Local Flow Patterns Research Articles

COMPUTATIONAL FLUID DYNAMICS FOR TISSUE ENGINEERING APPLICATIONS

Hydrodynamic cellular environment plays an important role in translating engineered tissue constructs into clinically useful grafts. However, the cellular fluid dynamic environment inside bioreactor systems is highly complex and it is normally impractical to experimentally characterize the local flow patterns at the cellular scale. Computational fluid dynamics (CFD) has been recognized as an invaluable and reliable alternative to investigate the complex relationship between hydrodynamic environments and the regeneration of engineered tissues at both the macroscopic and microscopic scales. This review describes the applications of CFD simulations to probe the hydrodynamic environment parameters (e.g., flow rate, shear stress, etc.) and the corresponding experimental validations. We highlight the use of CFD to optimize bioreactor design and scaffold architectures for improved ex-vivo hydrodynamic environments. It is envisioned that CFD could be used to customize specific hydrodynamic cellular environments to meet the unique requirements of different cell types in combination with advanced manufacturing techniques and finally facilitate the maturation of tissue-engineered constructs.

The Use of Topography as an Indicator of Ground Water Flow Direction and Its Implications for Due Diligence

Abstract Topography often is used as a quick and preliminary method to identify local and regional ground water flow patterns within the upper-most saturated interval. This approach usually is applied in designing shallow monitoring networks at properties or in areas where little to no site specific hydrogeologic data are available. Perhaps the most problematic application of this method is in due diligence. Critical business decisions regularly are made based on potential environmental impacts from surrounding parcels using topographic presumptions about upgradient or downgradient ground water flow directions. However, what most practitioners fail to realize is that topography is a reliable predictor of ground water flow only under a fairly narrow range of physiographic conditions. To assist in determining when topography should be used to evaluate ground water flow direction within the upper-most saturated interval, we have developed an evaluation matrix that allows the investigator, who may have only modest hydrogeologic training, to rapidly identify when topography can be appropriately and reliably used to determine shallow ground water flow directions. Limitations on the use of the matrix are provided as well as suggestions for obtaining site specific data that allow for a more definitive understanding of shallow ground water flow without the need for intrusive field work.

Mathematical Modeling of Fluid Flow in Bath during Combined Side and Top Blowing AOD Refining Process of Stainless Steel: Application of the Model and Results

AbstractThe mathematical model developed for the molten steel flow in the combined side and top blowing AOD refining process of stainless steel has been used to compute and analyze the flow fields of the liquid phases in the baths of the 120 t AOD converter and its water model unit with a 1/4 linear scale. The influence of the side tuyere number and the angle between each tuyere on the flows has been examined. The results demonstrate that the mathematical model can quite reliably and well model and predict the fluid flow in an AOD bath with the combined blowing. The liquid flow in an AOD converter bath with the combined blowing is resulted from the gas side blowing streams under the influence of a gas top blowing jet. The streams play a governing role on it; and the liquid in the whole bath is in active agitation and circulatory motion during the gas blowing process. The gas jet from the top lance does not change the essential features of the gas stirring and liquid flow in the bath, but can make the local flow pattern of the bath liquid obviously vary and its turbulent kinetic energy enhance. The changes in the tuyere position and number have similarly not altered the basic characteristics and patterns of the gas agitation and liquid flow and turbulent kinetic energy distribution in the bath. At a given tuyere number and gas side blowing rate or a given angular separation between each tuyere and gas side blowing rate, however, the variation of the angle between each tuyere or the tuyere number can locally change them. Using 6 tuyeres with 27° can reach the more uniform flow field and turbulent energy distribution of the liquid in the bath than taking 7 tuyeres with 18° or 22.5° and 6 tuyeres with 22.5°.

Hydrodynamic Complexity Induced by the Pulsing Flow Field in USP Dissolution Apparatus 4

USP dissolution Apparatus 4 can have a pulsing or non-pulsing flow, with most commercial apparatus employing a pulsing flow. Overall, a low velocity flow field is present, particularly in the larger 22.6-mm diameter cell. Dissolution data, computational fluid dynamics (CFD), and imaging methods are used and discussed to investigate the effects of low velocity pulsing flow on dissolution. Local velocity patterns, density gradients, and the effect of flow rate on particulate dispersion can all contribute to the observed dissolution rate. In silico simulation tools and imaging techniques have proved useful in determining local hydrodynamics and, in some cases, their effect on observed dissolution rates. Methods to investigate concentration gradients at the dissolving surface are required to investigate further the effect of hydrodynamics on the dissolution process. In conclusion, it is necessary to take local flow patterns and velocities into account, rather than overall flow rate alone, when interpreting dissolution rate data in the flow-through apparatus.

Temporal and seasonal variability of arsenic in drinking water wells in Matlab, southeastern Bangladesh: A preliminary evaluation on the basis of a 4 year study

Temporal and seasonal variability of As concentrations in groundwater were evaluated in As-affected areas of Matlab, southeastern Bangladesh. Groundwater samples from 61 randomly selected tubewells were analyzed for As concentrations over a period of three years and four months (from July 2002 to November 2005) and monitored seasonally (three times a year). The mean As concentrations in the sampled tubewells decreased from 153 to 123 μg/L during July 2002 to November 2005. Such changes were pronounced in tubewells with As concentration >50 μg/L than those with As concentrations <50 μg/L. Similarly, individual wells revealed temporal variability, for example some wells indicated a decreasing trend, while some other wells indicated stable As concentration during the monitoring period. The mean As concentrations were significantly higher in Matlab North compared with Matlab South. The spatial variations in the mean As concentrations may be due to the differences in local geological conditions and groundwater flow patterns. The variations in mean As concentrations were also observed in shallow (<40 m) and deep (>40 m) wells. However, to adequately evaluate temporal and seasonal variability of As concentration, it is imperative to monitor As concentrations in tubewells over a longer period of time. Such long-term monitoring will provide important information for the assessment of human health risk and the sustainability of safe drinking water supplies.

Visual Analysis of Flow Features Using Information Theory

Unsteady scientific data is one of the top challenges in visualization, because a huge amount of information must be displayed. ϵ-machines are an information-theoretic concept; they compress the dynamics in the data set to a finite-state machine, in which nodes represent local flow patterns and edges represent transitions between them. Several enhancements to the fundamental ϵ-machine representation can help users identify interesting time intervals, analyze the evolution of unusual local dynamics, and track features over time. Automatically abstracting information from the original data is a first step toward knowledge-assisted visualization. Successive findings from analysis can help provide subsequent users with knowledge gained in earlier research, resulting in a knowledge-assisted system for the analysis of unsteady-flow features based on information theory. This article is part of a special issue on knowledge-assisted visualization.

DNS study on viscoelastic effect in drag-reduced turbulent channel flow

The effect of viscoelastic contribution on wall-bounded turbulent flow of drag-reducing surfactant solution is investigated through direct numerical simulations (DNS). A series of DNS on turbulent channel flow is performed for different rheological properties at two different Reynolds numbers. It is found that high drag reduction is achieved by suppressing the turbulent contribution for the high Weissenberg number, and/or by decreasing the viscosity ratio and the effective viscosity. A highly drag-reduced turbulent flow at a high Reynolds number is mainly caused by the viscoelastic effect in the elastic layer, whereas the outer layer flow hardly affects the drag reduction. Moreover, we focus on the viscoelastic contribution term in the budget of Reynolds stress and its relation to the local flow pattern. It is shown that in the near-wall region of the highly drag-reduced flow a positive work done by viscoelastic stress is closely associated with vortex stretching that produces turbulent kinetic energy from stored elastic energy, whereas a negative one causes vortex compression.

Evaluation of Local Flow Conditions in Jailed Side Branch Lesions Using Computational Fluid Dynamics

Background and ObjectivesLesions of vascular bifurcation and their treatment outcomes have been evaluated by anatomical and physiological methods, such as intravascular ultrasound and fractional flow reserve (FFR). However, local changes in flow dynamics in lesions of bifurcation have not been well evaluated. This study aimed at evaluating changes in the local flow patterns of bifurcation lesions.Materials and MethodsEight (n=8) representative simulation-models were constructed: 1 normal bifurcation, 5 main-branch (MB) stenting models with various side-branch (SB) stenoses (ostial or non-ostial 75% diameter stenosis with 1- or 2-cm lesion lengths, ostial 75% diameter stenosis caused by carina shift), and 2 post-kissing models (no or 50% SB residual stenosis). Pressure, velocity, and wall shear stress (WSS) profiles around the bifurcation sites were investigated using computational fluid dynamics.ResultsPost-stenting models revealed significant pressure drop in the SB (FFR<0.75), excluding the carina shift model (FFR=0.89). In the post-kissing models, there was no significant pressure drop. All post-stenting models revealed eccentric low velocity flow patterns and areas of low WSS, primarily in the lateral wall on distal MB. Post-kissing angioplasty improved pressure drop in the SB but resulted in alteration of flow distribution in the MB. In the carina shift model, kissing ballooning resulted in deteriorated local flow conditions due to increased area of low velocity and WSS.ConclusionThis study suggests that the most commonly used bifurcation intervention strategy may cause local flow disturbances, which may partially explain high restenosis and event rates in patients with bifurcation lesions.

Measurements and CFD simulations of flow and dispersion in urban geometries

Dispersion in urban areas is determined by transport and mixing processes within the street network and ventilation into the external atmosphere. In order to improve the understanding of the exchange mechanisms within the urban canopy, wind tunnel and numerical investigations have been performed on two idealised typical urban configurations, the street intersection and the square. The numerical simulations were compared to the experimental values, in order to evaluate the reliability of the numerical approach in simulating local flow patterns and study the influence of the geometrical layout on mass and momentum exchange mechanisms.

Computational Modeling of Distal Protection Filters

To quantify the relationship between velocity and pressure gradient in a distal protection filter (DPF) and to determine the feasibility of modeling a DPF as a permeable surface using computational fluid dynamics (CFD). Four DPFs (Spider RX, FilterWire EZ, RX Accunet, and Emboshield) were deployed in a single tube representing the internal carotid artery (ICA) in an in vitro flow apparatus. Steady flow of a blood-like solution was circulated with a peristaltic pump and compliance chamber. The flow rate through each DPF was measured at physiological pressure gradients, and permeability was calculated using Darcy's equation. Two computational models representing the RX Accunet were created: an actual representation of the filter geometry and a circular permeable surface. The permeability of RX Accunet was assigned to the surface, and CFD simulations were conducted with both models using experimentally derived boundary conditions. Spider RX had the largest permeability while RX Accunet was the least permeable filter. CFD modeling of RX Accunet and the permeable surface resulted in excellent agreement with the experimental measurements of velocity and pressure gradient. However, the permeable surface model did not accurately reproduce local flow patterns near the DPF deployment site. CFD can be used to model DPFs, yielding global flow parameters measured with bench-top experiments. CFD models of the detailed DPF geometry could be used for "virtual testing" of device designs under simulated flow conditions, which would have potential benefits in decreasing the number of design iterations leading up to in vivo testing.

Plastic flow macrolocalization in aluminum and the Hall-Petch relation

The parameters of plastic deformation macrolocalization are compared to the parameters of the Hall-Petch relation for the flow stress in polycrystalline aluminum samples with a grain size of 0.008–5.000 mm. Two types of the dependence of the localized plastic deformation autowave length on the grain size and two versions of hardening according to the Hall-Petch relation are found in the grain size range under study. The boundary between these versions is shown to correspond to d ≈ 0.1 mm for both cases. A relation between localized plastic flow patterns and the Hall-Petch relation is revealed.

Subsoil Characteristics and Hydrogeology of the Export Processing Zone, Calabar-Southeastern Nigeria

The subsoil characterization and hydrogeological investigation of the Export Processing Zone (EPZ), Calabar Southeastern Nigeria was undertaken using geotechnical analysis of soils and water level monitoring. Geotechnical analysis of soils in the EPZ show that the grain size range from poorly graded (well sorted) to well graded (poorly sorted) soils with very little or no fines (0.1-5.30% passing 0.075mm sieve). Permeability values vary from 6.80 x10-3 cm/s to 9.43 x10-3 cm/s for the sandy clay to gravelly clay, to 7.00 x10-1cm/s to 8.70 x10-1cm/s for the coarse sand to gravelly sand. Soil consistency shows a plasticity range (7.00 – 13.00%) for the clayey soils with the predominantly sandy soil being non-plastic. The local groundwater migration in the area is towards the north easterly direction, meaning that the local flow pattern discharges towards area which has the lowest groundwater elevation. Following this, the implication is that subsurface water migration in the area will move towards and intersect the Calabar River flowing mostly in a southerly direction. This, by implication, will decrease the concentration of probable contaminants due to dispersion and other attenuation effects. Therefore, regular assessment of the subsoil characteristics and hydrogeology of the area will provide baseline data for planning in order to reduce the associated/attendant negative environmental hazards of export processing activities.

공조용 축류홴 설계 및 설계변수에 따른 성능과 소음비교

Axial fans for an air-conditioning unit are designed to equip the system with an expected flow-rate and low noise level by applying the blade design method of multi-sectioning and local camber generation. In this study, the distributions of chord length, stagger angle, and camber angle are globally and locally determined for the given specific speed, which is considered to be relatively high. The mock-up fans are observed to satisfy the aerodynamic performance and the noise level for the system simultaneously and discussed in terms of local flow patterns related to the emitted noise.

Aortic Regurgitation Dramatically Alters the Distribution of Atherosclerotic Lesions and Enhances Atherogenesis in Mice

Hemodynamics plays a critical role in atherogenesis, but the association between flow pattern and preferential localization of lesion is not fully understood. We developed a mouse model of aortic valve regurgitation (AR) to change the aortic flow pattern and observed the effects on plaque formation. High-frequency Doppler ultrasound imaging of 10 untreated C57BL/6J mice and 6 sham-treated low-density lipoprotein receptor-deficient (Ldlr(-/-)) mice revealed consistent antegrade blood flow throughout the aorta and oscillatory flow only along the lesser curvature of the aortic arch. Catheter-induced AR in 7 Ldlr(-/-) mice produced various degrees of diastolic retrograde flow throughout the aorta. After the mice were fed a cholesterol-enriched diet for 6 weeks, the burden of atherosclerotic lesions was increased 6-fold, with the naturally plaque-resistant descending aorta becoming susceptible. The AR severity correlated positively with the lesion burden in the descending thoracic and abdominal aorta but negatively with the lesions in the ascending aorta and aortic arch. This AR model is valuable for elucidating the relationship between hemodynamics and predisposition of the artery wall to atherosclerosis, because of the significant alterations in local flow patterns and the conversion of large regions in the descending aorta from lesion resistant to lesion prone.

Drivers of bacterial colonization patterns in stream biofilms

Dispersal and colonization are important for the assembly and biodiversity of microbial communities. While emigration as the initial step of dispersal has become increasingly understood in model bacterial biofilms, the drivers of dispersal and colonization in complex biofilms remain elusive. We grew complex biofilms in microcosms from natural surface water in laminar and turbulent flow, and investigated dispersal and colonization patterns of fluorescently labeled cells and microbeads in nascent and mature biofilms. Settling occurred in nonrandom spatial patterns governed by the interplay of local flow patterns and biofilm topography. Settling was higher in treatments with nascent biofilms, with fewer cells remaining in the water column than in treatments with mature biofilms. The flow regime had no effect on settling velocity, even though in mature biofilms the formation of streamers under turbulent flow enhanced particle trapping compared with the laminar flow treatment. Hence, small-scale variations in the flow pattern seemed to be more important than the overall flow regime. Furthermore, spatial analysis of the colonizer patterns suggests that bacteria have moved in the biofilm after settling. Our results show that colonization of biofilms in a model stream environment is a heterogeneous process differently affected by biological and physical factors.

Development and Verification of Unstructured Adaptive Mesh Technique with Edge Compatibility

In a design study of the large-sized sodium-cooled fast reactors in Japan (JSFR), one key issue to establish an economically superior design is suppression of a gas entrainment (GE) phenomenon at a free surface in the reactor vessel. However, the GE phenomenon is highly non-linear and too difficult to be evaluated theoretically. Therefore, we are developing a high-precision CFD method to evaluate the GE phenomenon accurately. The CFD method is formulated on an unstructured mesh to establish an accurate modeling for a complicated shape of the JSFR system. As a two-phase flow simulation method, a high-precision volume-of-fluid algorithm is employed in the CFD method. In addition, physically appropriate formulations at gas-liquid interfaces are introduced into the CFD method. The developed CFD method is already applied to the simulation of a GE phenomenon in a basic GE experiment and the simulation results show good agreement with experimental results. Therefore, it is confirmed that the proposed CFD method can reproduce a GE phenomenon. However, for the simulation of the GE phenomenon in the JSFR, we still have one problem on a mesh subdivision. Though a fine mesh subdivision has to be applied to the regions where the GE occurs, it is difficult to preliminarily know the regions because the GE occurrence is strongly affected by a local instant flow pattern, i.e. a vortex generation. Therefore, an adaptive mesh technique is necessary to apply a fine mesh subdivision automatically to only the local GE occurrence regions in the large-sized JSFR. In this study, as one part of an adaptive mesh development, a two-dimensional unstructured adaptive mesh technique is developed and verified. In the proposed two-dimensional adaptive mesh technique, each cell is isotropically subdivided to reduce distortions of the mesh. In addition, a connection cell is formed to eliminate the edge incompatibility between a refined and a non-refined cells. A connection cell has several subdivision patterns and one of them is selected to be compatible with adjacent cells on every cell edge. Finally, the present unstructured adaptive mesh technique is verified by solving well-known driven cavity problem. As the result, the present unstructured adaptive mesh technique succeeds in providing a high-precision solution, although we employ a poor-quality distorted mesh at the initial state. In addition, the simulation error on the unstructured adaptive mesh at the steady state is much less than the error on the structured mesh consisting of a larger number of cells.Copyright © 2009 by ASME

Scanning Laser Image Correlation (SLIC) Measurements in Zebra Fish Larvae

3003‐Pos Scanning Laser Image Correlation (SLIC) Measurements in Zebra Fish Larvae Molly J. Rossow, William W. Mantulin, Enrico Gratton. University of California Irvine, Irvine, CA, USA. Scanning Laser Image Correlation (SLIC) is a technique to measure the flow of small particles and to observe local flow patterns over an area. SLIC can be applied to situations including blood cells flowing through blood vessels or tracer particles flowing through microfluidic channels. The main advantage of SLIC over other flow measurement techniques is that SLIC can be scaled to measure flow in areas ranging from microns to centimeters wide. To accomplish this, an image is first acquired through laser scanning and analyzed with number and brightness analysis (N&B) to identify regions of flow. This is used as a guide to manually (or potentially automatically) select a pattern within the image, such as a line along the center of a channel, that is then scanned repeatedly with the laser beam. Since the entire image is not scanned in each measurement SLIC measurements can be obtained quickly and efficiently. The results of these scanned patterns are analyzed with the recently developed pair correlation technique to extract the rate of flow and to identify characteristic flow patterns such as turbulence, particles that adhere to the channel walls, and variable velocity along the length of the channel. In the work presented here we demonstrate the effectiveness of SLIC by measuring blood flow in a zebra fish model. With SLIC, we are able to obtain blood flow measurements equivalent to those obtained with other techniques. We are also able to map the rate of flow and to observe variability in flow rate over time. This indicates that SLIC has potential to measure blood flow in other animals as well and may hold potential as the basis of a medical device. This research was supported by the National Institutes of Health (PHS 5 P41‐RR003155.)

J0206-2-3 Multi-scale modeling of the cardiovascular system : integration of local flow patterns, arterial wave propagation and global hemodynamics

Definition of boundary conditions is a crucial step in three-dimensional (3-D) hemodynamic computation. An improper treatment of boundary conditions may result in misleading predictions no matter how precisely the geometric model has been constructed. In consideration of the fact that any arterial segment belongs to the cardiovascular system, a reasonable means of getting rid of difficulties in boundary condition definition is to place a 3-D hemodynamic model into a model which represents a broader cardiovascular area. In this study, we develop a multi-scale model by coupling models of dimensional levels. The model integrates 3-D flows, arterial wave propagation and global hemodynamics into a unique computational system. Benchmark tests demonstrate that the model is able to reasonably describe hemodynamic phenomena in various physiological conditions.

Drivers of bacterial colonization patterns in stream biofilms

Dispersal and colonization are important for the assembly and biodiversity of microbial communities. While emigration as the initial step of dispersal has become increasingly understood in model bacterial biofilms, the drivers of dispersal and colonization in complex biofilms remain elusive. We grew complex biofilms in microcosms from natural surface water in laminar and turbulent flow, and investigated dispersal and colonization patterns of fluorescently labeled cells and microbeads in nascent and mature biofilms. Settling occurred in nonrandom spatial patterns governed by the interplay of local flow patterns and biofilm topography. Settling was higher in treatments with nascent biofilms, with fewer cells remaining in the water column than in treatments with mature biofilms. The flow regime had no effect on settling velocity, even though in mature biofilms the formation of streamers under turbulent flow enhanced particle trapping compared with the laminar flow treatment. Hence, small-scale variations in the flow pattern seemed to be more important than the overall flow regime. Furthermore, spatial analysis of the colonizer patterns suggests that bacteria have moved in the biofilm after settling. Our results show that colonization of biofilms in a model stream environment is a heterogeneous process differently affected by biological and physical factors.

An In Vitro Perfusion System to Examine the Responses of Endothelial Cells to Simulated Flow and Inflammatory Stimulation

Atherosclerosis is a disease process which develops at the arterial branches and curvatures of medium to large arteries. Local haemodynamic flow patterns in these vessels play an essential role in the formation of atherosclerotic lesions. To simulate pro-atherogenic blood flow patterns, we have developed a perfusion system with the ability to simulate in vivo patterns of blood flow in vitro. In this system, human umbilical vein endothelial cells were seeded in y-shaped microslides, in which they were exposed to a variety of flow patterns. Besides being able to reproduce the disturbed flow involved in the development of pro-atherogenic events within the arterial wall, the system also permitted the assessment of the pre-conditioning/priming effect of oscillatory flow on endothelial cells. The system was further capable of integrating multi-endpoint assays relevant to cardiovascular disease. We show that oscillatory flow primed endothelial cells, making them more sensitive to subsequent treatments. The treatment of oscillatory flow primed cells with TNFalpha resulted in the detection of enhanced levels of pro-inflammatory and chemoattractant factors such as IL-8 and MCP-1. These measurements were facilitated by the small volumes of medium circulating within the perfusion system. Oscillatory flow also altered the characteristics of monocyte adhesion to the endothelial layer. In summary, this system allows the monitoring of multiple endpoints and biomarkers, and provides an alternative to the use of in vivo and ex vivo models of cardiovascular disease.