Distinct Flow Patterns Research Articles

Flow pattern and transition in gas–liquid–solid three phase spouted bed

The flow patterns and transitions in a gas–liquid–solid three phase spouted bed were investigated. Experiments were carried out in a cylindrical and a semi-cylindrical spouted bed with conical base. Glass beads and water were used as the solid phase and liquid phase, respectively. The changes in pressure drop and photographs were used to determine the flow patterns. Five distinct flow patterns were identified and described, i.e. fix bed (FB), grain spouting (GS), cluster spouting with slugging (CS-S), solid fixed with gas–liquid bubbling (SF-GLB), and slurry agitated bed (SA). Flow pattern maps of a three phase spouted bed (d=2.6mm, H0/Dt=1.86) under different liquid saturations were plotted. It was found that the flow pattern transition with the spouting gas velocity increasing at different liquid saturations (S) could be divided into three stages, i.e. transition from FB to GS when S<0.2, transition from FB to CS-S when 0.2<S<0.5, and transition from SF-GLB to SA when S>0.5. Increase in liquid saturation (S) always leads to increasing of the amplitude of pressure fluctuations. Besides, in stage I (GS), the minimum spouting velocity increases initially and then decreases apparently with increasing S. The range of stage I region becomes wider with lower initial bed height and particle diameter.

Vertically stratified two-phase flow in a curved channel: Insights from a domain perturbation analysis

In this work, we investigate the fully developed flow field of two vertically stratified fluids (one phase flowing above the other) in a curved channel of rectangular cross section. The domain perturbation technique is applied to obtain an analytical solution in the asymptotic limit of low Reynolds numbers and small curvature ratios (the ratio of the width of the channel to its radius of curvature). The accuracy of this solution is verified by comparison with numerical simulations of the nonlinear equations. The flow is characterized by helical vortices within each fluid, which are driven by centrifugal forces. The number of vortices and their direction of circulation varies with the parameters of the system (the volume fraction, viscosity ratio, and Reynolds numbers). We identify nine distinct flow patterns and organize the parameter space into corresponding flow regimes. We show that the fully developed interface between the fluids is not horizontal, in general, but is deformed by normal stresses associated with the circulatory flow. The results are especially significant for flows in microchannels, where the Reynolds numbers are small. The mathematical results in this paper include an analytical solution to two coupled biharmonic partial differential equations; these equations arise in two-phase, two-dimensional Stokes flows.

Rip currents under obliquely incident wind waves and tidal longshore currents

A field experiment on the nature of rip currents was conducted on the Dutch coast, which differs from previous rip current study sites because it is a wind-sea dominated environment with mostly obliquely incident waves and tidally-driven longshore currents. During the experiment three distinct flow patterns, obtained with GPS tracked drifter instruments, were observed: (1) a locally governed circulation cell, (2) an offshore current that is deflected shore parallel outside the surf zone and (3) a meandering longshore current. The transition from rip currents (flow patterns 1 and 2) to meandering longshore currents (flow pattern 3) occurred gradually within the tidal cycle with longshore currents prevalent at mid to high tide. Rip currents at this site appeared at depressions in the surf zone bar and typically occurred when the water level fell below NAP (equivalent to MSL), even in the presence of obliquely incident waves and tidally driven longshore currents. Hindcast simulations of the drifter experiments were performed with the numerical model XBeach and showed good agreement with field observations. The model was subsequently used to investigate the influence of tidal water level fluctuations, longshore currents and obliquely incident waves on rip currents.Rip currents were initiated when the water level dropped below a specific threshold with the magnitude of the rip current associated with the water level. The strength of the tidal current and its orientation with respect to the incident waves governed the offshore extent and orientation of the rip current. In contrast to other studies that suggest that rip currents solely occur under shore normal (or slightly oblique waves), in this study both observations and numerical model simulations indicate that rip currents can exist under large angles of wave incidence, when the rip channel is sufficiently wide and the wave height is small.

A thermodynamic basis for predicting falling-film mode transitions

Horizontal-tube, falling-film heat exchangers are used in many air-conditioning and refrigeration systems. Depending on the tube diameter and spacing, the flow rate, and fluid properties, when a liquid film falls over a series of horizontal tubes three distinct flow patterns can be manifested. These flow patterns are the droplet mode, the jet mode, and the sheet mode. A thermodynamic analysis is undertaken to predict the transitions between these modes. By seeking the condition corresponding to thermodynamic equilibrium between two neighboring modes, a scaling relation is developed for the transitional Reynolds number. This theoretical framework is used to provide the first explanation for the relationship between the transitional Reynolds number and modified Galileo number which has been previously based solely on experimental observations. This approach offers insight into the prevailing physics, and it suggests a tube-spacing effect on the mode transitions which has not previously been anticipated. Using limited data and prior results from the literature it is found that this effect is likely to exist. The implications of this thermodynamic approach to predicting two-phase flow patterns are discussed in terms of entropy generation minimization and transition hysteresis, as is its incompleteness.

Flow patterns and transitions in a rectangular three-phase bubble column

The flow patterns and transitions in a rectangular gas–liquid–solid three-phase bubble column were studied. The influence of solid volume fraction, particle size and particle density on the flow regime transitions of the three-phase bubble column was investigated experimentally. Experiments were carried out for solid volume fraction Vs=0.03–0.3, average particle size dp=48μm–270μm, particle density ρp=2500kg/m3–4800kg/m3, and superficial gas velocity Ug=0.007m/s–0.7m/s in a rectangular bubble column measured 0.8m tall, 0.1m long and 0.01m wide. Four distinct flow patterns and three transition points were observed in this experimental system, and the four flow regimes were discrete bubble regime, transition regime, bubble coalescence regime and strong turbulent regime, which were determined on the basis of criteria as well as schematic diagrams and typical flow pattern images obtained from a high-resolution digital charge couple device (CCD) camera. Typical flow patterns maps were plotted for illustrating flow regime transitions under different particle conditions. It was found out that particle volume fraction and particle density had an effect on the flow pattern transitions; when increasing the values of particle volume fraction and the particle density respectively, the values of the flow regime transition points all decreased. Particle size had a little effect on flow regime transition points when the particle size shifted in the range of 48μm to 150μm; when particle size was larger than 150μm and increased to 270μm, the operation ranges of the transition regime and the bubble coalescence regime decreased.

Observable Signatures of Classical T Tauri Stars Accreting in an Unstable Regime

We discuss key observational signatures of Classical T Tauri stars (CTTSs) accreting through Rayleigh-Taylor instability, which occurs at the interface between an accretion disk and a stellar magnetosphere. In this study, the results of global 3-D MHD simulations of accretion flows, in both stable and unstable regimes, are used to predict the variability of hydrogen emission lines and light curves associated with those two distinctive accretion flow patterns. In the stable regime, a redshifted absorption component (RAC) periodically appears in some hydrogen lines, but only during a fraction of a stellar rotation period. In the unstable regime, the RAC is present rather persistently during a whole stellar rotation period, and its strength varies non-periodically. The latter is caused by multiple accreting streams, formed randomly due to the instability, passing across the line of sight to an observer during one stellar rotation. This results in the quasi-stationarity appearance of the RAC because at least one of the accretion stream is almost always in the line of sight to an observer. In the stable regime, two stable hot spots produce a smooth and periodic light curve that shows only one or two peaks per stellar rotation. In the unstable regime, multiple hot spots formed on the surface of the star, produce the stochastic light curve with several peaks per rotation period.

Gas–liquid two-phase flow patterns in microchannels with reentrant cavities in sidewall

Two-phase flow patterns in two horizontal microchannels with uniform reentrant cavities in sidewall were experimentally studied, using deionized water and air as working fluids. The hydraulic diameter of microchannels in the constant rectangular cross-section is 133.3μm. Two structures (fan-shaped and triangular) of reentrant cavity are arranged to be mounted in sidewall with 0.25mm spacing to develop two new microchannels with variable cross-sections. The liquid and gas superficial velocities in the microchannels cover the 0.1–0.5m/s and 0.1–10m/s ranges, respectively. High-performance CCD digital imaging camera with a Nikon microscope is used to determine the flow patterns in the two new proposed microchannels. Two distinct flow patterns were observed: intermittent flow and separated flow. Evolution characteristic for different flow patterns along the flow direction are presented. Flow pattern maps using gas and liquid superficial velocities as coordinates and the correlations to describe flow pattern transition for the newly proposed microchannel test sections are obtained, respectively.

Bubble formation from a micro-pillar in a microchannel

Bubble characteristics downstream of a 150μm micro-pillar embedded inside a 225μm deep microchannel were experimentally studied. Nitrogen was issued through a slit along the pillar’s depth into liquid water flow. The gas flow was imaged and recorded by a high speed camera and the liquid flow field was obtained through a micro particle image velocimetry (μ-PIV) system. Gas flows through different slit angles were examined, and it was shown that the gas flow characteristics were significantly different for different slit angles. Three distinct bubble flow patterns were observed, which were dependent on the slit angle and liquid/gas flow rates. These patterns include discrete bubbles, attached ligament, and transitional flow. The liquid flow measurements were used to reveal the mechanisms controlling the bubble formation and flow patterns.

CFD simulation of flow regime maps in a slot‐rectangular spouted bed

The recognition of flow regimes is very important in the application of spouted beds. In this paper, two‐dimensional computer simulations of a slot‐rectangular spouted bed using FLUENT commercial software were utilised to construct flow regime maps. Two‐phase gas–solid flows in the bed were simulated with computational fluid dynamics (CFD) using the two‐phase Eulerian–Eulerian granular model. The numerical simulations were applied to predict different flow regimes. The constructed flow regime map for a bed containing solid particles with a diameter of 1.44 mm was in good agreement with the experimental map previously reported elsewhere. With this successful numerical mapping, a flow regime map for particles with a diameter of 3.77 mm was constructed for various superficial gas velocities and static bed heights. The map was composed of six distinct flow patterns, that is fixed bed, internal jet, jet‐in‐fluidised‐bed, spouting, incoherent spouting and slugging. The slugging flow regime occurred at large values for the static bed height and air inlet velocity, while the spouting regime arose by increasing the air inlet velocity at low values of static bed height. Unlike the spouting regime, large pressure drop fluctuations were observed in the incoherent spouting regime.

Three-dimensional instability in the flow past two side-by-side square cylinders

The onset of the three-dimensional (3D) instability in the flow past two side-by-side square cylinders has been studied via Floquet stability analysis. The gap between the two cylinders is the key parameter that alters flow topology, leading to change of stability characteristics. Depending on the gap, four distinct flow patterns have been identified for base flow. For each pattern of base flow, distinctive Floquet modes of 3D instability have been found. The spatial and temporal characteristics of the Floquet modes are similar to those of mode A or mode B of the single square cylinder case, namely, odd or even Reflection-Translation symmetries. Six distinct Floquet modes have been identified in total, and the spatial and temporal characteristics of each mode are described in detail. The criticality of Floquet modes, including their critical Reynolds number and the associated dominant spanwise wave number, is presented for each gap considered in the current study. For a small gap, the flow past the two cylinders resembles that of a single object, resulting in an approximately doubled characteristic length. Consequently, the critical Reynolds number becomes low with a small gap between the two cylinders. When the two cylinders are moderately apart, the interaction between the two wakes diminishes, and the criticality of 3D instabilities approaches that of the single cylinder case. Coexisting 3D instabilities for a specific base-flow topology are also noticed with a moderate gap. Neutral stability curves are computed and presented for some selected values of the gap.

Measurements of turbulence in a microscale multi-inlet vortex nanoprecipitation reactor

The microscale multi-inlet vortex reactor (MIVR) is designed for use in Flash NanoPrecipitation (FNP), a promising technique for producing nanoparticles within small particle size distribution. Fluid mixing is crucial in the FNP process, and due to mixing’s strong dependence upon fluid kinematics, investigating velocity and turbulence within the reactor is crucial to optimizing reactor design. To this end, microscopic particle image velocimetry has been used to investigate flow within the MIVR. Three Reynolds numbers are studied, namely, Rej = 53, 93 and 240. At Rej = 53, the flow is laminar and steady. Due to the strong viscous effects at this Reynolds number, distinct flow patterns are observed at different distances from the reactor top and bottom walls. The viscous effects also retard the tangential motions within the reactor, resulting in a weaker vortex than appears at the higher Reynolds numbers. As the Reynolds number is increased to 93, the flow becomes more homogeneous over the depth of the reactor due to weaker viscous effects, yet the flow is still steady. The diminishing effects of viscosity also result in a stronger vortex. At the highest Reynolds number investigated, the flow is turbulent. Turbulent statistics including tangential and radial velocity fluctuations and Reynolds shear stresses are analyzed for this case in addition to the mean velocity field. The tangential motions of the flow are strongest at Rej = 240. Both the tangential and radial velocity fluctuations increase as the flow spirals toward the center of the reactor. The magnitudes of the tangential and radial velocity fluctuations are similar, suggesting that the turbulence is locally isotropic.

Turbulent Flow Characteristics and Dynamics Response of a Vertical-Axis Spiral Rotor

The concept of a vertical-axis spiral wind rotor is proposed and implemented in the interest of adapting it to air flows from all directions and improving the rotor’s performance. A comparative study is performed between the proposed rotor and conventional Savonius rotor. Turbulent flow features near the rotor blades are simulated with Spalart-Allmaras turbulence model. The torque coefficient of the proposed rotor is satisfactory in terms of its magnitude and variation through the rotational cycle. Along the height of the rotor, distinct spatial turbulent flow patterns vary with the upstream air velocity. Subsequent experiments involving a disk generator gives an in-depth understanding of the dynamic response of the proposed rotor under different operation conditions. The optimal tip-speed ratio of the spiral rotor is 0.4–0.5, as is shown in both simulation and experiment. Under normal and relative-motion flow conditions, and within the range of upstream air velocity from 1 to 12 m/s, the output voltage of the generator was monitored and statistically analyzed. It was found that normal air velocity fluctuations lead to a non-synchronous correspondence between upstream air velocity and output voltage. In contrast, the spiral rotor’s performance when operating from the back of a moving truck was significantly different to its performance under the natural conditions.

Hemodynamic Features in Stenosed Coronary Arteries: CFD Analysis Based on Histological Images

Histological images from the longitudinal section of four diseased coronary arteries were used, for the first time, to study the pulsatile blood flow distribution within the lumen of the arteries by means of computational fluid dynamics (CFD). Results indicate a strong dependence of the hemodynamics on the morphology of atherosclerotic lesion. Distinctive flow patterns appear in different stenosed regions corresponding to the specific geometry of any artery. Results show that the stenosis affects the wall shear stress (WSS) locally along the diseased arterial wall as well as other adjacent walls. The maximum magnitude of WSS is observed in the throat of stenosis. Moreover, high oscillatory shear index (OSI) is observed along the stenosed wall and the high curvature regions. The present study is capable of providing information on the shear environment in the longitudinal section of the diseased coronary arteries, based on the models created from histological images. The computational method may be used as an effective way to predict plaque forming regions in healthy arterial walls.

Characterization of Shear-Sensitive Genes in the Normal Rat Aorta Identifies Hand2 as a Major Flow-Responsive Transcription Factor

ObjectiveShear forces play a key role in the maintenance of vessel wall integrity. Current understanding regarding shear-dependent gene expression is mainly based on in vitro or in vivo observations with experimentally deranged shear, hence reflecting acute molecular events in relation to flow. Our objective was to combine computational fluid dynamic (CFD) simulations with global microarray analysis to study flow-dependent vessel wall biology in the aortic wall under physiological conditions.Methods and ResultsMale Wistar rats were used. Animal-specific wall shear stress (WSS) magnitude and vector direction were estimated using CFD based on aortic geometry and flow information acquired by magnetic resonance imaging. Two distinct flow pattern regions were identified in the normal rat aortic arch; the distal part of the lesser curvature being exposed to low WSS and a non-uniform vector direction, and a region along the greater curvature being subjected to markedly higher levels of WSS and a uniform vector direction. Microarray analysis identified numerous novel mechanosensitive genes, including Trpc4 and Fgf12, and confirmed well-known ones, e.g. Klf2 and Nrf2. Gene ontology analysis revealed an over-representation of genes involved in transcriptional regulation. The most differentially expressed gene, Hand2, is a transcription factor previously shown to be involved in extracellular matrix remodeling. HAND2 protein was endothelial specific and showed higher expression in the regions exposed to low WSS with disturbed flow.ConclusionsMicroarray analysis validated the CFD-defined WSS regions in the rat aortic arch, and identified numerous novel shear-sensitive genes. Defining the functional importance of these genes in relation to atherosusceptibility may provide important insight into the understanding of vascular pathology.

Dynamical functional prediction and classification, with application to traffic flow prediction

Motivated by the need for accurate traffic flow prediction in transportation management, we propose a functional data method to analyze traffic flow patterns and predict future traffic flow. In this study we approach the problem by sampling traffic flow trajectories from a mixture of stochastic processes. The proposed functional mixture prediction approach combines functional prediction with probabilistic functional classification to take distinct traffic flow patterns into account. The probabilistic classification procedure, which incorporates functional clustering and discrimination, hinges on subspace projection. The proposed methods not only assist in predicting traffic flow trajectories, but also identify distinct patterns in daily traffic flow of typical temporal trends and variabilities. The proposed methodology is widely applicable in analysis and prediction of longitudinally recorded functional data.

Altered nitric oxide production mediates matrix-specific PAK2 and NF-κB activation by flow

Shear stress generated by distinct blood flow patterns modulates endothelial cell phenotype to spatially restrict atherosclerotic plaque development. Signaling through p21-activated kinase (PAK) mediates several of the deleterious effects of shear stress, including enhanced NF-κB activation and proinflammatory gene expression. Whereas shear stress activates PAK in endothelial cells on a fibronectin matrix, basement membrane proteins limit shear-induced PAK activation and inflammation through a protein kinase A-dependent pathway; however, the mechanisms underlying this regulation were unknown. We show that basement membrane proteins limit membrane recruitment of PAK2, the dominant isoform in endothelial cells, by blocking its interaction with the adaptor protein Nck. This uncoupling response requires protein kinase A-dependent nitric oxide production and subsequent PAK2 phosphorylation on Ser-20 in the Nck-binding domain. Of importance, shear stress does not stimulate nitric oxide production in endothelial cells on fibronectin, resulting in enhanced PAK activation, NF-κB phosphorylation, ICAM-1 expression, and monocyte adhesion. These data demonstrate that differential flow-induced nitric oxide production regulates matrix-specific PAK signaling and describe a novel mechanism of nitric oxide-dependent NF-κB inhibition.

On the effect of shear-thinning rheology on hemodynamic characteristics in basilar tip aneurysms with implication of two distinct flow patterns

Intra-aneurysmal hemodynamics such as wall shear stress and complex flow structures have been implicated as one of the important factors on the growth and risk of rupture of an aneurysm. In this study, the sensitivity of intra-aneurysmal blood flow dynamics to the shear-thinning rheological model is investigated by using the idealized geometries of a basilar tip aneurysm with two representative anterior-posterior (AP) tilting angles (2° and 30°). By choice of different rheological models, time-averaged hemodynamic factors such as wall shear stress, oscillatory shear index and relative residence time exhibited only minor effects. However, highly unstable flow present in idealized aneurysm model with 2° AP tilting angle facilitated an evident change in the instantaneous local flow dynamics with a considerable increase in effective viscosity. Nevertheless, the distinct hemodynamic phenotype, which characterizes the gross intraaneurysmal flow pattern, was independent of the choice of rheological model. This result suggests that the shear thinning viscous effect is of secondary importance in the gross hemodynamics in a basilar tip aneurysm but is appreciably enhanced on the instantaneous hemodynamics with unstable complex flow structures.

Distinctive flow pattern of wall shear stress and oscillatory shear index: similarity and dissimilarity in ruptured and unruptured cerebral aneurysm blebs

The difference in the hemodynamics of wall shear stress (WSS) and oscillatory shear index (OSI) between ruptured and unruptured aneurysms is not well understood. The authors investigated the hemodynamic similarities and dissimilarities in ruptured and thin-walled unruptured aneurysm blebs. Magnetic resonance imaging-based fluid dynamics analysis was used to calculate WSS and OSI, and hemodynamic and intraoperative findings were compared. The authors also compared ruptured and unruptured thin-walled blebs for the magnitude of WSS and OSI. Intraoperatively, 13 ruptured and 139 thin-walled unruptured aneurysm blebs were identified. Twelve of the ruptured (92.3%) and 124 of the unruptured blebs (89.2%) manifested low WSS and high OSI. The degree of WSS was significantly lower in ruptured (0.49 ± 0.12 Pa) than in unruptured (0.64 ± 0.15 Pa; p < 0.01) blebs. Ruptured and unruptured blebs shared a distinctive pattern of low WSS and high OSI. The degree of WSS at the rupture site was significantly lower than in the unruptured thin-walled blebs.

Computational Models of Transcranial Direct Current Stimulation

During transcranial direct current stimulation (tDCS), controllable dose parameters are electrode number (typically 1 anode and 1 cathode), position, size, shape, and applied electric current. Because different electrode montages result in distinct brain current flow patterns across the brain, tDCS dose parameters can be adjusted, in an application-specific manner, to target or avoid specific brain regions. Though the tDCS electrode montage often follows basic rules of thumb (increased/decreased excitability "under" the anode/cathode electrode), computational forward models of brain current flow provide more accurate insight into detailed current flow patterns and, in some cases, can even challenge simplified electrode-placement assumptions. With the increased recognized value of computational forward models in informing tDCS montage design and interpretation of results, there have been recent advances in modeling tools and a greater proliferation of publications. In addition, the importance of customizing tDCS for potentially vulnerable populations (eg, skull defects, brain damage/stroke, and extremes of age) can be considered. Finally, computational models can be used to design new electrode montages, for example, to improve spatial targeting such as high-definition tDCS. Pending further validation and dissemination of modeling tools, computational forward models of neuromodulation will become standard tools to guide the optimization of clinical trials and electrotherapy.

Visualization Experiments of a Specific Fuel Flow Through Quartz-glass Tubes Under both Sub- and Supercritical Conditions

The present work is a visualization study of a typical kerosene (RP-3) flowing through vertical and horizontal quartz-glass tubes under both sub- and supercritical conditions by a high speed camera. The experiments are accomplished at temperatures of 300-730 K under pressures from 0.107-5 MPa. Six distinctive two-phase flow patterns are observed in upward flow and the critical point of RP-3 is identified as critical pressure pc=2.33 MPa and critical temperature Tc=645.04 K and it is found that when the fluid pressure exceeds 2.33 MPa the flow can be considered as a single phase flow. The critical opalescence phenomenon of RP-3 is observed when the temperature is between 643.16 K and 648.61 K and the pressure is between 2.308 MPa and 2.366 MPa. The region filled by the critical opalescence in the upward flow is clearly larger than that in the downward flow due to the interaction between the buoyancy force and fluid inertia. Morecover, obvious layered flow phenomenon is observed in horizontal flow under supercritical pressures due to the differences of gravity and density.