Vertical layering of photochemical ozone during land-sea breeze transport

Summary Recent theoretical and experimental studies have shown how local wind fields, such as the land-sea breeze circulation in coastal areas, influence the transport of oxidant precursors and the residence time and re-entry of photochemical compounds. Simple models usually do not take into account the vertical layering of pollutants, assuming a uniform concentration distribution within the boundary layer during transport processes. The present study reports the results of a monitoring survey conducted in an industrial coastal area to assess the time evolution of the vertical layering of ozone subject to land-sea breeze circulation in the boundary layer.     

A numerical simulation of the transport of surface ozone along a Mediterranean coastal area

Summary A simple numerical simulation of daily summer circulation in W-NW and E-SE directions was applied to a coastal area of Greater Ravenna in Northern Italy to assess the influence on ozone levels of the transport mechanism in the land-sea breeze system. Field measurements of ozone concentration and micrometeorological data collected over a ten-year period (1978–1989) by a coastal monitoring station were processed to identify meteorological and climatic situations. The diurnal pattern of a ?pure? off-shore-in-shore circulation system was reconstructed by applying a simple model in which data were considered or rejected as a function of the prevailing wind direction. Only data collected under W-NW and E-SE wind direction conditions became part of the ?reconstructed wind circulation system?. The findings show that the relative concentration values and the time duration of the simulated transport phenomenon are in good agreement with the recorded episodes of transport and of severe continuous fumigation, occurring during the night, at the polluted area.     

Stability analysis of quota allocation access protocols in ringnetworks with spatial reuse

We consider a slotted ring that allows simultaneous transmissions of messages by different nodes, known as ring with spatial reuse. To alleviate fairness problems that arise in such networks, policies have been proposed that operate in cycles and guarantee that a certain number of packets, not exceeding a given number called a quota, will be transmitted by every node in every cycle. We provide sufficient and necessary stability conditions that implicitly characterize the stability region for such rings. These conditions are derived by extending a technique developed for some networks of queues satisfying a monotonicity property. Our approach to instability is novel and its peculiar property is that it is derived from the instability of a dominant system. Interestingly, the stability region depends on the entire distribution of the message arrival process and the steady-state average cycle lengths of lower dimensional systems, leading to a region with nonlinear boundaries, the exact computation of which is in general intractable. Next, we introduce the notions of essential and absolute stability region. An arrival rate vector belongs to the former region if the system is stable under any arrival distribution with this arrival vector, while it belongs to the latter if there exists some distribution with this rate vector for which the system is stable. Using a linear programming approach, we derive bounds for these stability regions that depend only on conditional average cycle lengths. For the case of two nodes, we provide closed-form expressions for the essential stability region     

Thiocarbonyls. X. The Isomeric 2,4,6-Tris(halophenyl)-1,3,5-thianes

Nine pairs of isomeric 2,4,6-tris(halophenyl)-1,3,5-trithianes have been assayed in the crude state by n.m.r. techniques, and separated and purified by chromatography. Contrary to previous reports, the α-(cis, trans)-isomers are the major products in most cases. These compounds are shown to exist as puckered chair trithiane structures, even in the more hindered α-(cis, trans)-o-halophenyl cases, by the clear resolution of axial and equatorial trithiane ring protons in a ratio of 2:1. An o-halogen on an axial phenyl group in the α-isomers causes the aromatic group to exert an anisotropic deshielding effect on adjacent axial protons, so as to cause the axial and equatorial proton peaks to appear as a singlet in some solvents. Melting point differences, in several cases quite large, from those previously reported have been observed for six of the eighteen triaryltrithianes reported.     

A Method for the Delay Analysis of Random Multiple-Access Algorithms Whose Delay Process is Regenerative

Random multiple-access algorithms are used to control the accessing of a common communication channel by a large population of bursty channel users. For such algorithms, the induced transmission delay is a key performance measure. A systematic method for finding the delay characteristics of random multiple-access algorithms, whose delay process is regenerative, is presented. The method uses a powerful result from the theory of regenerative processes, in effect, to reduce the problem of determining the delay moments to the problem of solving denumerable dimensional systems of linear equations. Techniques for finding tight bounds on the solutions of such systems are presented. The "0.487" algorithm is used to exemplify the method.     

Elastostatics of the orthotropic double-cantilever-beam fracture specimen

Crack-plane stresses and the stress intensity factor were determined in an orthotropic double-cantilever-beam configuration. The DCB fracture specimen was modeled as an infinite strip containing a semi-infinite crack at its midplane. Concentrated loads acted upon the crack surfaces, whereas the strip surfaces were traction free. Constitutive equations of an orthotropic body involving four independent material constants were considered. Fourier transforms and the Wiener-Hopf technique were utilized for an analytical solution within the context of the two-dimensional, linear theory of elasticity.ASME Conference, Advances in the Mechanics of Composite Materials and Structures, New Orleans 1990.     

Energy-Aware Broadcast Trees in Wireless Networks

In this paper we address the problem of broadcasting in wireless networks, so that the power consumed by any node is as small as possible. This approach is motivated by the fact that nodes in such networks often use batteries and, hence, it is important to conserve energy individually, so that they remain operational for a long time. We formulate the problem as a lexicographic node power optimization one. The problem is in general NP-complete. We provide an optimal algorithm which runs in polynomial time in certain cases. We also provide a heuristic algorithm whose performance relative to the optimal one is fairly satisfactory. We next show that these algorithms can also be used to solve the problem of broadcasting so that the residual energy of any node after the broadcast process is as large as possible. Finally, we discuss the issues of implementing the above algorithms distributively, as well as their multicast extensions.     

Emission of stress waves during fracture

Emission of both longitudinal and surface (Rayleigh) waves during fracture of plates under conditions of plane stress and plane strain were studied experimentally. The non-equilibrated tensile stress in the fractured section of the plate creates an elastic wave, which travels radially along the plate at the sound speed. Moreover, the high surface deformation around the crack tip, due to the high stress concentration there, propagates as a surface wave following fracture of this zone, at the respective Rayleigh wave speed with a circular wavefront. The influence of the thickness of the plate and the type of fracture (brittle or ductile) was examined and interesting results were derived, by utilizing a high speed photography technique.