Local Flow Patterns Research Articles

Pore‐scale and continuum modeling of gas flow pattern obtained by high‐resolution optical bench‐scale experiments

High‐resolution optical bench‐scale experiments were conducted in order to investigate local gas flow pattern and integral flow properties caused by point‐like gas injection into water‐saturated glass beads. The main goal of this study was to test the validity of the continuum approach for two‐fluid flow in macroscopic homogeneous media. Analyzing the steady state experimental gas flow pattern that satisfies the necessary coherence condition by image processing and calibrating the optical gas distribution by the gravimetrical gas saturation, it was found that a pulse‐like function yields the best fit for the lateral gas saturation profile. This strange behavior of a relatively sharp saturation transition is in contradiction to the widely anticipated picture of a smooth Gaussian‐like transition, which is obtained by the continuum approach. This transition is caused by the channelized flow structure, and it turns out that only a narrow range of capillary pressure is realized by the system, whereas the continuum approach assumes that within the representative elementary volume the whole spectrum of capillary pressures can be realized. It was found that the stochastical hypothesis proposed by Selker et al. (2007) that bridges pore scale and continuum scale is supported by the experiments. In order to study channelized gas flow on the pore scale, a variational treatment, which minimizes the free energy of an undulating capillary, was carried out. On the basis of thermodynamical arguments the geometric form of a microcapillary, macrochannel formation and a length‐scale‐dependent transition in gas flow pattern from coherent to incoherent flow are discussed.

Shape modification of an axial flow turbine nozzle to reduce erosion

PurposeThis paper aims to validate an axial turbine modified nozzle design, looking for a reduction of the nozzle erosion process during operation in power plants.Design/methodology/approachThe approach taken is numerical simulation using the computational fluid dynamics (CFD) tool, comparing original and proposed/modified nozzle designs.FindingsThe paper provides information about how to achieve a solution of the turbine operational problem (abrasive wear) by an analysis of flow patterns under a variety of conditions.Research limitations/implicationsIt does not give a detailed interpretation of flow behaviour due to the lack of validation data.Practical implicationsA very useful flow simulation methodology that can be used in industry is provided.Originality/valueThe proposed design modification of an axial turbine nozzle with the aid of CFD simulation has not been performed yet. This paper investigates the possibility of nozzle erosion reduction by modifying local flow patterns.

Hybrid modelling for simulation of scour and flow patterns in laboratory flumes

AbstractIn this study, a hybrid numerical–mathematical model is developed, which simulates the temporal evolution of local scour and flow patterns in laboratory flumes. The mathematical module calculates the shape of the scour hole by solving an integro‐parabolic equation. The numerical module simulates the two‐dimensional laterally averaged flow patterns in the corresponding scour hole. The hybrid model is verified by experimental data taken from the literature. The hybrid model predicted the temporal evolution of local scour and simulated the flow patterns in and around the scoured zone in good agreement with the experimental ones. The proposed hybrid model proved to be efficient when used in complex local scour and turbulent flow problems. Copyright © 2009 John Wiley & Sons, Ltd.

Node Mobility Control and Capacity in Wireless Ad Hoc Networks

The end-to-end throughput of stationary random wireless networks is scaled by Θ (1nlogn). This throughput scale can be changed into Θ(1) by making the flows to be local flow patterns. By exploiting the controllable mobility of mobile nodes in order to reduce the path length, it is possible to achieve the constant scale end-to-end capacity regardless of the node density. The shortened path length at the resultant network reduces the traffic delay and improves the energy efficiency at the same time. The mobility control for the constant throughput scale can be formulated as a optimization problem, and we propose a simple heuristic mobility control algorithm based on the analysis and the problem formulation. Extensive simulations show that the proposed mobility control algorithm improve the end-to-end capacity.

Diffusion of a passive scalar by convective flows under parametric disorder

We study transport of a weakly diffusive pollutant (a passive scalar) throughthermoconvective flow in a fluid-saturated horizontal porous layer heated from below underfrozen parametric disorder. In the presence of disorder (random frozen inhomogeneities ofthe heating or of macroscopic properties of the porous matrix), spatially localized flowpatterns appear below the convective instability threshold of the system without disorder.Thermoconvective flows crucially affect the transport of a pollutant along the layer,especially when its molecular diffusion is weak. The effective (or eddy) diffusivity alsoallows us to observe the transition from a set of localized currents to an almost everywhereintense ‘global’ flow. We present results of numerical calculation of the effectivediffusivity and discuss them in the context of localization of fluid currents and thetransition to a ‘global’ flow. Our numerical findings are in good agreement with theanalytical theory that we develop for the limit of a small molecular diffusivityand sparse domains of localized currents. Though the results are obtained for aspecific physical system, they are relevant for a broad variety of fluid dynamicalsystems.

Morning breakup of cold pools in complex terrain

Under fair weather conditions, local flow patterns in areas of complex topography are driven by diurnal heating and cooling. If the topography is basin-shaped, downslope flows occurring at night accumulate (pool) in the basin valley to form a stable layer of cold air. During the morning transition, this cold pool is destroyed by the onset of turbulent convection and upslope flow. A series of laboratory experiments was conducted to identify mechanisms responsible for the breakup of cold pools. An idealized V-shaped tank filled with thermally stratified water and heated with an approximately uniform bottom heat flux was used. Temperature measurements and dye visualization were used for flow diagnostics. Mechanisms of cold pool destruction were identified and placed in the context of previously proposed mechanisms. A new mechanism was identified, wherein a dominant intrusion emanating from the upslope flow plays a dynamically important role in cold pool destruction. The results are expected to help develop subgrid parameterizations for meso-scale weather forecasting models, which are notorious for giving poor predictions during the morning transition period in complex terrain.

Blood flow pattern and wall shear stress in the internal carotid arteries of healthy subjects

The carotid bifurcation is one of the most susceptible locations for atherosclerotic plaques. It is important to assess wall shear stress (WSS) and blood flow patterns in this area. To assess local WSS and blood flow patterns at the origin of the internal carotid artery (ICA). Twenty-eight ICAs of 16 healthy volunteers were studied. A cine phase-contrast magnetic resonance (MR) sequence combined with a three-dimensional paraboloid model was applied to calculate WSS values and to obtain flow parameters. WSS spatial distribution features and local flow patterns were analyzed. Mean WSS was 0.560 +/- 0.180 (range -0.745 to 3.563) N/m2 at the origin of 28 ICAs. Error points were found in six vessels. Four WSS spatial distribution patterns were classified according to the location of low WSS values. Reverse WSS was found in 20 vessels. The mean oscillatory shear index (OSI) for these vessels was 0.089 +/- 0.094 (range 1.00 x 10(-6) to 0.490). Complex flow status including reverse velocity, flow stagnancy, and high oscillating WSS was displayed in the end systole of this area. Flow patterns were variant and complicated at the origin of the ICAs. Different types of local WSS spatial distributions were found around the vessel circumference.

Effects of stream‐aquifer disconnection on local flow patterns

Disconnected stream‐aquifer systems are becoming increasingly common because of lowering groundwater tables. This work focuses on the pathways and rates of infiltration and seepage as streams transition from fully connected to disconnected conditions. HYDRUS‐2D simulations show for a connected stream, water infiltrates vertically then moves laterally below the top of the preflooding capillary fringe height, finally causing an upward displacement of antecedent water into the vadose zone. This contradicts the commonly held assumption that stream water moves laterally for some distance into the stream bank, forming a wedge above the antecedent water. Even for shallow disconnections (<1 m), there is an increase in infiltration losses from the stream, elimination of postflooding seepage, and increased lateral and vertical mixing of stream water and antecedent pore water.

Streamline topology near non-simple degenerate critical points in two-dimensional flow with symmetry about an axis

The local flow patterns and their bifurcations associated with non-simple degenerate critical points appearing away from boundaries are investigated under the symmetric condition about a straight line in two-dimensional incompressible flow. These flow patterns are determined via a bifurcation analysis of polynomial expansions of the streamfunction in the proximity of the degenerate critical points. The normal form transformation is used in order to construct a simple streamfunction family, which classifies all possible local streamline topologies for given order of degeneracy (degeneracies of order three and four are considered). The relation between local and global flow patterns is exemplified by a cavity flow.

Flow-sensitive four-dimensional magnetic resonance imaging: flow patterns in ascending aortic aneurysms

Pathological aortic flow patterns differ significantly from haemodynamics within the healthy aorta. Development and impact of pathological flow is largely unknown and might affect pathogenesis and the progression of thoracic aortic diseases. This study presents pathological blood-flow patterns within a series of six patients suffering from ascending aortic aneurysms investigated with high-detail flow-sensitive, four-dimensional (4D)-MRI and three-dimensional (3D) computer-aided flow-visualisation strategies. Data were acquired on a 3T magnetic resonance system (TRIO, Siemens, Erlangen, Germany) using a flow-sensitive 4D (time-resolved 3D) sequence protocol. Measurements were taken in synchrony with the cardiac cycle and under respiration control. After data pre-processing, blood-flow was visualised by means of systolic 3D streamlines and time-resolved 3D particle traces using the software EnSight (CEI, Apex, NC, USA) and homemade visualisation tools. We investigated six adult patients with ascending aortic aneurysms and one healthy individual and findings were compared to 3D-haemodynamics of the dilated ascending aorta described in current literature. Normal blood-flow in the healthy volunteer resulted in highest velocities of up to 1 ms in the ascending and descending aorta, a right-handed helical flow pattern featuring 0.5-1.5 revolutions within the ascending aorta was present. Two atherosclerotic aneurysms presented either increased right-handed helical flow with flow acceleration along the great curvature, or multiple vortical flows in the sinuses and middle of the ascending aorta. One Marfan-associated aneurysm exhibited increased vortical flow in the dilated sinuses. One pseudo-aneurysm at the proximal anastomosis of an earlier supracoronary aortic replacement showed extensive vortex formation inside the aneurysm's lumen. An aneurysm in a patient with a bicuspid aortic valve revealed one major vortex formation directly above the aortic valve. One aneurysm following congenital valvular stenosis and commissurotomy in childhood was characterised by helical diastolic backflow in the central ascending aorta and a vortex at the small curvature. Patients with ascending aortic aneurysms reveal considerable differences in local flow patterns among themselves and compared to healthy individuals. Further investigations are necessary to identify flow patterns predisposing to aortic aneurysm development or adverse events in the course of aortic disease.

Refrigerant Condensation Flow Regimes in Enhanced Tubes and Their Effect on Heat Transfer Coefficients and Pressure Drops

Flow regimes influence the heat and mass transfer processes during two-phase flow, implying that any statistically accurate and reliable prediction of heat transfer and pressure drop during flow condensation should be based on the analysis of the prevailing flow pattern. Many correlations for heat transfer coefficient and pressure drop during flow condensation completely ignored flow regime effects and treated flows as either annular or non-stratified flow or as stratified flow. This resulted in correlations of poor accuracy and limited validity and reliability. Current heat transfer coefficient, pressure drop, and void fraction models are based on the local flow pattern, though, resulting in deviations of around 20% from experimental data. There are, however, several inconsistencies and anomalies regarding these models, which are discussed in this paper. A generalized solution methodology for two-phase flow problems still remains an elusive goal, mainly because gas-liquid flow systems combine the complexities of turbulence with those of deformable vapor-liquid interfaces. The paper focuses on the state of the art in correlating flow condensation in micro-fin tubes and proposes flow regime-based correlations of heat transfer coefficient and pressure drop for refrigerant condensation in smooth, helical micro-fin, and herringbone micro-fin tubes.

Preliminary study of hemodynamic distribution in patient-specific stenotic carotid bifurcation by image-based computational fluid dynamics

Regions prone to atherosclerosis, such as bends and bifurcations, tend to exhibit a certain degree of non-planarity or curvature, and these geometric features are known to strongly influence local flow patterns. Recently, computational fluid dynamics (CFD) has been used as a means of enhancing understanding of the mechanisms involved in atherosclerotic plaque formation and development. To analyze flow patterns and hemodynamic distribution in stenotic carotid bifurcation in vivo by combining CFD with magnetic resonance angiography (MRA). Twenty-one patients with carotid atherosclerosis proved by digital subtraction angiography (DSA) and/or Doppler ultrasound underwent contrast-enhanced MR angiography of the carotid bifurcation by a 3.0T MR scanner. Hemodynamic variables and flow patterns of the carotid bifurcation were calculated and visualized by combining vascular imaging postprocessing with CFD. In mild stenotic cases, there was much more streamlined flow in the bulbs, with reduced or disappeared areas of weakly turbulent flow. Also, the corresponding areas of low wall shear stress (WSS) were reduced or even disappeared. As the extent of stenosis increased, stronger blood jets formed at the portion of narrowing, and more prominent eddy flows and slow back flows were noted in the lee of the stenosis. Regions of elevated WSS were predicted at the portion of stenosis and in the path of the downstream jet. Areas of low WSS were predicted on the leeward side of the stenosis, corresponding with the location of slowly turbulent flows. CFD combined with MRA can simulate flow patterns and calculate hemodynamic variables in stenotic carotid bifurcations as well as normal ones. It provides a new method to investigate the relationship of vascular geometry and flow condition with atherosclerotic pathological changes.

Microsatellite loci for the small hive beetle, Aethina tumida, a nest parasite of honey bees

Aethina tumida, a beetle parasite of honey bee colonies, has recently and dramatically expanded its range and now parasitizes honey bees on three continents. Polymorphic microsatellite loci for this beetle species will help map this continuing range expansion, and will also prove useful for exploring the mating system and local gene flow patterns for this important parasite. We describe 15 loci that are polymorphic in both the native and introduced ranges of this species, showing from two to 22 alleles.

Early postoperative morphology of the carotid artery following endarterectomy: systematic prospective studies with a high resolution ultrasound real-time imaging system

The ultrasonic morphology of the carotid artery following 55 endarterectomies in 50 patients was systematically studied using a high resolution ultrasound duplex system. The typical findings were: At the site of endarterectomy the new vessel wall showed: lack of the "sonographic tunica intima"; the thickness of the vessel wall was reduced with less pronounced ultrasonic structure compared with healthy subjects; the surface toward the lumen was less smooth; "ultrasonically soft" thrombotic layers with irregular surface, individual distribution (sometimes only spots) and thickness (0.5 to 4.0 mm) were seen in the majority of patients. The edge of the tunica intima was always visible in the common carotid (edge of endarterectomy), whereas in the internal carotid artery there was often an interference of the jaw. The change of caliber at the site of the edge of the tunica intima was dependant on the thickness of the "sonographic intima" as well as on the layer of the vessel wall in which endarterectomy was done. At the site of arterial clamping intramural haematomas, lesions of the tunica intima with thrombotic layers causing some stenosis were observed after 9 of 55 procedures. Vascular sutures were always visible as bright spots. A "normalisation" of the pulsatory motion of the vessel wall as well as "normalisation" of local flow patterns could be seen in dependence on the morphological "normalisation". The soft thrombotic layers of the new vessel wall were especially pronounced at sites of incomplete endarterectomy, in one case with change of the vessel geometry and in another case were the opposite carotid was operated on 3 days later. Obviously, a short phase of high instability of the endarterectomy site (3 days, development of the platelet-fibrin layer) is followed by a period of relative instability of the new vessel wall, which lasted for 4 to 8 weeks. In 5 patients with central neurological deficits following surgery, especially pronounced soft thrombotic layers of the new vessel wall including 2 complete occlusions and 1 near-occlusion were observed.

Physical modeling of the influence of bedrock topography and ablation on ice flow and meteorite concentration in Antarctica

Three‐dimensional laboratory physical experiments have been used to investigate the influence of bedrock topography and ablation on ice flow. Different models were tested in a Plexiglas box, where a transparent silicone simulating ice in nature was allowed to flow. Experimental results show how the flow field (in terms of both flow lines and velocity) and variations in the topography of the free surface and internal layers of the ice are strongly influenced by the presence and height of bedrock obstacles. In particular, the buttressing effect forces the ice to slow down, rise up, and avoid the obstacle; the higher the bedrock barrier, the more pronounced the process. Only limited uplift of internal layers is observed in these experiments. In order to exhume deep material embedded in the ice, ablation (simulated by physically removing portions of silicone from the model surface to maintain a constant topographic depression) must be included in the physical models. In this case, the analogue ice replenishes the area of material removal, thereby allowing deep layers to move vertically to the surface and severely altering the local ice flow pattern. This process is analogous to the ice flow model proposed in the literature for the origin of meteorite concentrations in blue ice areas of the Antarctic plateau.

A locally conservative variational multiscale method for the simulation of porous media flow with multiscale source terms

We present a variational multiscale mixed finite element method for the solution of Darcy flow in porous media, in which both the permeability field and the source term display a multiscale character. The formulation is based on a multiscale split of the solution into coarse and subgrid scales. This decomposition is invoked in a variational setting that leads to a rigorous definition of a (global) coarse problem and a set of (local) subgrid problems. One of the key issues for the success of the method is the proper definition of the boundary conditions for the localization of the subgrid problems. We identify a weak compatibility condition that allows for subgrid communication across element interfaces, a feature that turns out to be essential for obtaining high-quality solutions. We also remove the singularities due to concentrated sources from the coarse-scale problem by introducing additional multiscale basis functions, based on a decomposition of fine-scale source terms into coarse and deviatoric components. The method is locally conservative and employs a low-order approximation of pressure and velocity at both scales. We illustrate the performance of the method on several synthetic cases and conclude that the method is able to capture the global and local flow patterns accurately.

A Review of Flow Pattern-Based Predictive Correlations during Refrigerant Condensation in Horizontally Smooth and Enhanced Tubes

A state-of-the-art review of recent work on flow pattern-based predictive correlations during refrigerant condensation in horizontally smooth and enhanced tubes is presented. The review focuses primarily on correlations for heat transfer coefficient and pressure drop during flow condensation that ignored flow regime effects and treated flows as either annular or non-stratified flow, or as stratified flow. This resulted in correlations of poor accuracy and limited validity and reliability. Current heat transfer coefficient, pressure drop, and void fraction models are based on the local flow pattern, resulting in deviations of around 20% from experimental data. There are, however, several inconsistencies and anomalies regarding these models that are addressed in this paper. A generalized solution methodology for two-phase flow problems still remains an elusive goal, mainly because gas-liquid flow systems combine the complexities of turbulence with those of deformable vapor-liquid interfaces. The paper focuses on the state-of-the-art research in the field of enhanced heat transfer, and proposes future research areas to help pave the way for the development of a unified and accurate two-phase flow model.

Endothelial dysfunction and monocyte recruitment in cells exposed to non-uniform shear stress

Atherosclerosis results from a combination of local blood flow patterns and systemic risk factors. We investigated whether non-uniform shear stress at bifurcations induces pro-atherogenic endothelial dysfunction and monocyte recruitment. Bifurcating flow-through cell culture slides were used to expose HUVECs to laminar or non-uniform shear stress for 18 h at 10 dyne/cm(2). For the adhesion assay, HUVECs were subsequently perfused with medium containing THP-1 monocytes for 1 h. Protein expression was determined by immunofluorescence. In areas exposed to laminar shear stress, alignment of endothelial cells with the flow was observed, accompanied by upregulation of eNOS and downregulation of connective tissue growth factor (CTGF). In contrast, cells exposed to non-uniform shear stress near the outer walls of bifurcations were characterized by irregular, unaligned shape, induction of endothelin-1 and CTGF, as well as reduced eNOS expression. These atherogenic effects of non-uniform shear stress were prevented when cells were treated with statins (1 mumol/l) during flow. Under non-uniform shear stress, a slight induction of VCAM-1, ICAM-1, and E-/P-selectin was observed. In agreement with this, monocyte recruitment, which was nearly undetectable under laminar shear stress, was moderately induced by non-uniform shear stress (P<0.02). In conclusion, inhibition of antioxidative eNOS and upregulation of atherogenic proteins is the first step in non-uniform shear stress-mediated endothelial dysfunction, which in vivo in the presence of atherogenic risk factors may further enhance monocyte recruitment into the artery wall.

IMPACTS OF FLOOD EVENT ON CHANNEL MORPHOLOGIES AROUND RIVER RESTORATION STRUCTURES

Flood event has obvious and long-lasting impacts on channel morphologies, posing high risks of damage and/or failure of river restoration projects. This study evaluates the impacts of flood event on the channel morphologies around typical river restoration structures and provides possible management alternatives for a real restoration project. Both laboratorial experiments and numerical simulations have been carried out. The experiments are based on a large-scale physical model, being able to resolve local flow patterns and morphological variations with a high accuracy. The numerical model is formulated on unstructured meshes, allowing the exact representation of complex geometries and boundaries. It is shown that the numerical results are reasonably consistent with those of the experimental measurements. The numerical model might make a powerful tool and the results of this study would be of excellent references for future engineering designs and post-project assessments in the river restoration.

Plaques in the descending aorta: A new risk factor for stroke? Visualization of potential embolization pathways by 4D MRI

The combination of morphologic and hemodynamic information can help in assessing the risk of embolic stroke associated with thrombi and plaques in the descending aorta. For two acute stroke patients, the determination of individual embolic pathways using flow-sensitive four-dimensional (4D) MRI are reported. 3D visualization of local flow patterns, i.e., retrograde flow channels originating at the site of the atheroma, in conjunction with exact plaque localization, suggested potential embolization of high-risk plaques in the descending aorta although they are located downstream from the supraaortic arteries. Our findings indicate that taking plaques of the descending aorta into consideration may help improve the spectrum of pathologies considered as high-risk sources for brain ischemia.