91 research outputs found
EVALUATION OF SOIL-STRUCTURE INTERACTION MODELS USING DIFFERENT MODEL-ROBUSTNESS APPROACHES
The aim of this study is to show an application of model robustness measures for soilstructure interaction (henceforth written as SSI) models. Model robustness defines a measure for the ability of a model to provide useful model answers for input parameters which typically have a wide range in geotechnical engineering. The calculation of SSI is a major problem in geotechnical engineering. Several different models exist for the estimation of SSI. These can be separated into analytical, semi-analytical and numerical methods. This paper focuses on the numerical models of SSI specific macro-element type models and more advanced finite element method models using contact description as continuum or interface elements. A brief description of the models used is given in the paper. Following this description, the applied SSI problem is introduced. The observed event is a static loaded shallow foundation with an inclined load. The different partial models to consider the SSI effects are assessed using different robustness measures during numerical application. The paper shows the investigation of the capability to use these measures for the assessment of the model quality of SSI partial models. A variance based robustness and a mathematical robustness approaches are applied. These different robustness measures are used in a framework which allows also the investigation of computational time consuming models. Finally the result shows that the concept of using robustness approaches combined with other model–quality indicators (e.g. model sensitivity or model reliability) can lead to unique model–quality assessment for SSI models
High-fidelity view of the structure and fragmentation of the high-mass, filamentary IRDC G11.11-0.12
Star formation in molecular clouds is intimately linked to their internal
mass distribution. We present an unprecedentedly detailed analysis of the
column density structure of a high-mass, filamentary molecular cloud, namely
IRDC G11.11-0.12 (G11). We use two novel column density mapping techniques:
high-resolution (FWHM=2", or ~0.035 pc) dust extinction mapping in near- and
mid-infrared, and dust emission mapping with the Herschel satellite. These two
completely independent techniques yield a strikingly good agreement,
highlighting their complementarity and robustness. We first analyze the dense
gas mass fraction and linear mass density of G11. We show that G11 has a top
heavy mass distribution and has a linear mass density (M_l ~ 600 Msun pc^{-1})
that greatly exceeds the critical value of a self-gravitating, non-turbulent
cylinder. These properties make G11 analogous to the Orion A cloud, despite its
low star-forming activity. This suggests that the amount of dense gas in
molecular clouds is more closely connected to environmental parameters or
global processes than to the star-forming efficiency of the cloud. We then
examine hierarchical fragmentation in G11 over a wide range of size-scales and
densities. We show that at scales 0.5 pc > l > 8 pc, the fragmentation of G11
is in agreement with that of a self-gravitating cylinder. At scales smaller
than l < 0.5 pc, the results agree better with spherical Jeans' fragmentation.
One possible explanation for the change in fragmentation characteristics is the
size-scale-dependent collapse time-scale that results from the finite size of
real molecular clouds: at scales l < 0.5 pc, fragmentation becomes sufficiently
rapid to be unaffected by global instabilities.Comment: 8 pages, 8 figures, accepted to A&
A contact problem aplication for the local behaviour of soil pile interaction
In geotechnical engineering, the main parameter for the performance of structures such as reinforced walls or deep foundations is often the shaft bearing capacity. In numerical analysis, important advancements have been made on studying the behavior of the soil and the retaining structures separately. The performance of many geotechnical foundation systems depends on the shear behavior at the soil structure interface. For deep foundations, the main component that affects friction is the horizontal earth pressure. When a pile is getting axially loaded, the soil grain network at the interface, starts to move and rearrange. In conditions of axial cyclic loading a contractive behavior of soil can generally be observed as in [1] and [2]. This can be explained by the progressive densification and relaxation of the soil under cyclic shear at the soil pile interface, as well as the local refinement of the grain distribution by grain breakage and rearrangements. As the soil contracts and decreases in volume, the normal stress around the pile surface decreases and the soil pile friction degrades. This can lead to failure of the whole geotechnical foundation system. The purpose of the work presented in this paper is to analyze locally (at the element level) the contact behavior of a soil-pile contact problem. Therefore, a 2D shear test is modeled using the Finite Element Method. The formulation of a 4 nodded zero-thickness interface element of Beer [3] is chosen with a linear interpolation function. Four constitutive contact models adapted for contact problems have been implemented. The simple Mohr-Coulomb [4] and Clough and Duncan [5] models were chosen initially, due to the ease of implementation and few number of parameters needed. After, more complicated models in the framework of elasto-plasticity such as: Lashkari [6] and Mortara [7] were implemented for the first time into the finite element code of the shear test problem. They include other phenomena such as: relative density of soil, the stress level and sand dilatancy. From the results the relation between shear displacement and shear stress has been deduced. Finally, a discussion of the advantages and the drawbacks during computation of each model is given at the end
Directionally dependent strength and dilatancy behavior of soil–structure interfaces
Soil–structure interfaces typically exhibit a shear behavior that is independent of the direction of relative displacement due to symmetry in the solid material\u27s surface profile. This experimental study investigates the interface shear behavior of surfaces with asymmetric profiles inspired by the scales of snake skin. The results of shear box interface tests on two sandy soils indicate that the peak and residual interface shear strengths and dilatancy are greater when the soil is displaced against the sharp edges of the asperities (cranial direction) than when the soil is displaced along the asperities (caudal direction). The experimental results indicate that the effect of asperity geometry on the interface shear response can be captured with the ratio of asperity length to asperity height (L/H). Analysis of the stress–dilatancy behavior indicates that interfaces with a relatively short asperity length follow a classical flow rule developed for soils. However, the relationship between the mobilized stress ratio and the dilatancy rate is shown to be a function of the shearing direction and asperity geometry. Implementation of snake skin-inspired profiles on the surface of geotechnical structures may provide benefits in performance and efficiency during installation and service life. In general, the results of this study indicate the behavior of the soil-structure interfaces sheared in the cranial direction is similar to that of interfaces between soil and fully rough surfaces. In contrast, the behavior of the soil-structure interface sheared in the caudal direction shares characteristics with that of interfaces with smooth surfaces, including the mobilization of smaller a interface strength and dilation
Relationship between the column density distribution and evolutionary class of molecular clouds as viewed by ATLASGAL
We present the first study of the relationship between the column density
distribution of molecular clouds within nearby Galactic spiral arms and their
evolutionary status as measured from their stellar content. We analyze a sample
of 195 molecular clouds located at distances below 5.5 kpc, identified from the
ATLASGAL 870 micron data. We define three evolutionary classes within this
sample: starless clumps, star-forming clouds with associated young stellar
objects, and clouds associated with HII regions. We find that the N(H2)
probability density functions (N-PDFs) of these three classes of objects are
clearly different: the N-PDFs of starless clumps are narrowest and close to
log-normal in shape, while star-forming clouds and HII regions exhibit a
power-law shape over a wide range of column densities and log-normal-like
components only at low column densities. We use the N-PDFs to estimate the
evolutionary time-scales of the three classes of objects based on a simple
analytic model from literature. Finally, we show that the integral of the
N-PDFs, the dense gas mass fraction, depends on the total mass of the regions
as measured by ATLASGAL: more massive clouds contain greater relative amounts
of dense gas across all evolutionary classes.Comment: Accepted for publication in A&A (25th June 15) 23 pages, 12 figures.
Additional appendix figures will appear in the journal version of this pape
The structured environments of embedded star-forming cores. PACS and SPIRE mapping of the enigmatic outflow source UYSO 1
The intermediate-mass star-forming core UYSO 1 has previously been found to
exhibit intriguing features. While deeply embedded and previously only
identified by means of its (sub-)millimeter emission, it drives two powerful,
dynamically young, molecular outflows. Although the process of star formation
has obviously started, the chemical composition is still pristine. We present
Herschel PACS and SPIRE continuum data of this presumably very young region.
The now complete coverage of the spectral energy peak allows us to precisely
constrain the elevated temperature of 26 - 28 K for the main bulge of gas
associated with UYSO1, which is located at the interface between the hot HII
region Sh 2-297 and the cold dark nebula LDN 1657A. Furthermore, the data
identify cooler compact far-infrared sources of just a few solar masses, hidden
in this neighbouring dark cloud.Comment: accepted contribution for the forthcoming Herschel Special Issue of
A&A, 5 pages (will appear as 4-page letter in the journal), 6 figure file
Hypoplastische Modelle fĂĽr Boden-Bauwerkskontakte: Modellierung und Implementierung
The consideration of interfaces is an important issue when modelling the holistic global structural behaviour of geotechnical engineering structures. The most prominent example is the shaft friction of axial loaded piles and anchors. In this thesis, a stochastic assessment scheme was proposed and applied. This scheme was modified to take into account the special considerations for the assessment of interface models. Based on the assessment and a state-of-the-art review, theoretical considerations were used and a novel scheme was developed. This scheme uses reformulated mathematical operators as well as reduced stress and strain rate tensors, based on existing constitutive equations, to model interfaces. Shear stress mobilization and the volumetric behaviour are predicted more accurately, and the bearing behaviour of frictional contacts can be modelled in a better way. By using the novel scheme, an older hypoplastic interface model for granular interfaces was enhanced, and three different hypoplastic clay interface models were proposed. In addition to the theoretical constitutive model formulation, an implementation method was developed. This allowed a user-friendly implementation of advanced constitutive models as interface models using the capabilities of a commercial finite element software package. This concept was exemplary applied to various 3D boundary value problems and the benefits of such advanced hypoplastic interface model are discussed.Das Kontaktverhalten von geotechnischen Strukturen ist wichtig zur Berechnung des ganzheitlichen Strukturverhaltens bei der Berücksichtigung von Boden-Bauwerks-Interaktion. Bekannte Beispiele dafüur sind axial belastete Pfähle und Anker. In dieser Arbeit wurden verschiedene existierende Modelle zur Modellierung von Boden-Bauwerks-Kontakten mit Hilfe eines stochastischen Ansatzes untersucht und bewertet. Anhand dieser Bewertung und dem Stand der Forschung wurde basierend auf theoretischen Überlegungen ein Methode entwickelt. Diese erlaubt es, die Kontaktflächen mittels modifizierter mathematischen Operatoren und reduzierten Spannungs und Dehnungstensoren, basierend auf existierenden Kontinuums-Modelle zu berechnen. Dies führt zu verbesserten Modellierung der Scherspannungs-Mobilisierung und des volumentrischen Verhaltens der Kontaktzone. Basierend auf dem neuen Konzept wurde ein existierendes, hypoplastisches Modell für granulare Kontaktreibung verbessert und drei unterschiedliche hypoplastische Modelle für das Kontaktverhalten von feinkörnigen Böden entwickelt. Alle Modelle wurden verifiziert und mit experimentellen Daten validiert. Zusätzlich zur Formulierung der theoretischen Modelle wurde ein Konzept erarbeitet, mit dem die entwickelten Modelle in eine Finite-Elemente Software implementiert werden konnten. Hierzu werden existierende Boden-Kontinuumsmodelle benutzt. Die Implementierung dieser Modelle wurde mittels unterschiedlicher Randwertprobleme erfolgreich validiert und die Vorteile der Modelle sowie des Implementierungskonzeptes diskutiert. Die theoretischen Überlegungen und das benutzerfreundliche Implementierungsschema wird die Zugänglichkeit dieser zukunftsweisenden Modelle für Ingenieure verbessern. Hierdurch können die Ergebnisse der Modellierungen und die experimentelle Beobachtungen angeglichen werden, was die Aussagefähigkeit von Finite-Elemente Analysen weiter verbessern wird
Dust-temperature of an isolated star-forming cloud: Herschel observations of the Bok globule CB244
We present Herschel observations of the isolated, low-mass star-forming Bok
globule CB244. It contains two cold sources, a low-mass Class 0 protostar and a
starless core, which is likely to be prestellar in nature, separated by 90
arcsec (~ 18000 AU). The Herschel data sample the peak of the Planck spectrum
for these sources, and are therefore ideal for dust-temperature and column
density modeling. With these data and a near-IR extinction map, the MIPS 70
micron mosaic, the SCUBA 850 micron map, and the IRAM 1.3 mm map, we model the
dust-temperature and column density of CB244 and present the first measured
dust-temperature map of an entire star-forming molecular cloud. We find that
the column-averaged dust-temperature near the protostar is ~ 17.7 K, while for
the starless core it is ~ 10.6K, and that the effect of external heating causes
the cloud dust-temperature to rise to ~ 17 K where the hydrogen column density
drops below 10^21 cm^-2. The total hydrogen mass of CB244 (assuming a distance
of 200 pc) is 15 +/- 5 M_sun. The mass of the protostellar core is 1.6 +/- 0.1
M_sun and the mass of the starless core is 5 +/- 2 M_sun, indicating that ~ 45%
of the mass in the globule is participating in the star-formation process.Comment: Accepted for A&A Herschel Special Issue; 5 pages, 2 figure
The Ubiquity of Micrometer-Sized Dust Grains in the Dense Interstellar Medium
Cold molecular clouds are the birthplaces of stars and planets, where dense
cores of gas collapse to form protostars. The dust mixed in these clouds is
thought to be made of grains of an average size of 0.1 micrometer. We report
the widespread detection of the coreshine effect as a direct sign of the
existence of grown, micrometer-sized dust grains. This effect is seen in half
of the cores we have analyzed in our survey, spanning all Galactic longitudes,
and is dominated by changes in the internal properties and local environment of
the cores, implying that the coreshine effect can be used to constrain
fundamental core properties such as the three-dimensional density structure and
ages and also the grain characteristics themselves
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