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Discontinuous
Terms modified by Discontinuous Selected AbstractsHigh speed sliding of axonemal microtubules produced by outer arm dyneinCYTOSKELETON, Issue 2 2005Raviraja N. Seetharam Abstract To study dynein arm activity at high temporal resolution, axonemal sliding was measured field by field for wild type and dynein arm mutants of Tetrahymena thermophila. For wt SB255 cells, when the rate of data acquisition was 60 fps, about 5× greater than previously published observations, sliding was observed to be discontinuous with very high velocity sliding (average 196 ,m/sec) for a few msec (1 or 2 fields) followed by a pause of several fields. The sliding velocities measured were an order of magnitude greater than rates previously measured by video analysis. However, when the data were analyzed at 12 fps for the same axonemes, consistent with previous observations, sliding was linear as the axonemes extended several times their original length with an average velocity of ,10 ,m/sec. The pauses or stops occurred at approximately 200 and 300% of the initial length, suggesting that dynein arms on one axonemal doublet were initially active to the limit of extension, and then the arms on the next doublet became activated. In contrast, in a mutant where OADs are missing, sliding observed at 60 fps was continuous and slow (5 ,m/sec), as opposed to the discontinuous high-velocity sliding of SB255 and of the mutant at the permissive temperature where OADs are present. High-velocity step-wise sliding was also present in axonemes from an inner arm dynein mutant (KO6). These results indicate that the high-speed discontinuous pattern of sliding is produced by the mechanochemical activity of outer arm dynein. The rate of sliding is consistent with a low duty ratio of the outer arm dynein and with the operation of each arm along a doublet once per beat. Cell Motil. Cytoskeleton 60:96,103, 2005. © 2004 Wiley-Liss, Inc. [source] Development and fine structure of the yolk nucleus of previtellogenic oocytes in the medaka Oryzias latipesDEVELOPMENT GROWTH & DIFFERENTIATION, Issue 6 2000Hirokuni Kobayashi The development and fine structure of yolk nuclei in the cytoplasm of previtellogenic oocytes were examined by electron microscopy during several stages of oogenesis in the medaka, Oryzias latipes. Shortly after oogenesis starts, oocytes 20,30 ,m in diameter have much electron-dense (basophilic) cytoplasm, within which a continuous or discontinuous, irregular ring-shaped lower electron-dense area of flocculent appearance (LF) begins to emerge around the nucleus. The yolk nucleus is first recognized within an LF area as a few fragments of dense granular thread measuring 20,25 nm in width. The threads consist of two rows of very dense granules resembling ribosomes or ribonucleoprotein (RNP)-like particles in size and electron density. These thread-like fragments gradually increase in number and length until they assemble into a compact, spherical mass of complicated networks. Analysis of serial sections suggests that the yolk nucleus is a complicated mass of numerous, small deformed vacuoles composed of a single lamella with double layers of ribosomes or RNP-like granules, rather than a mass of granular threads. When oocytes develop to greater than 100 ,m in diameter, the yolk nucleus begins to fragment before dispersing throughout the surrounding cytoplasm, concomitantly with the disappearance of LF areas. At this stage of oogenesis, a restricted region of the granulosa cell layer adjacent to the yolk nucleus becomes somewhat columnar in morphology, fixing the vegetal pole region of the oocyte. [source] Electrophoretically mediated microanalysis with partial filling technique and indirect or direct detection as a tool for inhibition studies of enzymatic reactionELECTROPHORESIS, Issue 7-8 2004Magdaléna Telnarová Abstract The inhibition of the model enzyme, haloalkane dehalogenase from Sphingomonas paucimobilis, was investigated by a combination of electrophoretically mediated microanalysis with a partial filling technique, followed by indirect or direct detection. In this setup, part of the capillary is filled with a buffer suitable for the enzymatic reaction (20 mM glycine buffer, pH 8.6) whereas the rest of the capillary is filled with the background electrolyte optimal for separation of substrates and products. Two different background electrolytes and corresponding detection approaches were used to show the versatility of the developed method. The inhibition effect of 1,2-dichloroethane on the dehalogenation of brominated substrate 1-bromobutane was studied by means of 10 mM chromate , 0.1 mM cetyltrimethylammonium bromide (pH 9.2) in combination with indirect detection or 20 mM ,-alanine , hydrochloric acid (pH 3.5) in combination with direct detection. The method was used to estimate the inhibition constant KI (0.44 mM by indirect detection and 0.63 mM by of direct detection) and to determine the inhibition type. Compared to spectrophotometric and other discontinuous assays, the method is rapid, can be automated, and requires only small amount of reagents that is especially important in the case of enzymes and inhibitors. [source] Serum tumour necrosis factor-, levels in cancer patients are discontinuous and correlate with weight lossEUROPEAN JOURNAL OF CLINICAL INVESTIGATION, Issue 12 2000M. Bossola Tumour necrosis factor-, (TNF) has been regarded as a potential mediator of cancer cachexia. Assessment of TNF circulating levels in cancer patients and their correlation with weight loss has led to controversial results. We measured TNF circulating levels in 28 patients with gastrointestinal cancer and 29 controls with benign gastrointestinal diseases at different times (08.00 h, 14.00 h, 20.00 h) before operation. TNF activity was not detected in any of controls at any times. In cancer patients, TNF circulating levels were detectable in 18 cases (64·3%) and appeared to be discontinuous. TNF levels above the limit of detection were present in 15 patients (53·6%) at 08.00 h, in 14 (50%) at 014.00 h and in nine (32·1%) at 20.00 h. Mean TNF levels were 14·3 ± 4 pg mL,1 at 08.00 h, 16·7 ± 4·6 pg mL,1 at 14.00 h (P = 0·05) and 18·5 ± 10·2 pg mL,1 at 20.00 h (P < 0·05 vs. 08.00 h and 14.00 h). According to Spearman's analysis, the sum of TNF concentrations at the three times significantly correlated with the severity of weight loss (Spearman's correlation coefficient =,,0·420; P = 0·026). TNF concentrations were consistently and significantly higher in patients with severe weight loss than in those with moderate or light weight loss at 08.00 h (26·3 ± 8·3, 8·9 ± 4·2, 3·8 ± 2·1, respectively; P = 0·04 at one-way anova). TNF levels were higher in anorectic than in nonanorectic patients at any hour, but the differences were not statistically significant. The present study demonstrates that TNF is intermittently or discontinuously detectable in patients with gastrointestinal cancer and that its levels correlate with the severity of weight loss. [source] Development and evolution of adaptive polyphenismsEVOLUTION AND DEVELOPMENT, Issue 1 2003H. Frederik Nijhout SUMMARY Phenotypic plasticity is the primitive character state for most if not all traits. Insofar as developmental and physiological processes obey the laws of chemistry and physics, they will be sensitive to such environmental variables as temperature, nutrient supply, ionic environment, and the availability of various macro- and micronutrients. Depending on the effect this phenotypic plasticity has on fitness, evolution may proceed to select either for mechanisms that buffer or canalize the phenotype against relevant environmental variation or for a modified plastic response in which some ranges of the phenotypic variation are adaptive to particular environments. Phenotypic plasticity can be continuous, in which case it is called a reaction norm, or discontinuous, in which case it is called a polyphenism. Although the morphological discontinuity of some polyphenisms is produced by discrete developmental switches, most polyphenisms are due to discontinuities in the environment that induce only portions of what is in reality a continuous reaction norm. In insect polyphenisms, the environmental variable that induces the alternative phenotype is a token stimulus that serves as a predictor of, but is not itself, the environment to which the polyphenism is an adaptation. In all cases studied so far, the environmental stimulus alters the endocrine mechanism of metamorphosis by altering either the pattern of hormone secretion or the pattern of hormone sensitivity in different tissues. Such changes in the patterns of endocrine interactions result in the execution of alternative developmental pathways. The spatial and temporal compartmentalization of endocrine interactions has produced a developmental mechanism that enables substantial localized changes in morphology that remain well integrated into the structure and function of the organism. [source] A gene duplication led to specialized ,-aminobutyrate and ,-alanine aminotransferase in yeastFEBS JOURNAL, Issue 7 2007Gorm Andersen In humans, ,-alanine (BAL) and the neurotransmitter ,-aminobutyrate (GABA) are transaminated by a single aminotransferase enzyme. Apparently, yeast originally also had a single enzyme, but the corresponding gene was duplicated in the Saccharomyces kluyveri lineage. SkUGA1 encodes a homologue of Saccharomyces cerevisiae GABA aminotransferase, and SkPYD4 encodes an enzyme involved in both BAL and GABA transamination. SkPYD4 and SkUGA1 as well as S. cerevisiaeUGA1 and Schizosaccharomyces pombeUGA1 were subcloned, over-expressed and purified. One discontinuous and two continuous coupled assays were used to characterize the substrate specificity and kinetic parameters of the four enzymes. It was found that the cofactor pyridoxal 5,-phosphate is needed for enzymatic activity and ,-ketoglutarate, and not pyruvate, as the amino group acceptor. SkPyd4p preferentially uses BAL as the amino group donor (Vmax/Km = 0.78 U·mg,1·mm,1), but can also use GABA (Vmax/Km = 0.42 U·mg,1·mm,1), while SkUga1p only uses GABA (Vmax/Km = 4.01 U·mg,1·mm,1). SpUga1p and ScUga1p transaminate only GABA and not BAL. While mammals degrade BAL and GABA with only one enzyme, but in different tissues, S. kluyveri and related yeasts have two different genes/enzymes to apparently ,distinguish' between the two reactions in a single cell. It is likely that upon duplication ,200 million years ago, a specialized Uga1p evolved into a ,novel' transaminase enzyme with broader substrate specificity. [source] A finite element analysis of tidal deformation of the entire earth with a discontinuous outer layerGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2007H. L. Xing SUMMARY Tidal deformation of the Earth is normally calculated using the analytical solution with some simplified assumptions, such as the Earth is a perfect sphere of continuous media. This paper proposes an alternative way, in which the Earth crust is discontinuous along its boundaries, to calculate the tidal deformation using a finite element method. An in-house finite element code is firstly introduced in brief and then extended here to calculate the tidal deformation. The tidal deformation of the Earth due to the Moon was calculated for an geophysical earth model with the discontinuous outer layer and compared with the continuous case. The preliminary results indicate that the discontinuity could have different effects on the tidal deformation in the local zone around the fault, but almost no effects on both the locations far from the fault and the global deformation amplitude of the Earth. The localized deformation amplitude seems to depend much on the relative orientation between the fault strike direction and the loading direction (i.e. the location of the Moon) and the physical property of the fault. [source] On strike-slip faulting in layered mediaGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2002Maurizio Bonafede Summary We study the effects of structural inhomogeneities on the stress and displacement fields induced by strike-slip faults in layered media. An elastic medium is considered, made up of an upper layer bounded by a free surface and welded to a lower half-space characterized by different elastic parameters. Shear cracks with assigned stress drop are employed as mathematical models of strike-slip faults, which are assumed to be vertical and planar. If the crack is entirely embedded within the lower medium (case A), a Cauchy-kernel integral equation is obtained, which is solved by employing an expansion of the dislocation density in Chebyshev polynomials. If the crack is within the lower medium but it terminates at the interface (case B), a generalized Cauchy singularity appears in the integral kernel. This singularity affects the singular behaviour of the dislocation density at the crack tip touching the interface. Finally, the case of a crack crossing the interface is considered (case C). The crack is split into two interacting sections, each placed in a homogeneous medium and both open at the interface. Two coupled generalized Cauchy equations are obtained and solved for the dislocation density distribution of each crack section. An asymptotic study near the intersection between the crack and the interface shows that the dislocation densities for each crack section are bounded at the interface, where a jump discontinuity is present. As a corollary, the stress drop must be discontinuous at the interface, with a jump proportional to the rigidity contrast between the adjoining media. This finding is shown to have important implications for the development of geometrical complexities within transform fault zones: planar strike-slip faults cutting across layer discontinuities with arbitrary stress drop values are shown to be admissible only if the interface between different layers becomes unwelded during the earthquake at the crack/interface junction. Planar strike-slip faulting may take place only in mature transform zones, where a repetitive earthquake cycle has already developed, if the rheology is perfectly elastic. Otherwise, the fault cannot be planar: we infer that strike-slip faulting at depth is plausibly accompanied by en-echelon surface breaks in a shallow sedimentary layer (where the stress drop is lower than prescribed by the discontinuity condition), while ductile deformation (or steady sliding) at depth may be accommodated by multiple fault branching or by antithetic faulting in the upper brittle layer (endowed with lower rigidity but higher stress). [source] Internal structure of an alpine rock glacier based on crosshole georadar traveltimes and amplitudesGEOPHYSICAL PROSPECTING, Issue 3 2006Martin Musil ABSTRACT Rapid melting of permafrost in many alpine areas has increased the probability of catastrophic rock slides. In an attempt to provide critical structural information needed for the design and implementation of suitable mitigation procedures, we have acquired low frequency (22 MHz) cross-hole radar data from within a fast-moving rock glacier, an important form of alpine permafrost. Since the ice, rock and pockets of water and air found in the underground of high alpine areas have very different dielectric permittivities and electrical conductivities, the radar method was well-suited for investigating the structure and state of the rock glacier. Our interpretation of the radar velocities and attenuations was constrained by geomorphological observations, borehole lithological logs and the results of a surface seismic survey. The radar data revealed the existence of a discontinuous 7,11 m thick ice-rich zone distinguished by high velocities (0.14,0.17 m/ns) and low attenuations (0.04,0.09 m,1) and a thin underlying ice-free zone characterized by moderate velocities (0.11,0.12 m/ns) and low attenuations (0.04,0.09 m,1). Beneath these two zones, we observed a prominent band of high velocities (0.14,0.17 m/ns) and moderately high attenuations (0.10,0.20 m,1) associated with unconsolidated glacial sediments and numerous large air-filled voids, which in the past were probably filled with ice. At greater depths, the variably dry to water-saturated sediments were represented by generally lower velocities (0.08,0.10 m/ns) and higher attenuations (0.16,0.24 m,1). The bedrock surface was represented by an abrupt ,0.03 m/ns velocity increase. We speculate that the disappearance of ice, both laterally and with depth, occurred during the past one to two decades. [source] Spring phenology in boreal Eurasia over a nearly century time scaleGLOBAL CHANGE BIOLOGY, Issue 3 2008NICOLAS DELBART Abstract It has been widely reported that tree leaves have tended to appear earlier in many regions of the northern hemisphere in the last few decades, reflecting climate warming. Satellite observations revealed an 8-day advance in leaf appearance date between 1982 and 1991 in northern latitudes. In situ observations show that leaf appearance dates in Europe have advanced by an average of 6.3 days from 1959 to 1996. Modelling of leaf appearance on the basis of temperature also shows a marked advance in temperate and boreal regions from 1955 to 2002. However, before 1955, reported studies of phenological variations are restricted to local scale. Modelling, ground observations and satellite observations are here combined to analyse phenological variations in Eurasian taiga over nearly a century. The trend observed by remote sensing consists mainly in a shift at the end of the 1980s, reflecting a shift in winter and spring temperature. In western boreal Eurasia, a trend to earlier leaf appearance is evident since the mid-1930s, although it is discontinuous. In contrast, the strong advance in leaf appearance detected over Central Siberia using satellite data in 1982,1991 is strengthened by late springs in 1983,1984; moreover, in this region the green-up timing has displayed successive trends with opposite signs since 1920. Thus, such strong trend is not unusual if considered locally. However, the recent advance is unique in simultaneously affecting most of the Eurasian taiga, the leaf appearance dates after 1990 being the earliest in nearly a century in most of the area. [source] Dissolved organic matter in small streams along a gradient from discontinuous to continuous permafrostGLOBAL CHANGE BIOLOGY, Issue 9 2004Masayuki Kawahigashi Abstract The Yenisei river passes every type of permafrost regime, from south to north, being characterized by increasing continuity of the permafrost and by decreasing thickness of the active layer. We used that situation to test the hypothesis that amounts and properties of dissolved organic matter (DOM) in small streams draining forested catchments respond to different permafrost regimes. Water samples were taken from eight tributaries along the Yenisei between 67°30,N and 65°49,N latitude. The samples were analysed for dissolved organic carbon (DOC) and nitrogen (DON) and DOM was characterized by its chemical composition (XAD-8 fractionation, sugars, lignin phenols, amino acids, protein, UV and fluorescence spectroscopy), and its biodegradability. Most properties of the tributary waters varied depending on latitude. The higher the latitude, the higher were DOC, DON and the proportion of the hydrophobic fraction of DOC. The contribution of hexoses and pentoses to DOC were higher in southern tributaries; on the other hand, phenolic compounds were more abundant in northern tributaries. Mineralizable DOC ranged between 4% and 28% of total DOC. DOM in northern tributaries was significantly (P<0.05) less biodegradable than that in southern tributaries reflecting the differences in the chemical properties of DOM. Our results suggest that the differences in DOM properties are mainly attributed to differences of permafrost regime, affecting depth of active layer, soil organic matter accumulation and vegetation. Soil organic matter and vegetation determine the amount and composition of DOM produced in the catchments while the depth of the active layer likely controls the quantity and quality of DOM exported to streams. Sorptive interactions of DOM with the soil mineral phase typically increase with depth. The results imply that a northern shift of discontinuous permafrost likely will change in the long term the input of DOM into the Yenisei and thus probably into the Kara Sea. [source] Fragmentation and Sprawl: Evidence from Interregional AnalysisGROWTH AND CHANGE, Issue 3 2002John I. Carruthers Recent years have witnessed widespread expansion of state and regional planning programs in the United States. A major purpose of these efforts is to reduce urban sprawl,low density, discontinuous, suburban,style development, often characterized as the result of rapid, unplanned, and/or uncoordinated growth, by promoting jurisdictional cooperation and regulatory consistency across metropolitan areas. This paper evaluates the efficacy of this approach by examining the relationship between governmental fragmentation and several measurable outcomes of urban development: density, urbanized land area, property value, and public expenditures on infrastructure. The four dimensions are modeled in a simultaneous equations framework, providing substantive evidence on how fragmentation and other exogenous factors affect metropolitan growth patterns. Fragmentation is associated with lower densities and higher property values, but has no direct effect on public service expenditures; less fragmented metropolitan areas occupy greater amounts of land due to the extensive annexation needed to bring new development under the control of a central municipality. The findings of the analysis lend support to state and regional planning efforts aimed at increasing cooperation among local governments, but also suggest that further research is needed in order to evaluate whether or not they produce their intended effects. [source] Diagnostic difficulties in inflammatory bowel disease pathologyHISTOPATHOLOGY, Issue 2 2006R K Yantiss This review summarizes some of the common diagnostic problems encountered by pathologists when evaluating patients with chronic colitis and in whom inflammatory bowel disease (IBD) is either suspected or within the differential diagnosis. Both ulcerative colitis (UC) and Crohn's disease (CD) show characteristic, but non-specific, pathological features that may overlap and result in a diagnosis of ,indeterminate colitis' (IC). However, other reasons why pathologists may entertain a diagnosis of IC include failure to recognize or accept certain ,hardcore' histological features as indicative of CD, an attempt to classify cases of chronic colitis based on mucosal biopsy material or in the absence of adequate clinical and radiographic information, and the presence of other disease processes that mask, or mimic, IBD. In addition, some cases of UC may show unusual CD-like features, such as discontinuous or patchy disease, ileal inflammation, extracolonic inflammation, granulomatous inflammation in response to ruptured crypts, aphthous ulcers, or transmural inflammation. Furthermore, other forms of colitis, such as microscopic colitis, diverticulitis and diversion colitis may, on occasion, also show IBD-like changes. The clinical and pathological features that aid in the distinction between these entities, and others, are covered in detail in this review. [source] MEMORY, MEMORIALS, AND COMMEMORATION,HISTORY AND THEORY, Issue 3 2008ANITA KASABOVA ABSTRACT According to a popular view, the past is present here and now. This is presentism combined with endurantism: the past continuously persists through time to the present. By contrast, I argue that memories, memorials, and histories are of entities discontinuous with present experiences, and that the continuity between past and present in them is a construct. Memories, memorials, and histories are semantic means for dealing with the past. My presupposition that past and present are different is supported by grammar: as verbal tenses show, the past is not present here and now, for otherwise it would not be past. A failure to note this difference is a lack of chronesthesia, a sense of time specific to human beings. I argue that presentism fails to account for the temporal structures of memory and the changes in perspective as we switch from the present to a past situation. My account is perdurantist in the sense that it allows for temporal parts of things such as memorials or tombstones, as well as events such as wars or commemorations. But my main goal is to outline a semantic approach to the past: the tie between past and present actions and events is the semantic ground,consequence relation: a past event is the antecedent grounding a present situation, explaining why it is the case. In addition, I show how we refer to the past by means of two rhetorical figures of speech: synecdoche, using the (emblematic-) part-whole relation for relating the past to the present by transposing its sense; and anaphor, which has a deictic function,it points back toward the past. In references to the past, the deictic field is a scene visualized by the speaker and addressees: the deictic field is transposed from a perceptual to an imaginary space. [source] 4. THE MATERIAL PRESENCE OF THE PASTHISTORY AND THEORY, Issue 3 2006EWA DOMANSKA ABSTRACT This article deals with the material presence of the past and the recent call in the human sciences for a "return to things." This renewed interest in things signals a rejection of constructivism and textualism and the longing for what is "real," where "regaining" the object is conceived as a means for re-establishing contact with reality. In the context of this turn, we might wish to reconsider the (ontological) status of relics of the past and their function in mediating relations between the organic and the inorganic, between people and things, and among various kinds of things themselves for reconceptualizing the study of the past. I argue that the future will depend on whether and how various scholars interested in the past manage to modify their understanding of the material remnants of the past, that is, things as well as human, animal, and plant remains. In discussing this problem I will refer to Martin heidegger's distinction between an object and a thing, to bruno latour's idea of the agency of things and object-oriented democracy, and to Don Ihde's material hermeneutics. To illustrate my argument I will focus on some examples of the ambivalent status of the disappeared person (dead or alive) in argentina, which resists the oppositional structure of present versus absent. In this context, the disappeared body is a paradigm of the past itself, which is both continuous with the present and discontinuous from it, which simultaneously is and is not. Since there are no adequate terms to analyze the "contradictory" or anomalous status of the present-absent dichotomy, I look for them outside the binary oppositions conventionally used to conceptualize the present-absent relationship in our thinking about the past. for this purpose I employ Algirdas Julien Greimas's semiotic square. [source] Topographic controls on shallow groundwater dynamics: implications of hydrologic connectivity between hillslopes and riparian zones in a till mantled catchmentHYDROLOGICAL PROCESSES, Issue 16 2010J. M. Detty Abstract Hydrologic connectivity is regarded as one of the key controls in determining catchment rainfall,run-off response and has been linked to the export of solutes from uplands to streams. We sought to identify the patterns of hydrologic connectivity within a small forested watershed by monitoring the shallow groundwater fluctuations of simple topographically defined landform sequences (footslope,backslope,shoulder). A spatially distributed instrument network was employed to continuously measure hydrometric responses of the shallow subsurface during seasonal wet-up from summer through winter in a small till mantled research catchment. We demonstrate that the spatial patterns of shallow water table extent and duration, and therefore hydrologic connectivity, had a strong seasonal signature. During the low antecedent soil moisture conditions typically associated with the growing season, water tables were patchy, discontinuous, and only the wettest near-stream footslope areas were consistently hydrologically connected with the stream network. During the dormant season, footslopes and backslopes maintained water tables that persisted between storm events and were almost continuously connected with the stream network. In the largest storm events, the typically driest landforms (shoulder slopes) established shallow transient water tables, suggesting that nearly the entire catchment was temporarily hydrologically connected with the stream network. In addition, we found significant differences (p < 0·05) in the magnitude and duration of groundwater responses to rainfall among landform groups both seasonally and during events. These results have implications for using a similarity approach in representing characteristic hydrologic responses of topographically defined watershed elements, determining hydrologic connectivity between watershed elements, as well as for understanding solute transport in catchments. Copyright © 2010 John Wiley & Sons, Ltd. [source] Soil water repellency in a Japanese cypress plantation restricts increases in soil water storage during rainfall eventsHYDROLOGICAL PROCESSES, Issue 17 2007Masahiro Kobayashi Abstract Forest soils in Japan are often water repellent. Substantial water repellency frequently occurs and impedes water infiltration into the soil matrix, but continuous overland flow is not necessarily observed because forest soils usually have macropores through which the water can enter the subsoil. Although this flow pattern may influence the manner of water storage in forest soils at the solum scale, field evidence has not yet indicated this process. We monitored soil water storage during natural rainfall events in a 60-cm deep solum using time domain reflectometry (TDR) moisture sensors, and observed stained flow patterns in the soil following simulated rainfall containing a colour dye, on a slope planted with Japanese cypress (Chamaecyparis obtusa). The surface soil at the research plot exhibited strong water repellency at water contents lower than the threshold critical water content of 0·29 m3 m,3. Under dry antecedent moisture conditions, increases in soil water storage were small compared to the cumulative rainfall, despite the low wetness of the soil matrix. In contrast, under moderate moisture conditions, increases in the water content corresponded to the cumulative rainfall. Under dry conditions, rainwater may have entered the subsoil at a few limited locations connected with continuous vertical macropores, such as decayed root channels or interstructural voids. Therefore, the water seemed to bypass a large part of the soil matrix away from the macropores. Such preferential water flow was confirmed by the stained flow patterns after the rainfall simulation. The flow patterns visualized by the dye were discontinuous and scattered under dry conditions and diffuse under moderate moisture conditions. Repellency induced preferential flow led to restricted increases in solum scale water storage during rainfall events, reflecting a physical nonequilibrium in soil water storage. Copyright © 2007 John Wiley & Sons, Ltd. [source] A second order discontinuous Galerkin method for advection on unstructured triangular meshesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 4 2003H. J. M. Geijselaers Abstract In this paper the advection of element data which are linearly distributed inside the elements is addressed. Across element boundaries the data are assumed discontinuous. The equations are discretized by the Discontinuous Galerkin method. For stability and accuracy at large step sizes (large values of the Courant number), the method is extended to second order. Furthermore the equations are enriched with selective implicit terms. This results in an explicit and local advection scheme, which is stable and accurate for Courant numbers less than .95 on unstructured triangle meshes. Results are shown of some pure advection test problems. Copyright © 2003 John Wiley & Sons, Ltd. [source] A discontinuous enrichment method for the efficient solution of plate vibration problems in the medium-frequency regimeINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 2 2010Paolo Massimi Abstract A discontinuous enrichment method (DEM) is presented for the efficient discretization of plate vibration problems in the medium-frequency regime. This method enriches the polynomial shape functions of the classical finite element discretization with free-space solutions of the biharmonic operator governing the elastic vibrations of an infinite Kirchhoff plate. These free-space solutions, which represent flexural waves and decaying modes, are discontinuous across the element interfaces. For this reason, two different and carefully constructed Lagrange multiplier approximations are introduced along the element edges to enforce a weak continuity of the transversal displacement and its normal derivative, and discrete Lagrange multipliers are introduced at the element corners to enforce there a weak continuity of the transversal displacement. The proposed DEM is illustrated with the solution of sample plate vibration problems with different types of harmonic loading in the medium-frequency regime, away from and close to resonance. In all cases, its performance is found to be significantly superior to that of the classical higher-order finite element method. Copyright © 2010 John Wiley & Sons, Ltd. [source] Simultaneous untangling and smoothing of moving gridsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 7 2008Ezequiel J. López Abstract In this work, a technique for simultaneous untangling and smoothing of meshes is presented. It is based on an extension of an earlier mesh smoothing strategy developed to solve the computational mesh dynamics stage in fluid,structure interaction problems. In moving grid problems, mesh untangling is necessary when element inversion happens as a result of a moving domain boundary. The smoothing strategy, formerly published by the authors, is defined in terms of the minimization of a functional associated with the mesh distortion by using a geometric indicator of the element quality. This functional becomes discontinuous when an element has null volume, making it impossible to obtain a valid mesh from an invalid one. To circumvent this drawback, the functional proposed is transformed in order to guarantee its continuity for the whole space of nodal coordinates, thus achieving the untangling technique. This regularization depends on one parameter, making the recovery of the original functional possible as this parameter tends to 0. This feature is very important: consequently, it is necessary to regularize the functional in order to make the mesh valid; then, it is advisable to use the original functional to make the smoothing optimal. Finally, the simultaneous untangling and smoothing technique is applied to several test cases, including 2D and 3D meshes with simplicial elements. As an additional example, the application of this technique to a mesh generation case is presented. Copyright © 2008 John Wiley & Sons, Ltd. [source] On the L2 and the H1 couplings for an overlapping domain decomposition method using Lagrange multipliersINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 3 2007P.-A. Guidault Abstract In this paper, a comparison of the L2 and the H1 couplings is made for an overlapping domain decomposition method using Lagrange multipliers. The analysis of the local equations arising from the formulation of the coupling of two mechanical models shows that continuous weight functions are required for the L2 coupling term whereas both discontinuous and continuous weight functions can be used for the H1 coupling. The choice of the Lagrange multiplier space is discussed and numerically studied. The paper ends with some numerical examples of an end-loaded cantilever beam and a cracked plate under tension and shear. It is shown that the continuity enforced with the H1 coupling leads to a link with a flexibility that can be beneficial for coupling a very coarse mesh with a very fine one. To limit the effect of the volume coupling on the global response, a narrow coupling zone is recommended. In this case, volume coupling tends to a surface coupling, especially with a L2 coupling. Copyright © 2006 John Wiley & Sons, Ltd. [source] Hybrid and enhanced finite element methods for problems of soil consolidationINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 2 2007X. X. Zhou Abstract Hybrid and enhanced finite element methods with bi-linear interpolations for both the solid displacements and the pore fluid pressures are derived based on mixed variational principles for problems of elastic soil consolidation. Both plane strain and axisymmetric problems are studied. It is found that by choosing appropriate interpolation of enhanced strains in the enhanced method, and by choosing appropriate interpolations of strains, effective stresses and enhanced strains in the hybrid method, the oscillations of nodal pore pressures can be eliminated. Several numerical examples demonstrating the capability and performance of the enhanced and hybrid finite element methods are presented. It is also shown that for some situations, such as problems involving high Poisson's ratio and in other related problems where bending effects are evident, the performance of the enhanced and hybrid methods are superior to that of the conventional displacement-based method. The results from the hybrid method are better than those from the enhanced method for some situations, such as problems in which soil permeability is variable or discontinuous within elements. Since all the element parameters except the nodal displacements and nodal pore pressures are assumed in the element level and can be eliminated by static condensation, the implementations of the enhanced method and the hybrid method are basically the same as the conventional displacement-based finite element method. The present enhanced method and hybrid method can be easily extended to non-linear consolidation problems. Copyright © 2006 John Wiley & Sons, Ltd. [source] Strong and weak arbitrary discontinuities in spectral finite elementsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 8 2005A. Legay Abstract Methods for constructing arbitrary discontinuities within spectral finite elements are described and studied. We use the concept of the eXtended Finite Element Method (XFEM), which introduces the discontinuity through a local partition of unity, so there is no requirement for the mesh to be aligned with the discontinuities. A key aspect of the implementation of this method is the treatment of the blending elements adjacent to the local partition of unity. We found that a partition constructed from spectral functions one order lower than the continuous approximation is optimal and no special treatment is needed for higher order elements. For the quadrature of the Galerkin weak form, since the integrand is discontinuous, we use a strategy of subdividing the discontinuous elements into 6- and 10-node triangles; the order of the element depends on the order of the spectral method for curved discontinuities. Several numerical examples are solved to examine the accuracy of the methods. For straight discontinuities, we achieved the optimal convergence rate of the spectral element. For the curved discontinuity, the convergence rate in the energy norm error is suboptimal. We attribute the suboptimality to the approximations in the quadrature scheme. We also found that modification of the adjacent elements is only needed for lower order spectral elements. Copyright © 2005 John Wiley & Sons, Ltd. [source] Time continuity in cohesive finite element modelingINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 5 2003Katerina D. Papoulia Abstract We introduce the notion of time continuity for the analysis of cohesive zone interface finite element models. We focus on ,initially rigid' models in which an interface is inactive until the traction across it reaches a critical level. We argue that methods in this class are time discontinuous, unless special provision is made for the opposite. Time discontinuity leads to pitfalls in numerical implementations: oscillatory behavior, non-convergence in time and dependence on nonphysical regularization parameters. These problems arise at least partly from the attempt to extend uniaxial traction,displacement relationships to multiaxial loading. We also argue that any formulation of a time-continuous functional traction,displacement cohesive model entails encoding the value of the traction components at incipient softening into the model. We exhibit an example of such a model. Most of our numerical experiments concern explicit dynamics. Copyright © 2003 John Wiley & Sons, Ltd. [source] Numerical approximation of a thermally driven interface using finite elementsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 11 2003P. Zhao Abstract A two-dimensional finite element model for dendritic solidification has been developed that is based on the direct solution of the energy equation over a fixed mesh. The model tracks the position of the sharp solid,liquid interface using a set of marker points placed on the interface. The simulations require calculation of the temperature gradients on both sides of the interface in the direction normal to it; at the interface the heat flux is discontinuous due to the release of latent heat during the solidification (melting) process. Two ways to calculate the temperature gradients at the interface, evaluating their interpolants at Gauss points, were proposed. Using known one- and two-dimensional solutions to stable solidification problems (the Stefan problem), it was shown that the method converges with second-order accuracy. When applied to the unstable solidification of a crystal into an undercooled liquid, it was found that the numerical solution is extremely sensitive to the mesh size and the type of approximation used to calculate the temperature gradients at the interface, i.e. different approximations and different meshes can yield different solutions. The cause of these difficulties is examined, the effect of different types of interpolation on the simulations is investigated, and the necessary criteria to ensure converged solutions are established. Copyright © 2003 John Wiley & Sons, Ltd. [source] Practical evaluation of five partly discontinuous finite element pairs for the non-conservative shallow water equationsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 6 2010Richard Comblen Abstract This paper provides a comparison of five finite element pairs for the shallow water equations. We consider continuous, discontinuous and partially discontinuous finite element formulations that are supposed to provide second-order spatial accuracy. All of them rely on the same weak formulation, using Riemann solver to evaluate interface integrals. We define several asymptotic limit cases of the shallow water equations within their space of parameters. The idea is to develop a comparison of these numerical schemes in several relevant regimes of the subcritical shallow water flow. Finally, a new pair, using non-conforming linear elements for both velocities and elevation (P,P), is presented, giving optimal rates of convergence in all test cases. P,P1 and P,P1 mixed formulations lack convergence for inviscid flows. P,P2 pair is more expensive but provides accurate results for all benchmarks. P,P provides an efficient option, except for inviscid Coriolis-dominated flows, where a small lack of convergence is observed. Copyright © 2009 John Wiley & Sons, Ltd. [source] Some results on the accuracy of an edge-based finite volume formulation for the solution of elliptic problems in non-homogeneous and non-isotropic mediaINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 3 2009Darlan Karlo Elisiário de Carvalho Abstract The numerical simulation of elliptic type problems in strongly heterogeneous and anisotropic media represents a great challenge from mathematical and numerical point of views. The simulation of flows in non-homogeneous and non-isotropic porous media with full tensor diffusion coefficients, which is a common situation associated with the miscible displacement of contaminants in aquifers and the immiscible and incompressible two-phase flow of oil and water in petroleum reservoirs, involves the numerical solution of an elliptic type equation in which the diffusion coefficient can be discontinuous, varying orders of magnitude within short distances. In the present work, we present a vertex-centered edge-based finite volume method (EBFV) with median dual control volumes built over a primal mesh. This formulation is capable of handling the heterogeneous and anisotropic media using structured or unstructured, triangular or quadrilateral meshes. In the EBFV method, the discretization of the diffusion term is performed using a node-centered discretization implemented in two loops over the edges of the primary mesh. This formulation guarantees local conservation for problems with discontinuous coefficients, keeping second-order accuracy for smooth solutions on general triangular and orthogonal quadrilateral meshes. In order to show the convergence behavior of the proposed EBFV procedure, we solve three benchmark problems including full tensor, material heterogeneity and distributed source terms. For these three examples, numerical results compare favorably with others found in literature. A fourth problem, with highly non-smooth solution, has been included showing that the EBFV needs further improvement to formally guarantee monotonic solutions in such cases. Copyright © 2008 John Wiley & Sons, Ltd. [source] Anisotropic mesh adaption for time-dependent problems,INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 9 2008S. Micheletti Abstract We propose a space,time adaptive procedure for a model parabolic problem based on a theoretically sound anisotropic a posteriori error analysis. A space,time finite element scheme (continuous in space but discontinuous in time) is employed to discretize this problem, thus allowing for non-matching meshes at different time levels. Copyright © 2008 John Wiley & Sons, Ltd. [source] Improving Eulerian two-phase flow finite element approximation with discontinuous gradient pressure shape functionsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2005A. H. Coppola-Owen Abstract In this paper we present a problem we have encountered using a stabilized finite element method on fixed grids for flows with interfaces modelled with the level set approach. We propose a solution based on enriching the pressure shape functions on the elements cut by the interface. The enrichment is used to enable the pressure gradient to be discontinuous at the interface, thus improving the ability to simulate the behaviour of fluids with different density under a gravitational force. The additional shape function used is local to each element and the corresponding degree of freedom can therefore be condensed prior to assembly, making the implementation quite simple on any existing finite element code. Copyright © 2005 John Wiley & Sons, Ltd. [source] Performance of finite volume solutions to the shallow water equations with shock-capturing schemesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 10 2002K. S. Erduran Abstract Numerical methods have become well established as tools for solving problems in hydraulic engineering. In recent years the finite volume method (FVM) with shock capturing capabilities has come to the fore because of its suitability for modelling a variety of types of flow; subcritical and supercritical; steady and unsteady; continuous and discontinuous and its ability to handle complex topography easily. This paper is an assessment and comparison of the performance of finite volume solutions to the shallow water equations with the Riemann solvers; the Osher, HLL, HLLC, flux difference splitting (Roe) and flux vector splitting. In this paper implementation of the FVM including the Riemann solvers, slope limiters and methods used for achieving second order accuracy are described explicitly step by step. The performance of the numerical methods has been investigated by applying them to a number of examples from the literature, providing both comparison of the schemes with each other and with published results. The assessment of each method is based on five criteria; ease of implementation, accuracy, applicability, numerical stability and simulation time. Finally, results, discussion, conclusions and recommendations for further work are presented. Copyright © 2002 John Wiley & Sons, Ltd. [source] | |||||||||||||||||||||||||||||||||||||||||||