Contribution to flashover modelling: Development of a validated numerical model for ignition of non-contiguous wood samples

A computational model of flashover is presented that closely follows the experimental setup at CNRS-ENSMA-Poitiers. A propane burner with thermal power of 55 kW is used as a primary source of fire and square beech wood samples (30 mm×30 mm×5 mm) as fire spread targets. The computational model describes the wood pyrolysis with a progress variable. Using the conservation of heat fluxes at the solid–gas interface, the thermal diffusion in the wood samples is coupled with the convective and the radiative heat transfer in the ambient gas phase. The incoming heat flux at the upper surface of the wood samples reaches values between 20 and 30 kW/m². With the ignition and subsequent combustion of the pyrolysis volatiles, the heat flux increases by approx. 12 kW/m². The results show that the ignition of the wood samples is triggered at an approx. surface temperature of 650 K. Due to large local variations in incident heat flux, significant differences in the ignition times of the wood samples are observed. The comparison of the calculated and the experimentally measured temperature shows a good agreement for the first wood sample and the model predicts the ignition time very well. But for the second and the third wood samples the model overpredicts the temperature, which leads to a premature ignition of these wood samples.     

Absorption mass transfer in the presence of axial diffusion in convective—diffusive flows

The effect of axial diffusion, usually neglected in mass transfer calculations, in investigated for the absorption process into a liquid film. The absorption takes place within a finite length of the film and the surface resistance is assumed to be constant. For such a system, several upstream and down-stream boundary conditions are analysed and the results are presented in terms of the overall Sherwood number.It is found that the effect of axial diffusion is appreciable for very short absorbing lengths or relatively low flow velocities. For long absorbing lengths, all cases considered are insensitive to the upstream and downstream boundary conditions.     

Ventilation as a means of air-conditioning power saving in reinforced concrete telephone-exchange buildings-analysis and directions for design

The paper presents a computer-aided analysis of the combined use of ventilation and structural heat capacity and insulation as a means of saving air-conditioning power in digital telephone exchanges. It is shown that such savings can be effected by increasing air change with outdoor cool air at night, and when feasible, also in daytime, through the air change unit, in a conventional air-conditioning cycle. The total thermal time constant (TTTC) method of calculation used enables the evaluation of daily or seasonal heat-flow balance for a given indoor temperature, considering the tendency of the heat capacity of the building and of envelope components to flatten the fluctuations of this temperature.     

Partitioning a sequence into few monotone subsequences

In this paper we consider the problem of finding sets of long disjoint monotone subsequences of a sequence of numbers. We give an algorithm that, after preprocessing time, finds and deletes an increasing subsequence of size (if it exists) in time . Using this algorithm, it is possible to partition a sequence of numbers into monotone subsequences in time . Our algorithm yields improvements for two applications: The first is constructing good splitters for a set of lines in the plane. Good splitters are useful for two dimensional simplex range searching. The second application is in VLSI, where we seek a partitioning of a given graph into subsets, commonly refered to as the pages of a book, where all the vertices can be placed on the spine of the book, and each subgraph is planar. Received: 23 July 1990 / 19 June 1997     

On the Limitations of Universally Composable Two-Party Computation Without Set-Up Assumptions

The recently proposed universally composable security framework for analyzing security of cryptographic protocols provides very strong security guarantees. In particular, a protocol proven secure in this framework is guaranteed to maintain its security even when run concurrently with arbitrary other protocols. It has been shown that if a majority of the parties are honest, then universally composable protocols exist for essentially any cryptographic task in the plain model (i.e., with no set-up assumptions beyond that of authenticated communication). When honest majority is not guaranteed, general feasibility results are known only when given a trusted set-up, such as in the common reference string model. Only little was known regarding the existence of universally composable protocols in the plain model without honest majority, and in particular regarding the important special case of two-party protocols. We study the feasibility of universally composable two-party function evaluation in the plain model. Our results show that in this setting, very few functions can be securely computed in the framework of universal composability. We demonstrate this by providing broad impossibility results that apply to large classes of deterministic and probabilistic functions. For some of these classes, we also present full characterizations of what can and cannot be securely realized in the framework of universal composability. Specifically, our characterizations are for the classes of deterministic functions in which (a) both parties receive the same output, (b) only one party receives output, and (c) only one party has input.     

Effects of specific brain lesions on the thermal responses of rats to D-amphetamine

A streptomycete strain (FERM-P1185), isolated from soil, secreted a slime on glucose-asparagine agar, and produced viscous growth in liquid media containing peptone as nitrogen source. A purified polysaccharide isolated from the culture broth was composed of glucose and mannose units (molar ratio 1-87:1). Periodate oxidation, Smith degradation, methylation analysis, infrared and 13c nuclear magnetic resonance spectra indicated that this mannoglucan had a linear structure consisting of alpha-1,3- and alpha-1,4-linked glucopyranose and mannopyranose units.     

Wavelet-based analysis of transient electromagnetic wave propagation in photonic crystals

Photonic crystals and optical bandgap structures, which facilitate high-precision control of electromagnetic-field propagation, are gaining ever-increasing attention in both scientific and commercial applications. One common photonic device is the distributed Bragg reflector (DBR), which exhibits high reflectivity at certain frequencies. Analysis of the transient interaction of an electromagnetic pulse with such a device can be formulated in terms of the time-domain volume integral equation and, in turn, solved numerically with the method of moments. Owing to the frequency-dependent reflectivity of such devices, the extent of field penetration into deep layers of the device will be different depending on the frequency content of the impinging pulse. We show how this phenomenon can be exploited to reduce the number of basis functions needed for the solution. To this end, we use spatiotemporal wavelet basis functions, which possess the multiresolution property in both spatial and temporal domains. To select the dominant functions in the solution, we use an iterative impedance matrix compression (IMC) procedure, which gradually constructs and solves a compressed version of the matrix equation until the desired degree of accuracy has been achieved. Results show that when the electromagnetic pulse is reflected, the transient IMC omits basis functions defined over the last layers of the DBR, as anticipated.     

Robust linear dimensionality reduction

We present a novel family of data-driven linear transformations, aimed at finding low-dimensional embeddings of multivariate data, in a way that optimally preserves the structure of the data. The well-studied PCA and Fisher's LDA are shown to be special members in this family of transformations, and we demonstrate how to generalize these two methods such as to enhance their performance. Furthermore, our technique is the only one, to the best of our knowledge, that reflects in the resulting embedding both the data coordinates and pairwise relationships between the data elements. Even more so, when information on the clustering (labeling) decomposition of the data is known, this information can also be integrated in the linear transformation, resulting in embeddings that clearly show the separation between the clusters, as well as their internal structure. All of this makes our technique very flexible and powerful, and lets us cope with kinds of data that other techniques fail to describe properly.     

Efficient adaptive collect algorithms

Deterministic collect algorithms are presented that are adaptive to total contention and are efficient with respect to both the number of registers used and the step complexity. One of them has optimal O(k) step and O(n) space complexities, but assumes that processes’ identifiers are in O(n), where n is the total number of processes in the system and k is the total contention. The step complexity of an unrestricted name space variant of this algorithm remains O(k), but its space complexity increases to O(n ²).     

Routing with a clue

We suggest a new simple forwarding technique to speed up IP destination address lookup. The technique is a natural extension of IP, requires 5 bits in the IP header (IPv4, 7 in IPv6), and performs IP lookup nearly as fast as IP/Tag switching but with a smaller memory requirement and a much simpler protocol. The basic idea is that each router adds a "clue" to each packet, telling its downstream router where it ended the IP lookup. Since the forwarding tables of neighboring routers are similar, the clue either directly determines the best prefix match for the downstream router, or provides the downstream router with a good point to start its IP lookup. The new scheme thus prevents repeated computations and distributes the lookup process across the routers along the packet path. Each router starts the lookup computation at the point its upstream neighbor has finished. Furthermore, the new scheme is easily assimilated into heterogeneous IP networks, does not require routers coordination, and requires no setup time. Even a flow of one packet enjoys the benefits of the scheme without any additional overhead. The speedup we achieve is about 10 times faster than current standard techniques. In a sense, this paper shows that the current routers employed in the Internet are clue-less; namely, it is possible to speed up the IP lookup by an order of magnitude without any major changes to the existing protocols