect of dimerization in the crystal on the electronic energy levels. Several tautomeric states were identified in the dimer of 1,3-bis(pyridin-2-yl)propan-2-one. A model is proposed in which four of these dimer states are populated in the crystal at ambient temperatures. The crystallographic data were treated according to this four-state dimer model, suggesting that the free energy of the OH-NH dimers is higher than that of the OH-OH dimers by 120 +/- 10 cal mol(-1) and that the NH-NH dimers are yet higher in free energy by 50 +/- 10 cal mol(-1).