Velocity Increases (velocity + increase)

Distribution by Scientific Domains

Selected Abstracts

The role of vegetation patterns in structuring runoff and sediment fluxes in drylands

Juan Puigdefábregas
Abstract The dynamics of vegetation-driven spatial heterogeneity (VDSH) and its function in structuring runoff and sediment fluxes have received increased attention from both geomorphological and ecological perspectives, particularly in arid regions with sparse vegetation cover. This paper reviews the recent findings in this area obtained from field evidence and numerical simulation experiments, and outlines their implications for soil erosion assessment. VDSH is often observed at two scales, individual plant clumps and stands of clumps. At the patch scale, the local outcomes of vegetated patches on soil erodibility and hydraulic soil properties are well established. They involve greater water storage capacity as well as increased organic carbon and nutrient inputs. These effects operate together with an enhanced capacity for the interception of water and windborne resources, and an increased biological activity that accelerates breakdown of plant litter and nutrient turnover rates. This suite of relationships, which often involve positive feedback mechanisms, creates vegetated patches that are increasingly different from nearby bare ground areas. By this way a mosaic builds up with bare ground and vegetated patches coupled together, respectively, as sources and sinks of water, sediments and nutrients. At the stand scale within-storm temporal variability of rainfall intensity controls reinfiltration of overland flow and its decay with slope length. At moderate rainfall intensity, this factor interacts with the spatial structure of VDSH and the mechanism of overland flow generation. Reinfiltration is greater in small-grained VDSH and topsoil saturation excess overland flow. Available information shows that VDSH structures of sources and sinks of water and sediments evolve dynamically with hillslope fluxes and tune their spatial configurations to them. Rainfall simulation experiments in large plots show that coarsening VDSH leads to significantly greater erosion rates even under heavy rainfall intensity because of the flow concentration and its velocity increase. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Internal structure of an alpine rock glacier based on crosshole georadar traveltimes and amplitudes

Martin 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]

A Broad Diagnostic Battery for Bedside Transcranial Doppler to Detect Flow Changes With Internal Carotid Artery Stenosis or Occlusion

Ioannis Christou MD
ABSTRACT Background and Purpose. The authors establish accuracy parameters of a broad diagnostic battery for bedside transcranial Doppler (TCD) to detect flow changes due to internal carotid artery (ICA) stenosis or occlusion. Methods. The authors prospectively studied consecutive patients with stroke or transient ischemic attack referred for TCD. TCD was performed and interpreted at bedside using a standard insonation protocol. A broad diagnostic battery included major criteria: collateral flow signals, abnormal siphon or terminal carotid signals, and delayed systolic flow acceleration in the middle cerebral artery. Minor criteria included a unilateral decrease in pulsatility index (, 0.6 or , 70% of contralateral side), flow diversion signs, and compensatory velocity increase. Angiography or carotid duplex ultrasound (CDU) was used to grade the degree of carotid stenosis using North American criteria. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of TCD findings were determined. Results. Seven hundred and twenty patients underwent TCD, of whom 517 (256 men and 261 women) had angiography and/or CDU within 8.8 ± 0.9 days. Age was 63.1 ± 15.7 years. For a 70% to 99% carotid stenosis or occlusion, TCD had sensitivity of 79.4%, specificity of 86.2%, PPV of 57.0%, NPV of 94.8%, and accuracy of 84.7%. For a 50% to 99% carotid stenosis or occlusion, TCD had sensitivity of 67.5%, specificity of 83.9%, PPV of 54.5%, NPV of 90.0%, and accuracy of 81.6%. TCD detected intracranial carotid lesions with 84.9% accuracy and extracranial carotid lesions with 84.4% accuracy (sensitivity of 88% and 79%, specificity of 85% and 86%, PPV of 24% and 54%, and NPV of 99% and 95%, respectively). The prevalence of the ophthalmic artery flow reversal was 36.4% in patients with , 70% stenosis or occlusion. If present, this finding indicated a proximal ICA lesion location in 97% of these patients. Conclusions. In symptomatic patients, bedside TCD can accurately detect flow changes consistent with hemodynamically significant ICA obstruction; however, TCD should not be a substitute for direct carotid evaluation. Because TCD is sensitive and specific for , 70% carotid stenosis or occlusion in both extracranial and intracranial carotid segments, it can be used as a complementary test to refine other imaging findings and detect tandem lesions. [source]

An experimental study on the ripple,dune transition

André Robert
Abstract Flume experiments were conducted on different bed stages across the ripple,dune transition. As flow velocity increases, an initially flat bed surface (made of fairly uniform sandy material) is gradually transformed into a two-dimensional rippled bed. With further increase in velocity, two-dimensional ripples are replaced by irregular, linguoid ripples. As the average velocity necessary for the ripple,dune transition to occur is imposed on the bed surface, these non-equilibrium linguoid ripples are further transformed into larger, two-dimensional dunes. For each of these stages across the transition, a concrete mould of the bed was created and the flow structure above each fixed bed surface investigated. An acoustic Doppler velocimeter was used to study the flow characteristics above each bed surface. Detailed profiles were used along a transect located in the middle of the channel. Results are presented in the form of spatially averaged profiles of various flow characteristics and of contour maps of flow fields (section view). They clearly illustrate some important distinctions in the flow structure above the different bedform types associated with different stages during the transition. Turbulence intensity and Reynolds stresses gradually increase throughout the transition. Two-dimensional ripples present a fairly uniform spatial distribution of turbulent flow characteristics above the bed. Linguoid ripples induce three-dimensional turbulence structure at greater heights above the bed surface and turbulence intensity tends to increase steadily with height above bed surface in the wake region. A very significant increase in turbulence intensity and momentum exchange occurs during the transition from linguoid ripples to dunes. The turbulent flow field properties above dunes are highly dependent on the position along and above the bed surface and these fields present a very high degree of spatial variability (when compared with the rippled beds). Further investigations under natural conditions emphasizing sediment transport mechanisms and rates during the transition should represent the next step of analysis, together with an emphasis on quadrant analysis. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Grain transport mechanics in shallow overland flow

ECOHYDROLOGY, Issue 3 2009
S.N. Prasad
Abstract A physical model based on continuum multiphase flow is described to represent saltating transport of grains in shallow overland flows. The two-phase continuum flow of water and sediment considers coupled StVenant-type equations. The interactive cumulative effect of grains is incorporated by a dispersive stress term. The mean fluid thrust on the particle in the saltation layer of grains is expressed in terms of a slip velocity. The continuum model leads to the unexpected, but an interesting result is that particle velocity increases with the solid concentration. This increase predicts monotonic behaviour leading to overestimates of particle velocity at higher sediment concentration. To improve the predictions, grain dynamic equations, which incorporate bed collision, are analysed. The analysis leads to an improved model for predicting saltation height. Incorporation of the results in the continuum model yields a velocity-concentration relationship that is consistent with experimental observations for increasing concentration. Laboratory flume experiments explore the evaluation of various parameters from the measured particle velocities by photonic probes. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Chemical modification of pyroclastic rock by hot water: an experimental investigation of mass transport at the fluid,solid interface

Abstract Hydrothermal water,(pyroclastic) rock interactions were examined using flow-through experiments to deduce the effect of mass transport phenomena on the reaction process. A series of experiments were conducted over the temperature range 75,250°C, with a constant temperature for each experiment, and at saturated vapour pressure, to estimate the apparent rate constants as a function of temperature. Based on the chemistry of analysed solutions, the water,rock interaction in the experiments was controlled by diffusion from the reaction surface and by the existence of a surface layer at the rock,fluid interface, which regulated the chemical reaction rate. The reaction progress depended to a high degree on flow velocity and temperature conditions, with element abundances in the fluid significantly affected by these factors. Mass transport coefficients for diffusion from the rock surface to the bulk solution have been estimated. Ca is selectively depleted under lower temperature conditions (T < 150°C), whereas Na is greatly depleted under higher temperature conditions (T > 150°C), and K reaction rates are increased when flow velocity increases. Using these conditions, specific alkali and alkali earth cations were selectively leached from mineral surfaces. The ,surface layer' comprised a 0.5,1.8 mm boundary film on the solution side (the thickness of this layer has no dependence on chemical character) and a reaction layer. The reaction layer was composed of a Si, Al-rich cation-leached layer, whose thickness was dependent on temperature, flow velocity and reaction length. The reaction layer varied in thickness from about 10,4 to 10,7 mm under high temperature/low fluid velocity and low temperature/high fluid velocity conditions, respectively. [source]

Effects of chemical reaction, heat and mass transfer on non-linear MHD flow over an accelerating surface with heat source and thermal stratification in the presence of suction or injection

S. P. Anjali Devi
Abstract An approximate numerical solution for the steady MHD laminar boundary-layer flow over an accelerating vertical surface with suction or injection in the presence of species concentration and mass diffusion has been obtained by solving the governing equations using R.K. Gill method. The fluid is assumed to be viscous, incompressible and electrically conducting with a magnetic field applied transversely to the direction of the flow. It has been observed that in the presence of mass diffusion: (i) in the case of suction, the velocity decreases and the temperature distribution and concentration of the fluid increase and for injection, the velocity increases and the temperature distribution and concentration of the fluid decrease with increase of thermal stratification parameter, (ii) in the presence of thermal stratification parameter in both the cases of suction and injection, the skin friction and rate of mass transfer decrease and the rate of heat transfer of the fluid increases with increase of chemical reaction effects, (iii) in the cases of suction and injection, an increase in the strength of magnetic field leads to fall in the velocity and rise in the temperature and concentration of the fluid along the surface. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Cerebral blood flow velocity increases during a single treatment with the molecular adsorbents recirculating system in patients with acute on chronic liver failure

Lars E. Schmidt
The aim of this uncontrolled pilot study is to determine the effect of treatment with the molecular adsorbents recirculating system (MARS) on cerebral perfusion in patients with acute on chronic liver failure (AOCLF). In 8 patients (median age, 44 years; range, 35 to 52 years) admitted with AOCLF, a single 10-hour MARS treatment was performed. Hepatic encephalopathy (HE) was graded according to the Fogarty criteria. Changes in cerebral perfusion were determined by transcranial Doppler as mean flow velocity (Vmean) in the middle cerebral artery. Arterial ammonia and bilirubin levels were monitored as a measure of the capability of the MARS to remove water-soluble and protein-bound toxins. During MARS treatment, HE grade improved in 3 patients and remained unchanged in 5 patients (P = .11). Vmean increased from 42 cm/sec (range, 26 to 59 cm/sec) to 72 cm/sec (range, 52 to 106 cm/sec; P < .05), whereas arterial ammonia level decreased from 88 ,mol/L (range, 45 to 117 ,mol/L) to 71 ,mol/L (range, 26 to 98 ,mol/L; P < .05) and bilirubin level from 537 ,mol/L (range, 324 to 877 ,mol/L) to 351 ,mol/L (range, 228 to 512 ,mol/L; P < .05). In conclusion, cerebral perfusion is increased and levels of ammonia and bilirubin are reduced during MARS treatment in patients with AOCLF. [source]

Borehole deformation measurements and internal structure of some rock glaciers in Switzerland

Lukas Arenson
Abstract In order to understand the mechanical processes that influence the deformation patterns of active rock glaciers, information about local horizontal and vertical deformations as well as knowledge of the internal structure and the temperature distribution is necessary. Results from borehole deformation measurements of three sites in the Swiss Alps show that despite different internal structures, similar phenomena can be observed. In contrast to temperate glaciers, permafrost within rock glaciers has distinct shear zones where horizontal and vertical differential movements are concentrated. In addition, a reduction in volume can be caused by compressive flow due to the presence of air voids within the permafrost. The flow velocity depends on the temperature, the surface and bedrock slopes of the rock glacier, and the composition of the ice-rich frozen ground. Within degrading permafrost, the ice content decreases, the creep velocity increases and the shear zone rises towards the surface, where seasonal temperature changes and the presence of liquid water might also influence deformation. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Are long subglacial bedforms indicative of fast ice flow?

BOREAS, Issue 3 2002
It has been suggested that extremely long subglacial bedforms (e.g. attenuated drumlins and mega-scale glacial lineations) record former areas of fast-flowing ice and that bedform elongation ratio is a useful proxy for ice velocity. Despite the availability of much data pertaining to the measurement and analysis of subglacial bed-forms, these assumptions have rarely been explicitly addressed in detail. In this paper, we demonstrate that long subglacial bedforms (length:width ratios 10:1) are indicative of fast ice flow. Using satellite imagery, we mapped over 8000 lineaments associated with a highly convergent flow pattern near Dubawnt Lake, District of Keewatin, Canada. This flow pattern is unusual in that it displays a large zone of convergence feeding into a main ,trunk' and then diverging towards the inferred ice margin. The ,bottleneck' pattern is taken to record an increase and subsequent decrease in ice velocity and we analysed transverse and longitudinal variations in bedform morphometry. The main trunk of the flow pattern (down-ice of the convergent zone) is characterized by mega-scale glacial lineations of great length (up to 13 km) and high elongation ratios (up to 43:1). The down-ice variations in elongation ratio reflect exactly what we would expect from a terrestrial ice stream whose velocity increases in the onset zone passes through a maximum in the main trunk and slows down as the ice diverges at the terminus. It is suggested that any unifying theory of drumlin formation must be able to account for the association between long subglacial bedforms and fast ice flow, although it is not assumed that fast ice flow always produces attenuated bedforms. A further implication of this work is that many more ice streams may be identified on the basis of attenuated subglacial bedforms, radically altering our views on the flow dynamics of former ice sheets. [source]

A Practical Method to Estimate the Bed Height of a Fluidized Bed of Fine Particles

M. Zhang
Abstract Knowledge of both dense bed expansion and freeboard solids inventory are required for the determination of bed height in fluidized beds of fine particles, e.g., Fluidized Catalytic Cracking (FCC) catalysts. A more accurate estimation of the solids inventory in the freeboard is achieved based on a modified model for the freeboard particle concentration profile. Using the experimentally determined dense bed expansion and the modified freeboard model, a more practical method with improved accuracy is provided to determine the bed height both in laboratory and industrial fluidized beds of FCC particles. The bed height in a fluidized bed can exhibit different trends as the superficial gas velocity increases, depending on the different characteristics of the dense bed expansion and solids entrainment in the freeboard. The factors that influence the bed height are discussed, showing the complexity of bed height and demonstrating that it is not realistic to determine the bed height by a generalized model that can accurately predict the dense bed expansion and freeboard solids inventory simultaneously. Moreover, a method to determine the bed height, based on axial pressure fluctuation profiles, is proposed in this study for laboratory fluidized beds, which provides improved accuracy compared to observation alone or determining the turning points in the axial pressure profiles, especially in high-velocity fluidized beds. [source]

Investigation of Fluid and Coarse-Particle Dynamics in a Two-Dimensional Spouted Bed

T. Swasdisevi
Abstract The aerodynamics of particles and gas flow in a two-dimensional spouted bed (2DSB) with draft plates is investigated with the aid of the discrete element method. The geometry of the 2DSB with draft plates is set as close as possible to the experimental apparatus of Kudra [1] and Kalwar [2]. The physical properties of the coarse particles are similar to those of shelled corn. The calculated minimum spouting velocity and pressure drop agree well with the correlations of Kudra [1] and Kalwar [2]. In the spout region, the particle vertical velocities are found to decrease as the height increases. The fluid velocity in the downcomer region decreases as the superficial gas velocity increases. The particle circulation rate increases when the friction coefficient decreases or the separation height increases. At the minimum spouting velocity, the bed height does not affect the particle circulation rate in the 2DSB with draft plates. The draft plates not only reduce the minimum spouting velocity and pressure drop but also increase the maximum spoutable bed height. The effect of taking out the draft plates on the spouting phenomenon is investigated and the effect of putting in a deflector on the possible breakage of the particles is also estimated. [source]