35 research outputs found
Diffusion in an absorbing porous medium : from microscopic geometry to macroscopic transport
No full textThesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 71-73).Two physical models of diffusion in absorbing porous media are proposed on two length scales. One models diffusion in the pore space of a random medium with absorbing interfaces while the other is a reaction diffusion model where particles are absorbed in the bulk. Typical particle traveling distances and a bulk absorption coefficient are described in terms of general geometrical characteristics of a random medium and the analytical relations are found to compare well with numerical experiments. For the case of geometries consisting of randomly placed cubes, absorption in the bulk scales with the solid fraction to the two-thirds power. The statistical distribution of reaction rates in these models is found to be inversely related to the reaction rate. A quasi-static Monte-Carlo model is also investigated. The more complex problem of microbial extracellular enzyme distributions in marine sediment was an inspiration for this work.by David C. Forney, III.S.M
At limits of life: multidisciplinary insights reveal environmental constraints on biotic diversity in continental Antarctica
No full textData source: Supporting information, http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0044578#s5Multitrophic communities that maintain the functionality of the extreme Antarctic terrestrial ecosystems, while the simplest of any natural community, are still challenging our knowledge about the limits to life on earth. In this study, we describe and interpret the linkage between the diversity of different trophic level communities to the geological morphology and soil geochemistry in the remote Transantarctic Mountains (Darwin Mountains, 80uS). We examined the distribution and diversity of biota (bacteria, cyanobacteria, lichens, algae, invertebrates) with respect to elevation, age of glacial drift sheets, and soil physicochemistry. Results showed an abiotic spatial gradient with respect to the diversity of the organisms across different trophic levels. More complex communities, in terms of trophic level diversity, were related to the weakly developed younger drifts (Hatherton and Britannia) with higher soil C/N ratio and lower total soluble salts content (thus lower conductivity). Our results indicate that an increase of ion concentration from younger to older drift regions drives a succession of complex to more simple communities, in terms of number of trophic levels and diversity within each group of organisms analysed. This study revealed that integrating diversity across multi-trophic levels of biotic communities with abiotic spatial heterogeneity and geological history is fundamental to understand environmental constraints influencing biological distribution in Antarctic soil ecosystems.Catarina Magalhães, Mark I. Stevens, S. Craig Cary, Becky A. Ball, Bryan C. Storey, Diana H. Wall, Roman Tűrk and Ulrike Ruprech
Microbial aspects of anaerobic methane oxidation with sulfate as electron acceptor
No full textAnaerobic oxidation of methane (AOM) is an important methane sink in the ocean but the microbes responsible for AOM are as yet resilient to cultivation. It was shown that AOM was coupled to sulfate reduction (SR) and this gave rise to current research which aims to develop a biotechnological process in which methane is used an electron donor for SR. This thesis describes the microbial analysis of an enrichment capable of high rate AOM (286 µmol.gdry weight-1.day-1) coupled to SR using a novel submerged membrane bioreactor system. Initially AOM rates were extremely low (0.004 mmol L-1 d-1), but AOM and SR increased exponential over the course of 884 days to 0.60 mmol L-1 d-1. The responsible organisms doubled every 3.8 months. By constructing a clone library with subsequent sequencing and fluorescent in situ hybridization (FISH), we showed that the responsible methanotrophs belong to the ANME-2a subgroup of anaerobic methanotrophic archaea, and that sulfate reduction is most likely performed by sulfate reducing bacteria commonly found in association with other ANME related archaea in marine sediments. Another relevant portion of the bacterial sequences can be clustered within the order of Flavobacteriales but their role remains to be elucidated. FISH analyses showed that the ANME-2a cells occur as single cells without close contact to the bacterial syntrophic partner. Incubation with 13C labeled methane showed substantial incorporation of 13C label in the bacterial C16 fatty acids (bacterial; 20, 44 and 49%) and in archaeal lipids, archaeol and hydroxyl-archaeol (21 and 20%, respectively). This confirms that both archaea and bacteria are responsible for the anaerobic methane oxidation in a bioreactor enrichment inoculated with Eckernförde bay sediment. To unravel the pathway of this syntrophic conversion, the effect of possible intermediates on AOM and SR was assessed. To investigate which kind of waste and process streams can be treated by the methanotrophic sulfate-reducing enrichment, the effect of environmental conditions and different substrates was assessed. The optimum pH, salinity and temperature for SR with methane by the enrichment were 7.5, 30‰ and 20°C, respectively. The biomass had a good affinity for sulfate (Km 75 KPa) and AOM was completely inhibited at 2.4 (±0.1) mM sulfide. The enrichment utilized sulfate, thiosulfate, sulfite and elemental sulfur as alternative electron acceptors for methane oxidation and formate, acetate and hydrogen as alternative electron donors for sulfate reduction. As a co-substrate for methane oxidation only methanol stimulated the conversion of 13C labeled CH4 to 13CO2 in batch incubations of Eckernförde bay sediment, other possible co-substrates had a negative effect on the AOM rate. The research described in this thesis shows the possibility of enriching slow growing methane oxidizing communities but also shows the difficulties in applying this process for a biotechnological purpose because of the extreme slow doubling times and the lack of understanding of the metabolic routes used by these organisms. <br/
Occurrence, distribution and abundance of cetaceans in Onslow Bay, North Carolina, USA
No full textIn this paper the occurrence, distribution and abundance of cetaceans in offshore waters of Onslow Bay, North Carolina, USA is described. Between June 2007 and June 2010 monthly aerial and shipboard line-transect surveys were conducted along ten 74km transects placed perpendicular to the shelf break. In total 42,676km of aerial trackline (218 sightings) and 5,209km of vessel trackline (100 sightings) were observed. Seven species of cetaceans were observed, but the fauna was dominated strongly by common bottlenose and Atlantic spotted dolphins. Both species were present year-round in the study area. Using photo-identification techniques, five bottlenose dolphins and one spotted dolphin were resighted during the three-year period. In general, the abundance of cetaceans in Onslow Bay was low and too few sightings were made to estimate monthly abundances for species other than bottlenose and spotted dolphins. Maximum monthly abundances of bottlenose and spotted dolphins were 4,100 (95% CI: 1,300–9,400) in May 2010 and 6,000 (95% CI: 2,500–17,400) in March 2009, respectively. Bottlenose dolphins were found throughout the study area, although they were encountered most frequently just off the shelf break. In contrast, spotted dolphins exhibited a strong preference for waters over the continental shelf and were not encountered beyond the shelf break.Peer reviewe
Computation of General Inner-Outer and Spectral Factorizations
No full textIn this paper, we solve two problems in linear systems theory: the computation of the inner-outer and spectral factorizations of a continuous-time system considered in the most general setting. We show that these factorization problems rely essentially on solving for the stabilizing solution a standard algebraic Riccati equation of order usually much smaller than the McMillan degree of the transfer function matrix of the system. The proposed procedures are completely general, being applicable for a polynomial/proper/improper system whose transfer function matrix could be rank deficient and could have poles/zeros on the imaginary axis or at infinity. As an application we discuss the extension to the case of rational matrices of the complete orthogonal decomposition of a constant matrix. Numerical refinements and examples illustrating the proposed approach, are discussed in detail
Single- and Multi-carrier Quadrature Amplitude Modulation: Principles and Applications for Personal Communications, WATM and Broadcasting: 2nd
No full textSingle- and Multi-carrier Quadrature Amplitude Modulation Principles and Applications for Personal Communications, WLANs and Broadcasting L. Hanzo Department of Electronics and Computer Science, University of Southampton, UK W. Webb Motorola, Arlington Heights, USA formerly at Multiple Access Communications Ltd, Southampton, UK T. Keller Ubinetics, Cambridge Technology Centre, Melbourn, UK formerly at Department of Electronics and Computer Science, University of Southampton, UK Motivated by the rapid evolution of wireless communication systems, this expanded second edition provides an overview of most major single- and multi-carrier Quadrature Amplitude Modulation (QAM) techniques commencing with simple QAM schemes for the uninitiated through to complex, rapidly-evolving areas, such as arrangements for wide-band mobile channels. Targeted at the more advanced reader, the multi-carrier modulation based second half of the book presents a research-orientated outlook using a variety of novel QAM-based arrangements. * Features six new chapters dealing with the complexities of multi-carrier modulation which has found applications ranging from Wireless Local Area Networks (WLAN) to Digital Video Broadcasting (DVB) * Provides a rudimentary introduction for readers requiring a background in the field of modulation and radio wave propagation * Discusses classic QAM transmission issues relevant to Gaussian channels * Examines QAM-based transmissions over mobile radio channels * Incorporates QAM-related orthogonal techniques, considers the spectral efficiency of QAM in cellular frequency re-use structures and presents a QAM-based speech communications system design study * Introduces Orthogonal Frequency Division Multiplexing (OFDM) over both Gaussian and wideband fading channels By providing an all-encompassing self-contained treatment of single- and multi- carrier QAM based communications, a wide range of readers including senior undergraduate and postgraduate students, practising engineers and researchers alike will all find the coverage of this book attractive
Adaptive minimum symbol-error-rate decision feedback equalization for multilevel pulse-amplitude modulation
No full textThe design of decision feedback equalizers (DFEs) is typically based on the minimum mean square error (MMSE) principle, as this leads to effective adaptive implementation in the form of the least mean square algorithm. It is well-known, however, that in certain situations the MMSE solution can be distinctly inferior to the optimal minimum symbol error rate (MSER) solution. We consider the MSER design for multi-level pulse-amplitude modulation. Block-data adaptive implementation of the theoretical MSER DFE solution is developed based on the Parzen window estimate of probability density function. Furthermore, a sample-by-sample adaptive MSER algorithm, called the least symbol error rate (LSER), is derived for adaptive equalization application. The proposed LSER algorithm has a complexity that increases linearly with the equalizer length. Computer simulation is employed to evaluate the proposed alternative MSER design for equalization application with multi-level signalling schemes
Correction: Epidemiology and outcomes of early-onset AKI in COVID-19-related ARDS in comparison with non-COVID-19-related ARDS: insights from two prospective global cohort studies (Critical Care, (2023), 27, 1, (3), 10.1186/s13054-022-04294-5)
No full textFollowing publication of the original article [1], the authors identified that the collaborating authors part of the collaborating author group CCCC Consortium was missing. The collaborating author group is available and included as Additional file 1 in this article
Asymptotic Bayesian Decision Feedback Equalizer Using a Set of Hyperplanes
No full textWe present a signal space partitioning technique for realizing the optimal Bayesian decision feedback equalizer (DFE). It is known that, when the signal to noise ratio (SNR) tends to infinity, the decision boundary of the Bayesian DFE is asymptotically piecewise linear and consists of several hyperplanes. The proposed technique determines these hyperplanes explicitly and uses them to partition the observation signal space. The resulting equalizer is made up of a set of parallel linear discriminant functions and a Boolean mapper. Unlike the existing signal space partitioning technique of Kim and Moon, which involves complex combinatorial search and optimization in design, our design procedure is simple and straightforward, and guarantees to achieve the asymptotic Bayesian DFE
