Flow Stagnation Research Articles

Comprehensive numerical assessment of molten iron–slag trickle flow and gas countercurrent in complex coke bed by Eulerian–Lagrangian approach

The transient behavior of immiscible two-liquid interfaces initiated by molten iron–slag trickle flow was investigated using a Lagrangian particle-based smoothed particle hydrodynamics (SPH) model to track the movements of both liquid phases directly. The transient behavior of the associated gas flow was simulated using a Eulerian grid-based model dependent on the gas permeability characteristics exhibited by the molten iron–slag counterflow. As an example of the assessments conducted, the sliding behavior of molten iron down the slag surface at different between-phase interfacial tensions was observed in the unsteady state. While the effect of wetting between the molten slag and coke on the gas flow was found to be insignificant, decreasing the liquid–liquid interfacial tension promoted mixing between the two liquids, which in turn hindered the rapid dropping of the molten iron and resulted in a significant stagnation of gas flow.

A New Look at the Electron Diffusion Region in Asymmetric Magnetic Reconnection

AbstractA new look at the structure of the electron diffusion region in collision less magnetic reconnection is presented. The research is based on a particle‐in‐cell simulation of asymmetric magnetic reconnection, which includes a temperature gradient across the current layer in addition to density and magnetic field gradient. We find that none of X‐point, flow stagnation point, and local current density peak coincide. Current and energy balance analyses around the flow stagnation point and current density peak show consistently that current dissipation is associated with the divergence of nongyrotropic electron pressure. Furthermore, the same pressure terms, when combined with shear‐type gradients of the electron flow velocity, also serve to maintain local thermal energy against convective losses. These effects are similar to those found also in symmetric magnetic reconnection. In addition, we find here significant effects related to the convection of current, which we can relate to a generalized diamagnetic drift by the nongyrotropic pressure divergence. Therefore, only part of the pressure force serves to dissipate the current density. However, the prior conclusion that the role of the reconnection electric field is to maintain the current density, which was obtained for a symmetric system, applies here as well. Finally, we discuss related features of electron distribution function in the electron diffusion region (EDR). Specifically, we analyze both new crescent substructures as well as outer, higher energy crescents generated by accelerated magnetospheric particles.

Neutrophil granulocytes promote flow stagnation due to dynamic capillary stalls following experimental stroke

Flow stagnation of peri-ischemic capillaries due to dynamic leukocyte stalls has been described to be a contributor to ongoing penumbral injury in transient brain ischemia, but has not been investigated in permanent experimental stroke so far. Moreover, it is discussed that obstructing neutrophils are involved in this process; however, their contribution has not yet been proven.Here, we characterize the dynamics of neutrophil granulocytes in two models of permanent stroke (photothrombosis and permanent middle cerebral artery occlusion) using intravital two-photon fluorescence microscopy. Different to previous studies on LysM-eGFP+ cells we additionally apply a transgenic mouse model with tdTomato-expressing neutrophils to avoid interference from additional immune cell subsets.We identify repetitively occurring capillary stalls of varying duration promoted by neutrophils in both models of permanent cerebral ischemia, validating the suitability of our new transgenic mouse model in determining neutrophil occlusion formation in vivo. Flow cytometric analysis of peripheral blood (PB) and brain tissue from mice subjected to photothrombosis reveal an increase in the total proportion of neutrophils, with selective upregulation of endothelial adherence markers in the PB.In conclusion, the dynamic microcirculatory stall phenomenon that is described after transient ischemia followed by reperfusion also occurs after permanent small- or large-vessel stroke and is clearly attributable to neutrophils.

Release of colloids in saturated porous media under transient hydro-chemical conditions: A pore-scale study

The deposition and consequent release of colloids pose a significant challenge to the environment, groundwater quality and human health. Subsurface soil contains numerous types of colloids that exhibit a diverse range of interaction favorability with the soil grains and their impact on release behavior remains unclear. The objective of this study was to investigate, at the pore-scale, the impact of colloid interaction favorability on colloid deposition and subsequent release in response to perturbations of flow rate and solution chemistry in saturated porous media. Pore-scale experiments were conducted using a micromodel that is geometrically representative of a real sand-stone rock. Favorable, medium-favorable and unfavorable colloids (i.e., repulsion absent, repulsion at long-range of separation distances and attraction absent with the micromodel surface, respectively) were deposited in the micromodel and then a series of colloid release experiments were conducted at different conditions including increasing the flow rate, decreasing the ionic strength and increasing the solution pH. Favorable colloids exhibited extensive deposition on the collector center where the flow streamlines are parallel to the collector surface, as adhesion forces overcome hydrodynamic forces. However, at medium and high ionic strength, deposition in Forward Flow Stagnation Zone (FFSZ) was dominant for unfavorable colloids as the hydrodynamic forces are negligible. Pore-scale images showed that, upon perturbations in flow rate and solution chemistry, colloids that were initially deposited on collector centers were more susceptible to release as compared to colloids that were initially deposited in FFSZs. The negligible hydrodynamic drag forces in FFSZ and deep primary minimum interaction at short separation distances were the major factors that hindered the release of colloids in FFSZ under transient hydro-chemical conditions. The intensity of colloid deposition and release decreases as the favorability of colloids decreases and as the ionic strength decrease for unfavorable colloids. This study provides a clear insight to the pore-scale colloid deposition and release mechanisms during transient hydro-chemical conditions that help in the modeling of environmental and engineering applications including managed aquifer recharge, groundwater contamination and wastewater treatment processes.

Fluid-structure coupled biotransport processes in aortic valve disease

Biological transport processes near the aortic valve play a crucial role in calcific aortic valve disease initiation and bioprosthetic aortic valve thrombosis. Hemodynamics coupled with the dynamics of the leaflets regulate these transport patterns. Herein, two-way coupled fluid–structure interaction (FSI) simulations of a 2D bicuspid aortic valve and a 3D mechanical heart valve were performed and coupled with various convective mass transport models that represent some of the transport processes in calcification and thrombosis. Namely, five different continuum transport models were developed to study biochemicals that originate from the blood and the leaflets, as well as residence-time and flow stagnation. Low-density lipoprotein (LDL) and platelet activation were studied for their role in calcification and thrombosis, respectively. Coherent structures were identified using vorticity and Lagrangian coherent structures (LCS) for the 2D and 3D models, respectively. A very close connection between vortex structures and biochemical concentration patterns was shown where different vortices controlled the concentration patterns depending on the transport mechanism. Additionally, the relationship between leaflet concentration and wall shear stress was revealed. Our work shows that blood flow physics and coherent structures regulate the flow-mediated biological processes that are involved in aortic valve calcification and thrombosis, and therefore could be used in the design process to optimize heart valve replacement durability.

Visualization of flow characteristics between the ribbed plates via particle image velocimetry

Heat transfer is considerably influenced by flow stagnation, separation and reattachment regions due to the ribbed plates. Placing the ribs such as fins, turbulators that trigger the flow separation, enhances the heat transfer inside the channel by increasing the turbulence intensity. The flow separation is caused by disturbing the thermal and hydrodynamic development lengths. Moreover, these ribs also make an impact that increases the heat transfer by enlarging the heat transfer area. However, the ribs lead to the increment of the required pumping power in the meantime due to the increasing pressure loss in such systems. This aforementioned method is used for the heat exchangers, the solar collectors, the cooling of electronic devices. The investigation of the flow characteristics is very crucial to understand the heat transfer mechanism in the ducts for this reason. In the present paper, the flow characteristics between the plates have been experimentally re-searched. Particle image velocimetry system in the open water channel of Selcuk University Advanced Technology Research and Application Center has been used. The smooth plates have been taken as the reference model and used for the comparison with the plates having the rectangular cross-sectional ribs. The ribs with various heights of 0.1 ? h? = h/H ? 0.3 have been symmetrically placed on the internal surfaces of the plates via several spacing values of 0.5 ? S? = S/H ? 1 for varying Reynolds numbers as 10000 ? Re ? 20000. As a result, the flow characteristics have been given in terms of the contour graphics for velocity vector field, velocity components and vorticity.

How to Avoid Periprocedural Thrombotic Complications

Administration of antithrombotic agents in neuroendovascular therapy is important to prevent periprocedural thromboembolic complications. Both anticoagulants that prevent red thrombi formed by flow stagnation in the catheter and antiplatelets that prevent white thrombi triggered by the implanted foreign body, such as coils and stents, are needed. With the progress of endovascular techniques and development of new devices, periprocedural antiplatelet therapy has been used more aggressively in recent years. Currently, the standard antiplatelet management in many neuroendovascular treatments is dual antiplatelet therapy. Perioperative antithrombotic therapy may reduce ischemic complications, but it presents the potential risk of hemorrhagic complications. Moreover, long-term dual therapy does not reduce the risk of stroke recurrence and is associated with an increased risk of major bleeding events. For patients at high risk for hemorrhagic complications, the physician should undertake measures, such as reducing the dose or discontinuing it early.

Two-phase flow measurements and observations in a cooling panel of the reactor cavity cooling system

The analysis of the flow behavior within a cooling panel of a scaled water-cooled reactor cavity cooling system has been performed supported by high-resolution measurements of the void fraction at the risers’ outlet. The results have shown a stable, symmetric distribution of the flow through the risers during subcooled boiling conditions, characterized by small, slow bubbles with average void fraction lower than 0.3 for all risers. As the temperature increased to saturation, flow distribution appeared strongly asymmetrical and unstable, with peaks of void fraction as high as 0.9. Large, faster bubbles were observed during these peaks, immediately followed by flow of subcooled liquid water. Flow stagnation and inversion has also been observed. The high resolution void fraction results, complemented by the measurements of the coolant flow rate and temperatures, not only have further described the behavior of the system under hypothetical accident conditions, but represent a unique set of data to support the validation of system-level codes, and more advanced computational tools.

Measurement of the gas flow stagnation temperature by the method of two identical thermocouples in the short-duration aerodynamic facilities

Investigations of the temperature and heat flux distributions over the external surface of flying vehicles are important for the development of hypersonic planes. For obtaining correct results, experiments are performed in short-duration high-enthalpy wind tunnels. Determination of the stagnation temperature of the flow in such facilities is a challenging task due to short start-up times and high flow temperature. In this paper a new method is proposed for determining the stagnation temperature of a gas flow in the short-duration aerodynamic wind tunnels with stepwise changes in its parameters. The method is based on using two identical thermocouples with different initial temperatures. It is shown that in the case of jump-like changes in flow parameters the instrument function of the difference of the thermocouple readings coincides with the instrument function of each thermocouple, and the instrument function itself is actually the normalized temperature difference. The instrument function obtained in the study allows temperature retrieval by the method of deconvolution. This approach is highly sensitive to random noise of experimental data; therefore, regularization methods are applied for high-frequency noise suppression. A procedure for choosing the regularization parameter for temperature reconstruction is proposed. The research novelty of the article is that the proposed method allows exact reproduction of the stepwise function and instrument function without any additional assumptions, while the measurements and dynamic calibration are performed in one experiment. Applicability of the method for obtaining the stagnation temperature in the short-duration high-enthrall wind tunnel IT-302 M ITAM SB RUS is demonstrated.

Long Island enhanced aerosol event during 2018 LISTOS: Association with heatwave and marine influences.

The co-occurrence of enhancement in aerosol concentration, temperatures, and ozone mixing ratio was observed between June 29 and July 4, 2018 (enhanced period, EP) on Long Island (LI) and the greater NYC metropolitan area during part of the 2018 Long Island Sound Tropospheric Ozone Study (LISTOS). Two aerosol formation pathways were identified during the EP, the first being the condensation of semi- and intermediate volatility oxidation products of anthropogenic volatile organic compounds (AVOCs) under stagnant synoptic flow conditions, high temperatures and afternoon sea-breeze circulation. While this first pathway was prevalent, the most abundant organic aerosol factor was less oxidized oxygenated organic aerosol or LO-OOA. The second formation pathway occurred during a period of more persistent (synoptic) on-shore flow transporting more aged aerosol which consisted of an internal mixture of more oxidized oxygenated organic aerosol (MO-OOA), methanesulfonic acid (MSA) and sulfate. It was estimated that 35% of the sulfate observed during the mature period (an average of about 1.2μgm-3) originated from oceanic dimethyl sulfide (DMS) emissions. These two formation pathways helped elucidate the sources of fine particle pollution, highlighted the interaction between human emissions and natural DMS emission, and will help our understanding of pollution affecting other urban areas adjacent to large bodies of water during hot and stagnant periods.

Stagnation of glymphatic interstitial fluid flow and delay in waste clearance in the SOD1-G93A mouse model of ALS

Overexpression and mislocalization of aquaporin-4 (AQP4) in the SOD1G93A mouse model of amyotrophic lateral sclerosis (ALS) have previously been reported. However, how alterations of AQP4 affect interstitial bulk flow in the brain and spinal cord, the so-called glymphatic system, is unclear. Here, we report an enhanced accumulation of disease-associated SOD1 species including SOD1 oligomers in SOD1G93A;AQP4−/− mice compared with SOD1G93A mice during ALS disease progression, as analyzed by sandwich ELISA. By directly injecting SOD1 oligomers into the spinal cord parenchyma, we observed a significantly larger delay in clearance of biotinylated or fluorescent-labeled SOD1 oligomers in AQP4−/− mice than in wild-type mice. Furthermore, when we injected the fluorescent-labeled tracer protein ovalbumin into the cisterna magna and analyzed the tracer distribution in the cervical spinal cord, approximately 35 % processing ability was found to be reduced in SOD1G93A mice compared to wild-type mice. These results suggest that the glymphatic system is abnormal and that waste clearance is delayed in SOD1G93A mice.

Transport of particles in a turbulent rough-wall pipe flow

Abstract

Fluid dynamics simulations show that facial masks can suppress the spread of COVID-19 in indoor environments

The coronavirus disease outbreak of 2019 has been causing significant loss of life and unprecedented economic loss throughout the world. Social distancing and face masks are widely recommended around the globe to protect others and prevent the spread of the virus through breathing, coughing, and sneezing. To expand the scientific underpinnings of such recommendations, we carry out high-fidelity computational fluid dynamics simulations of unprecedented resolution and realism to elucidate the underlying physics of saliva particulate transport during human cough with and without facial masks. Our simulations (a) are carried out under both a stagnant ambient flow (indoor) and a mild unidirectional breeze (outdoor), (b) incorporate the effect of human anatomy on the flow, (c) account for both medical and non-medical grade masks, and (d) consider a wide spectrum of particulate sizes, ranging from 10 µm to 300 µm. We show that during indoor coughing some saliva particulates could travel up to 0.48 m, 0.73 m, and 2.62 m for the cases with medical grade, non-medical grade, and without facial masks, respectively. Thus, in indoor environments, either medical or non-medical grade facial masks can successfully limit the spreading of saliva particulates to others. Under outdoor conditions with a unidirectional mild breeze, however, leakage flow through the mask can cause saliva particulates to be entrained into the energetic shear layers around the body and transported very fast at large distances by the turbulent flow, thus limiting the effectiveness of facial masks.

A comparison of methods for finding magnetic nulls in simulations and in situ observations of space plasmas

Context.Magnetic nulls are ubiquitous in space plasmas, and are of interest as sites of localised energy dissipation or magnetic reconnection. As such, a number of methods have been proposed for detecting nulls in both simulation data and in situ spacecraft data from Earth’s magnetosphere. The same methods can be applied to detect stagnation points in flow fields.Aims.In this paper we describe a systematic comparison of different methods for finding magnetic nulls. The Poincaré index method, the first-order Taylor expansion (FOTE) method, and the trilinear method are considered.Methods.We define a magnetic field containing fourteen magnetic nulls whose positions and types are known to arbitrary precision. Furthermore, we applied the selected techniques in order to find and classify those nulls. Two situations are considered: one in which the magnetic field is discretised on a rectangular grid, and the second in which the magnetic field is discretised along synthetic “spacecraft trajectories” within the domain.Results.At present, FOTE and trilinear are the most reliable methods for finding nulls in the spacecraft data and in numerical simulations on Cartesian grids, respectively. The Poincaré index method is suitable for simulations on both tetrahedral and hexahedral meshes.Conclusions.The proposed magnetic field configuration can be used for grading and benchmarking the new and existing tools for finding magnetic nulls and flow stagnation points.

Particle-in-cell simulations of asymmetric reconnection driven by laser-powered capacitor coils

During magnetic reconnection, such as in the magnetopause of magnetized planets, the upstream plasma conditions between the two inflow regions are usually different. In this paper, we demonstrate that such a kind of asymmetric reconnection can be studied in the laboratory using the recently proposed experimental scheme where reconnection is driven by laser-powered capacitor coils. Two-dimensional particle-in-cell simulations on the plane in a cylindrical coordinate are conducted to study magnetic reconnection with the inflow along the direction. Magnetic reconnection is found to be asymmetric with a stronger magnetic field in the inner (small ) inflow region and a weaker magnetic field in the outer (large ) inflow region due to the cylindrical symmetric geometry. Electron crescent velocity distributions are observed near the flow stagnation point while ion crescent velocity distributions are observed in the region with Larmor electric field. The out-of-plane Hall magnetic field is asymmetric between the two inflow regions with a larger spatial scale in the outer inflow region. This asymmetric Hall magnetic field configuration is different from that in previous studies. The typical reconnection rate increases with a stronger driver and the highest rate is around 0.2, where is the typical value of the magnetic field and is the Alfven speed. This study provides a new method to experimentally study asymmetric reconnection in the laboratory and has potential applications regarding magnetic reconnection in the magnetopause of magnetized planets.

Abstract 13136: Left Atrial Appendage Structural Characteristics Predict Thrombus Formation

Introduction: Atrial fibrillation remains highly prevalent in the US with over 6 million diagnosed individuals. Discordant atrial contraction inherent to atrial fibrillation results in stagnant blood flow and increases the risk for thrombus formation particularly within the left atrial appendage (LAA). Anticoagulation is the mainstay of intracardiac thrombus prevention and is initiated based on the CHAD 2 S 2 -VASc risk score that does not account for LAA morphology. The relationship between LAA structural characteristics and thrombus development remains poorly understood. Hypothesis: We hypothesized that in patients with atrial fibrillation, LAA structural characteristics correlate with the presence of thrombus. Methods: We performed a retrospective case-control study of 151 subjects with a history of atrial fibrillation who underwent transesophageal echocardiogram (TEE) to evaluate the LAA for pre-cardioversion indication or Watchman screening evaluation. Subjects discovered to have LAA thrombus on TEE defined the study case group (n = 74) while control subjects did not have LAA thrombus (n = 77). Baseline demographics and echocardiographic characteristics were collected for all subjects. Results: Subjects without thrombus were older than those with thrombus (thrombus: 68.6 ± 13.1 years, no thrombus: 74.7 ± 9.2 years, p = 0.001 ). LAA ostial cross-sectional area was smaller in subjects with thrombus than those without thrombus (thrombus: 251.1 ± 117.4 mm 2 , no thrombus: 332.9 ± 171.2 mm 2 , p < 0.001 ). This finding remained significant in both unadjusted and adjusted logistic regression models ( Table 1 ). CHA 2 DS 2 -VASc score did not significantly differ in those with or without thrombus (thrombus: 4.4 ± 1.8, no thrombus: 4.3 ± 1.7, p = 0.822). Conclusions: Smaller LAA ostial cross-sectional area correlates with thrombus formation. Consideration of LAA structural characteristics may improve precision of anticoagulation management.

A swept fin-induced flow field with different height mounting gaps

In order to apply the air fin successfully and ensure the maneuverability of hypersonic vehicle, a key problem to be studied urgently is the heat flux brought by the fin mounting gap. The appearance of mounting gap and fin shaft can induce many complex flow structures which need more attentions to be investigated. Under Ma 6, Nano-tracer-based Planar Laser Scattering (NPLS) and Temperature Sensitive Paints (TSP) were applied to visualize and measure transient flow structures and heat flux distribution of a swept fin-induced flow field with different height mounting gaps. Complementarily, Reynolds-averaged N-S equations were solved with k-ω SST turbulent model. The heat flux distribution results of numerical simulation and TSP observed the change of high heat flux region with different mounting gap, both in position and magnitude. The streamlines based on Computational Fluid Dynamics (CFD) and flow visualization results obtained by NPLS revealed the cause of high heat flux region. The high heat flux region in this flow field is mainly related to the reattachment of vortex and flow stagnation. The increase of gap height can lead to stronger gap overflow and shaft-induced horseshoe vortex, which are source of the high heat flux around the fin. The case with the highest mounting gap (4 mm) en-counters the most severe aerodynamic heating, both on the surface of fin and plate. Thus, under the premise of ensuring the flexibility of the fin, the gap should be set as small as possible.

Effects of 3D contraction on pebble flow uniformity and stagnation in pebble beds

Pebble flow characteristics can be significantly affected by the configuration of pebble bed, especially for HTGR pebble beds. How to achieve a desired uniform flow pattern without stagnation is the top priority for reactor design. Pebbles flows inside some specially designed pebble bed with arc-shaped contraction configurations at the bottom, including both concave-inward and convex-outward shapes are explored based on discrete element method. Flow characteristics including pebble retention, residence-time frequency density, flow uniformity as well as axial velocity are investigated. The results show that the traditionally designed pebble bed with cone-shape bottom is not the most preferred structure with respect to flow pattern for reactor design. By improving the contraction configuration, the flow performance can be significantly enhanced. The flow in the convex-shape configuration featured by uniformity, consistency and less stagnation, is much more desirable for pebble bed design. In contrast, when the shape is from convex-forward to concave-inward, the flow shows more nonuniformity and stagnation in the corner although the average cross-section axial velocity is the largest due to the dominant middle pebbles.

Inline force on human body due to non-impulsive wave overtopping at a vertical seawall

Wave overtopping endangers pedestrians on a seawall’s crest area, but quantitative assessment of the risk is still a challenge. In this study, a circular cylinder is taken as a schematisation of human body. Physical model tests and numerical simulations were conducted to investigate the inline force on the cylinder due to overtopping flow at a vertical seawall. It is found that the overtopping flow is well reproduced by the numerical model. When the overtopping flow encounters the cylinder, a high pressure zone is developed on the front surface of the cylinder due to the local flow stagnation, while a shallow wake flow occurs behind the cylinder due to the blockage effect. The asymmetric pressure distribution produces the inline force on the cylinder. When the cylinder is further moved inland, the depth of the local overtopping flow it faces is smaller. However, the inline force does not decay accordingly, which is due to the variation of the flow velocity along the seawall’s crest. Finally, a predictor is developed for desktop computation of the maximum inline force. A prototype-scale numerical case study is carried out to demonstrate the capability of this predictor.

Numerical simulation of non-Newtonian hydrodynamics in an annular channel with tool joint

The paper presents the simulations of the local and integral characteristics of an annular flow with tool joints, depending on problem parameters. The structure of the flow in the vicinity of the coupling, in the areas of stagnant and reverse flow, is studied in detail. The characteristic length of separation and stabilization zones, i.e. the distance behind the joint where the flow reaches steady-state again is investigated. The sizes of pipes and tool joints in the study match those used while drilling. The numerical modelling is performed in 3D steady-state formulation for both laminar and turbulent regimes using original in-house software.