The effects of grain geometry on waterflooding and viscous fingering in micro-fractures and porous media from a lattice Boltzmann method study

The study of fluid fronts formed in porous media is important for enhanced oil recovery. The purposes of this study are to simulate waterflooding and to investigate influential factors on the fluid front movement through a micro-fracture and through simple porous media with different grain geometries. This study used the Shan–Chen form of the Lattice Boltzmann Method (LBM). An increase in the velocity is found to result in viscous fingering, whereas an increase in the wettability of the displacing fluid and the dynamic viscosity ratio creates a piston form of the fluid front. In porous media with the same porosities, various geometries act differently as obstacles against fluid flow from the inlet to the outlet. By enlarging the cross-sectional area of grains in the fluid paths and making them more tortuous, narrower and more twisted films of viscous fingering are formed. The sweep efficiency was also determined under various conditions: with a fixed capillary number, neutral wettability and different viscosity ratios; and with a fixed capillary number, viscosity ratio of (1/3) and wet or non-wet conditions. In all cases, the best sweep efficiency was obtained with grains of diamond geometry. Generally, the least sweep efficiency occurs with grains of star geometry. Simulation results verified the strength and accuracy of LBM predictions.     

Pore Direction in Relation to Anisotropy of Mechanical Strength in a Cubic Starch Compact

The purpose of this research was to evaluate the relation between preferential direction of pores and mechanical strength of cubic starch compacts. The preferential pore direction was quantified in SEM images of cross sections of starch compacts using a previously described algorithm for determination of the quotient of transitions (Q). This parameter and the mechanical strength were evaluated in compacts of different porosities. Starch was chosen as a model compound for materials with ductile behaviour of which tablets with low porosities can be made and which shows some elastic recovery after compaction. At medium and high porosity Q was significantly higher in the images providing a side view of the compact than in the images providing a top view (0.973 vs. 0.927 and 0.958 vs. 0.874 at 0 mm from the side of the compact and 0.956 vs. 0.854 and 0.951 vs. 0.862 at 3.5 mm), indicating that the pores were mainly oriented in the direction perpendicular to the direction of compression. This was accompanied by a lower crushing force in this direction. This could be explained by considering the pores as cracks which propagate through the sample during crushing. For both directions the crushing force decreased with increasing porosity. The yield strength of the compacts also decreased with increasing porosity, but this parameter was not dependent on the direction of crushing when the porosity was below 10%. The results show that pore direction significantly influences the crushing force but does not influence the yield strength, at porosities below 10%.     

The mechanisms of the vulnerable window: the role of virtual electrodes and shock polarity

Vulnerability and defibrillation are mechanistically dependent upon shock strength, polarity, and timing. We have recently demonstrated that shock-induced virtual electrode polarization (VEP) may induce reentry. However, it remains unclear how the VEP mechanism may explain the vulnerable window and polarity dependence of vulnerability. We used a potentiometric dye and optical mapping to assess the anterior epicardial electrical activity of Langendorff-perfused rabbit hearts (n = 7) during monophasic shocks (+/-100 V and +/-200 V, duration of 8 ms) applied from a transvenous defibrillation lead at various coupling intervals. Arrhythmias were induced in a coupling interval and shock polarity dependent manner: (i) anodal and cathodal shocks induced arrhythmias in 33.2 +/- 30.1% and 53.1 +/- 39.3% cases (P < 0.01), respectively, and (ii) the vulnerable window was located near the T-wave. Optical maps revealed that VEP was also modulated by the coupling interval and shock polarity. Recovery of excitability produced by negative polarization, known as de-excitation, and the resulting reentry was more readily achieved during the relative refractory period than the absolute refractory period. Furthermore, anodal shocks produced wavefronts propagating in an inward direction with respect to the electrode, whereas cathodal shocks propagated in an outward direction. Wavefronts produced by anodal shocks were more likely to collide and annihilate each other than those caused by cathodal shocks. The probability of degeneration of the VEP-induced phase singularity into a sustained arrhythmia depends upon the gradient of VEP and the direction of the VEP-induced wavefront. The VEP gradient depends upon the coupling interval, while the direction depends upon shock polarity; these factors explain the vulnerable window and polarity-dependence of vulnerability, respectively.     

Microvasculature of the feline stomach

The feline gastric microvasculature was studied by corrosion of the injected vascular bed, which allowed evaluation of the vascular pattern of the different tunics. The serosal pattern consisted of numerous interconnected capillary vessels, forming a delicate network. Submucosal arteries supplied the muscular tunic through numerous anastomosing vessels that followed the direction of the smooth muscle fibers. The entire mucosal tunic was supplied by arterioles derived from the submucosal plexus; these gave rise to capillaries that surrounded the gastric glands and terminated in a diffuse, anastomosing subepithelial capillary network. Venules coursed through the mucosa in a perpendicular manner, forming submucosal veins.     

亚热带不同树种土壤水源涵养功能

亚热带地区由于大面积的砍伐使天然林被人工林所代替,对森林土壤水源涵养功能造成了很大影响。树种可以通过自身特性来改变土壤物理结构进而影响土壤持水能力,因此合理选择树种对区域水源涵养具有重要意义。然而,立地条件的差异往往会对实验结果产生影响。为减少立地条件的差异,2012年2月在土壤发育和经营历史相同的林地上建立了中亚热带常见树种同质园。2019年8月测定了种植12个树种后不同土层(0—10、10—20、20—30、30—40 cm和40—50 cm)的土壤容重、含水量、总/毛管/非毛管孔隙度、最大/毛管/非毛管持水量和蓄水量。结果表明,种植不同树种7年后,土壤容重、含水量、总/毛管/非毛管孔隙度、最大/毛管/非毛管持水量和蓄水量均在表层(0—20 cm)土壤中差异显著,而在深层(20—50 cm)土壤中差异不显著。土壤孔隙度、持水量和蓄水量均与土壤容重呈显著负相关关系,而与土壤含水量呈显著正相关关系。与其他树种相比,种植鹅掌楸、枫香和全缘叶栾树等落叶阔叶树种可在较短时间内增加土壤孔隙度,提高土壤持水量和蓄水量。因此,在亚热带人工林经营管理中,可在杉木、马尾松纯林中适当引入鹅掌楸、枫香和全缘叶栾树等落叶阔叶树种,提升亚热带森林土壤水源涵养功能。     

The entrainment of sediments by the turbulent flow of water

This paper attempts to clarify the role of fluid turbulence in the entrainment process of particles from the flat bed of an alluvial channel. A 6 mm diameter sphere, placed in an array of similar particles on the bed of a laboratory flume, was connected to a 3-component force transducer. Dynamic measurements of longitudinal, lift and lateral force components were made with the particle at a number of positions above the bed. Detailed measurements of velocity and turbulence were also made. The velocity and force spectral measurements showed similarities in the longitudinal (streamwise) direction but the lift force component spectra were significantly different.An analytical model of the entrainment process is proposed, in which the impulse required to lift a sediment particle from the channel bed may be calculated. The experimental data has been used to evaluate the constants in the analytical expressions, and a relationship between the forces and turbulence intensity established. The analysis shows that the impulse required to entrain a particle may be expressed as a function of a dimensionless entrainment parameter, the turbulence intensity, and the initial angle of repose of the particle.     

Mechanisms of superiority of ascending ramp waveforms: new insights into mechanisms of shock-induced vulnerability and defibrillation

Monophasic ascending ramp (AR) and descending ramp (DR) waveforms are known to have significantly different defibrillation thresholds. We hypothesized that this difference arises due to differences in mechanisms of arrhythmia induction for the two waveforms. Rabbit hearts (n = 10) were Langendorff perfused, and AR and DR waveforms (7, 20, and 40 ms) were randomly delivered from two line electrodes placed 10 mm apart on the anterior ventricular epicardium. We optically mapped cellular responses to shocks of various strengths (5, 10, and 20 V/cm) and coupling intervals (CIs; 120, 180, and 300 ms). Optical mapping revealed that maximum virtual electrode polarization (VEP) was reached at significantly different times for AR and DR of the same duration (P < 0.05) for all tested CIs. As a result, VEP for AR were stronger than for DR at the end of the shock. Postshock break excitation resulting from AR generated faster propagation and typically could not form reentry. In contrast, partially dissipated VEP resulting from DR generated slower propagation; the wavefront was able to propagate into deexcited tissue and thus formed a shock-induced reentry circuit. Therefore, for the same delivered energy, AR was less proarrhythmic compared with DR. An active bidomain model was used to confirm the electrophysiological results. The VEP hypothesis explains differences in vulnerability associated with monophasic AR and DR waveforms and, by extension, the superior defibrillation efficacy of the AR waveform compared with the DR waveform.     

The capillary system of the rat heart in occlusion of the coronary artery

In 74 white rats by means of non-injection++ method morphofunctional state of the myocardial capillary bed has been studied in dynamics at occlusion of descending branch of the left coronary artery. During the first week after the operation blood supply of the areas, adjoining the necrosis, increases at the expense of dilatation and some increase in number of functioning capillaries, that results in enlargement of the exchanging surface and capacity of the capillary bed. Beginning from the 12th day, the value of all these parameters decreases, however, they do not reach their initial level. By the end of the experiment (45 days) the number of the functioning capillaries somewhat decreases, but the capillary diameters remain increased. By that time in the myocardial areas, adjoining the necrosis a parviansiform capillary network without a definite orientation, concerning muscle fibers, has been formed.     

An electrostatic model of a membrane ion pump

The model is based upon an ion channel with an electric dipolar structure. With simplifying assumptions it is possible to calculate that a typical channel, 1 nm in diameter and 5 nm long, could contain at most two or three univalent cations at a time. The channel ion binding sites have an effective affinity for ions from the fluid bathing the negative end of the channel, several orders of magnitude higher than their affinity for ions from the fluid bathing the positive end of the channel. The approach of an external, positively charged body to the negative end of the channel, is sufficient to convert the two- or three-channel ion sites with high affinity for ions from the fluid bathing this end into very low affinity sites for the same ions that now have access only to the fluid bathing the other end of the channel. The change in affinity and fluid access requires no molecular or electrical change in the channel structure other than the passive superposition of the electrostatic potential of the dipolar channel and that of the charged body. An oscillating electric field externally applied to an electric dipolar channel is shown to result in the unidirectional pumping of cations in the direction of the channel dipole even against large adverse ion concentration gradients. The energy required must be supplied by the sources of the electric field. By using two such channels in close proximity, one selective for K+ ions with its dipole moment pointing into a cell and the other selective for Na+ ions with its dipole moment pointing out from the cell, it is possible to construct a model pump with calculated properties that simulate many of those measured for Na+-K+-ATPase, with both physiological and artificial ionic concentrations.     

Continuous beds (monoliths): stationary phases for liquid chromatography formed using the hydrophobic interaction-based phase separation mechanism

The pioneering research work published by Hjertén et al. [J. Chromatogr. 473 (1989) 273] in 1989 dealing with development and application of the continuous bed (monolithic) technique as an attractive alternative for the classical packed columns in chromatography, stimulated further investigations in this direction. The research data published since that time on the development and application of the continuous beds formed using hydrophobic interaction-based phase separation mechanism are reviewed. Some innovative species of the beds, such as polyrotaxane beds or nonparticulate restricted-access materials for direct analysis of the biological fluids in the capillary format are also discussed. Characteristic features and practical details of the continuous bed technique are revealed. Due to many advantages, the continuous bed technique became a competitor with the traditional packings in capillary or chip-based microanalysis. The importance of the continuous bed morphology on the chromatographic characteristics is shown. The applicability of modern microscopic analysis to evaluate the morphology of the continuous beds is demonstrated.     

The brain Renin-angiotensin system controls divergent efferent mechanisms to regulate fluid and energy balance

The renin-angiotensin system (RAS), in addition to its endocrine functions, plays a role within individual tissues such as the brain. The brain RAS is thought to control blood pressure through effects on fluid intake, vasopressin release, and sympathetic nerve activity (SNA), and may regulate metabolism through mechanisms which remain undefined. We used a double-transgenic mouse model that exhibits brain-specific RAS activity to examine mechanisms contributing to fluid and energy homeostasis. The mice exhibit high fluid turnover through increased adrenal steroids, which is corrected by adrenalectomy and attenuated by mineralocorticoid receptor blockade. They are also hyperphagic but lean because of a marked increase in body temperature and metabolic rate, mediated by increased SNA and suppression of the circulating RAS. β-adrenergic blockade or restoration of circulating angiotensin-II, but not adrenalectomy, normalized metabolic rate. Our data point to contrasting mechanisms by which the brain RAS regulates fluid intake and energy expenditure.     

Screen-less expanded bed column: new approach for the recovery and purification of a malaria transmission blocking vaccine candidate from Pichia pastoris

An experimental malaria transmission blocking vaccine antigen, Pfs25H, expressed and secreted from Pichia pastoris was recovered and purified using a screenless expanded bed column equipped with a rotating fluid distribution system. This column was able to accommodate feed stock, containing 30% biomass, at a flow rate of 300–400 cm/h without affecting column stability. This capability is three times higher than the capability of the expanded bed column currently in use, which is equipped with a perforated plate fluid distribution system; this design could accommodate biomass concentrations of only up to 10%. The screen-less design did not affect the binding capacity, purification level or process yield and, therefore, shorten the process. Purified Pfs25H of 6.4 g were recovered from 37 l of Pichia pastoris culture in one step.     

Degradation of supercoiled plasmid DNA within a capillary device

Supercoiled plasmid DNA is susceptible to fluid stress in large-scale manufacturing processes. A capillary device was used to generate controlled shear conditions and the effects of different stresses on plasmid DNA structure were investigated. Computational fluid dynamics (CFD) analysis was employed to characterize the flow environment in the capillary device and different analytical techniques were used to quantify the DNA breakage. It was found that the degradation of plasmid DNA occurred at the entrance of the capillary and that the shear stress within the capillary did not affect the DNA structure. The degradation rate of plasmids was well correlated with the average elongational strain rate or the pressure drop at the entrance region. The conclusion may also be drawn that laminar shear stress does not play a significant role in plasmid DNA degradation.     

On the temperature-dependency of optimal nectar concentrations for birds.

In addition to the crucial need for energy balance, the metabolically-intense hummingbirds must maintain osmotic homeostasis by regulating salt and fluid balance. Hypothetically, flowers which have coevolved with pollination by hummingbirds could provide both energy and water balance simultaneously if they produced nectars of appropriate concentrations which depend upon environmental temperature. To the extent that this has not completely adjusted, hummingbirds exposed to cold and hot extremes in temperature will have problems of water excess or deficiency, respectively.     

The water-salt balance and renal function in space flights and in model experiments

Study of a condition of mineral and water-electrolyte metabolism, function of kidneys, and their hormonal regulation during model experiments (hypokinesia, bed rest, immersion etc.), and also in space flights and in readaptation period, has shown a major role of water-electrolyte homeostasis during general adaptation of humans and animals to new conditions of life and to conditions of weightlessness in particular. The change in regulation of volumes of fluid milieu in an initial period of weightlessness was shown to be the consequence of redistribution of blood and hemodynamics of the shifts resulting in change of production of volume-regulation hormones, formation of negative water balance, and redistribution of fluid in the organism among various fluid compartments. At later stages of flight or long-term hypokinesia, a change of water-electrolyte homeostasis occurs with a decrease in the kidneys excretion of sodium, and diuresis, but with an increased excretion of calcium and production of ADH and RAAS hormones. Following returning to earth gravitation, the majority of astronauts have adaptive reactions, compensating for the loss extracellular fluid and mineral substances and formation of "earth" water-electrolyte homeostasis. For estimation of water-electrolyte homeostasis and the functions of kidneys in astronauts, various functional loading tests have been developed. The developed system of preventive maintenance is successfully used for abolition of adverse changes at various stages of space flight and in readaptation period.     

Mechanistic interpretation of the influence of lipid phase transitions on transport functions.

In an attempt to understand the mechanism by which a structural change of membrane lipids affects transport functions, the temperature dependence of transport rate has been measured to below the low temperature end of the fluid in equilibrium ordered phase transition of the membrane lipids. The unsaturated fatty acid requiring Escherichia coli strain T105 was supplemented with either trans-delta9-octadecenoate or trans-delta9-hexadecenoate or supplemented with and subsequently starved for cis-delta9-octadecenoate. Fluid in equilibrium ordered phase transitions measured in whole cells using the fluorescence probe N-phenyl-1-naphthylamine were compared with the temperature dependence of beta-glucoside and beta-galactoside transport. In addition to the previously observed downward "break" in the Arrhenius plot of transport rate which occurred near the middle of the phase transition temperature range, a second upward "break" was observed which could be correlated with the low-temperature end of the phase transition. These experiments are interpreted in terms of a partitioning of transport proteins between ordered and fluid domains which is described by a lateral distribution coefficient, k. This distribution coefficient varies with the membrane lipid composition as well as with the transport system. Values for k suggest a 2-20-fold preference for the partitioning of transport proteins into the fluid parts of the membrane.     

Tissue pressures and fluid dynamics in the kidney.

The kidney has several characteristics which make renal pressures and fluid dynamics unique when compared to other organs. Renal blood flow is roughly 100 times that of skeletal muscle. The renal circulation consists of two distinct capillary beds in series: a high pressure system in the glomerulus that favors filtration and a low pressure system in the peritubule network that favors reabsorption. The hydrostatic pressure in the glomerular capillary is 4-6 times higher than the hydrostatic pressure in the peritubule capillary so that approximately 25% of the plasma is filtered. The bulk of the filtrate is subsequently reabsorbed by the peritubule capillary network. Micropuncture techniques have been used to obtain quantitative measurements of the pressures and fluid dynamics of the peritubule microcirculation. The net force for uptake of all the fluid reabsorbed by a single proximal tubule up to the point of micropuncture is 21 mm Hg acting over a capillary bed with a permeability surface area product of 2 nl/min per mm Hg. In contrast to subcutaneous tissue and muscle, the renal interstitial fluid pressure is positive. The consequence of a positive interstitial fluid pressure is that normal lymph flow is relatively high and changes in interstitial fluid pressure have relatively little effects on lymph flow.     

Movement of a spherical cell in capillaries using a boundary element method

This study aims to investigate the translation and rotation of a spherical particle in capillaries and overcomes limitations in previous studies by using a boundary element method. The capillary, a straight cylindrical tube, is filled with a Newtonian viscous fluid. A spherical particle is arbitrarily positioned in the capillary either co-centrically or eccentrically and is free to translate and rotate. Flow in the capillary is first assumed to be caused solely by the movement of the sphere under the gravity. When a steady state is reached, the net force and torque on the sphere are zero. The translating velocity and rotation of the particle are calculated from equilibrium equations. For a co-centric sphere, our result agrees to Bohlin's analytical solution (Bohlin, 1960) and the difference is less than 1%. For an eccentrically positioned sphere in the tube, there are no analytical solutions unless the eccentricity is infinitesimal. Results by boundary element method (BEM) give an improved estimations on the velocity and rotation of the sphere than earlier results by a boundary singularity method (BSM), particularly when the clearance between the tube and the sphere becomes small. Movement of a spherical particle in a capillary driven by a pressure gradient is further investigated, which has closer relevance to movement of blood cells in capillaries. The current study using BEM enables investigation on cell movement in close proximities of the capillary wall.