Diffusion Flow Research Articles

An improvement to continuous random walk model for sediment diffusion in inhomogeneous turbulent flows

An improvement to continuous random walk model for sediment diffusion in inhomogeneous turbulent flows

Impact of double diffusivity on the hyperbolic tangent model conveying nano fluid flow over the wedge

The significant thing in our analysis is to inspect the double-diffusivity convection for the hyperbolic tangent nanofluid model, under the spotlight effects of activation energy and viscosity over the wedge-shaped geometry. Activation energy enunciates chemical reaction and also Buongiorno model is applied to highlight the important features of the thermophoresis and Brownian diffusion coefficient. The convection of double diffusivity is illustrated through Soret and Dufour parameter. The mathematical modeling of demarcated flow produces a collection of non-linear partial differential equations, which are consequently transmuted into ordinary differential equations using suitable transformation. The graphical results for these ordinary differential equations are created using the Bvp4c approach. The effect of pertinent parameters on fluid flow, thermal variation, concentration and double diffusivity profiles are examined. The velocity field generally behaves as the decreasing function of thermal and nanofluid Grashof number but surges along the Solutal Grashof number. The Brownian diffusion coefficient surges the temperature profile and minors the concentration and double-diffusivity. Soret diffusivity surges the concentration distribution when fluid crosses the edge of the wedge. The nanofluid Lewis number particularly slows down the double-diffusivity profile. The graphical sketches of local skin friction, Nusselt number, Sherwood number, isothermal contour and streamlines flow elaborated our analyses more precisely.

Enhancing heat dissipation and mass transfer of oxygen gas flow channel in a proton exchange membrane fuel cell using multiobjective topology optimization

To enhance the heat dissipation and mass transfer from a proton exchange membrane fuel cell (PEMFC) stack, a multi-objective topology optimization model considering structural deformation, heat transfer, mass diffusion, and gas flow process is proposed herein. The average molar concentration of oxygen and average temperature in the cathode catalyst layer (CL) are considered as the optimization objectives, and the mechanical, mass transfer, and gas flow properties are treated as constraints. The results reveal that compared with a straight gas channel, the topology-optimized configuration leads to an improvement in oxygen molar concentration by 10.36% and reduction in average temperature by 2.26 K. A higher power-dissipation constraint leads to a more complex three-dimensional channel structure with more branches. A larger structural displacement constraint results in a wider and flatter flow channel topology. The average oxygen molar concentration of topologically optimized configurations increases ranging from 5.18 to 14.96%, and the average temperature in the CL decreases about 2.2 K, compared with that of straight gas channel configurations, when the velocity at inlet varies from 0.05 to 0.3 m/s. These findings can aid in the design of PEMFCs.

Analysis on the Mechanism of Rotating Stall Inner a Pump Turbine in Pump Mode Based on the Proper Orthogonal Decomposition

Abstract An in-depth understanding of stall behavior is essential to improve the safe and stable operation of the pump turbine. In this research, the detached eddy simulation (DES) is used to investigate the various characteristics and propagation mechanism of the rotating stall of the hump instability region. The frequency characteristics and spatial intensity distribution of the stall cells are studied by adopting the proper orthogonal decomposition (POD) method. The results indicate that the energy loss at 0.80QDES and 0.75QDES are induced by the rotating stall with three cells that occur in the diffuser flow channel, and the corresponding high amplitude frequencies are St0.0074 and St0.0111, respectively. The effect of the stall cells is particularly severe near the hub. When the flow rate is further reduced to 0.68QDES, the three rotating stall cells develop into two symmetrically distributed stable stall cells with no definite characteristic frequencies, and the blocking phenomenon is concentrated near the shroud. With the stall cells losing their rotational characteristics, they have less impact on the flow field and contribute less energy. This paper also reveals that the energy loss in the diffuser contributes significantly to the hump formation on the performance curve.

The Relationship of Aerosol Properties and Ice‐Nucleating Particle Concentrations in Beijing

AbstractThis paper investigates immersion mode ice‐nucleating particle (INP) concentrations measured with a continuous flow diffusion chamber at temperatures of −20°C, −25°C, and −30°C in Beijing from 4 May to 4 June 2018. Aerosol conditions in the experiment period are classified into four types: clean, light PM2.5 pollution, heavy PM2.5 pollution, and dust. The PM mass concentrations and the aerosol size distributions are distinctly different in the four aerosol conditions. INP concentration increases as temperature decreases from −20°C to −30°C for all the four aerosol conditions. At a given temperature, INP concentrations progressively increase as the aerosol condition changes from clean to light PM2.5 pollution and to heavy PM2.5 pollution condition. At −25°C and −30°C, the dust condition has a much higher INP concentration than the other aerosol conditions. The INP concentration is closely related to the number concentration of aerosols with diameters of 1.0–2.5 μm (). Using a similar method as in the previous parameterization of INP concentrations for global and fairly clean aerosol conditions, a new parameterization of the INP concentration based on is provided for the complicated aerosol conditions with high aerosol number concentrations in Beijing.

Ecological risk assessment of phthalates in the urban channels of São Vicente, Brazil

Phthalate acid esters (PAEs) are a group of plasticising chemical compounds used in the plastic industry worldwide that are well known for their environmental contamination and toxicological effects as endocrine disruptors. In this work, the occurrence and preliminary ecological risk of six PAEs – namely, dimethyl phthalate (DMP), diethyl phthalate (DEP), di-n-octyl phthalate, butyl benzyl phthalate, di-n-butyl phthalate (DBP) and di(2-ethylhexyl) phthalate – were investigated, for the first time, in five urban drainage channels located along the poor nearby neighbourhoods on the outskirts of São Vicente City, São Paulo, Brazil, whose diffuse loads flow continuously to the estuarine waters of this island. Of these, three congeners, DMP (<limit of detection (LOD), 0.023 μg/l), DEP (<LOD, 0.033 μg/l) and DBP (<LOD, 0.042 μg/l), were consistently quantified using gas chromatography coupled with tandem mass spectrometry. Specifically, the DBP occurrence is a cause of environmental concern for the São Vicente estuary, because the results suggest moderate risk for algae and fishes and high ecological risk for crustaceans after acute exposure. Therefore, even at low concentrations, the occurrence of PAEs cannot be neglected in this coastal zone. In addition, the present study reinforces the need for further ecotoxicological studies (particularly with tropical marine organisms) to assess the toxicity of these bioactive compounds.

The Use of Microfluidic Platforms with Raman Spectroscopy for Investigating the Co-Precipitation of Metals and Radionuclides in Carbonates

In the context of long-term safety assessments of deep geological repositories for radio-active wastes, a rigorous understanding of the retention of radionuclides such as 226Ra due to co-precipitation with carbonate and sulphate minerals is important for a realistic prediction of radionuclide migration behaviour in the repository near and far field. The co-precipitation of 226Ra in sulphate minerals, in particular barite, has been studied experimentally and numerically in detail throughout the last decade to establish the thermodynamic properties and mixing behaviour of its solid solutions over a wide range of temperatures. However, so far, few studies have been dedicated to the incorporation of 226Ra into carbonates, and little is known about the mixing behaviour of 226Ra and calcium carbonate phases such as calcite, aragonite, or vaterite. The aim of the work presented here was to develop and explore innovative microfluidic experiments in combination with in situ Raman spectroscopy that can be used to investigate co-precipitation processes of radionuclides in carbonate minerals, using stable Ba as a chemical analogue for 226Ra in the first step, due to their similar ionic radii. Different microfluidic set-ups were developed to address co-precipitation in bulk solution as well as in confinement or under diffusive flow regimes. It could be shown by XRD and SEM-EDX analyses that high amounts of Ba can be incorporated into the lattice of calcite when formed via an amorphous precursor phase, suggesting that the formation of calcium carbonates can contribute to the retention of 226Ra in the repository environment, which will be verified in future studies using the presented microfluidic platforms.

In-vivo Estimation of Intramuscular and Diffusive Limitations to O2 Flow Limitations in Resistance and Endurance Trained Individuals by Near Infra-red Spectroscopy

Introduction: Compared with resistance training (RT), endurance training (ET) induces structural changes within skeletal muscles that include increased capillary density and increased oxidative enzyme activity. The adaptations in ET of oxidative capacity are thought to be greater than those of capillary density, causing the location of O2 flow limitation to move from the intramuscular (O2 utilization limited) to the extramuscular (O2supply limited) compartment. We recently proposed a new approach to estimate oxidative capacity and O2diffusion limitation in vivo by near-infrared spectroscopy (NIRS), using the premise that diffusion limitation is exposed by an experimentally-imposed reduction in O2 supply. We therefore evaluated whether this technique could distinguish muscular adaptations between ET and RT individuals. We hypothesized that both muscle oxidative capacity and diffusion limitation would be greater in ET than RT. Methods: Twenty-one young male resistance (RT, n=9, 26±4 yrs) and endurance (ET, n=12, 27±7 yrs) trained individuals volunteered. RT or ET status was characterized using: maximal isometric voluntary torque (MVT) of knee extensors; incremental cycle-ergometry to the limit of tolerance for peak oxygen uptake (V̇O2peak); vastus lateralis fractional O2extraction at V̇O2peak (ΔHHbMbpeak), expressed as percentage of ΔHHbMb during sustained occlusion; and by V̇O2 kinetics (τV̇O2) during moderate intensity constant work-rate exercise. Vastus lateralis muscle V̇O2recovery rate constant ( k) following brief knee extensor exercise was assessed using NIRS and repeated transient arterial occlusions. k was assessed twice, with arterial occlusions manipulated to maintain tissue saturation index within a 10% range in either LOW or HIGH conditions. Results: MVT was greater in RT than ET (328±43 vs. 255±40N, P<0.001). V̇O2peak was lower in RT than ET (38.8±4.6 vs. 55.6±3.8ml∙kg-1∙min-1, P<0.001). ΔHHbMbpeak was lower in RT than ET (57±11 vs. 91±6%, P<0.001). τV̇O2 was lower in RT than ET (39.8±7.6 vs. 24.0±4.4s, P<0.001). kHIGH was lower in RT than ET (1.49±0.25 vs. 3.04±0.71min-1, P=0.001) and the difference in k (Δ k = kHIGH – kLOW) was less in RT than ET (0.29±0.16 vs 0.80±0.69 min-1, P=0.045). Conclusions: As hypothesized, non-invasive assessment of muscle functional indices of oxidative and diffusive O2 flow in vivo revealed significantly greater oxidative capacity (greater kHIGH) and a greater diffusion limitation (greater Δ k) in ET individuals compared with RT. The new NIRS method allows independent assessment of limitations in muscle oxidative and diffusive O2 flow in resistance and endurance trained individuals. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Generation of a Vertical Fine Structure by Internal Waves on the Sea Shelf

Free internal waves are considered in the Boussinesq approximation with account of horizontal turbulent viscosity and diffusion in a two-dimensional flow in the sea of constant depth. In the linear approximation the boundary-value problem for the amplitude of vertical velocity has complex coefficients and is solved numerically using the third-order implicit Adams method. The vertical component of the Stokes drift velocity and the vertical wave fluxes of mass and salt are found in the second order in the wave amplitude. These fluxes lead to generation of a vertical fine structure that has an irreversible nature.

On the Role of Kinetic Alfven Waves in the Magnetosheath Ion Thermalization Around the Night‐Side Magnetopause

AbstractThe solar wind ion transport across the magnetopause is one of the main sources of plasma for the Earth's magnetotail. Such a transport is supported by various dynamic processes at the flank magnetopause, where wave‐particle interactions play a crucial role in ion flow thermalization and diffusion across magnetic field surfaces of the magnetopause tangential discontinuity. In this paper we numerically model such ion thermalization by the most intense electromagnetic waves observed in the magnetosheath, kinetic Alfven waves. We aim to develop an approach for long‐term simulations of ion scattering by waves and ion dynamics around realistic magnetopause magnetic fields. This approach is based on a combination of test particle simulations and stochastic differential equations modeling ion diffusion in velocity space. We demonstrate that for realistic magnetopause configuration and wave characteristics, the magnetosheath ion flow can be substantially thermalized around the magnetopause. This result explains observations of ion energy conservation across the flank magnetopause: kinetic and thermal energies of flowing magnetosheath ions approximately equal to the thermal energy of stagnant magnetospheric ions.

A multiphase Cahn–Hilliard system with mobilities and the numerical simulation of dewetting

We propose in this paper a new multiphase Cahn–Hilliard model with doubly degenerate mobilities. We prove by a formal asymptotic analysis that it approximates with second order accuracy the multiphase surface diffusion flow with mobility coefficients and surface tensions. To illustrate that it lends itself well to numerical approximation, we propose a simple and effective numerical scheme together with a very compact Matlab implementation. We provide the results of various numerical experiments to show the influence of mobility and surface tension coefficients. Thanks to its second order accuracy and its good suitability for numerical implementation, our model is very handy for tackling notably difficult surface diffusion problems. In particular, we show that it can be used very effectively to simulate numerically the dewetting of thin liquid tubes on arbitrary solid supports without requiring nonlinear boundary conditions.

Delineation of preferential flow pathways in a tropical crystalline rock aquifer in Tarkwa, Ghana using integrated hydrogeophysical methods

Abstract In Ghana, crystalline rock aquifers with secondary hydraulic features and preferential flow pathways serve as very important aquifers. Protecting and managing these geological porous media require identifying preferential flow pathways and hydraulic characteristics. In this study, preferential flow pathways and hydraulic parameters were characterised for two boreholes (Hilly and Valley) located in Tarkwa, Ghana via the integrated use of borehole dilution testing (BDT), slug testing, and geological mapping. The geological fieldwork mapped a relatively sparsely fractured Sandstone Unit overlying a heavily foliated and fissile Phyllite Unit. Geology influenced groundwater flow in boreholes. The BDT confirmed the geology by showing a stratified flow system with preferential flow pathways. The Hilly/recharge area borehole shows relatively low downward flow, whereas the Valley/transition area borehole is dominated by the concentrated diffuse horizontal flow. These flow patterns are in agreement with the borehole locations and their dominant geologies. The estimated hydraulic conductivity, transmissivity, and storage coefficient ranged between 5.1 × 10−4–7.7 × 10−2 m/d, 4.0 × 10−2–3.2 m2/d, and 10−7–10−5, respectively. The cheap integrated hydrogeophysical methods used in this study are applicable for characterising, protecting, and managing other crystalline aquifers in the West African sub-region and other tropical terrains.

Peridynamic model of deformation and failure for rock material under the coupling effect of multi-physical fields

This paper presents a thermo-hydro-mechanical coupling peridynamic (PD) model to simulate the deformation and failure of rock materials and the diffusion of the flow and temperature subjected to the coupling effect of multi-physical fields. By using the peridynamic least squares minimization (PD LSM) method, the thermal conduction and the flow diffusion equation in the coupling of thermo-mechanical, hydro-mechanical, and thermo-hydro-mechanical are refined in the PD form. The equation of motion for thermo-hydro-mechanical coupling is built in the framework of the bond-based PD model. The verification of the coupling PD model is demonstrated by comparison against the analytical solutions and previous numerical results in the problems of steady thermal conduction, thermal deformation, transient flow, hydraulic fracturing, thermal consolidation, and hydraulic fracturing considering the heat effect. The comparison results show that the proposed thermo-hydro-mechanical coupling PD model can capture the deformation and failure of rock materials under the coupling effect of multi-physical fields.

The permeability effect on the diffusion and correlation process of fluid flow in porous media

This study numerically examines the fluid flow in porous media used in desalination. This topic is of significant current interest in the fields of science, engineering and technology, particularly in the process of water filtration. Moreover, in this work, we are attracted to correlation and diffusion process of the porous medium below the effect of permeability, dynamic pressure and friction coefficient. These parameters characterized the fluid flow in porous medium. To study numerically this phenomenon, many models have been proposed. However, we developed our investigation by using the Langevin dynamics model. Furthermore, this dynamic framework is based on Newton’s second law and Darcy’s law. Hence, we modeled the medium as a set of random diameters pores, which are dispersed randomly. The obtained results indicate that the average velocity time evolution presents an exponential profile with two different states: the first is transient and the second is permanent. The two regimes are separated by a cross-over time. Furthermore, time evolution exhibits an increasing profile versus the permeability, then it presents a decreasing profile versus the friction coefficient. Likewise, the diffusion process and correlation are tested for different parameters, especially the permeability and the dynamic pressure. We remark that the effective diffusion coefficient decreases exponentially with permeability and increases linearly with dynamic pressure. Hence, the fluid flow correlation in porous media presents a Gaussian function profile, with standard deviation function which increases exponentially with permeability.

The characteristics of flow patterns in the shell-and-plate heat exchanger

The flow path of the fluid in the passage of the shell-and-plate heat exchanger shows the characteristics of first dispersing and then gathering, which significantly affects the efficiency of the heat exchanger. Compared with the traditional rectangular plate heat exchanger (PHE), an in-depth understanding of the flow characteristics in SPHE is insufficient. The properties of the flows between the adjacent plates were numerically studied in the present study. The single-phase fluid passes rapidly between the staggered contact points, and there is noticeable pressure fluctuation at the entrance and exit. In addition, the flow diffusion and convergence cause noticeable differences in local velocity and pressure variation. For two-phase flows, three flow patterns are defined, and a subregional discussion on the flow pattern in the passage and their pressure gradient and void fraction are also provided. The void fraction is also introduced to determine the flow patterns, i.e., the dominant flow pattern is bubbly flow when α<0.4, 0.4≤α≤0.7 is for slug flow, and the primary type is film flow when α>0.7. By analyzing the pressure drop, an empirical equation of pressure drop prediction is proposed.

Heat transfer efficacy and flow progress of tripartite diffusion in magneto-radiative Reiner-Philippoff nanofluid in porous Darcy-Forchheimer substance with heat source

When describing natural phenomena like the movement of pollutants, subterranean water, acidulent rain, and other phenomena, it is sometimes necessary to have more than one solute distributed in fluid mixes. The steadiness triple diffusive boundary layer natural convective flow along a vertical stretchable plate embedded in a Darcy-Forchheimer porous material filled with a salty Reiner-Philippoff nanofluid with two salts is examined in this research. The accompanying nanofluid is thought to convective heat the surface. Through a Buongiorno concept, the regulating equations additionally incorporate the contributions of Brownian motion and thermophoresis factors. A solver of the bvp4c approach is used to produce the numerically calculated after employing similar solutions in the MATLAB® program. On both salts and nanofluid parameters, the impacts of the electromagnetic, thermal radiative fluxing, and heat generating parameters on flowing and heat transference are assessed. It has been discovered that adding salts and nanoparticles accelerates the rate of heat transfer. The rate of heat transference of pseudo-plastic nanofluids is lower than that of dilatant nanofluids. Whilst the mass transition of nanofluid is quite the opposite. Darcy-Forchheimer porousness material and existence heat generating impact occurs the salts rates of mass transfers is diminished.

A Combined Approach Using Water Quality Indexes and Statistical Analyses to Assess the Urban Surface Runoff: a Case Study in São Paulo Coastal Zone, Brazil

Although the most conventional methods to determine the quality of an aquatic ecosystem are aimed to evaluate its physicochemical, microbiological and ecotoxicological characteristics, the monitoring of a large number of environmental variables can represent a high cost for developing countries like Brazil. However, a combined approach using water quality indexes and statistical analyses may help us to monitor water quality through a previous selection of a few key environmental variables. In this context, the objective of this study was to highlight the set of environmental variables most useful to assess the urban channels water quality of Guarujá, Brazil, whose diffuse loads flow continuously into four tourist beaches (Tombo, Enseada, Perequê and Iporanga). For this purpose, three different methods were applied: (i) an existent published large data set (30 environmental variables: 28 physicochemical/microbiological, plus 2 ecotoxicological) obtained during a previous monitoring of the urban drainage channels of these beaches; (ii) the potential risks for the aquatic local fauna and flora through established water quality indexes, such as IMPC, TSI and ALPI; and (iii) through statistical methods such as Cohen D test, Standardized Odds Ratio and Logistic Regression, the set of environmental variables with the greatest potential to cause acute and chronic toxicity in urban channels waters. This combined approach using water quality indexes and statistical analyses was effective to successfully reduce the number of environmental variables needed to assess the ecological status of this coastal area. Moreover, this combined approach was useful to gather intuitive and user-friendly environmental information that could help decision makers (i.e., public authorities and environmental agencies) to plan and perform low-cost and effective monitoring plans in different coastal zones worldwide, namely in developing countries such as Brazil.

Effects of extended mixing processes on fresh, hardened and durable properties of cement systems incorporating fly ash

Specifications that correspond with system performance may guarantee the addition of value. Most specifications for ready-mixed concrete address limits on discharge time and truck-drum revolution counts. These limits have been developed for conventional concrete. As the uses of supplementary cementing materials (SCMs) become ubiquitous, it is important to determine whether these specifications are applicable to SCMs, that is, systems containing fly ash. This paper presents results of the effects of mixing time and mixer revolution counts on characteristics of lab-made pastes and mortars containing 20% and 50% fly ash. Their characteristics assessed include time-variant ion concentrations, setting time, flow, compressive strength, porosity, and apparent chloride diffusivity coefficient. Results indicate that with increasing mixing time and mixer revolution counts, mixtures with a replacement of fly ash exhibit improved both fresh and hardened characteristics. When mixed for 60 min or 25,505 revolution count, the 28-day compressive strengths of mixtures containing 20% and 50% fly ash are 50% to 100% higher than the neat cement. Fly ash is suggested to adopt in the extended mixing processes of cement systems.

Influence of air inlet distance on coal dust pollution characteristics in working environment during tunneling based on CFD method

Coal dust is one of the pollutants generated during the coal tunneling process. However, the air inlet distance will increase with the advancement of the working face, resulting in the redistribution of the air flow field, further affecting the pollution characteristics of coal dust. This effect weakens the effectiveness of dust removal equipment. Therefore, transient CFD modeling was used to study the influence of air inlet distance on air flow, dust migration distance, and dust concentration distribution in the breathing zone and achieve the highest dust reduction efficiency. Results showed that the air flow formed two reflows in the tunnel under forced ventilation. When the air inlet distance increased from 6.0 m to 8.4 m, wind flow diffusion distance (along the return side coal wall) Lp and air inlet distance d can be described as Lp = 34.272–7.108d + 0.570d2 . Furthermore, the dust diffusion time to the outlet increased from 35 s to 40 s, and the dust migration distance increased as a power function of air inlet distance. Meanwhile, the dust migration distance in the first 10 s accounted for 45–50% of that for the entire time period. In addition, the dust migration distance decreased only after 25 s, and the dust concentration in the return side breathing zone gradually increased. Finally, air curtains and dust removal fans were recommended for dust control, and the dust in the daily tunneling process can be effectively controlled when da = 15.0 m and de = 8.0 m. The dust reduction efficiency was 85.29–99.90%. The research results provide technical support for the dust outlet installation of dust extraction fans during tunneling and guidance for dust control in the similar working environment.

Assessing large landscape patterns of potential fire connectivity using circuit methods

ContextMinimizing negative impacts of wildfire is a major societal objective in fire-prone landscapes. Models of fire connectivity can aid in understanding and managing wildfires by analyzing potential fire spread and conductance patterns. We define ‘fire connectivity’ as the landscape’s capacity to facilitate fire transmission from one point on the landscape to another.ObjectivesOur objective was to develop an approach for modeling fire connectivity patterns representing potential fire spread and relative flow across a broad landscape extent, particularly in the management-relevant context of fuel breaks.MethodsWe applied an omnidirectional circuit theory algorithm to model fire connectivity in the Great Basin of the western United States. We used predicted rates of fire spread to approximate conductance and calculated current densities to identify connections among areas with high spread rates. We compared existing and planned fuel breaks with fire connectivity patterns.ResultsFire connectivity and relative flow outputs were characterized by spatial heterogeneity in the landscape’s capacity to transmit fire. We found that existing fuel break networks were denser in areas with relatively diffuse and impeded flow patterns, rather than in locations with channelized flow.ConclusionsThis approach could be paired with traditional fire behavior and risk analyses to better understand wildfire spread as well as direct strategic placement of individual fuel breaks within larger networks to constrain fire spread. Thus, our findings may offer local- to landscape-level support for management actions that aim to disrupt fire spread and mitigate the costs of fire on the landscape.