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  • 1
    Language: English
    In: Advances in Water Resources, 2011, Vol.34(2), pp.314-325
    Description: ► Stochastic reconstruction with a combination of multi-point statistics. ► Good rendition of connectivity with Minkowski functions and Chord length distributions. ► Transport behavior compares well between reference media and reconstructed media. ► Pressure field tends to bridge local discontinuities within highly conductive regions. Flow and transport in porous media is determined by its structure. Beside spatial correlation, especially the connectivity of heterogeneous conductivities is acknowledged to be a key factor. This has been demonstrated for well defined random fields having different topological properties. Yet, it remains an open question which morphological measures carry sufficient information to actually predict flow and transport in porous media. We analyze flow and transport in classical, two-dimensional random fields showing different topology and we determine a selection of structural characteristics including classical two-point statistics, chord-length distribution and Minkowski functions (four-point statistics) including the Euler number as a topological measure. Using the approach of simulated annealing for global optimization we generate analog random fields that are forced to reproduce one or several of theses structural characteristics. Finally we evaluate in how far the generated analogons reproduce the original flow and transport behavior as well as some more elaborate structural characteristics including percolation probabilities and the pair connectivity function. The results confirm that two-point statistics is insufficient to capture functional properties since it is not sensitive to connectivity. In contrast, the combination of Minkowski functions and chord length distributions carries sufficient information to reproduce the breakthrough curve of a conservative solute. Hence, global topology provided by the Euler number together with local clustering provided by the chord length distribution seems to be a powerful condensation of structural complexity with respect to functional properties.
    Keywords: Simulated Annealing ; Solute Transport ; Minkowski Functionals ; Chord Length Distribution ; Local Percolation Probability ; Pair Connectivity Function ; Engineering
    ISSN: 0309-1708
    E-ISSN: 1872-9657
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  • 2
    Language: English
    In: Advances in Water Resources, August 2012, Vol.44, pp.101-112
    Description: ► New averaging approach that conserves hydraulic non-equilibrium during rapid infiltration of water. ► New indicators to describe hydraulic non-equilibrium quantitatively. ► Direct link between front morphology and hydraulic non-equilibrium. ► Insights into how structural connectivity affects hydraulic non-equilibrium. ► Shortcomings of an upscaled Richards model extended by hydraulic non-equilibrium. Water infiltration into heterogeneous, structured soil leads to hydraulic non-equilibrium across the infiltration front. That is, the water content and pressure head are not in equilibrium according to some static water retention curve. The water content increases more rapidly in more conductive regions followed by a slow relaxation towards an equilibrium state behind the front. An extreme case is preferential infiltration into macropores. Since flow paths adapt to the structural heterogeneity of the porous medium, there is a direct link between structure and non-equilibrium. The aim of our study is to develop an upscaled description of water dynamics which conserves the macroscopic effects of non-equilibrium and which can be directly linked to structural properties of the material. A critical question is how to define averaged state variables at the larger scale. We propose a novel approach based on flux-weighted averaging of pressure head, and compare its performance to alternative methods for averaging. Further, we suggest some meaningful indicators of hydraulic non-equilibrium that can be related to morphological characteristics of infiltration fronts in quantitative terms. These methods provide a sound basis to assess the impact of structural connectivity on hydraulic non-equilibrium. We demonstrate our approach using numerical case studies for infiltration into two-dimensional heterogeneous media using three different structure models with distinct differences in connectivity. Our results indicate that an increased isotropic, short-range connectivity reduces non-equilibrium, whereas anisotropic structures that are elongated in the direction of flow enforce it. We observe a good agreement between front morphology and effective hydraulic non-equilibrium. A detailed comparison of averaged state variables with results from an upscaled model that includes hydraulic non-equilibrium outlines potential improvements in the description of non-equilibrium dynamics including preferential flow in simplified, upscaled models based on Richards equation.
    Keywords: Transient Flow ; Upscaling ; Pressure Head Averaging ; Hydraulic Non-Equilibrium ; Preferential Flow ; Connectivity ; Engineering
    ISSN: 0309-1708
    E-ISSN: 1872-9657
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  • 3
    Language: English
    In: Journal of Contaminant Hydrology, December 2016, Vol.195, pp.31-39
    Description: Engineered nanoparticles released into soils may be coated with humic substances, potentially modifying their surface properties. Due to their amphiphilic nature, humic coating is expected to affect interaction of nanoparticle at the air-water interface. In this study, we explored the roles of the air-water interface and solid-water interface as potential sites for nanoparticle attachment and the importance of hydrophobic interactions for nanoparticle attachment at the air-water interface. By exposing Ag nanoparticles to soil solution extracted from the upper soil horizon of a floodplain soil, the mobility of the resulting “soil-aged” Ag nanoparticles was investigated and compared with the mobility of citrate-coated Ag nanoparticles as investigated in an earlier study. The mobility was determined as a function of hydrologic conditions and solution chemistry using column breakthrough curves and numerical modeling. Specifically, we compared the mobility of both types of nanoparticles for different unsaturated flow conditions and for pH = 5 and pH = 9. The soil-aged Ag NP were less mobile at pH = 5 than at pH = 9 due to lower electrostatic repulsion at pH = 5 for both types of interfaces. Moreover, the physical flow field at different water contents modified the impact of chemical forces at the solid-water interface. An extended Derjaguin-Landau-Verwey-Overbeek (eDLVO) model did not provide satisfactory explanation of the observed transport phenomena unlike for the citrate-coated case. For instance, the eDLVO model assuming sphere-plate geometry predicts a high energy barrier (〉 90 ) for the solid-water interface, indicating that nanoparticle attachment is less likely. Furthermore, retardation through reversible sorption at the air-water interface was probably less relevant for soil-aged nanoparticles than for citrate-coated nanoparticles. An additional cation bridging mechanism and straining within the flow field may have enhanced nanoparticle retention at the solid-water interface. The results indicate that the mobility of engineered Ag nanoparticles is sensitive to solution chemistry, especially pH and the concentration of multivalent cations, and to the unsaturated flow conditions influencing particle interaction at biogeochemical interfaces.
    Keywords: Unsaturated Transport ; Water Dynamics ; Cation Bridging ; Amphiphilic ; Edlvo ; Engineering ; Environmental Sciences ; Geography
    ISSN: 0169-7722
    E-ISSN: 1873-6009
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  • 4
    Language: English
    In: AEUE - International Journal of Electronics and Communications, 2006, Vol.60(1), pp.13-19
    Description: A federation approach for security in future distributed service delivery platforms for mobile users offers some key advantages over an integrated solution relying on a common choice of a standardized authentication technology. By agreeing on an exchange protocol for security assertions rather than on the detailed security mechanisms, Service Aggregators and Access Network Operators will be able to federate their customer offerings flexibly and jointly offer services. The consumer will find formerly separate offerings combined, and Service Operators will enjoy open interfaces towards the network's service delivery platform. Through the use of Security Assertion Markup Language (SAML), standardized assertion statements can be made not only for the user's identity, but also on attributes and authorizations associated with it. This will allow a seamless personalized service experience offering single sign-on across separate operational domains. An example from automobile telematics is used to illustrate the concepts.
    Keywords: Security ; Assertion ; Single Sign-on ; Saml ; Open Service Provisioning ; Telematics ; Automotive ; Engineering
    ISSN: 1434-8411
    E-ISSN: 1618-0399
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  • 5
    Language: English
    In: Wireless Personal Communications, 1999, Vol.11(1), pp.45-62
    Description: One of the most widely used buzzwords in mobile communications of the recent years is Space Division Multiple Access (SDMA). The introduction of an additional space domain multiple access component is likely to boost system capacity, due to the spatial reuse of physical channels within one cell. While early approaches of SDMA system modelling show capacity to be gained in the order of 300 to 400%, there remains some more in depth system analysis to be done, as inhomogeneities in the user distribution are expected to likely cause dramatic drops in the additional capacity gained by SDMA. The spatial distribution of terminals directly influences the number of channels to be gained by spatial multiplexing. Therefore, it is not until detailed models for user distribution, user mobility and the traffic load generated by users are combined with a model of the SDMA radio subsystem, that realistic approximations for these capacity gains are possible. In this paper, we discuss the issues in modelling and simulation of SDMA systems. We present an approach for integrated SDMA system modelling, discuss analytic teletraffic dimensioning methods for macro- and microcellular environments and their relevance to SDMA systems, and present some early simulation results.
    Keywords: SDMA ; smart antenna systems ; capacity ; traffic models ; mobility models ; simulation
    ISSN: 0929-6212
    E-ISSN: 1572-834X
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  • 6
    Book
    Book
    Chichester, UK: John Wiley & Sons, Ltd
    Language: English
    Description: Since the publication of the first edition the number of GSM subscribers has exploded and it is now deployed in more than 140 countries worldwide. Revised and updated "GSM Switching, Services, and Protocols" now features the abundant new services and applications that GSM can provide. By focusing on...
    Keywords: Global system for mobile communications; Mobile communication systems; Wireless communication systems;
    ISBN: 9780471499039
    ISBN: 047149903X
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  • 7
    Language: German
    In: Informationstechnik,
    Description: Das Mobilfunksystem GSM (Global System for Mobile Communication) wurde von Anfang an sehr erfolgreich in ganz Europa und auch in vielen außereuropäischen Ländern eingesetzt. Es ist mittlerweile zu einem wirklich globalen System herangewachsen - der GSM-Anteil am Weltmarkt der digitalen Mobilfunksysteme betrug Ende 1998 über 60% und die Teilnehmerzahlen hatten sich innerhalb eines Jahres verdoppelt. Das Buch stellt auf der Grundlage der europäischen GSM-Standards die technischen Grundlagen und die Systemarchitektur von GSM vor. Dabei liegen die Schwerpunkte auf der Darstellung der Protokolle, der Mobilvermittlungstechnik und der Dienstrealisierung. Die wichtigsten Prinzipien der GSM-Übertragungstechnik werden ebenfalls behandelt. Die erweiterte und aktualisierte Neuauflage enthält zusätzlich vor allem eine ausführliche Beschreibung des General Packet Radio Service (GPRS) und einen aktuellen Überblick zum Stand der Entwicklung der dritten Mobilfunkgeneration UMTS/IMT-2000. Angesprochen sind Studierende der Elektrotechnik, Informationstechnik und Informatik von Universitäten und Fachhochschulen, Anwender und Praktiker aus der Industrie, von Netzbetreibern und aus der Forschung, die sich einen fundierten Einblick in die Funktionsweise des komplexen Systems GSM verschaffen wollen und vor allem am Zusammenspiel der verschiedenen Funktionen interessiert sind. Für dieses Werk stehen unter einem Link auf der Homepage des Buches (http://www.lkn.ei.tum.de/gsm_buch/errata_Aufl2.html) Errata-Webseiten zur Verfügung.
    Keywords: Engineering ; Engineering ; Engineering, General ; Engineering
    ISBN: 9783322918581
    ISBN: 3322918580
    ISBN: 9783322940636
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