Tortuous Channels Research Articles

A stepped leader model for lightning including charge distribution in branched channels

The stepped leader process in negative cloud-to-ground lightning plays a vital role in lightning protection analysis. As lightning discharge usually presents significant branched or tortuous channels, the charge distribution along the branched channels and the stochastic feature of stepped leader propagation were investigated in this paper. The charge density along the leader channel and the charge in the leader tip for each lightning branch were approximated by introducing branch correlation coefficients. In combination with geometric characteristics of natural lightning discharge, a stochastic stepped leader propagation model was presented based on the fractal theory. By comparing simulation results with the statistics of natural lightning discharges, it was found that the fractal dimension of lightning trajectory in simulation was in the range of that observed in nature and the calculation results of electric field at ground level were in good agreement with the measurements of a negative flash, which shows the validity of this proposed model. Furthermore, a new equation to estimate the lightning striking distance to flat ground was suggested based on the present model. The striking distance obtained by this new equation is smaller than the value estimated by previous equations, which indicates that the traditional equations may somewhat overestimate the attractive effect of the ground.

Management of renal arteriovenous malformations: A pictorial review.

BackgroundArteriovenous malformations (AVMs) are communications between an artery and a vein outside the capillary level. This pathologic communication may be either a fistula, a simple communication between a single artery and a dilated vein, or a more complex communication, a nidus of tortuous channels between one or more arteries/arterioles and one or more draining veins. The latter type of lesion is most frequently seen in the extremities; in the kidney they tend to appear more rarely. The most common clinical presentation of renal arteriovenous malformations (RAVMs) is haematuria. Percutaneous treatment with selective endovascular techniques offers a minimally invasive, nephron-sparing option in comparison to the more invasive surgical approaches. The purpose of this pictorial review is to highlight the general lines of management and to show the range of imaging findings of the percutaneous treatment of RAVMs.MethodsThe imaging characteristics of a selection of cases of percutaneously managed congenital RAVMs are presented and the most common lines of approach are discussed.ConclusionThe imaging spectrum of diagnosis and percutaneous treatment of RAVMs is presented in order to aid interpretation and endovascular management.Teaching points• Renal arteriovenous malformations are very rare lesions.• Clinical expression is usually haematuria.• Diagnosis is made with CT or MRI but the gold standard is digital subtraction angiography.• Catheter-directed treatment with the use of coils or liquid embolics is minimally invasive, safe and effective.

Intestinal lymphangiectasia and reversible high liver stiffness

Primary intestinal lymphangiectasia (PIL) is a protein-losing enteropathy characterized by tortuous and dilated lymph channels of the small bowel. The main symptoms are bilateral lower limb edema, serosal effusions, and vitamin D malabsorption resulting in osteoporosis. We report here a case of long-lasting misdiagnosed PIL with a peculiar liver picture, characterized by a very high stiffness value at transient elastography, which decreased with clinical improvement. The complex interplay between lymphatic and hepatic circulatory system is discussed.

Transient laminar heat transfer simulations in periodic zigzag channels

At sufficiently high Reynolds numbers, laminar flows in tortuous channels become unsteady. A computational fluid dynamics methodology is developed to study transient, laminar flow and heat transfer in a periodic zigzag channel with a semi-circular cross-section. The computational domain consists of seven repeating zigzag units with smoothly joined inlet and outlet sections. Reynolds numbers ranging from 400 to 800 and Prandtl numbers ranging from 0.7 to 20 are examined for constant wall heat flux and constant temperature thermal boundary conditions. Simulation results show that the flow reaches a “developed” state after around three units, where the local velocities fluctuate with time but give well defined average heat transfer rates and pressure loss. The power spectra of the velocity at monitor points located periodically along the channel also become very similar. Significant heat transfer enhancement is observed in the transient regime studied, which is accompanied by a modest pressure-drop penalty, both of which increase with increasing Reynolds number. Vortex structures are visualized at different simulation times and Reynolds numbers and it is found that with increasing Reynolds number, vortices with smaller length-scale are generated, which contribute largely to the enhancement of heat transfer. The results for different Prandtl numbers show that the heat transfer enhancement is proportional to Pr1/3. The Nusselt number for the T boundary condition is found to be always higher than that for the H2 boundary condition.

A two-staged successful transcatheter embolization for multiple pulmonary arteriovenous fistulas

Pulmonary arteriovenous fistulas (PAVF) are abnormal connections between pulmonary arteries and veins through aneurysmal sac or dilated tortuous channels without capillary bed. The clinical presentation includes exercise intolerance and hypoxemia in childhood. Nowadays, transcatheter closure methods using coils, balloons and several devices have become widely accepted treatment modalities, rather than surgical intervention, in childhood. The Amplatzer ® vascular plug is frequently used in the management of various pathologies of both peripheral and central vascular structures. In this article, we report a three-year-old boy who underwent a successful two-stage closure of multiple PAVF using an Amplatzer vascular plug (I and II) device.

The Viscosity of Silica Nanoparticle Dispersions in Permeable Media

Summary The potential application of nanoparticle dispersions as formation-stimulation agents, contrast agents, or simply as tracers in the upstream oil and gas industry requires knowledge of the flow properties of these nanoparticles. The modeling of nanoparticle transport in hydrocarbon reservoirs requires a comprehensive understanding of the rheological behavior of these nanofluids. Silica nanoparticles have been commonly used because of their low-cost fabrication and cost-effective surface modification. The aqueous silica-nanoparticle dispersions show Newtonian behavior under steady shear measurements controlled by a rheometer, as discussed by Metin et al. (2011b). The viscosity of nanoparticle dispersions depends strongly on the particle concentration, and that this correlation can be depicted by a unified rheological model (Metin et al. 2011b). In addition, during flow in permeable media, the variation of shear associated with complex pore morphology and the interactions between the nanoparticles and tortuous flow channels can affect the viscosity of nanoparticle dispersion. The latter is particularly important if the concentration of nanoparticles in dispersion may change because of nanoparticle adsorption on mineral/fluid and oil/water interfaces or by mechanical trapping of nanoparticles. In this paper, the flow of silica-nanoparticle dispersions through different permeable media is investigated. The rheological behaviors of the dispersions are compared with those determined by use of a rheometer. We established a correlation between the nanoparticle concentration and dispersion viscosity in porous media for various nanoparticle sizes. The effects of pore structure and shear rate are also studied. We have confirmed that the concept of effective maximum packing fraction can be applied to describe the viscosity of aqueous nanoparticle dispersions in both bulk flow and flow in porous media with high permeability and regular pore structures, but not at low permeability because of mechanical trapping. Our work provides new insight to engineering nanoparticle rheology for subsurface applications.

Theoretical description on the interface-enhanced conductivity of SDC/LiNa-carbonate composite electrolytes

SDC (Ce0.8Sm0.2O1.9)/LiNa-carbonates composite electrolytes for intermediate temperature SOFCs have been prepared by infiltrating molten LiNaCO3 into the pre-sintered porous SDC pellets with distinctive porosity structures. XRD, SEM and Mercury Porosimetry were employed to structurally characterize the SDC pellets and the composite electrolytes, while AC impedance spectroscopy was used to measure their ionic conductivities. It has been found that the conductivities of SDC-carbonate composites are strongly affected by the porosity structure parameters of SDC pellets including the tortuosity of pore channels and their specific interface area, depending on the dimensions of SDC grains and the built-up pores. According to the ideas of charge transport in tortuous pore channels, a theoretical description on the interface-enhanced conductivities of SDC-carbonates composites has been achieved. It agrees well with the experimental data and reveals that the conductivity of composite electrolytes may be linearly increased with the specific SDC-carbonate interface area, but inversely proportional to the square of tortuosity as well.

Experimental Evidence for Self-Limiting Reactive Flow through a Fractured Cement Core: Implications for Time-Dependent Wellbore Leakage

We present a set of reactive transport experiments in cement fractures. The experiments simulate coupling between flow and reaction when acidic, CO(2)-rich fluids flow along a leaky wellbore. An analog dilute acid with a pH between 2.0 and 3.15 was injected at constant rate between 0.3 and 9.4 cm/s into a fractured cement core. Pressure differential across the core and effluent pH were measured to track flow path evolution, which was analyzed with electron microscopy after injection. In many experiments reaction was restricted within relatively narrow, tortuous channels along the fracture surface. The observations are consistent with coupling between flow and dissolution/precipitation. Injected acid reacts along the fracture surface to leach calcium from cement phases. Ahead of the reaction front, high pH pore fluid mixes with calcium-rich water and induces mineral precipitation. Increases in the pressure differential for most experiments indicate that precipitation can be sufficient to restrict flow. Experimental data from this study combined with published field evidence for mineral precipitation along cemented annuli suggests that leakage of CO(2)-rich fluids along a wellbore may seal the leakage pathway if the initial aperture is small and residence time allows mobilization and precipitation of minerals along the fracture.

Retrograde percutaneous recanalization of chronic total occlusion of the left anterior descending artery through an extremely tortuous right ventricular branch

We report a case of chronic total occlusion (CTO) of the left anterior descending artery, which was treated by percutaneous coronary intervention using a retrograde approach via an extremely tortuous right ventricular branch of the right coronary artery. New concept guidewires and a channel dilator were advanced by delicate manipulation into the distal site of the CTO and a successful percutaneous recanalization was performed. These new concept guidewires may facilitate the retrograde approach for CTOs via tortuous collateral channels.

Disruption of the Retinal Parafoveal Capillary Network in Type 2 Diabetes before the Onset of Diabetic Retinopathy

To establish, using adaptive optics scanning laser ophthalmoscopy (AOSLO), that the retinal parafoveal capillary network is altered before the onset of diabetic retinopathy in adult patients with type 2 diabetes. AOSLO videos were acquired in the parafoveal region of one eye from control subjects and from patients with type 2 diabetes and no retinopathy. Detailed images of the parafoveal capillary network were generated with custom motion contrast enhancement algorithms. The combination of AOSLO images and videos enabled the simultaneous assessment of several features of the parafoveal capillary network. Arteriovenous (AV) channels were identified by finding the least tortuous capillary channels connecting terminal arterioles to postcapillary venules. Measures of capillary dropout and capillary hemodynamics were also quantified. The average tortuosity of AV channels was 26% higher in patients with type 2 diabetes when compared with controls, even though there were no signs of diabetic retinopathy in any of the eyes that were assessed (P < 0.05). In addition, the metrics of capillary dropout showed small changes (between 3% and 7%), leukocyte speed 14% lower, and pulsatility 25% higher, but none of these differences was statistically significant. It is often difficult to find consistent changes in the retinal microvasculature due to large intersubject variability. However, with a novel application of AOSLO imaging, it is possible to visualize parafoveal capillaries and identify AV channels noninvasively. AV channels are disrupted in type 2 diabetes, even before the onset of diabetic retinopathy.

Direct pore-level observation of permeability increase in two-phase flow by shaking

[1] Increases in permeability of natural reservoirs and aquifers by passing seismic waves have been well documented. If the physical causes of this phenomenon can be understood, technological applications would be possible for controlling the flow in hydrologic systems or enhancing production from oil reservoirs. The explanation of the dynamically increased mobility of underground fluids must lie at the pore level. The natural fluids can be viewed as two-phase systems, composed of water as the wetting phase and of dispersed non-wetting globules of gas or organic fluids, flowing through tortuous constricted channels. Capillary forces prevent free motion of the suspended non-wetting droplets, which tend to become immobilized in capillary constrictions. The capillary entrapment significantly reduces macroscopic permeability. In a controlled experiment with a constricted capillary channel, we immobilize the suspended ganglia and test the model of capillary entrapment: it agrees precisely with the experiment. We then demonstrate by direct optical pore-level observation that the vibrations applied to the wall of the channel liberate the trapped ganglia if a predictable critical acceleration is reached. When the droplet begins to progressively advance, the permeability is restored. The mobilizing acceleration in the elastic wave, needed to “unplug” an immobile flow, is theoretically calculated within a factor of 1–5 of the experimental value. Overcoming the capillary entrapment in porous channels is hypothesized to be one of the principal pore-scale mechanisms by which natural permeabilities are enhanced by the passage of elastic waves.

Pore-scale mass and reactant transport in multiphase porous media flows

AbstractReactive processes associated with multiphase flows play a significant role in mass transport in unsaturated porous media. For example, the effect of reactions on the solid matrix can affect the formation and stability of fingering instabilities associated with the invasion of a buoyant non-wetting fluid. In this study, we focus on the formation and stability of capillary channels of a buoyant non-wetting fluid (developed because of capillary instabilities) and their impact on the transport and distribution of a reactant in the porous medium. We use a combination of pore-scale numerical calculations based on a multiphase reactive lattice Boltzmann model (LBM) and scaling laws to quantify (i) the effect of dissolution on the preservation of capillary instabilities, (ii) the penetration depth of reaction beyond the dissolution/melting front, and (iii) the temporal and spatial distribution of dissolution/melting under different conditions (concentration of reactant in the non-wetting fluid, injection rate). Our results show that, even for tortuous non-wetting fluid channels, simple scaling laws assuming an axisymmetrical annular flow can explain (i) the exponential decay of reactant along capillary channels, (ii) the dependence of the penetration depth of reactant on a local Péclet number (using the non-wetting fluid velocity in the channel) and more qualitatively (iii) the importance of the melting/reaction efficiency on the stability of non-wetting fluid channels. Our numerical method allows us to study the feedbacks between the immiscible multiphase fluid flow and a dynamically evolving porous matrix (dissolution or melting) which is an essential component of reactive transport in porous media.

Polymeric membranes conditioning for sensors applications: mechanism and influence on analytes detection

In this work, we studied an ion-exchange membrane based on an inert polymer skeleton in which it is dispersed and anchored a molecule with charged groups able to discriminate and bind positive or negatively charged ions present in a sample. In order to be ready to work, electromembranes need a complex procedure called activation or conditioning. Although most of the known literature looks at the subject from an electrochemical point of view, we put forward a structural approach. Membrane conditioning, in fact, is considered a required step to improve sensor performances and to allow the collection of reproducible data. Even if this operation is carefully followed by all the operators working with sensors equipped with a membrane, it looks like that a thoroughly explanation of the working mechanism and a detailed balance of cost and gains has still not been carried out. As a consequence, we suggest a bulk or membrane approach, where the landscape is mainly characterized by the long-range structure of the membrane itself. Our findings suggest that membrane conditioning has to be carried out carefully and the advantages of this pre-treatment can be appreciated especially for very low concentration measurements. The need for the conditioning mainly results from the necessity of a complete permeation of all the different tortuous channels constituting the membrane itself.

A Case of Pulmonary Lymphangiomatosis

Pulmonary lymphangiomatosis is a rare disorder involving the entire intrathoracic lymphatic system from the mediastinum to the pleura. Pulmonary lymphangiomatosis mostly occurs in children and young adults without gender predilection. Although it is pathologically benign, it shows a progressive and fatal course with variable initial presentation. We now report a case of pulmonary lymphangiomatosis in a 35-year-old man. He presented with hemoptysis 6 months previously. Chest x-ray and a chest computed tomography scan showed diffuse interstitial thickening with left pleural effusion. Chylothorax was confirmed by thoracentesis. Lymphangiography showed dilated and tortuous lymphatic channels. Surgical lung biopsy revealed proliferation of complex anastomosing lymphatic channels. He was diagnosed with pulmonary lymophangiomatosis. Closed thoracostomy and chemical pleurodesis were done and the dyspnea was reduced.

Structure-reactivity relationship for aromatics transalkylation and isomerization process with TNU-9, MCM-22 and ZSM-5 zeolites, and their industrial implications

TNU-9 is a medium pore zeolite with a complex tridimensional channel system. Its catalytic properties have been studied in some reactions that involve the BTX fraction, such as benzene and toluene alkylation with methanol, ethanol or isopropanol. These reactions use in practice medium pore zeolites such as ZSM-5 (MFI) or MCM-22 (MWW), and the selectivities obtained with TNU-9 are compared and analyzed from the point of view of the zeolite structure and pore topology, as well as from its possible industrial application. For benzene alkylation to give ethylbenzene (EB), TNU-9 is an active and selective catalyst with selectivities to EB much higher than ZSM-5 and close to those of the industrially relevant MCM-22. However, olefin oligomerization within the pores of TNU-9 occurs in a larger extension than with MCM-22, leading not only to some lower selectivity, but also to a faster catalyst deactivation. In the case of cumene formation, again the selectivity of TNU-9 to the desired product is higher than ZSM-5 but lower than MCM-22, owing to the formation of larger amounts of n-propylbenzene in the former as a result of its pore topology. Nevertheless, TNU-9 may be worth to be studied further for the disproportionation of toluene to give xylenes. In this case, its comparison with ZSM-5 appears of interest. By studying the alkylation of toluene with alcohols of different chain length, i.e. methanol, ethanol and isopropanol, it has been found that TNU-9 has an antagonistic behaviour, closer to ZSM-5 or MCM-22 depending on the activity or selectivity parameters considered that could be explained by its complex topology, formed by 10-ring tortuous channels with crosses as in ZSM-5, and large cavities as in MCM-22.

Perfusion and characterization of an endothelial cell-seeded modular tissue engineered construct formed in a microfluidic remodeling chamber

Tissue engineered constructs containing tortuous endothelial cell-lined perfusion channels were formed by randomly assembling endothelial cell-seeded submillimeter-sized collagen cylinders (modules) into a microfluidic perfusion chamber. The interconnected void space produced by random module packing created flow channels that were lined with endothelial cells. The effect of perfusion (0.5 mL min −1, Re ∗ = 27.78 and shear stress = 0.16 dyn cm −2) through the tortuous channels on construct remodeling and endothelium quiescence was studied. Over time, modules fused at their points of contact and as they contracted, decreased the internal void space, which reduced the overall perfusion through the construct. As compared to static controls, perfusion caused a transient increase in activation (ICAM-1 and VCAM-1 expression) after 1 h followed by a decrease after 24 h. Proliferation (by BrdU) was reduced significantly, while KLF2, which is upregulated with atheroprotective laminar shear stress, was upregulated significantly after 24 h. VE-cadherin became discontinuous and was significantly downregulated after 24 h, which was likely caused by the dismantling of the endothelial cell adherens junctions during remodeling. Collectively, these outcomes suggest that flow through the construct did not drive the endothelial cells towards an inflamed, “atherosclerotic like” disturbed flow pathology.

The characteristics of thunder on Titan.

Even though Huygens has not detected acoustic signatures of Titanian lightning, electric discharge processes are thought to be very plausible in Titan’s thick atmosphere dominated by nitrogen and methane. When correlated with electromagnetic signatures, thunder will very likely confirm the occurrence of lightning, both during a probe’s descent and/or once it has landed. The presentation has two parts. In the first part, the mechanisms of Titanian thunder generation are discussed, in the context of the most up-to-date predictions of lightning on Titan, as well as relevant Cassini-Huygens data. In the second part, the propagation of thunder in Titan’s troposphere is addressed. Here, the structure, composition, and ambient conditions of the satellite’s atmosphere will be presented and accounted for. Current Titan lightning models predict cloud-to-ground discharge channels of ∼20 km. Long-range Titanian thunder simulations are developed based on the far-field superposition of approximately 7000 N-waves arising from 3-m “elementary” sources. Two lightning geometries will be considered: linear and tortuous. (Tortuous channels are synthesized as a Gaussian distribution in the orientations of the 7000 elementary sources.) For illustrative purposes, to show the effects of atmospheric filtering, the thunder waveform described above will also be propagated in Venus’ atmosphere.

Using cellular automata for porous media simulation

A cellular automata (CA) approach is proposed for simulating a fluid flow through porous materials with tortuous channels at pore level. The approach aims to combine CA methods both for constructing computer representation of porous material morphology and for simulating fluid flow through it. Morphology representation is obtained using CA whose evolution exhibits self-organization and results in a stable configuration. The latter is then used for Lattice Gas CA application to simulate fluid flow through a porous material specimen and compute its permeability properties. Special boundary conditions are introduced allowing for different smoothness of solid pore walls surface. The model has been tested on a small 2D fragment in a PC and then implemented to investigate a porous carbon electrode of a hydrogen fuel cell on 128 processors of a multiprocessor cluster.

A cellular-automata method for studying porous media properties

A cellular-automata (CA) approach for investigating properties of porous media with tortuous channels and different smoothness of pore walls is proposed. This approach is aimed at combining two different CA models: the first one is intended for constructing the morphology of a porous material; the second, for simulating a fluid flow through it. The porous media morphology is obtained as a result of evolution of a cellular automaton, forming a “steady pattern.” The result is then used for simulating a fluid flow through a porous medium by applying the Lattice Gas CA model. The method has been tested on a small fragment of a porous material and implemented for investigating a carbon electrode of a hydrogen fuel cell on a multiprocessor cluster.

Dynamic cross flow filtration with Novoflow's single shaft disk filters

Dynamic cross flow filtration means rotating filter elements for this purpose. Rotating filter elements allow for decoupling of overflow velocity from transmembrane pressure (TMP). Considerable energy savings are realized compared to conventional tubular systems, because there is no need for constant circulation (pumping) of the feed over the membranes in order to generate shear forces. The combination of centrifugal and shear forces in dynamic cross flow modules improves control of cover layer build-up which extends operating life. Novoflow has specialized on compact and modular single shaft designs and has developed a proprietary process of disk manufacturing from almost any flat membranes. Applications in many fields have proven the economic viability of the concept. Dynamic cross flow technology has shown advantages for the application of high performance membranes. These are very thin and hard membranes with absolute pore sizes and open outflow, i.e. no drainage layers and support structure. In the absence of complex support structures (with tortuous channels), flux performances of high performance membranes reach by orders of magnitude higher levels at much lower filtration pressures (TMP) than conventional membranes. On the other hand, this higher performance causes higher concentration polarization and fast cover layer build-up on the concentrate/retentate side. Novoflow's dynamic cross flow filtration design has proven to be an ideal way to remove the accelerated cover layer build-up for high performance membranes.