Near-wall effects in micro scale Couette flow and heat transfer in the Maxwell-slip regimes

The effects of modified transport characteristics within an extremely thin layer adjacent to the fluid–solid interfaces are investigated for fully developed laminar micro-scale Couette flows with slip boundary conditions. The wall-adjacent layer effects are incorporated into the continuum-based mathematical model by imposing variable viscosity and thermal conductivity values close to the channel walls, for solving the momentum and energy conservation equations. Analytical expressions for the velocity profiles are derived and are subsequently utilized to obtain the temperature variations within the parallel plate channel, as a function of the significant system parameters. It is revealed that the variations in effective viscosity and thermal conductivity values within the wall-adjacent layer have profound influences on the fluid flow and the heat transfer characteristics within the channel, with an interesting interplay with the wall slip boundary conditions. These effects cannot otherwise be accurately captured by employing classical continuum based models for microscale Couette flows that do not take into account the alterations in effective transport properties within the wall adjacent layers.     

Thermally developing electroosmotic transport of nanofluids in microchannels

A theoretical analysis is presented in this work to assess the influence of nanofluids on thermally developing and hydrodynamically developed electroosmotic transport in parallel plate microchannels (Graetz problem). The hydraulic diameters of the microchannels are assumed to be beyond a certain threshold limit, so that the electric double layers formed adjacent to the plates do not overlap with each other. The volumetric heating arising from the conduction currents in the flow is modeled using Ohm’s law. The viscous generation terms in the energy equation are neglected, based on the earlier findings that the consequent effects are negligible as compared to the Joule heating effects in electroosmotically driven microchannel flows. Closed form expressions for the pertinent temperature distributions and the Nusselt number variations are obtained by employing the method of separation of variables in conjunction with an eigen value formulation, in order to assess the influence of volume fraction of the dispersed nano-particles on the overall rates of convective transport. It is revealed that the effects of nano-particles in the fluid turn out to be significant in the thermal entrance region only, especially for higher Peclet number values. The implications of the incorporation of nanofluids are demonstrated to be somewhat non-trivial in nature, and are strongly determined by the effective Peclet number values obtained on the basis of the phase-integral values of the thermo-physical properties and the pertinent flow parameters.     

Natural convection boundary layer of a non-Newtonian fluid about a permeable vertical cone embedded in a porous medium saturated with a nanofluid

An analysis was performed to study the effect of uniform transpiration velocity on free convection boundary-layer flow of a non-Newtonian fluid over a permeable vertical cone embedded in a porous medium saturated with a nanofluid. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. The governing partial differential equations are transformed into a set of non-similar equations and solved numerically by an efficient implicit, iterative, finite-difference method. Comparisons with previously published work are performed and excellent agreement is obtained. A parametric study of the physical parameters is conducted and a representative set of numerical results for the velocity, temperature, and volume fraction profiles as well as the local Nusselt and Sherwood numbers is illustrated graphically to show interesting features of the solutions.     

Transport of lead in the Mersey Estuary: The development of a novel approach to deriving partition coefficients

Modelling heavy metals in estuarine environments is extremely complex for various reasons; one of the primary complicating factors is that metals exist in two phases, dissolved and particulate bound. Dynamic changes in water chemistry, and in particular salinity, affect the partitioning of metals between the two phases and hence make it difficult to determine the fractions of each phase. A relatively simple approach was developed to relate variations in partition coefficient for Pb to salinity fluctuations in the Mersey Estuary. The functional relationship developed between partition coefficient and salinity departs from the traditional exponential type curve, providing a more realistic relationship.A numerical model was then developed for predicting the transport and distribution of Pb about the Mersey Estuary. The model couples transport of metals throughout the water along with incorporating the chemical processes controlling how lead is fractioned between dissolved and particulate phases through the newly developed partition coefficient relationship. Model predictions of dissolved Pb along the longitudinal axis of the estuary were compared with measurements of Pb for two events; very good correlation was obtained between the model results and the data. The approach is compared with approaches adopted by other researchers. Also results are presented for the determination of partition coefficients for a second metal, Ni, using the author’s approach. These results are used to support the approach developed by the authors.     

Lie group analysis of stagnation-point flow of a nanofluid

This article concerns with a steady two-dimensional boundary layer flow of an electrically conducting incompressible nanofluid over a stretching sheet in a porous medium with internal heat generation/absorption. The transport model includes the effect of Brownian motion with thermophoresis in the presence of chemical reaction and magnetic field. Lie group analysis is applied to the governing equations. The transformed self similar non-linear ordinary differential equations along with the boundary conditions are solved numerically. The influences of various relevant parameters on the flow field, temperature and nanoparticle volume fraction as well as wall heat flux and wall mass flux are elucidated through graphs and tables.     

Homotopy perturbation method for couple stresses effect on MHD peristaltic flow of a non-Newtonian nanofluid

Microsystem Technologies - An analytical study is presented for couple stresses effects on MHD peristaltic transport of a non-Newtonian Jeffery nanofluid. The fluid flows through a porous media...     

FITOVERT: A dynamic numerical model of subsurface vertical flow constructed wetlands

This paper introduces a mathematical model (FITOVERT) specifically developed to simulate the behaviour of vertical subsurface flow constructed wetlands (VSSF-CWs). One of the main goals of the development of FITOVERT was to keep the complexity of the model to an acceptable level, so as to provide a practical tool for design and operation optimization. The dynamic formulation of the model allows to simulate the typical non stationary feeding-emptying operation of VSSF-CWs. FITOVERT is able to describe the water flow through porous media in unsaturated conditions, combined with evapotranspiration; its biochemical module describes the degradation of both organic matter and nitrogen; the transport in the liquid phase is implemented for both dissolved and particulate components; the oxygen transport in the gaseous phase of the soil and its exchange with the liquid phase are also considered. As a main advantage, compared to the few currently available dedicated numerical models, FITOVERT is able to handle the porosity reduction due to bacteria growth and accumulation of particulate components, so that the clogging process is also simulated as an effect of the pore size reduction on the hydraulic conductivity of the simulated system. The performance of the model was firstly analyzed by comparison with hydrodynamic tests recorded in an experimental VSSF-CW pilot plant: tracer test were carried out in three different saturation conditions (fully saturated, partially saturated, and completely drained). FITOVERT proved to accurately simulate the hydraulic behaviour of VSSF-CWs in both saturated and unsaturated conditions. The needs for model improvements and further calibration are finally discussed.     

A stochastic model for thermal transport of nanofluid in porous media: Derivation and applications

In this paper, a stochastic thermal transport model is developed for nanofluid flowing through porous media. This model incorporates the influences of nanoparticle migration on convective heat transfer of the colloidal solution. We show that Lévy flight movement patterns of nanoparticles result in the derived model using fractional derivative for the diffusion term. The new thermal transport model is then applied to the mixed convective problem which is solved using finite difference method. Numerical results show that the smaller values of Lévy index $\gamma$ lead to larger Nusselt numbers, thus the occurrence of long jumps for nanoparticles increases the heat transport of nanofluids. The effects of other involved physical parameters are also presented and discussed.     

Investigating the sensitivity of health benefits to focussed PM2.5 emission abatement strategies

Modelling, pollution monitoring and epidemiological studies all have a role to play in developing effective policies to improve air quality and human health. Epidemiological studies have shown that of particular importance are the effects of fine particulate matter, PM₁₀ and PM_2.5 which can penetrate into human lungs. At present it is not clear which components of PM are responsible for health effects although toxicological studies have identified several potential factors. Hence, based on WHO guidance, current legislation has focused on the total mass, with the EC setting limit values on total PM₁₀, followed by target reductions for population exposure to PM_2.5 in urban agglomerations. Trends in measured concentrations at selected urban monitoring stations are required as evidence for achievement of these reductions. This paper addresses these issues at the borough level in London using the integrated assessment model UKIAM, developed originally for application at the national scale, with illustrations comparing abatement of two contrasting sources – domestic combustion and road transport. The former, dominated by natural gas generating NO_X emissions, contributes to longer range secondary PM formation extending beyond the city. The latter is an important source of black carbon as a primary pollutant causing local exposure, as well as NO_X. WHO data is used in relation to impacts of particle concentrations by mass, and response functions for black carbon are taken from the literature. The results show that from a city perspective there are enhanced benefits from reducing the road transport emissions, especially with regard to potential toxicity of black carbon. The scenarios modelled also highlight the spatial variations of benefits across London, and illustrate deviations from trends as represented by limited monitoring data from the different boroughs, together with the influence upon exposure of mobile population within the city.     

Numerical study of thermodiffusion effects on boundary layer flow of nanofluids over a power law stretching sheet

This paper deals with the triple-diffusive boundary layer flow of nanofluid over a nonlinear stretching sheet. In this model, where binary nanofluid is used, the Brownian motion, thermophoresis, and cross-diffusion are classified as the main mechanisms, which are responsible for the enhancement of the convection features of the nanofluid. The boundary layer equations governed by the partial differential equations are transformed into a set of ordinary differential equations with the help of group theory transformations, which is introduced by Blasius (The boundary layers in fluids with little friction, 1950). The variational finite element method is used to solve these ordinary differential equations. We have examined the effects of different controlling parameters, namely the Brownian motion parameter, the thermophoresis parameter, modified Dufour number, nonlinear stretching parameter, Prandtl number, regular Lewis number, Dufour Lewis number, and nanofluid Lewis number on the flow field and heat transfer characteristics. The physics of the problem is well explored for the embedded material parameters through tables and graphs. The present study has many applications in coating and suspensions, movement of biological fluids, cooling of metallic plate, melt-spinning, heat exchangers technology, and oceanography.     

Aggregate growth and breakup in particulate suspension flow through a micro-nozzle

A computational study is reported on the growth of aggregates in flow of a particulate suspension through a micro-nozzle. The study employs a soft-sphere discrete element method (DEM) with van der Waals adhesion force between the particles in two-dimensional, incompressible channel flow. A new computational approach for particle transport in complex domains is developed which uses a background Cartesian grid for efficient flow field interpolation at the particle locations, together with a level-set method to represent the nozzle boundaries in the particle computation. Three mechanisms for the growth or breakup of particulate aggregates in the micro-nozzle are examined: (1) enhanced particle collision due to lateral compression as fluid elements pass through the nozzle, (2) stretching of aggregates due to axial stretching of fluid elements, and (3) collision and intermittent adhesion of particles to the nozzle wall. The first of these mechanisms leads to aggregate growth, and the second to aggregate breakup. The wall collision and adhesion mechanism can enhance either aggregate growth or breakup, but it is found in most cases to be a primary agent in the breakup of incident aggregates as part of the aggregate attaches to the nozzle wall and is torn from the remainder of the aggregate due to the high shear near the walls. Simplified models for these processes are developed and used to interpret the trends observed in the DEM simulations. The effects of particle adhesion parameter, particle size and density, particle concentration, and nozzle geometry are examined. It is found that passage of a particulate suspension through a nozzle can lead to either a substantial decrease in aggregate size or a modest increase under different conditions, depending in part on the size of the incident aggregates.     

Onset of double-diffusive convection of a sparsely packed micropolar fluid in a porous medium layer saturated with a nanofluid

The onset of convection of a sparsely packed micropolar fluid in a porous medium layer saturated by a nanofluid is examined by using a linear and nonlinear stability analyses. The Darcy–Brinkman–Forchheimer model is employed for the porous medium layer. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. The critical Rayleigh number, wave number for stationary and oscillatory modes and frequency of oscillations are obtained analytically using linear theory, and the nonlinear analysis is made with minimal representation of the truncated Fourier series analysis involving only two terms. The effect of various parameters on the stationary and oscillatory convections is shown pictorially. The dependence of stationary or oscillatory convection on the porous parameter and parameters involved in micropolar fluids is also discussed. We also study the effect of time on transient Nusselt number and Sherwood number which are found to be oscillatory when time is small. However, when time becomes very large, both the transient Nusselt value and Sherwood value approach to their steady-state values.     

COMPUTER GRAPHICS IN SIMULATION OF CARDIOVASCULAR TRANSPORT PHENOMENA

Simulation is a necessary tool if we are to understand better the complexities involved in cardiovascular transport. While some of the phenomena modeled can be described analytically, perusal of the equations alone often doesn't result in full appreciation of the model system. It therefore becomes pertinent to utilize computer graphics in order to enhance simulation of physiologic transport processes. Graphic representation not only facilitates interaction between the investigator and the simulation, it provides a juxtaposition of the model to the real system, as well as a simplification of relationships between various features of the model.Increased mathematical sophistication required in the investigation of cardiovascular transport phenomena often makes traditional graphic representation cumbersome. Therefore several different types of graphics have been utilized, including 2-, 3-, and 4-dimensional displays. The methods and algorithms for these displays have been generalized to make them easy to use over a broad spectrum of applications. In some cases we have generated motion pictures of sequential model solutions which have increased and accelerated model comprehension, as well as been valuable for teaching purposes.     

Enhanced natural convection in an isosceles triangular enclosure filled with a nanofluid

Natural convection is studied in an isosceles triangular enclosure with a heat source located at its bottom wall and filled with an Ethylene Glycol–Copper nanofluid. This paper examines the effects of pertinent parameters such as the Rayleigh number, the solid volume fraction, the heat source location, and the enclosure apex angle on the thermal performance of the enclosure. The thermal performance of the enclosure is improved with an increase in the Rayleigh number and solid volume fraction. The results also show that the variation of heat transfer rate with respect to the enclosure apex angle and heat source position and dimensions is different at low and high Rayleigh numbers. A comparison is also presented between the results obtained from the modified and original Maxwell models. The results show that the heat transfer is generally higher based on the modified Maxwell model.     

Unravel the landscape and pulses of cycling activities from a dockless bike-sharing system

The recent boom of sharing economy along with its technological underpinnings have brought new opportunities to urban transport ecosystems. Today, a new mobility option that provides station-less bike rental services is emerging. While previous studies mainly focus on analyzing station-based systems, little is known about how this new mobility service is used in cities. This research proposes an analytical framework to unravel the landscape and pulses of cycling activities from a dockless bike-sharing system. Using a four-month GPS dataset collected from a major bike-sharing operator in Singapore, we reconstruct the temporal usage patterns of shared bikes at different places and apply an eigendecomposition approach to uncover their hidden structures. Several key built environment indicators are then derived and correlated with bicycle usage patterns. According to the analysis results, cycling activities on weekdays possess a variety of temporal profiles at both trip origins and destinations, highlighting substantial variations of bicycle usage across urban locations. Strikingly, a significant proportion of these variations is explained by the cycling activeness in the early morning. On weekends, the overall variations are much smaller, indicating a more uniform distribution of temporal patterns across the city. The correlation analysis reveals the role of shared bikes in facilitating the first- and last-mile trips, while the contribution of the latter (last-mile) is observed to a limited extent. Some built environment indicators, such as residential density, commercial density, and number of road intersections, are correlated with the temporal usage patterns. While others, such as land use mixture and length of cycling path, seem to have less impact. The study demonstrates the effectiveness of eigendecomposition for uncovering the system dynamics. The workflow developed in this research can be applied in other cities to understand this new-generation system as well as the implications for urban design and transport planning.     

Thermal instability in a rotating porous layer saturated by a non-Newtonian nanofluid with thermal conductivity and viscosity variation

The stability of a non-Newtonian nanofluid saturated horizontal rotating porous layer subjected to thermal conductivity and viscosity variation is investigated using linear and nonlinear stability analyses. The model used for the non-Newtonian nanofluid includes the effects of Brownian motion and thermophoresis. The Darcy law for the non-Newtonian nanofluid of the Oldroyd type is used to model the momentum equation. The linear theory based on the normal mode method, and the criteria for both stationary and oscillatory modes are derived analytically. A weak nonlinear analysis based on the minimal representation of truncated Fourier series method containing only two terms is used to compute the concentration and thermal Nusselt numbers. The results obtained during the analysis are presented graphically.     

Electroosmotic flows with simultaneous spatio-temporal modulations in zeta potential: cases of thick electrical double layers beyond the Debye Hückel limit

We devise a mathematical model for analyzing the effects of spatio-temporal perturbations in zeta potential on electroosmotic transport in narrow fluidic confinements, considering thick electrical double layer limits. The spatial perturbations in zeta potential may be attributed to surface charge patterning, either designed or manifested as a natural artifact of the surface inhomogeneities. The time-dependent variations in zeta potential, as considered in this work, may stem from the temporal perturbations in the bulk ionic concentrations in the end-channel reservoirs or ‘wells’. Overcoming the simplifications routinely employed in the literature, we develop here an improved analytical formalism, without imposing any constraints on the magnitude of the zeta potential. Using these solutions, we highlight the possibilities of obtaining designed rotationalities in the flow structure with simultaneous spatial variations in the zeta potential and temporal variations in the well concentrations. We show that such combinations of spatial and temporal variations, in effect, render the flow system to be capable of shedding vortex structures that are not otherwise obtainable with spatial variations in zeta potential alone.     

Constraints on using AVHRR composite index imagery to study patterns of vegetation cover in boreal forests

A wide range of techniques are being developed to map vegetation cover types using multi-date imagery from the Advanced Very High Resolution Radiometer. To date, these techniques do not account for severe constraints which exist for the world's boreal forest. Using composite AVHRR imagery collected over Alaska, we demonstrate how several factors influence the time-series normalized vegetation difference index (NDVI) signatures developed for the boreal forests in this region, including the effects of: (1) clouds and atmospheric haze; (2) climate variations on plant phenology; (3) fire on forest succession; and (4) forest stand patch size with respect to system resolution. Based on the analysis of AVHRR composite data from Alaska, the results of this study show: (1) clouds and haze have distinct effects on the intra-seasonal NDVI signature; (2) there are significant interseasonal variations in NDVI signatures caused by variations in the length of the growing season as well as variations in precipitation and moisture during the growing season; (3) disturbances affect large areas in interior Alaska and forest succession after fire results in significant variations in the inter-seasonal NDVI signatures; and (4) much of the landscape in interior Alaska consists of heterogeneous patches of forest which are much smaller than the resolution cell size of the AVHRR sensor, resulting in significant sub-pixel mixing. Based on these findings, the overall conclusion of this study is scientists using AVHRR to map land cover types in boreal regions must develop approaches which account for these sources of variation.     

基于Modelica的弹道导弹三自由度仿真

基于面向对象思想在Modelica语言平台上开发了弹道导弹三自由度仿真系统。在导弹三自由度动力学模型基础上,通过系统分解、功能实现和系统搭建3个步骤建立了Modelica语言平台上的仿真模型。所建立的仿真模型充分利用了Modelica语言的优势,具有较好的通用性和灵活性。最后,将建立的Modelica仿真模型与相应的C＋语言版本仿真系统进行了对比,结果表明全弹道仿真的结果位置误差在10m以内。     

Modeling of coupled momentum, heat and solute transport during DNA hybridization in a microchannel in the presence of electro-osmotic effects and axial pressure gradients

An integrated thermofluidic analysis of DNA hybridization, in the presence of combined electrokinetically and/or pressure-driven microchannel flows, is presented in this work. A comprehensive model is developed that combines bulk and surface transport of momentum, heat and solute with the pertinent hybridization kinetics, in a detailed manner. Results confirm that electrokinetic accumulation of DNA occurs within a few seconds or minutes, as compared to passive hybridization that could sometimes take several hours. Further, it is observed that by increasing the accumulation time, significantly higher concentration of DNA can be achieved at the capture probes. However, this eventually tends to attain a saturation state, due to a lesser probability of successful hybridization on account of a prior accumulation of target DNA molecules on the capture probe strands. While favorable pressure gradients augment DNA hybridization rates that are otherwise established by the electro-osmotic transport, adverse pressure gradients of comparable magnitude may turn out to be much less consequential in retarding the same. Such effects can be of potential significance in the designing of a microfluidic arrangement to achieve the fastest rate of DNA hybridization.