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J. G. Wouchuk - One of the best experts on this subject based on the ideXlab platform.
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Linear theory of Richtmyer-Meshkov like flows
Plasma Physics and Controlled Fusion, 2016Co-Authors: J. G. Wouchuk, F Cobos-camposAbstract:The hydrodynamic flow generated by rippled shocks and Rarefactions (Richtmyer–Meshkov like flows) is presented. When a corrugated shock travels inside an homogeneous fluid, it leaves pressure, density and velocity perturbations in the compressed fluid. The velocity perturbations generated in the composed fluid are inherently rotational. Vorticity is an important quantity in order to determine the asymptotic rate of growth in the linear stage. The size of the strongest vortices generated by the rippled shocks is analyzed as a function of the shock Mach number for different boundary conditions downstream. Comparison to experiments and simulations is provided for the RMI in the shock and Rarefaction reflected cases and the validity of the growth law is emphasized.
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linear perturbation growth at the trailing edge of a Rarefaction wave
Physics of Plasmas, 2003Co-Authors: J. G. Wouchuk, Ramiro CarreteroAbstract:An analytic model for the perturbation growth inside a Rarefaction wave is presented. The objective of the work is to calculate the growth of the perturbations at the trailing edge of a simple expanding wave in planar geometry. Previous numerical and analytical works have shown that the ripples at the Rarefaction tail exhibit linear growth asymptotically in time [Yang et al., Phys. Fluids 6, 1856 (1994), A. Velikovich and L. Phillips, ibid. 8, 1107 (1996)]. However, closed expressions for the asymptotic value of the perturbed velocity of the trailing edge have not been reported before, except for very weak Rarefactions. Explicit analytic solutions for the perturbations growing at the Rarefaction trailing edge as a function of time and also for the asymptotic perturbed velocity are given, for fluids with γ<3. The limits of weak and strong Rarefactions are considered and the corresponding scaling laws are given. A semi-qualitative discussion of the late time linear growth at the trailing edge ripple is pres...
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Linear perturbation growth at the trailing edge of a Rarefaction wave
Physics of Plasmas, 2003Co-Authors: J. G. Wouchuk, Ramiro CarreteroAbstract:An analytic model for the perturbation growth inside a Rarefaction wave is presented. The objective of the work is to calculate the growth of the perturbations at the trailing edge of a simple expanding wave in planar geometry. Previous numerical and analytical works have shown that the ripples at the Rarefaction tail exhibit linear growth asymptotically in time [Yang et al., Phys. Fluids 6, 1856 (1994), A. Velikovich and L. Phillips, ibid. 8, 1107 (1996)]. However, closed expressions for the asymptotic value of the perturbed velocity of the trailing edge have not been reported before, except for very weak Rarefactions. Explicit analytic solutions for the perturbations growing at the Rarefaction trailing edge as a function of time and also for the asymptotic perturbed velocity are given, for fluids with γ
Tarek F Antonios - One of the best experts on this subject based on the ideXlab platform.
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142 Onset of preeclampsia is preceded by structural capillary Rarefaction
Heart, 2012Co-Authors: Vivek Nama, Isaac Manyonda, Joseph Onwude, Tarek F AntoniosAbstract:Introduction Microvascular Rarefaction, defined as reduced vascular density, is a consistent finding in hypertension. Functional and structural capillary Rarefaction occurs in individuals with sustained and borderline essential hypertension, and in their normotensive offspring. Women who develop preeclampsia are at increased risk of hypertension in later life. We hypothesised that capillary Rarefaction precedes the onset of preeclampsia and could play a role in its pathogenesis. Methods In this longitudinal cohort study we recruited 322 Caucasian women, of which 305 subjects completed the study. We used intravital video-microscopy to measure basal (ie, functional) and maximal (ie, structural) skin capillary densities according to a well-validated protocol and measured plasma angiogenic and anti-angiogenic factors. Subjects were studied at five consecutive visits. Results Preeclampsia occurred in 16 women (mean onset at 35.6±4.8 weeks) and 272 women had normal pregnancy. In women with normal pregnancy significant structural reduction in capillary density occurred at 27–32 weeks, which had resolved by the puerperium (mean change: −2.2 capillaries/field, 95% CI −3.6 to −0.7). In contrast, in women who developed preeclampsia, more significant structural Rarefaction was observed earlier at 20–24 weeks (mean change: −6.1 capillaries/field, 95% CI −9.2 to −2.9), which persisted into the puerperium. We also found that the change in soluble Endoglin from 11–16 weeks to 27–32 weeks was significantly correlated with the change in structural capillary density. Conclusions This is the first study to show that significant structural capillary Rarefaction precedes the onset of preeclampsia. Capillary Rarefaction could play a role in the pathogenesis of this disease.
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Structural capillary Rarefaction and the onset of preeclampsia.
Obstetrics and gynecology, 2012Co-Authors: Vivek Nama, Isaac Manyonda, Joseph Onwude, Tarek F AntoniosAbstract:OBJECTIVE:To estimate if reduced capillary density (ie, capillary Rarefaction) precedes the onset of preeclampsia and if it could play a role in its pathogenesis. Capillary Rarefaction is a consistent finding in essential hypertension.METHODS:In this longitudinal cohort study, we recruited 322 conse
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effect of modest salt reduction on skin capillary Rarefaction in white black and asian individuals with mild hypertension
Hypertension, 2010Co-Authors: Maciej Marciniak, Nirmala D Markandu, Tarek F Antonios, Graham A MacgregorAbstract:Microvascular Rarefaction occurs in hypertension. We carried out a 12-week randomized double-blind crossover trial to determine the effect of a modest reduction in salt intake on capillary Rarefaction in 71 whites, 69 blacks, and 29 Asians with untreated mildly raised blood pressure. Both basal and maximal (during venous congestion) skin capillary density were measured by capillaroscopy at the dorsum and the side of the fingers. In addition, we used orthogonal polarization spectral imaging to measure skin capillary density at the dorsum of the fingers and the hand web. With a reduction in salt intake from 9.7 to 6.5 g/day, there was an increase in capillary density (capillaries per millimeter squared) from 10121 to 10623 (basal) and 10822 to 11522 (maximal) at the dorsum, and 10125 to 10726 (basal) and 11026 to 11626 (maximal) at the side of the fingers (P0.001 for all). Orthogonal polarization spectral imaging also showed a significant increase in capillary density both at the dorsum of the fingers and the web. Subgroup analysis showed that most of the changes were significant in all of the ethnic groups. Furthermore, there was a significant relationship between the change in 24-hour urinary sodium and the change in capillary density at the side of the fingers. These results demonstrate that a modest reduction in salt intake, as currently recommended, improves both functional and structural capillary Rarefactions that occur in hypertension, and a larger reduction in salt intake would have a greater effect. The increase in capillary density may possibly carry additional beneficial effects on target organs. (Hypertension. 2010;56:253-259.)
Marcin Molenda - One of the best experts on this subject based on the ideXlab platform.
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development of a Rarefaction wave at discharge initiation in a storage silo dem simulations
Particuology, 2018Co-Authors: Rafal Kobylka, Józef Horabik, Marcin MolendaAbstract:Abstract The generation of a Rarefaction wave at the initiation of discharge from a storage silo is a phenomenon of scientific and practical interest. The effect, sometimes termed the dynamic pressure switch, may create dangerous pulsations of the storage structure. Owing to the nonlinearity, discontinuity, and heterogeneity of granular systems, the mechanism of generation and propagation of stress waves is complex and not yet completely understood. The present study conducted discrete element simulations to model the formation and propagation of a Rarefaction wave in a granular material contained in a silo. Modeling was performed for a flat-bottom cylindrical container with diameter of 0.1 or 0.12 m and height of 0.5 m. The effects of the orifice size and the shape of the initial discharging impulse on the shape and extent of the Rarefaction wave were examined. Positions, velocities, and forces of particles were recorded every 10 −5 s and used to infer the location of the front of the Rarefaction wave and loads on construction members. Discharge through the entire bottom of the bin generates a plane Rarefaction wave that may be followed by a compaction wave, depending on the discharge rate. Discharge through the orifice generates a spherical Rarefaction wave that, after reflection from the silo wall, travels up the silo as a sequence of Rarefaction–compaction cycles with constant wavelength equal to the silo diameter. During the travel of the wave along the bin height, the wave amplitude increases with the distance traveled. Simulations confirmed earlier findings of laboratory and numerical (finite element method) experiments and a theoretical approach, estimating the speed of the front of the Rarefaction wave to range from 70 to 80 m/s and the speed of the tail to range from 20 to 60 m/s.
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Development of a Rarefaction wave at discharge initiation in a storage silo—DEM simulations
Particuology, 2018Co-Authors: Rafał Kobyłka, Józef Horabik, Marcin MolendaAbstract:Abstract The generation of a Rarefaction wave at the initiation of discharge from a storage silo is a phenomenon of scientific and practical interest. The effect, sometimes termed the dynamic pressure switch, may create dangerous pulsations of the storage structure. Owing to the nonlinearity, discontinuity, and heterogeneity of granular systems, the mechanism of generation and propagation of stress waves is complex and not yet completely understood. The present study conducted discrete element simulations to model the formation and propagation of a Rarefaction wave in a granular material contained in a silo. Modeling was performed for a flat-bottom cylindrical container with diameter of 0.1 or 0.12 m and height of 0.5 m. The effects of the orifice size and the shape of the initial discharging impulse on the shape and extent of the Rarefaction wave were examined. Positions, velocities, and forces of particles were recorded every 10 −5 s and used to infer the location of the front of the Rarefaction wave and loads on construction members. Discharge through the entire bottom of the bin generates a plane Rarefaction wave that may be followed by a compaction wave, depending on the discharge rate. Discharge through the orifice generates a spherical Rarefaction wave that, after reflection from the silo wall, travels up the silo as a sequence of Rarefaction–compaction cycles with constant wavelength equal to the silo diameter. During the travel of the wave along the bin height, the wave amplitude increases with the distance traveled. Simulations confirmed earlier findings of laboratory and numerical (finite element method) experiments and a theoretical approach, estimating the speed of the front of the Rarefaction wave to range from 70 to 80 m/s and the speed of the tail to range from 20 to 60 m/s.
Ramiro Carretero - One of the best experts on this subject based on the ideXlab platform.
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linear perturbation growth at the trailing edge of a Rarefaction wave
Physics of Plasmas, 2003Co-Authors: J. G. Wouchuk, Ramiro CarreteroAbstract:An analytic model for the perturbation growth inside a Rarefaction wave is presented. The objective of the work is to calculate the growth of the perturbations at the trailing edge of a simple expanding wave in planar geometry. Previous numerical and analytical works have shown that the ripples at the Rarefaction tail exhibit linear growth asymptotically in time [Yang et al., Phys. Fluids 6, 1856 (1994), A. Velikovich and L. Phillips, ibid. 8, 1107 (1996)]. However, closed expressions for the asymptotic value of the perturbed velocity of the trailing edge have not been reported before, except for very weak Rarefactions. Explicit analytic solutions for the perturbations growing at the Rarefaction trailing edge as a function of time and also for the asymptotic perturbed velocity are given, for fluids with γ<3. The limits of weak and strong Rarefactions are considered and the corresponding scaling laws are given. A semi-qualitative discussion of the late time linear growth at the trailing edge ripple is pres...
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Linear perturbation growth at the trailing edge of a Rarefaction wave
Physics of Plasmas, 2003Co-Authors: J. G. Wouchuk, Ramiro CarreteroAbstract:An analytic model for the perturbation growth inside a Rarefaction wave is presented. The objective of the work is to calculate the growth of the perturbations at the trailing edge of a simple expanding wave in planar geometry. Previous numerical and analytical works have shown that the ripples at the Rarefaction tail exhibit linear growth asymptotically in time [Yang et al., Phys. Fluids 6, 1856 (1994), A. Velikovich and L. Phillips, ibid. 8, 1107 (1996)]. However, closed expressions for the asymptotic value of the perturbed velocity of the trailing edge have not been reported before, except for very weak Rarefactions. Explicit analytic solutions for the perturbations growing at the Rarefaction trailing edge as a function of time and also for the asymptotic perturbed velocity are given, for fluids with γ
Giovanni Bacaro - One of the best experts on this subject based on the ideXlab platform.
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Incorporating spatial autocorrelation in Rarefaction methods: Implications for ecologists and conservation biologists
Ecological Indicators, 2016Co-Authors: Giovanni Bacaro, Duccio Rocchini, Alfredo Altobelli, Michela Cameletti, Daniela Ciccarelli, Stefano Martellos, Michael W. Palmer, Carlo Ricotta, Samuel M. Scheiner, Enrico TordoniAbstract:Recently, methods for constructing Spatially Explicit Rarefaction (SER) curves have been introduced in the scientific literature to describe the relation between the recorded species richness and sampling effort and taking into account for the spatial autocorrelation in the data. Despite these methodological advances, the use of SERs has not become routine and ecologists continue to use Rarefaction methods that are not spatially explicit. Using two study cases from Italian vegetation surveys, we demonstrate that classic Rarefaction methods that do not account for spatial structure can produce inaccurate results. Furthermore, our goal in this paper is to demonstrate how SERs can overcome the problem of spatial autocorrelation in the analysis of plant or animal communities. Our analyses demonstrate that using a spatially-explicit method for constructing Rarefaction curves can substantially alter estimates of relative species richness. For both analyzed data sets, we found that the rank ordering of standardized species richness estimates was reversed between the two methods. We strongly advise the use of Spatially Explicit Rarefaction methods when analyzing biodiversity: the inclusion of spatial autocorrelation into Rarefaction analyses can substantially alter conclusions and change the way we might prioritize or manage nature reserves.
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The spatial domain matters: spatially constrained species Rarefaction in a Free and Open Source environment
Ecological Complexity, 2012Co-Authors: Giovanni Bacaro, Duccio Rocchini, Anne Ghisla, Matteo Marcantonio, Markus Neteler, Alessandro ChiarucciAbstract:Abstract Rarefaction curves represent a powerful method for comparing species richness among habitats on an equal-effort basis. Three assumptions are required to correctly perform Rarefaction analysis: (i) data collection should be a representative sample of the community under study, (ii) individuals are randomly dispersed, and (iii) species are independently dispersed. However, the community structure is spatially organized, and these criteria cannot be guaranteed. Recently, Chiarucci et al. (2009) proposed a new type of Rarefaction, named Spatially Constrained Rarefaction (SCR), which allows to include the autocorrelated structure of the samples in the construction of a Rarefaction curve. Here we present an easy-to-use procedure to calculate Spatially Constrained Rarefaction curve in the R environment.
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Discovering and rediscovering the sample-based Rarefaction formula in the ecological literature
Community Ecology, 2008Co-Authors: Alessandro Chiarucci, Giovanni Bacaro, Duccio Rocchini, Lorenzo FattoriniAbstract:Rarefaction has long represented a powerful tool for detecting species richness and its variation across spatial scales. Some authors recently reintroduced the mathematical expression for calculating sample-based Rarefaction curves. While some of them did not claim any advances, others presented this formula as a new analytical solution. We provide evidence about formulations of the sample-based Rarefaction formula older than those recently proposed in ecological literature.
