Rapid Flow Conditions Research Articles

On the use of a unified constitutive model for modelling slope large deformations with material point method

Landslides and debris flows can have devastating effects, particularly when movement involves a transition from solid-like to fluid-like process. The elastoplastic constitutive model has a limited ability to describe the entire movement process; thus, this paper establishes a unified constitutive model comprising a hypoplastic model and Bagnold model within a GPU-accelerated material point method (MPM) framework. The model is validated first through low-speed direct shear tests to demonstrate its ability to describe solid-state friction behaviour, then collision behaviour under rapid flow conditions is examined using high-speed annular shear tests. The validity of the unified model for geotechnical engineering large deformations is subsequently verified using column collapse, where the superiority of the unified model over the traditional constitutive model in describing the large deformation process is demonstrated. An index, the dynamic stress ratio, is comprehensively analysed over the whole column failure process and the dynamic stress ratio is found to correlate well with the second-order work conversion criterion, distinguishing the transition of movement from solid-like to fluid-like stage. Finally, the constitutive model is applied to a hazard assessment of the Qianjiangping and Tatopani landslides in which the applicability and stability of the model along with the MPM framework are further demonstrated.

Kinetic Theory of Polydisperse Granular Mixtures: Influence of the Partial Temperatures on Transport Properties-A Review.

It is well-recognized that granular media under rapid flow conditions can be modeled as a gas of hard spheres with inelastic collisions. At moderate densities, a fundamental basis for the determination of the granular hydrodynamics is provided by the Enskog kinetic equation conveniently adapted to account for inelastic collisions. A surprising result (compared to its molecular gas counterpart) for granular mixtures is the failure of the energy equipartition, even in homogeneous states. This means that the partial temperatures (measuring the mean kinetic energy of each species) are different to the (total) granular temperature T. The goal of this paper is to provide an overview on the effect of different partial temperatures on the transport properties of the mixture. Our analysis addresses first the impact of energy nonequipartition on transport which is only due to the inelastic character of collisions. This effect (which is absent for elastic collisions) is shown to be significant in important problems in granular mixtures such as thermal diffusion segregation. Then, an independent source of energy nonequipartition due to the existence of a divergence of the flow velocity is studied. This effect (which was already analyzed in several pioneering works on dense hard-sphere molecular mixtures) affects to the bulk viscosity coefficient. Analytical (approximate) results are compared against Monte Carlo and molecular dynamics simulations, showing the reliability of kinetic theory for describing granular flows.

Thrombus Formation and Propagation in the Onset of Cardiovascular Events.

Ischemic cardiovascular disease is a major cause of morbidity and mortality worldwide and thrombus formation on disrupted atherosclerotic plaques is considered to trigger its onset. Although the activation of platelets and coagulation pathways has been investigated intensively, the mechanisms of thrombus formation on disrupted plaques have not been understood in detail. Platelets are thought to play a central role in the formation of arterial thrombus because of rapid flow conditions; however, thrombus that develops on disrupted plaques consistently includes large amounts of fibrin in addition to aggregated platelets. While, thrombus does not always become large enough to completely occlude the vascular lumen, indicating that the propagation of thrombus is also critical for the onset of cardiovascular events. Various factors, such as vascular wall thrombogenicity, altered blood flow and imbalanced blood hemostasis, modulate thrombus formation and propagation on disrupted plaques. Pathological findings derived from humans and experimental animal models of atherothrombosis have identified important factors that affect thrombus formation and propagation, namely platelets, extrinsic and intrinsic coagulation factors, proinflammatory factors, plaque hypoxia and blood flow alteration. These findings might provide insight into the mechanisms of thrombus formation and propagation on disrupted plaques that lead to the onset of cardiovascular events.

Factor VIII and von Willebrand factor – too sweet for their own good

Although factor VIII (FVIII) and von Willebrand factor (VWF) are products of two distinct genes, they circulate in plasma as a tight non-covalent complex. Moreover, they both play a critical role in the haemostatic process, a fact that is illustrated by the severe bleeding tendency associated with the functional absence of either protein. FVIII is an essential cofactor for coagulation factor IX, while VWF is pertinent to the recruitment of platelets to the injured vessel wall under conditions of rapid flow. FVIII and VWF have in common that they are heavily glycosylated: full-length FVIII contains 20 N-linked and at least seven O-linked glycans, while VWF contains 12 N-linked and 10 O-linked glycans. Three decades of research have revealed that the carbohydrate structures of FVIII and VWF contribute to many of the steps that can be distinguished in the life-cycle of these proteins, including biosynthesis/secretion, function and clearance. In this review, several of these aspects will be discussed. In addition, the interaction of the FVIII/VWF complex with two families of carbohydrate-binding proteins, i.e. Galectins and Siglecs, and their potential physiological relevance will be discussed.

Hydrogeochemistry and Water Quality Evaluation along the Flow Path in the Unconfined Aquifer of the Düzce Plain, North‐western Turkey

Abstract:The Düzce Plain has a multi‐aquifer system, which consists of a near surface unconfined aquifer, along with first and second deeper confined aquifers. Hydrochemical evolution and water quality are related to infiltration of the precipitation, recharge from the formations surrounding the plain, flow path of groundwater and the relationship between surface and groundwater. The groundwater in the unconfined aquifer flows towards the Efteni Lake and the Büyük Melen River. Surface waters are divided into two main hydrochemical facies in the study area: (a) Ca2+–HCO3−; and (b) Ca2+, Mg2+–HCO3, SO2−4. The groundwater has generally three main hydrochemical facies: (a) Ca2+–HCO−; (b) Ca2+, Mg2+–HCO−3; and (c) Ca2+, Mg2+–HCO−3, CI−. The hydrochemical facies “a” and “b” dominate within shallow depths in recharge areas under rapid flow conditions, while hydrochemical facies “c” characterizes shallow and mixed groundwater, which dominate intermediate or discharge areas (near Efteni Lake and Büyük Melen River) during low flow conditions and agricultural contamination. Calcium and bicarbonate ions, total hardness and electrical conductivity of total dissolved solids (EC‐TDS) values increase along the groundwater flow path; but these parameters remain within the limits specified by the standards set for industrial and agricultural usages.

Von Willebrand Factor, von Willebrand Factor-Cleaving Protease, and Shear Stress

von Willebrand factor (VWF) is a multimeric plasma glycoprotein (GP) involved in platelet adhesion at the site of vascular damage, which acts as a bridge between the injured subendothelium and the platelet receptors. The multimeric structure of VWF allows it to support multiple interactions with platelets and endothelial components under high shear stress. Rapid flow conditions induce a conformational transition of the VWF molecule, thus allowing its functional binding domains to be exposed. A specific VWF-cleaving protease (ADAMTS-13) physiologically down regulates the multimeric size of newly released and circulating VWF in order to prevent unwanted platelet thrombus formation. The occurrence of microvascular platelet aggregation in thrombotic thrombocytopenic purpura, which is caused by an ADAMTS-13 deficiency, well-demonstrates the important role of the protease in regulating the adhesive activity of VWF. Better knowledge of VWF function would contribute to the development of novel anti-thrombotic strategies based on the selective inhibition of the VWF interaction with platelet receptors and endothelial components in areas of the circulation characterised by elevated fluid dynamic forces.

Granular mixtures modeled as elastic hard spheres subject to a drag force

Granular gaseous mixtures under rapid flow conditions are usually modeled as a multicomponent system of smooth inelastic hard disks (two dimensions) or spheres (three dimensions) with constant coefficients of normal restitution alpha{ij}. In the low density regime an adequate framework is provided by the set of coupled inelastic Boltzmann equations. Due to the intricacy of the inelastic Boltzmann collision operator, in this paper we propose a simpler model of elastic hard disks or spheres subject to the action of an effective drag force, which mimics the effect of dissipation present in the original granular gas. For each collision term ij, the model has two parameters: a dimensionless factor beta{ij} modifying the collision rate of the elastic hard spheres, and the drag coefficient zeta{ij}. Both parameters are determined by requiring that the model reproduces the collisional transfers of momentum and energy of the true inelastic Boltzmann operator, yielding beta{ij}=(1+alpha{ij})2 and zeta{ij} proportional, variant1-alpha{ij}/{2}, where the proportionality constant is a function of the partial densities, velocities, and temperatures of species i and j. The Navier-Stokes transport coefficients for a binary mixture are obtained from the model by application of the Chapman-Enskog method. The three coefficients associated with the mass flux are the same as those obtained from the inelastic Boltzmann equation, while the remaining four transport coefficients show a general good agreement, especially in the case of the thermal conductivity. The discrepancies between both descriptions are seen to be similar to those found for monocomponent gases. Finally, the approximate decomposition of the inelastic Boltzmann collision operator is exploited to construct a model kinetic equation for granular mixtures as a direct extension of a known kinetic model for elastic collisions.

Chute Performance and Design for Rapid Flow Conditions

AbstractMany industrial chute applications are characterised by rapid flow conditions in which the bulk solid stream thickness or depth is less than the chute width. Under these conditions, it is possible to describe the stream flow by means of a lumped parameter model which takes into account the frictional drag around the chute boundaries as well as making allowance for inter‐particle friction. Equations of motion to describe the chute flow are presented and their application to the determination of chute profiles to achieve optimum flow is illustrated. By means of design examples, the problems associated with the feeding of bulk solids onto belt conveyors and conveyor transfers are discussed. Criteria for the selection of the most appropriate chute geometry to minimise chute wear and belt wear at the feed point are presented. The determination of optimum chute profiles to achieve specified performance criteria is outlined.

Solder wetting kinetics in narrow V-grooves

Experiments are performed to observe capillary flow in grooves cut into copper surfaces. Flow kinetics of two liquids, 1-heptanol and eutectic Sn Pb solder, are modeled with modified Washburn kinetics and compared to flow data. It is shown that both liquids flow parabolically in narrow V-grooves, and the data scale as predicted by the modified Washburn model. The early portions of the flow kinetics are characterized by curvature in the length vs time relationship which is not accounted for in the modified Washburn model. This effect is interpreted in terms of a dynamic contact angle. It is concluded that under conditions of rapid flow, solder spreading can be understood as a simple fluid flow process. Slower kinetics, e.g. solder droplet spreading on flat surfaces, may be affected by subsidiary chemical processes such as reaction.

Hydrochemical evolution and water quality along the groundwater flow path in the Sandıklı plain, Afyon, Turkey

An unconfined aquifer system suggests an open system in the study area. Hydrochemical evolution is related to the flow path of groundwater. The groundwaters are divided into two hydrochemical facies in the study area, 1) Ca–Mg–HCO3 and 2) Ca–Mg–SO4HCO3. Facies 1 has shallow (young) waters which dominate in recharge areas during rapid flow conditions, whereas facies 2 may show shallow and mixed waters which dominate intermediate or discharge areas during low flow conditions. Ionic concentrations, TDS, EC and water quality are related to groundwater residence time and groundwater types. The groundwaters in the plain are chemically potable and suitable for both domestic and agricultural purposes.

High-speed platelet adhesion under conditions of rapid flow.

The recognition of exposed collagen by circulating platelets is an initial step in the formation of the hemostatic plug or a thrombus after vascular injury. Theoretical calculations of the speed of platelet function required for effective hemostasis have suggested very short reaction times. However, it is not known how fast platelets can adhere to collagen under arterial flow conditions or which membrane proteins are involved. We have used a continuous-flow, microaffinity column linked to a resistive-particle counter to detect platelet adhesion. Adhesion of human platelets to native type I collagen was extremely rapid, with exponential half-times as short as 240 ms, and was nearly complete by 2 s. This RGD-independent process was not associated with platelet aggregation or secretion. The monoclonal antibody 6F1 directed against the glycoprotein Ia/IIa complex inhibited adhesion, suggesting that this complex plays an important role in the initial phases of platelet-collagen interaction under flow conditions. In addition, divalent cations were required for adhesion, as indicated by inhibition with EDTA in plasma and the dependence on Mg2+ for washed platelets.

Inorganic Calcite Morphology: Roles of Fluid Chemistry and Fluid Flow

ABSTRACT Evaluation of calcite crystal habits and fabric and their relation to precipitating fluids in spelean environments indicates that habit variations are controlled primarily by degree of supersaturation. At less than 6 times supersaturation, crystal habits are limited to those with shallow to steep rhombohedral forms (i.e., {1011}, {4041}). With increasing supersaturation, scalenohedrons, extremely steep rhombohedrons, pinacoids and hexagonal prisms are developed. Slight elongation due to differences in crystal dimensions of these forms may arise (i.e., scalenohedrons versus shallow rhombohedrons). At extreme supersaturation (> 12 times), curvature of crystal faces or curved surfaces appear. The development of fibrous versus equant fabrics is the result of changes or modification of reactant supply rates and the number of crystallites (nuclei) present. Under rapid flow conditions, growth rates are accelerated and greater numbers of nuclei are present; thus, compromise boundaries parallel to c-axis and perpendicular to flow (and substrate) are rapidly attained. In the absence of flow, fewer crystallites are present, and larger, well developed crystals or branching forms dictated by saturation conditions arise. In the absence of flow at low supersaturation, elongate composite crystals, and hence fibrous fabrics, may result from migration of the fluid-crystal interface as a result of changing or oscillating water levels. A flow control mechanism can be applied to numerous geological conditions in which transitions from fibrous to equant crystal morphologies are observed.

Effects of Metals and Metal Oxides on the Leaching of Nuclear Waste Glasses

ABSTRACTThe effects of the presence of various metals and metal oxides on the leaching of nuclear waste glasses in water can be considerable even under rapid flow conditions. The systems studied here include SRL TDS-131 glass in the presence of lead and stainless steel, and PNL 76-68 glass in the presence of aluminum, alumina and their mixtures. Lead specifically suppresses dissolved silica, but does not inhibit dissolution of defense waste glass as a whole. Stainless steel specifically reduces the levels of dissolved Ca and Sr. Al and Al2 O3 exercise a general reducing effect on the rates of dissolution of commercial waste glass and the presence of alumina may have a beneficial role in enhancing waste package durability.

Validation studies for the use of the microsphere method in cats and young minipigs.

Radioactive microspheres are suitable for measuring cardiac output, its distribution and organ blood flow if certain criteria are met. Cardiac output may be determined with the reference flow method if microspheres do not recirculate. In case 10 micron microspheres were extracted completely by the lungs. The use of microspheres for the determination of cardiac output is therefore not limited by recirculation of microspheres down to this size. Under baseline conditions we found no preferential streaming with 50 micron spheres in young minipigs and with 25 and 15 micron spheres in cats At high cardiac output values induced by the administration of 0.4 mg/kg of Dihydralazine in cats, 25 micron but not 15 micron microspheres showed a small, non significant tendency to stream preferentially into the upper body. Thus, under rapid flow conditions 15 micron microspheres may be preferable to larger ones from the rheological point of view. Under baseline conditions the distribution of cardiac output to a wide range of organs was very similar when comparing 25 and 15 micron microspheres in cats and 50 and 15 micron microspheres in minipigs, whereas 10 micron microspheres were poorly extracted by most organ in cats. 15 micron microspheres were found to be suitable for the determination of cardiac output and its distribution in both species provided that bronchial and arteriovenous shunt flows can be neglected.

Mechanism of Rapid Filtration in a Uniform Filter Bed

A radiotracer study has been carried out with cesium‐137 labeled vermiculite particles to study the pattern of removal in a deep sand filter under rapid flow conditions. Sized sand and anthracite were used as bed media, and suspended particles were sized between 1 and 120 microns. By comparing the activity ratios in the upper bed levels, curves were obtained relating the filter coefficient to particle size and bed pore sizes. By analyzing the curves in relation to the various theories proposed to account for filter action, it was shown that interstitial sieving was the dominant mechanism down to a nominal pore‐particle ratio of 4. Below this ratio, van der Waals forces were shown to be adequate to explain residual removal effects, and the effective range of these forces was calculated.

Fall in pressure in radial artery during reactive hyperemia.

Under conditions of rapid flow, the arterial bed between the aorta and radial artery offers an appreciable resistance to flow. During reactive hyperemia in normal subjects, approximately 30 per cent of the energy of pressure may be lost between the aorta and radial artery. Hypertension produced in normal subjects by infusion of levarterenol increases the gradient in pressure during reactive hyperemia between the aorta and radial artery. Hypertension in patients with hypertensive disease does not increase the gradient during reactive hyperemia beyond that which is produced in normal subjects.