Showing papers on "Transport phenomena published in 2020"

Physicochemical hydrodynamics of droplets out of equilibrium

Abstract: Droplets abound in nature and technology. In general, they are multicomponent, and, when out of equilibrium, have gradients in concentration, implying flow and mass transport. Moreover, phase transitions can occur, in the form of evaporation, solidification, dissolution or nucleation of a new phase. The droplets and their surrounding liquid can be binary, ternary or contain even more components, with several in different phases. Since the early 2000s, rapid advances in experimental and numerical fluid dynamical techniques have enabled major progress in our understanding of the physicochemical hydrodynamics of such droplets, further narrowing the gap from fluid dynamics to chemical engineering and colloid and interfacial science, arriving at a quantitative understanding of multicomponent and multiphase droplet systems far from equilibrium, and aiming towards a one-to-one comparison between experiments and theory or numerics. This Perspective discusses examples of the physicochemical hydrodynamics of droplet systems far from equilibrium and the relevance of such systems for applications. Droplets in general are multicomponent and experience gradients in concentration, often leading to transport phenomena and phase transitions. This Perspective discusses recent progress on the physicochemical hydrodynamics of such droplet systems and their relevance for many important applications.

Study of Activation Energy on the Movement of Gyrotactic Microorganism in a Magnetized Nanofluids Past a Porous Plate

Abstract: The present study deals with the swimming of gyrotactic microorganisms in a nanofluid past a stretched surface. The combined effects of magnetohydrodynamics and porosity are taken into account. The mathematical modeling is based on momentum, energy, nanoparticle concentration, and microorganisms’ equation. A new computational technique, namely successive local linearization method (SLLM), is used to solve nonlinear coupled differential equations. The SLLM algorithm is smooth to establish and employ because this method is based on a simple univariate linearization of nonlinear functions. The numerical efficiency of SLLM is much powerful as it develops a series of equations which can be subsequently solved by reutilizing the data from the solution of one equation in the next one. The convergence was improved through relaxation parameters in the study. The accuracy of SLLM was assured through known methods and convergence analysis. A comparison of the proposed method with the existing literature has also been made and found an excellent agreement. It is worth mentioning that the successive local linearization method was found to be very stable and flexible for resolving the issues of nonlinear magnetic materials processing transport phenomena.

Transport Phenomena in Nano/Molecular Confinements.

Abstract: The transport of fluid and ions in nano/molecular confinements is the governing physics of a myriad of embodiments in nature and technology including human physiology, plants, energy modules, water collection and treatment systems, chemical processes, materials synthesis, and medicine. At nano/molecular scales, the confinement dimension approaches the molecular size and the transport characteristics deviates significantly from that at macro/micro scales. A thorough understanding of physics of transport at these scales and associated fluid properties is undoubtedly critical for future technologies. This compressive review provides an elaborate picture on the promising future applications of nano/molecular transport, highlights experimental and simulation metrologies to probe and comprehend this transport phenomenon, discusses the physics of fluid transport, tunable flow by orders of magnitude, and gating mechanisms at these scales, and lists the advancement in the fabrication methodologies to turn these transport concepts into reality. Properties such as chain-like liquid transport, confined gas transport, surface charge-driven ion transport, physical/chemical ion gates, and ion diodes will provide avenues to devise technologies with enhanced performance inaccessible through macro/micro systems. This review aims to provide a consolidated body of knowledge to accelerate innovation and breakthrough in the above fields.

Perpendicular Transport of Energetic Particles in Magnetic Turbulence

Abstract: Scientists have explored how energetic particles such as solar energetic particles and cosmic rays move through a magnetized plasma such as the interplanetary and interstellar medium since more than five decades. From a theoretical point of view, this topic is difficult because the particles experience complicated interactions with turbulent magnetic fields. Besides turbulent fields, there are also large scale or mean magnetic fields breaking the symmetry in such systems and one has to distinguish between transport of particles parallel and perpendicular with respect to such mean fields. In standard descriptions of transport phenomena, one often assumes that the transport in both directions is normal diffusive but non-diffusive transport was found in more recent work. This is in particular true for early and intermediate times where the diffusive regime is not yet reached. In recent years researchers employed advanced numerical tools in order to simulate the motion of those particles through the aforementioned systems. Nevertheless, the analytical description of the problem discussed here is of utmost importance since analytical forms of particle transport parameters need to be known in several applications such as solar modulation studies or investigations of shock acceleration. The latter process is directly linked to the question of what the sources of high energy cosmic rays are, a problem which is considered to be one of the most important problems of the sciences of the 21st century. The present review article discusses analytical theories developed for describing particle transport across a large scale magnetic field as well as field line random walk. A heuristic approach explaining the basic physics of perpendicular transport is also presented. Simple analytical forms for the perpendicular diffusion coefficient are proposed which can easily be incorporated in numerical codes for solar modulation or shock acceleration studies. Test-particle simulations are also discussed together with a comparison with analytical results. Several applications such as cosmic ray propagation and diffusive shock acceleration are also part of this review.

Numerical study of slip and radiative effects on magnetic Fe3O4-water-based nanofluid flow from a nonlinear stretching sheet in porous media with Soret and Dufour diffusion

Abstract: Increasingly sophisticated techniques are being developed for the manufacture of functional nanomaterials. A growing interest is also developing in magnetic nanofluid coatings which contain magnetite nanoparticles suspended in a base fluid and are responsive to external magnetic fields. These nanomaterials are “smart” and their synthesis features high-temperature environments in which radiative heat transfer is present. Diffusion processes in the extruded nanomaterial sheet also feature Soret and Dufour (cross) diffusion effects. Filtration media are also utilized to control the heat, mass and momentum characteristics of extruded nanomaterials and porous media impedance effects arise. Magnetite nanofluids have also been shown to exhibit hydrodynamic wall slip which can arise due to non-adherence of the nanofluid to the boundary. Motivated by the multi-physical nature of magnetic nanomaterial manufacturing transport phenomena, in this paper, we develop a mathematical model to analyze the collective influence of hydrodynamic slip, radiative heat flux and cross-diffusion effects on transport phenomena in ferric oxide (Fe3O4-water) magnetic nanofluid flow from a nonlinear stretching porous sheet in porous media. Hydrodynamic slip is included. Porous media drag is simulated with the Darcy model. Viscous magnetohydrodynamic theory is used to simulate Lorentzian magnetic drag effects. The Rosseland diffusion flux model is employed for thermal radiative effects. A set of appropriate similarity transformation variables are deployed to convert the original partial differential boundary value problem into an ordinary differential boundary value problem. The numerical solution of the coupled, multi-degree, nonlinear problem is achieved with an efficient shooting technique in MATLAB symbolic software. The physical influences of Hartmann (magnetic) number, Prandtl number, Richardson number, Soret (thermo-diffusive) number, permeability parameter, concentration buoyancy ratio, radiation parameter, Dufour (diffuso-thermal) parameter, momentum slip parameter and Schmidt number on transport characteristics (e.g. velocity, nanoparticle concentration and temperature profiles) are investigated, visualized and presented graphically. Flow deceleration is induced with increasing Hartmann number and wall slip, whereas flow acceleration is generated with greater Richardson number and buoyancy ratio parameter. Temperatures are elevated with increasing Dufour number and radiative parameter. Concentration magnitudes are enhanced with Soret number, whereas they are depleted with greater Schmidt number. Validation of the MATLAB computations with special cases of the general model is included. Further validation with generalized differential quadrature (GDQ) is also included.

Effect of asymmetrical heat rise/fall on the film flow of magnetohydrodynamic hybrid ferrofluid.

Abstract: The movement of the ferrous nanoparticles is random in the base fluid, and it will be homogeneous under the enforced magnetic field. This phenomenon shows a significant impact on the energy transmission process. In view of this, we inspected the stream and energy transport in magnetohydrodynamic dissipative ferro and hybrid ferrofluids by considering an uneven heat rise/fall and radiation effects. We studied the Fe3O4 (magnetic oxide) and CoFe2O4 (cobalt iron oxide) ferrous particles embedded in H2O-EG (ethylene glycol) (50–50%) mixture. The flow model is converted as ODEs with suitable similarities and resolved them using the 4th order Runge-Kutta scheme. The influence of related constraints on transport phenomena examined through graphical illustrations. Simultaneous solutions explored for both ferro and hybrid ferrofluid cases. It is found that the magnetic oxide and cobalt iron oxide suspended in H2O-EG (ethylene glycol) (50–50%) mixture effectively reduces the heat transfer rate under specific conditions.

Transfer of mass and momentum at rough and porous surfaces

Abstract: The surface texture of materials plays a critical role in wettability, turbulence and transport phenomena. In order to design surfaces for these applications, it is desirable to characterise non-smooth and porous materials by their ability to exchange mass and momentum with flowing fluids. While the underlying physics of the tangential (slip) velocity at a fluid–solid interface is well understood, the importance and treatment of normal (transpiration) velocity and normal stress is unclear. We show that, when the slip velocity varies at an interface above the texture, a non-zero transpiration velocity arises from mass conservation. The ability of a given surface texture to accommodate a normal velocity of this kind is quantified by a transpiration length . We further demonstrate that normal momentum transfer gives rise to a pressure jump. For a porous material, the pressure jump can be characterised by so-called resistance coefficients . By solving five Stokes problems, the introduced measures of slip, transpiration and resistance can be determined for any anisotropic non-smooth surface consisting of regularly repeating geometric patterns. The proposed conditions are a subset of the effective boundary conditions derived from formal multi-scale expansion. We validate and demonstrate the physical significance of the effective conditions on two canonical problems – a lid-driven cavity and a turbulent channel flow, both with non-smooth bottom surfaces.

Lattice Boltzmann Method for Fluid-Thermal Systems: Status, Hotspots, Trends and Outlook

Abstract: Great advances have been made with the lattice Boltzmann (LB) method for complicated fluid phenomena and fundamental thermal processes over the past three decades. This paper presents a systematic overview of the LB method from 1990 to 2018, based on bibliometric analysis and the Science Citation Index Expanded (SCI-E) database. The results show that China took the leading position in this field, followed by the USA and UK. The Chinese Academy of Sciences had the most publications, while the Los Alamos National Laboratory was first as far as highest average citation per paper and h-index are concerned. Physical Review E was the most productive journal and “Mechanics” was the most frequently used subject category. Keyword analysis indicated that recent research has focused on the natural convection and heat transfer of nanofluid or multiphase flow in complex porous media. Hydrothermal treatment of nanofluid with shape factor on the conditions, such as variable magnetic fields, thermal radiation and slipping boundary, were the research hotspots. Further research perspectives mainly explore the multiscale models for coupling multiple transport phenomena, morphology optimization of porous parameters, new nanoparticles with shape factor, multicomponent LB method considering Knudsen diffusion effect, LB-based hybrid methods, radiation performance or boiling-heat transfer of nanofluid, and the active control of droplets, may continue to attract more attention. Moreover, some new applications, such as phase change of metal foam, erosion induced by nanaofluid, anode circulating, 3D modeling in thermal systems with vibration, and magnetohydrodynamics microfluid devices, could be of interest going forward.

A mathematical study of natural convection flow through a channel with non-singular kernels: An application to transport phenomena

Abstract: In this manuscript, we have obtained closed form solution using Laplace transform, inversion algorithm and convolution theorem. The study of mass transfer flow of an incompressible fluid is carried out near vertical channel. Recently, new classes of differential operators have been introduced and recognized to be efficient in capturing processes following the decay law and the crossover behaviors. For the study of heat and mass transfer, we applied the newly differential operators say Atangana-Baleanu ( ABC ) and Caputo-Fabrizio ( CF ) to model such flow. This model for temperature, concentration and velocity gradient is presented in dimensionless form. The obtained solutions have been plotted for various values physical parameters like α , D f , G m , G r , S c and P r on temperature and velocity profile. Our results suggest that for the variation of time the velocity behavior for CF and ABC are reversible. Finally, an incremental value of prandtl number is observed for decrease in the velocity field which reflects the control of thickness of momentum and enlargement of thermal conductivity. Further, dynamical analysis of fluid with memory effect are efficient for ABC as compared to CF.

Analysis of MHD Carreau fluid flow over a stretching permeable sheet with variable viscosity and thermal conductivity

Abstract: An Analysis has been carried out to study the magnetohydrodynamic boundary layer flow of Carreau fluid with variable thermal conductivity and viscosity over stretching/shrinking sheet. The thermal conductivity and viscosity is considered to be vary linearly with temperature. By applying suitable similarity transformations, the constitutive equation of Carreau fluid along with energy and transport equations transformed to set of ordinary differential equations. The obtained problem is solved analytically by Homotopy Analysis Method (HAM). As increasing magnetic force, skin friction and heat transfer rate are decreased in the stretching case, while opposite effects are seem in the shrinking case. Similarly, increase in the Lewis number leads to a reduction in the concentration profile. furthermore, increasing the power index and the Weissenberg number leads to an increase in skin friction and a heat transfer rate in the case of stretching, while both are reduced in case of shrinking For validity purpose the problem is also solved numerically using BVP4C (Matlab routine). Analysis of results show that analytical and numerical solutions are in excellent agreement. Furthermore, the impacts of different fluid parameters such as the Weissenberg number W e 2 , Magnetic parameter M 2 , Suction parameter s , Prandtl number P r , Lewis number L e , Stretching/ Shrinking parameter B and the heat flux constants on the velocity, temperature and concentration profiles are investigated graphically.

Hall current, viscous and Joule heating effects on steady radiative 2-D magneto-power-law polymer dynamics from an exponentially stretching sheet with power-law slip velocity : a numerical study

Abstract: A mathematical model is developed for 2-D laminar, incompressible, electrically conducting non-Newtonian (Power-law) fluid boundary layer flow along an exponentially stretching sheet with power-law slip velocity conditions in the presence of Hall currents, transverse magnetic field and radiative flux. The secondary flow has been induced with appliance of Hall current. The distinguish features of Joule heating and viscous dissipation are included in the model since they are known to arise in thermal magnetic polymeric processing. Rosseland’s diffusion model is employed for radiation heat transfer. The non-linear partial differential equations describing the flow (mass, primary momentum, secondary momentum and energy conservation) are transformed into non-linear ordinary differential equations by employing local similarity transformations. The non-dimensional nonlinear formulated set of equations is numerically evaluated with famous shooting algorithm by using MATLAB software. The validation of simulated numerical results has been completed with generalized differential quadrature (GDQ). Extensive visualization of primary and secondary velocities and temperature distributions for the effects of the emerging parameters is presented for both pseudo-plastic fluids (n=0.8) and dilatant fluids (n=1.2). The study is relevant to the manufacturing transport phenomena in electro-conductive polymers (ECPs).

Transport phenomena in electrolyte solutions: Nonequilibrium thermodynamics and statistical mechanics

Abstract: Author(s): Fong, KD; Bergstrom, HK; McCloskey, BD; Mandadapu, KK | Abstract: The theory of transport phenomena in multicomponent electrolyte solutions is presented here through the integration of continuum mechanics, electromagnetism, and nonequilibrium thermodynamics. The governing equations of irreversible thermodynamics, including balance laws, Maxwell's equations, internal entropy production, and linear laws relating the thermodynamic forces and fluxes, are derived. Green–Kubo relations for the transport coefficients connecting electrochemical potential gradients and diffusive fluxes are obtained in terms of the flux–flux time correlations. The relationship between the derived transport coefficients and those of the Stefan–Maxwell and infinitely dilute frameworks are presented, and the connection between the transport matrix and experimentally measurable quantities is described. To exemplify the application of the derived Green–Kubo relations in molecular simulations, the matrix of transport coefficients for lithium and chloride ions in dimethyl sulfoxide is computed using classical molecular dynamics and compared with experimental measurements.

Computational fluid dynamic simulation of two-fluid non-Newtonian nanohemodynamics through a diseased artery with a stenosis and aneurysm.

Abstract: This article presents a two-dimensional theoretical study of hemodynamics through a diseased permeable artery with a mild stenosis and an aneurysm present. The effect of metallic nanoparticles on the blood flow is considered, motivated by drug delivery (pharmacology) applications. Two different models are adopted to mimic non-Newtonian characteristics of the blood flow; the Casson (viscoplastic) fluid model is deployed in the core region and the Sisko (viscoelastic) fluid model employed in the peripheral (porous) region. The revised Buongiorno two-component nanofluid model is utilized for nanoscale effects. The blood is considered to contain a homogenous suspension of nanoparticles. The governing equations are derived by extending the Navier-Stokes equations with linear Boussinesq approximation (which simulates both heat and mass transfer). Natural (free) double-diffusive convection is considered to simulate the dual influence of thermal and solutal buoyancy forces. The conservation equations are normalised by employing appropriate non-dimensional variables. The transformed equations are solved numerically using the finite element method with the variational formulation scheme available in the FreeFEM++ code. A comprehensive mesh-independence study is included. The effect of selected parameters (thermophoresis, Brownian motion, Grashof number, thermo-solutal buoyancy ratio, Sisko parameter ratio, and permeability parameter) on velocity, temperature, nanoparticle concentration, and hemodynamic pressure have been calculated for two clinically important cases of arteries with stenosis and an aneurysm. Skin-friction coefficient, Nusselt number, volumetric flow rate, and resistance impedance of blood flow are also computed. Colour contours and graphs are employed to visualize the simulated blood flow characteristics. It is observed that by increasing the thermal buoyancy parameter, i.e. Grashof number (Gr), the nanoparticle concentration and temperature decrease, whereas velocity increases with an increment in the Brownian motion parameter (Nb). Furthermore, velocity decreases in the peripheral porous region with elevation in the Sisko material ratio (m) and permeability parameter (k'). The simulations are relevant to transport phenomena in pharmacology and nano-drug targeted delivery in haematology.

Further Discussion on the Significance of Quartic Autocatalysis on the Dynamics of Water Conveying 47 nm Alumina and 29 nm Cupric Nanoparticles

Abstract: Improvement of product performance, efficiency, and reliability is a major concern of experts, scientists, and technologists dealing with the dynamics of water conveying nanoparticles on objects with nonuniform thickness either coated or sprayed with the catalyst However, little is known on the significance of quartic autocatalysis as it affects the dynamics of water conveying alumina and cupric nanoparticles In this study, comparative analysis between the dynamics of water conveying 29 nm CuO and 47 nm $$\hbox {Al}_2\hbox {O}_3$$ on an upper horizontal surface of a paraboloid of revolution is modeled and presented In the transport phenomena, migration of nanoparticles due to temperature gradient, the haphazard motion of nanoparticles, and diffusion of motile microorganisms were incorporated into the mathematical models Due to the inherent nature of the thermophysical properties of the two nanofluids, viscosity, density, thermal radiation, and heat capacity of the two nanofluids were incorporated in the mathematical model The nonlinear partial differential equations that model the transport phenomenon were transformed, non-dimensionalized and parameterized The corresponding boundary value problems were converted to an initial value problem using the method of superposition and solved numerically The concentration of the catalyst increases significantly with buoyancy at a larger magnitude of space-dependent internal heat source in the flow of 29 nm CuO–water nanofluid Negligible migration of nanoparticles due to temperature gradient decreases the concentration of the fluid throughout the domain

Numerical study of transport phenomena in a nanofluid using fractional relaxation times in Buongiorno model

Abstract: Convective phenomena in a nanofluid flow under the influence of gravitational and magnetic body forces is analyzed in this communication. Nanofluid is of viscoelastic nature that preserves viscosity as well as elasticity. In order to capture the more realistic behavior of the convective phenomena Caputo fractional derivative and fractional relaxation time are introduced in the Buongiorno nanofluid model. Fractional derivative and relaxation time are used for controlled flow mechanism and to overcome infinite propagation speed for the temperature and concentration. The proposed model will also help to understand the hereditary and memory properties of the viscoelastic nanofluid. In order to more closely analyze the buoyancy forces nonlinear convection is introduced in the mathematical modeling of the flow problem. Finite difference-finite element numerical computations are carried out for the governing nonlinear partial differential equations. Quantities of physical interest are computed and discussed for the fractional model. The proposed fractional model can be used to realistically simulate various flow problems in polymer and chemical industries.

Double-diffusive transport and thermodynamic analysis of a magnetic microreactor with non-Newtonian biofuel flow

Abstract: Magnetic microfuel-reforming is a promising method of biofuel processing in diesel engines. However, the complex interactions amongst the non-Newtonian biofuel flow, magnetic field and reactor have hindered understanding of their influences upon the transport phenomena in the system. To resolve this issue, the transport of heat and mass in a porous microreactor containing a Casson rheological fluid and subject to a magnetic field is investigated analytically. The system is assumed to host a homogenous and uniformly distributed endothermic/exothermic chemical reaction. Two-dimensional analytical solutions are developed for the temperature and concentration fields as well as the Nusselt number and local entropy generations, and the results are rigorously validated. It is demonstrated that changes in the non-Newtonian characteristics of the fluid and altering the magnetic and thermal radiation properties can lead to bifurcation of temperature gradient on the surface of the porous medium. The general behaviour of such bifurcation is dominated by the exothermicity (or endothermicity) of the chemical reaction in the fluid phase. It is also shown that variations in the Casson fluid parameter and changes in the intensity and incident angle of the magnetic field can modify the Nusselt number considerably. The extent of these modifications is found to be heavily dependent upon the wall thickness and diminishes as the walls become thicker. Further, the total entropy generation is shown to be highly sensitive to the wall thickness and increases by intensifying the magnetic field, provided that the microreactor walls are thin.

Electrokinetic membrane pumping flow model in a microchannel

Abstract: A microfluidic pumping flow model driven by electro-osmosis mechanisms is developed to analyze the flow characteristics of aqueous electrolytes. The pumping model is designed based on a single propagative rhythmic membrane contraction applied on the upper wall of a microchannel. The flow lubrication theory coupled with a nonlinear Poisson–Boltzmann equation is used to model the microchannel unsteady creeping flow and to describe the distribution of the electric potential across the electric double layer. A generic solution is obtained for the Poisson–Boltzmann equation without the Debye–Huckel linearization. The effects of zeta potential, Debye length, and electric field on the potential distribution, pressure distribution, velocity profiles, shear stress, and net flow rate are computed and interpreted in detail. The results have shown that this electrokinetic membrane pumping model can be used to understand microlevel transport phenomena in various physiological systems. The proposed model can also be integrated with other microfluidic devices for moving microvolume of liquids in artificial capillaries used in modern biomedical applications.

Anomalous hydrodynamic transport in interacting noncentrosymmetric metals

Abstract: The authors propose a hydrodynamic theory for noncentrosymmetric highly-conductive metals, which suggests a nontrivial analogy between the electron fluids and chiral fluids, and thereby predicts a variety of anomalous transport phenomena such as asymmetric Poiseuille flow.

Magnetohydrodynamics-based pumping flow model with propagative rhythmic membrane contraction

Abstract: A novel magnetohydrodynamics (MHD)-based pumping flow model is proposed to study the magnetic property in transient flow of viscous fluids through finite length channel where the upper channel wall is derived to describe the propagative membrane mode of rhythmic contractions. The flow is generated by the pressure difference due to propagative membrane contraction. Inlet and outlet pumping flow mechanisms are applied during the compression and expansion phases. This model is developed based on low Reynolds number flow to considering the microscale transport phenomena in biomedical sciences. Closed-form solutions for velocity fields, pressure, volumetric flow rate, wall shear stress and stream functions are derived under the lubrication analysis. Salient features of the flow analysis and pumping performances are illustrated with the aid of graphical results under the effects of time variation, membrane shape parameter and Hartmann number. Contour plots for velocity fields, stream function and shear stress are prepared for better visualization and analysis. It is inferred that the pressure along the channel length is more with increasing the magnetic field property in both phases (expansion and compression) of membrane contractions. Maximum pressure difference occurs at the membrane contractions, which represents the pumping mechanism. This pumping model can be utilized to design the novel biomedical MHD micropumps for wide-ranging biomedical applications.

Influence of Inlet Turbulence Intensity on Transport Phenomenon of Modified Diamond Cylinder: A Numerical Study

Abstract: Flows around bluff cylinder have received the attention of many researchers over the years. Therefore, the purpose of this paper was to study the effect of turbulence intensity on the transport phenomena over modified diamond cylinders which is investigated in this work. The bluff cylinders considered are of diamond shape and extruded diamond shape. The hydraulic diameter of bluff bodies is taken as the non-dimensional length scale. The simulation is done to cover cross-flow covering the laminar and turbulent regime with the Reynolds number reaching up to 10,000, while the inlet turbulent intensity is varied between 5 and 20%. The influence of turbulent intensity on enhancing heat transfer from the body has been emphasized in this work. The transition SST models along with governing equations (continuity, momentum, and energy equations) are solved numerically with ANSYS Fluent 19.2. The simulation results are validated with established correlations, and excellent agreement is found. This work demonstrates that the transition SST model can effortlessly bridge all flow regimes for predicting the heat transfer. The study computes the influence of inlet turbulence intensity on augmenting heat transfer from the bluff cylinders. The pressure and drag coefficients are found to be unaffected by the inlet turbulent intensity.

Suitability of 2D modelling to evaluate flow properties in 3D porous media

Abstract: The employment of 2D models to investigate the properties of 3D flows in porous media is ubiquitous in the literature. The limitations of such approaches are often overlooked. Here, we assess to which extent 2D flows in porous media are suitable representations of 3D flows. To this purpose, we compare representative elementary volume (REV) scales obtained by 2D and 3D numerical simulations of flow in porous media. The stationarity of several quantities, namely porosity, permeability, mean and variance of velocity, is evaluated in terms of both classical and innovative statistics. The variance of velocity, strictly connected to the hydrodynamic dispersion, is included in the analysis in order to extend conclusions to transport phenomena. Pore scale flow is simulated by means of a Lattice Boltzmann model. The results from pore scale simulations point out that the 2D approach often leads to inconsistent results, due to the profound difference between 2D and 3D flows through porous media. We employ the error in the evaluation of REV as a quantitative measure for the reliability of a 2D approach. Moreover, we show that the acceptance threshold for a 2D representation to be valid strongly depends on which flow/transport quantity is sought.

Generalized input-output method to quantum transport junctions. I. General formulation

Abstract: The interaction of electrons with atomic motion critically influences charge transport properties in molecular conducting junctions and quantum dot systems, and it is responsible for a plethora of transport phenomena. Nevertheless, theoretical tools are still limited to treat simple model junctions in specific parameter regimes. In this paper, which forms the first paper of a series, we put forward a generalized input-output method (GIOM) for studying charge transport in molecular junctions accounting for strong electron-vibration interactions and including electronic and phononic environments. The method radically expands the scope of the input-output theory, which was originally put forward to treat quantum optic problems. Based on the GIOM, we derive a Langevin-type equation of motion for system operators, which possess a great generality and accuracy, and permits the derivation of a stationary charge current expression involving only two types of transfer rates. Furthermore, we devise the so-called polaron transport in electronic resonance approximation, which allows us to feasibly simulate electron dynamics in generic tight-binding models with strong electron-vibration interactions.