Local Flow Patterns Research Articles

Hydropedological investigations with ground-penetrating radar (GPR): Estimating water-table depths and local ground-water flow pattern in areas of coarse-textured soils

In coarse-textured soils, ground-penetrating radar (GPR) can provide continuous, high-resolution records that chart the depth to water tables. The use of this information can increase confidence in hydropedological site assessments and reduce the number of wells needed for water-table depth and ground-water flow determinations. Ground-penetrating radar was used to map spatial and temporal variations in water-table depths and ground-water flow patterns within an unconfined aquifer located beneath an eolian landscape in northwestern Indiana. Compared with the data collected at a limited number of wells, the greater number of observations with GPR provided more comprehensive site coverage and mapped more intricate local ground-water flow patterns. Over a 2-year period, GPR revealed systematic temporal and spatial variations in water-table depths and local ground-water flow patterns.

Structural hierarchy developed in injection molding of nylon 6/clay/carbon black nanocomposites

The influence of platelet-type clay nanoparticle (nanoplatelet) on the structural evolution in injection-molded nylon 6)/carbon composites was investigated. In the absence of nanoplatelets, the nylon 6/CB systems were found to exhibit unoriented structure with nylon 6 crystalline regions exhibiting exclusively α crystal form throughout the thickness of the samples. However, inclusion of nanoplatelets induces substantial local orientation of the nylon 6 chains in the molded parts in all processing conditions and compositions. In these clay/nylon 6/CB ternary nano systems, nylon 6 matrix was found to be exclusively in γ crystal form at the skin regions and α crystal form fraction increases towards the core of the molded parts as a result of decrease in cooling rate with depth during the solidification stage. Two nanoplatelet orientation behaviors were identified: (i) in the absence of irregular shaped CB, the nanoplatelets align parallel with one another following the local flow patterns. The latter behavior also causes enhanced orientation of the nylon 6 chains undergoing substantial shear amplification trapped in between the nanoplatelets (ii) in the CB-enriched regions, nanoplatelets though still remaining parallel to one another, are randomized by following the local contours of irregular shaped CB aggregates. These CB aggregates themselves were found to organize to form trains in larger scale as a result of flow alignment.

Gas–solids flow patterns in a novel dual-loop FCC riser

A novel FCC process for maximizing iso-paraffins (MIP) has been developed by Research Institute of Petroleum Processing, SINOPEC. The MIP process is capable of reducing olefin content and increasing iso-paraffin content of gasoline to meet the new specification of gasoline of olefin content less than 35 vol.%. The MIP reactor is a combination of a conventional FCC riser (section I) and a dense circulating fluidized bed (section II), acting as two different reaction zones, respectively. This paper is devoted to improving the understanding of gas-solids flow patterns in the MIP riser. Experiments were conducted in a 10.2 m high two-stage riser with diameters of 45 mm in lower section and 90 mm in upper section. FCC catalysts were carried by atmospheric air in the experiments. The overall and local flow patterns were determined by using the axial pressure profiles measurements and radial local optic fiber probe measurements of particles velocity and voidage at different riser height. The measurements revealed that a much complex axial and radial profiles of voidage and particle velocity existed in the riser. Significant velocity fluctuations and a much lower voidage existed at the inlet of section II due to the influences of diffuser and particles feeding above the diffuser. The influences of gas velocity and additional particles feeding on the pressure drop and solids circulating rate were different in section I and in section II. In section II, these influences weakened with the height increased.

Natural geochemical concentrations and fluxes of Cu, Th and U in Finland

Concentrations of U, Th and Cu were studied as a part of the coordinated IAEA (International Atomic Energy Agency) research project on ‘The use of selected safety indicators (concentrations; fluxes) in the assessment of radioactive waste disposal’. Uranium and Th served as analogues of radionuclides in the waste matrix (spent fuel) and Cu is a construction material in the Finnish disposal approach in granitic rock. Release and migration of these components through the geosphere to the biosphere is thought to be caused and influenced by corrosion, oxidation and dissolution and subsequent transport and retardation processes in groundwater in hydraulically conducting rock fractures. The weathering rates for Cu, U and Th were calculated on the basis of the weathering rate of base cations and the concentrations of these elements in the parent soils. The mean weathering rate for Cu in till soils is 0.21 mg m −2 a −1 . The mean values of estimated weathering rates for Th and U are 0.17 mg m −2 a −1 and 0.054 mg m −2 a −1 , respectively. Analytical data and flux measurements from groundwater monitoring of selected springs are used to calculate the quantity of dissolved elements discharging from the aquifer. Typical fluxes for five small springs were 0.023–0.46 kg Cu km −2 a −1 and 0.003–0.43 kg U km −2 a −1 . The estimated fluxes of Cu and U in headwater streams were calculated on the basis of concentration data and the rate of stream flow. The estimated fluxes in 30 streams are 0.44–1418 g/month for Cu and 0.08–285 g/month for U. From the point of view of the use of natural elemental concentrations and fluxes as natural safety indicators in the evaluation of nuclear waste repository, the basis of this complementary or alternative concept has been corroborated by the results of this work. Elemental concentration data can be better understood in relation to the underlying geology and the prevailing geochemistry and by correlation to observed elemental fluxes. The comparison of the spatial distribution of elements in different matrices gives a qualitative estimation of the dominating fluxes. Widespread correlation between different compartments indicates mobile element fluxes from the geosphere to the biosphere. Also fixation in sinks, in particular, for U was observed. The geochemical cycles are dominated by near-surface weathering processes. Fluxes are also controlled by small-scale, local groundwater flow patterns; contributions from greater depths were found to be negligible.

Effects of localized horizontal flow patterns on eigenfrequencies of stellar global modes

The main goal of this paper is to estimate the significance of effects the flow profile U0(z) induces on computed global oscillation spectra. We solve numerically the eigenvalue problem for the described basic state configuration with reference to solar conditions as an example of a typical star. Obtained results show that the considered shear flows do not only affect the eigenfrequencies ? but they also have an impact on spatial profiles of eigensolutions of linearized physical quantities. Such flows may even allow for local amplification of perturbation amplitudes at positions where the resonant Doppler condition ? = kU0(z) is satisfied. For typical subsonic flow speeds this Doppler resonant wave amplification can be effective only in the case of the non acoustic gravity modes with sufficiently large horizontal wave numbers k.

Experimental Study on Local Mass Transfer in a Simplified Bifurcation Model: Potential Role in Atherosclerosis

Local mass transfer coefficients and flow patterns were examined in an idealized human aortic bifurcation model. The objectives of this study are to gain further insights on the convective mass transfer process and its possible role in the localization of atherosclerotic lesions. The laser photochromic tracer method provided velocity and wall shear stress estimates in the plane of symmetry of a UV-transparent Plexiglas bifurcation model. Steady flow data were acquired at Reynolds numbers of 500, 600, and 750. A novel copper electrodeposition technique was used to obtain time-averaged convective local mass transfer coefficients in a model identical to that used in the flow experiments. The laminar flow mass transfer data for the trunk of the bifurcation are in good agreement with the analytical Levesque solution. At the bifurcation, higher mass transfer coefficients along the inner wall and lower ones along the outer wall were observed. Further, mass transfer and wall shear stress follow similar patterns both on the inner and outer walls in that StSc 2/3 and C(f)/2 demonstrate analogous behavior. Lower transfer rates of momentum and mass occurred along the outer wall of the branches where lesions tend to develop.

Hydrology and Geostatistics of a Vermont, USA Kettlehole Peatland

The ability to predict the response of peatland ecosystems to hydrologic changes is imperative for successful conservation and remediation efforts. We studied a 1.25-ha Vermont kettlehole bog for one year (September 2001–October 2002) to identify hydrologic controls, temporal and spatial variability in flow regimes, and to link hydrologic processes to density of the carnivorous plant ( Sarracenia purpurea), an ombrotrophic bog specialist. Using a spatial array of nested piezometers, we measured surface and subsurface flow in shallow peat and surrounding mineral soil. Our unique sampling array was based on a repeated measures factorial design with: (1) incremental distances from a central kettlehole pond; (2) equal distances between piezometers; and (3) at three depths from the peat surface. Local flow patterns in the peat were controlled by snowpack storage during winter and spring months and by evapotranspiration and pond water elevation during summer and fall months. Hydraulic head values showed a local reversal within the peat during spring months which was reflected in higher chemical constituent concentrations in these wells. On a regional scale, higher permeable soils diverted groundwater beneath the peatland to a nearby wetland complex. Horizontal water gradient magnitudes were larger in zones where the peatland was perched above regional groundwater and smaller in zones where a hydraulic connection existed between the peatland and the regional groundwater. The density of pitcher plants ( S. purpurea) is strongly correlated to the distance from a central pond, [Fe 3+], [Na +], [Cl −], and [ SO 4 2 − ] . The pH, conductivity, and [Ca 2+] had significant effects of depth and time with horizontal distance correlations between 20 and 26 m. The pH samples had temporal correlations between 27 and 79 days. The link between pitcher plants and ion chemistry; significant effects of peatland chemistry on distance, depth, and time; and spatial and temporal correlations are important considerations for peatland restoration strategies.

Natural examples of olivine lattice preferred orientation patterns with a flow-normal a-axis maximum.

Tectonic plate motion is thought to cause solid-state plastic flow within the underlying upper mantle and accordingly lead to the development of a lattice preferred orientation of the constituent olivine crystals. The mechanical anisotropy that results from such preferred orientation typically produces a direction of maximum seismic wave velocity parallel to the plate motion direction. This has been explained by the existence of an olivine preferred orientation with an 'a-axis' maximum parallel to the induced mantle flow direction. In subduction zones, however, the olivine a axes have been inferred to be arranged roughly perpendicular to plate motion, which has usually been ascribed to localized complex mantle flow patterns. Recent experimental work suggests an alternative explanation: under conditions of high water activity, a 'B-type' olivine preferred orientation may form, with the a-axis maximum perpendicular to the flow direction. Natural examples of such B-type preferred orientation are, however, almost entirely unknown. Here we document widespread B-type olivine preferred orientation patterns from a subduction-type metamorphic belt in southwest Japan and show that these patterns developed in the presence of water. Our discovery implies that mantle flow above subduction zones may be much simpler than has generally been thought.

Application of Arbitrary Lagrange Euler Formulations to Flow-Induced Vibration Problems

Most classical fluid force identification methods rely on mechanical structure response measurements associated with convenient data processes providing turbulent and fluid-elastic forces responsible for possible vibrations and damage. These techniques provide good results; however, they often involve high costs as they rely on specific modelings fitted with experimental data. Owing to recent improvements in computational fluid dynamics, numerical simulation of flow-induced structure vibration problems is now practicable for industrial purposes. As far as flow structure interactions are concerned, the main difficulty consists in estimating numerically fluid-elastic forces acting on mechanical components submitted to turbulent flows. The point is to take into account both fluid effects on structure motion and conversely dynamic motion effects on local flow patterns. This requires a code coupling to solve fluid and structure problems in the same time. This ability is out of limit of most classical fluid dynamics codes. That is the reason why recently an improved numerical approach has been developed and applied to the fully numerical prediction of a flexible tube dynamic response belonging to a fixed tube bundle submitted to cross flows. The methodology consists in simulating at the same time thermo-hydraulics and mechanics problems by using an Arbitrary Lagrange Euler (ALE) formulation for the fluid computation. Numerical results turn out to be consistent with available experimental data and calculations tend to show that it is now possible to simulate numerically tube bundle vibrations in presence of cross flows. Thus a new possible application for ALE methods is the prediction of flow-induced vibration problems. The full computational process is described in the first section. Classical and improved ALE formulations are presented in the second part. Main numerical results are compared to available experimental data in section 3. Code performances are pointed out in terms of mesh generation process and code coupling method.

On Recent Advances in Modeling of Two-Phase Flow and Heat Transfer

General thermal design methods for two-phase heat exchangers are emerging that are based on local two-phase flow patterns and the flow structure of the two-phases. These methods promise to be much more accurate and reliable for predicting two-phase heat transfer coefficients and pressure drops than the older, statistically-derived empirical design methods that completely ignore flow regime effects or simply treat flows as stratified (gravity-controlled) or nonstratified (shear-controlled) flows, which greatly limits their accuracy, validity, and reliability and often results in prediction errors surpassing 100% within their supposed range of application. These new flow pattern and flow structure types of design methods are particularly suited for use in modern heat exchanger design software, which are typically incremental and hence require local methods that capture the real trends in experimental data. The status of these new developments is reviewed here for intube two-phase flow and heat transfer processes.

Evaluation of a CFD porous model for calculating ventilation in explosion hazard assessments

Abstract In the past, gas explosion assessment relied on worst case scenarios. A more realistic approach is to look at the probability of explosions and their likely severity. The most flexible way of investigating many different scenarios is to estimate a ventilation flow, feed this into a flammable volume calculation and then calculate the explosion severity. The procedure allows many parameters to be varied efficiently. A Computational Fluid Dynamics porous model is evaluated for modelling the ventilation flow through congested regions, including a new method that has been developed to derive the resistance. Comparison with velocity measurements from a large scale model of an offshore module showed that overall the CFD model performs very well, especially considering that the homogenous porosity block does not model any of the internal obstructions and therefore would not predict any local flow effects. This gives confidence that the overall flow pattern is sufficiently close to the local flow patterns, to be used in explosion assessments. The porous approximation in CFX is found to underpredict the turbulence intensity in the obstacle array compared to the explosion model EXSIM. Improving the turbulence prediction in the porous model would be valuable, so a relatively simple method of increasing the turbulence in porous regions is proposed. The CFD model will provide the non-uniform natural ventilation flowfields of complex regions for future explosion assessments at a hierarchy of levels.

Impact of shuttle rates on local grain flows.

Investment in shuttle elevators and the ability of these facilities to utilize the more competitive rates in attracting grain have the potential to influence future local grain flow patterns. This analysis provides an estimate of how shuttle train rates will impact grain flows in North Dakota's grain gathering network. In comparative terms, 2% of the elevators may originate up to 32% of the average annual flows of the grain produced in the state grains. Increased concentration of bushels has implications for local roads, elevators, short-line railroads, bridge infrastructure and local communities.

Numerical simulation of carotid hemodynamics in patients with rotary blood pump cardiac assist.

In recipients of rotary blood pumps for cardiac assist, the pulsatility of arterial flow is considerably diminished. This influences the shear stress patterns and streamlines in the arterial bed, with potential influence on washout and subsequent plaque growth. To study these effects, a three-dimensional computer simulation of the carotid bifurcation at various levels of flow pulsatility was performed. The results showed that as expected pulsatile shear stress varied considerably, whereas local mean shear stress levels were nearly identical for all degrees of pulsatility. Particle residence time in the carotid bulb did only increase for less than 15%, with secondary washout patterns contributing to good washout also in nonpulsatile conditions. It is concluded that also under continuous pump support the local flow patterns in the carotids provide sufficient washout and fluid exchange to prevent excessive plaque growth.

Role of Integrins in Endothelial Mechanosensing of Shear Stress

The focal pattern of atherosclerotic lesions in arterial vessels suggests that local blood flow patterns are important factors in atherosclerosis. Although disturbed flows in the branches and curved regions are proatherogenic, laminar flows in the straight parts are atheroprotective. Results from in vitro studies on cultured vascular endothelial cells with the use of flow channels suggest that integrins and the associated RhoA small GTPase play important roles in the mechanotransduction mechanism by which shear stress is converted to cascades of molecular signaling to modulate gene expression. By interacting dynamically with extracellular matrix proteins, the mechanosensitive integrins activate RhoA and many signaling molecules in the focal adhesions and cytoplasm. Through such mechanotransduction mechanisms, laminar shear stress upregulates genes involved in antiapoptosis, cell cycle arrest, morphological remodeling, and NO production, thus contributing to the atheroprotective effects. This review summarizes some of the recent findings relevant to these mechanotransduction mechanisms. These studies show that integrins play an important role in mechanosensing in addition to their involvement in cell attachment and migration.

A case for strain-induced fluid flow as a regulator of BMU-coupling and osteonal alignment.

Throughout life, human bone is renewed continuously in a tightly controlled sequence of resorption and formation. This process of bone remodeling is remarkable because it involves cells from different lineages, collaborating in so-called basic multicellular units (BMUs) within small spatial and temporal boundaries. Moreover, the newly formed (secondary) osteons are aligned to the dominant load direction and have a density related to its magnitude, thus creating a globally optimized mechanical structure. Although the existence of BMUs is amply described, the cellular mechanisms driving bone remodeling-particularly the alignment process-are poorly understood. In this study we present a theory that explains bone remodelling as a self-organizing process of mechanical adaptation. Osteocytes thereby act as sensors of strain-induced fluid flow. Physiological loading produces stasis of extracellular fluid in front of the cutting cone of a tunneling osteon, which will lead to osteocytic disuse and (continued) attraction of osteoclasts. However, around the resting zone and the closing cone, enhanced extracellular fluid flow occurs, which will activate osteocytes to recruit osteoblasts. Thus, cellular activity at a bone remodeling site is well related to local fluid flow patterns, which may explain the coordinated progression of a BMU.

Blood flow patterns in an anatomically realistic coronary vessel: influence of three different reconstruction methods

Many clinical studies suggest that local blood flow patterns are involved in the location and development of atherosclerosis. In coronary diseases, this assumption should be corroborated by quantitative information on local hemodynamic parameters such as pressure, velocity or wall shear stress. Nowadays, computational fluid dynamics (CFD) algorithms coupled to realistic 3-D reconstructions of such vessels make these data accessible. Nevertheless, they should be carefully analysed to avoid misinterpretations when the physiological parameters are not all considered. As an example, we propose here to compare the flow patterns calculated in a coronary vessel reconstructed by three different methods. In the three cases, the vessel trajectory respected the physiology. In the simplest reconstruction, the coronary was modelled by a tube of constant diameter while in the most complex one, the cross-sections corresponded to the reality. We showed that local pressures, wall shear rates and velocity profiles were severely affected by the geometrical modifications. In the constant cross-section vessel, the flow resembled to that of Poiseuille in a straight tube. On the contrary, velocity and shear rate exhibited sudden local variations in the more realistic vessels. As an example, velocity could be multiplied by 5 as compared to Poiseuille's flow and area of very low wall shear rates appeared. The results obtained with the most complex model clearly outlined that, in addition to a proper description of the vessel trajectory, the section area changes should be carefully taken into account, confirming assumptions already highlighted before the rise of commercially available and efficient CFD softwares.

Aspects of atmospheric flow and dispersion of air pollutants in a mountainous basin

The variety of physicogeographic characteristics, the complex topography and channeling of the synoptic/regional flow are among the main reasons for the formulation of local flow patterns, and consequently for the dispersion conditions in a mountainous area. In this paper, an attempt is made to investigate the mechanisms responsible for these phenomena and to explain the dispersion of air pollutants in a mountainous basin, especially the impingement of plumes released by elevated stacks. A coupled atmospheric and Lagrangian dispersion model was used for the analysis. As it was found significant, mesoscale circulation, such as lake effects in mountainous terrain, up/downslope and valley winds, was calculated. It was proved that the plume impingement, along with pollutant recirculation, could explain the nighttime maximum air-pollutant concentrations measured in the city of Kozani.

Numerical Study of Dense Fluid Flow in Narrow Pores

Unsteady-state dense flows in narrow slitlike pores that result from pulse perturbations of the initial steady state are analyzed for the case of argon flow in a carbon pore whose walls have microasperities. The flow is modeled using a new microhydrodynamic approach in which the dissipation coefficients are calculated according to the lattice gas model. This method enables one to analyze the molecular concentration and velocity distributions as functions of the distance from the pore wall. It is shown how a local flow pattern is affected by microasperities, which reduce the pore width. The evolution of the concentration and longitudinal flow velocity fields is traced from the instant of perturbation until the establishment of a quasi-steady state.

A modeling of the sea breeze and its impacts on ozone distribution in northern Taiwan

A high-resolution air pollution numerical model system (APOPS) is applied to simulate the sea/land breeze and its impacts on the ozone distribution in northern Taiwan. The system can successfully simulate local flow patterns such as sea/land breezes and mountain-valley wind. The predicted surface ozone concentrations also agree with observed surface ozone values ( Wang, Z., et al., Tellus, 52B, 2000, 1189). The sea/land breezes in northern Taiwan play a significant role in the distribution of ozone and transport of ozone from the urban to the coastal and mountain areas. The sea breeze is a weak system, extending vertically to a height of less than 1 km with the wind speed less than 4 ms -1. The land breeze can transport the photochemically produced ozone and its precursors over the sea. The accumulated ozone on the sea can return to the land in the daytime with the sea breeze. This kind of transport tends to contribute significantly to high-ozone episodes in clean coastal and mountain regions.

Spatial heterogeneity of myocardial circulation and energy metabolism

Within the left ventricular myocardium, substantial differences can be observed in terms of both perfusion and energy turnover. In addition to the small transmural gradient from the subepi--to the subendocardium (1:1.2), more recent high-resolution studies reveal a major patchwork-pattern, e.g., in terms of flow. Adjacent 200 microliters areas can differ more than 3-fold in local perfusion. Low flow and high flow areas (< 50% or > 150% of mean flow, respectively) represent up to 1/5 of the left ventricular myocardium. This local flow pattern is temporally stable for at least days and possibly weeks. Low and high flow areas also differ in local energy metabolism. High flow areas are characterized by enhanced glucose phosphorylation and fatty acid permeability, resulting in increased uptake of these substrates. This is the basis for the recent finding that high flow areas are characterized by an enhanced turnover of the citric acid cycle and thus of local O2 consumption. Since local O2 supply and consumption are closely coupled, low flow areas display no biochemical signs of ischemia. Reducing local flow by 50% results in a similar rise of adenosine or lactate in low and high flow areas. Following complete cessation of perfusion, high flow areas display a greater risk of infarction, indicating enhanced energy demand. Further studies are needed to elucidate the molecular basis of this spatial heterogeneity and to test whether the 3-fold differences in local energy turnover within the myocardial wall also translate into comparable variations of local contractility.