Fluid-wall Interaction Research Articles

Influence of the distensibility of large arteries on the longitudinal impedance: application for the development of non-invasive techniques to the diagnosis of arterial diseases.

BackgroundThis study shows that the arterial longitudinal impedance constitutes a hemodynamic parameter of interest for performance characterization of large arteries in normal condition as well as in pathological situations. For this purpose, we solved the Navier–Stokes equations for an incompressible flow using the finite element analysis method and the Arbitrary Lagrangian Eulerian (ALE) formulation. The mathematical model assumes a two-dimensional flow and takes into account the nonlinear terms in the equations of fluid motion that express the convective acceleration, as well as the nonlinear deformation of the arterial wall. Several numerical simulations of the blood flow in large vessels have been performed to study the propagation along an arterial vessel of a pressure gradient pulse and a rate flow pulse. These simulations include various deformations of the wall artery leading to parietal displacements ranging from 0 (rigid wall) to 15% (very elastic wall) in order to consider physiological and pathological cases.ResultsThe results show significant changes of the rate flow and the pressure gradient wave as a function of aosc, the relative variation in the radius of the artery over a cardiac cycle. These changes are notable beyond a critical value of aosc equal to 0.05. This critical value is also found in the evolution of the longitudinal impedance. So, above a variation of radius of 5%, the convective acceleration, created by the fluid-wall interactions, have an influence on the flow detectable on the longitudinal impedance.ConclusionsThe interpretation of the evolution of the longitudinal impedance shows that it could be a mean to test the performance of large arteries and can contribute to the diagnosis of parietal lesions of large arteries. For a blood vessel with a wall displacement higher than 5% similar to those of large arteries like the aorta, the longitudinal impedance is substantially greater than that obtained in the absence of wall displacement. This study also explains the effects of convective acceleration, on the shape of the decline of the pressure gradient wave and shows that they should not be neglected when the variation in radius is greater than 5%.

Fluid transport in nanochannels induced by temperature gradients

We investigate the mechanisms of fluid transport driven by temperature gradients in nanochannels through molecular dynamics simulations. It is found that the fluid-wall interaction is critical in determining the flow direction. In channels of very low surface energy, where the fluid-wall binding energy ε(fw) is small, the fluid moves from high to low temperature and the flow is induced by a potential ratchet near the wall. In high surface energy channels, however, the fluid is pumped from low to high temperature and the pressure drop caused by the temperature gradient is the major driving force. In addition, as the fluid-wall interaction is strengthened, the flow flux assumes a maximum, where ε(fw) is close to the lower temperature T(L) of the channel and ε(fw)/kT(L) ≈ 1 is roughly satisfied.

A Model for Wall Slip Prediction of Confined n-Alkanes: Effect of Wall-Fluid Interaction Versus Fluid Resistance

It is shown via molecular dynamics simulations that the occurrence of wall slip for linear alkanes confined between geometrically smooth surfaces depends on the wall-fluid interactions and the chain length of the lubricant molecules. A wall slip model based on the competition between these two factors is introduced. A surface parameter accounts for the wall-fluid interaction and commensurability, and is valid for both canonical and complex crystal lattices: this quantity is then linked to the shear stress transferred to the fluid molecules. The lubricant internal cohesion under confinement is described by a bulk viscosity term. Finally, a semi-analytical law for wall slip prediction including both the fluid viscosity and the surface characterization parameter is proposed.

Nanoscale Poiseuille Flows of Liquid Argon

In nanoscale flow systems, the flow motion is affected by many parameters, some of which may play different roles under different conditions. In this work, we investigate the flux of liquid argon in nanoscale Poiseuille flows through molecular dynamics simulations. By illustrating the flux as a function of a dimensionless number, which represents the effective surface effect on the fluid, we show that the fluid motion in nanochannels under small external forces can be categorized into two regimes based on the role of the temperature. For large external forces, a bimodal behavior in the flux is observed as the fluid-wall interaction is varied. The underlying mechanisms that govern the flow fashions are discussed.

The Influence of Solid Wall-Fluid Molecular Interaction on Transport Properties of Gases in a Mini/micro Channel

The physical model and analytical method are put forward for considering the molecular interaction between solid wall and gas fluid when dealing with convective heat transfer in macro/mini/micro channels based on the boundary layer theory concept, the molecular kinetic theory of gases, structural chemistry and continuum hypothesis. The influence rule of wall-fluid intermolecular forces to the transport properties of gases located in boundary layer region is studied applying proposed models. The gas density variation distribution equation including the wall-fluid molecular interaction is derived with continuum media integral approach. The theoretical results show that the fluid diffusion is independent of the wall-fluid interaction but visosity and heat conductivity not. According to the gas molecular density distribution function and molecular dynamics, new formulae were developed for calculating viscidity coefficient and thermal conductivity with wall-fluid interacting effect for a fluid. The research results provide scientific reference for further study and exploitation on fluid flow and heat transfer of mini/micro channels. In addition, the formulae offered in this paper to compute the transport properties of gases are also suitable for fine analysis of boundary layer in macro-scale channels.

The Comparison and Analysis of Pore Size Distribution between Cotton Yarn and Fabric

To obtain more detailed and accurate pore size distribution for cotton fabric, this paper focuses on comparing size distribution between the yarn and fabric. The result shows that there are great difference as well as relationship in size distribution between the yarn and fabric. The size in fabric has 2 orders of magnitude including that in yarn with just 1 order of magnitude. The pore size ratio data also illustrate that the pore in yarn account for 33.3% of that in fabric, suggesting that the pore in yarn has a certain impact on the wicking behavior of fabric. However, the graphic exhibit that the size distribution shape in yarn is different from that in fabric, which may be owing to the fabric weaving configuration, implying that wicking liquid movement is affected not just by capillary effect along the yarn but also by interlacing points and fluid-wall interaction in the complex channels of fabric.

On the validity of the Navier-Stokes equations for nanoscale liquid flows: The role of channel size

In this work, we investigate the validity of the Navier-Stokes (NS) equations for nanoscale liquid flows through molecular dynamics simulations. We focus on the role of channel size by considering the fluid-wall interaction. Liquid flows between two planar parallel walls driven by an external force with channel size ranging from 2 to 80 nm are studied. The volumetric flux is computed and the dependence of the volumetric flux on the channel size is explained both qualitatively and quantitatively. It is found that the flow is sensitive to the fluid-wall binding energy and the classical fluid mechanics falls apart in small nanochannels. However, the wall effects become insignificant and the NS equations are valid when the channel size is larger than about 150 molecular diameters (∼ 50 nm).

Nanoscale spiral flow in a cylindrical channel

We study the behavior of pressure-driven flow in a cylindrical nanochannel patterned with different wettability using molecular dynamics simulations. We consider the flow perpendicular or parallel to the periodic stripes of wetting and nonwetting materials. For both geometries, we observe a sharp increase in the longitudinal velocity of the flow as wetting stripe width decreases, which is clearly distinguishable from the case of shear-driven flow. When the channel wall is spirally patterned with distinct wetting stripes, the transverse velocity of the flow is generated in the channel and its generation depends strongly on the degree of molecular ordering above the wetting stripes, which can be controlled by helical angle, wetting stripe width, the relative area of the wetting region, and the strength of the fluid-wall interaction.

Measurement of Near Wall Fluid Interaction with an Atomised Spray

Measurement of Near Wall Fluid Interaction with an Atomised Spray

CFD simulation of flow through heart: a perspective review

The heart is an organ which pumps blood around the body by contraction of muscular wall. There is a coupled system in the heart containing the motion of wall and the motion of blood fluid; both motions must be computed simultaneously, which make biological computational fluid dynamics (CFD) difficult. The wall of the heart is not rigid and hence proper boundary conditions are essential for CFD modelling. Fluid–wall interaction is very important for real CFD modelling. There are many assumptions for CFD simulation of the heart that make it far from a real model. A realistic fluid–structure interaction modelling the structure by the finite element method and the fluid flow by CFD use more realistic coupling algorithms. This type of method is very powerful to solve the complex properties of the cardiac structure and the sensitive interaction of fluid and structure. The final goal of heart modelling is to simulate the total heart function by integrating cardiac anatomy, electrical activation, mechanics, metabolism and fluid mechanics together, as in the computational framework.

Performance and Analysis of Blood Flow through Carotid Artery

The present paper a finite element implementation of a model of the arterial blood flow through the carotid artery with the effects of magnetic to considering fluid–wall interactions are investigated. The Navier–Stokes equations are used as the governing equations for the blood flow while an elastic compliant model is used for the arterial wall. The reduced one dimensional model solves the momentum and continuity equations in compliant tubes so as to reproduce the propagation of the pressure pulse in the arterial model. The obtained results adequately reproduce the general flow patterns reported in the literature. The results obtained in the investigation are in reasonably good agreement with experimental findings existing in the literature. The effects of a magnetic field have been used to control the flow, which may be useful in certain hypertension cases, etc.

Characterizing stability of “click” modified glass surfaces to common microfabrication conditions and aqueous electrolyte solutions

Microfluidic and nanofluidic systems are dominated by fluid-wall interactions due to enormous surface-area-to-volume ratios in these devices. Therefore, strategies to control wall properties in a reliable and repeatable manner can be important for device operation. Chemical modification of surfaces provides one such method. However, the stability of the surface adhered layers under fabrication and likely device operating conditions have not been evaluated in depth. This paper presents the stability analysis of three surface layers used in the 'click' chemistry methodology for surface modification. The three surface layers have bromo, amine, and methyl termination on glass surfaces. All three surface groups are exposed to various wet and dry conditions including acid, base, solvent, electrolyte buffer solutions, oxidative plasmas, UV light, and thermal processing conditions. Contact angle measurements, X-ray photoelectron spectroscopy, and atomic force microscopy were used to quantify the stability of the adhered surface layers. The data show that the brominated surface was stable to most test conditions, while both the amine and methyl surface layers were stable to a narrower set of test conditions.

Experimental analysis on MR fluid channel flow dynamics with complex fluid–wall interactions

MR fluid plugging performance by aggregation of magnetized particles in MR fluid is recently expected to be one of the most promising applications in medical or safety devices, such as blood flow control, steam issuing shut-down valve and fuel supply control for automobile. In this study, dynamic response of MR fluid plugging and its breakdown in a pressure mode with complex fluid–wall interactions was experimentally investigated, considering the effects of magnetic flux density, wall surface structure, wall permeability and wall elasticity of tube. Higher endurance pressure is obtained for wall surface groove structure and for steel wall due to a strong anchoring effect by rigid cluster formation in a concave region and strong MR fluid column formation in a channel core region, respectively. Furthermore, MR fluid plugging performance and the fluid storage characteristic of PVA tube as a bio-material was clarified. Because of the large radial expansion of the tube at the applied magnetic region in a pressure mode, PVA tube shows unique characteristics, such as storing MR fluid under magnetic field and MR fluid jet issuing under releasing magnetic field.

Relationship between induced fluid structure and boundary slip in nanoscale polymer films

The molecular mechanism of slip at the interface between polymer melts and weakly attractive smooth surfaces is investigated using molecular dynamics simulations. In agreement with our previous studies on slip flow of shear-thinning fluids, it is shown that the slip length passes through a local minimum at low shear rates and then increases rapidly at higher shear rates. We found that at sufficiently high shear rates, the slip flow over atomically flat crystalline surfaces is anisotropic. It is demonstrated numerically that the friction coefficient at the liquid-solid interface (the ratio of viscosity and slip length) undergoes a transition from a constant value to the power-law decay as a function of the slip velocity. The characteristic velocity of the transition correlates well with the diffusion velocity of fluid monomers in the first fluid layer near the solid wall at equilibrium. We also show that in the linear regime, the friction coefficient is well described by a function of a single variable, which is a product of the magnitude of surface-induced peak in the structure factor and the contact density of the adjacent fluid layer. The universal relationship between the friction coefficient and induced fluid structure holds for a number of material parameters of the interface: fluid density, chain length, wall-fluid interaction energy, wall density, lattice type and orientation, thermal or solid walls.

Ocellar lamprophyre dyke bearing mineralization, Wadi Nugrus, Eastern Desert, Egypt: Geology, mineralogy and geochemical implications

The ocellar lamprophyre dyke (ENE-WSW) is recorded at Wadi Nugrus, Eastern Desert, Egypt. It cuts porphyritic biotite granites and varies in thickness from 0.5 to 1.5 m and up to 3 km in length. The lamprophyre dyke has been altered, and it is characterized by porphyritic and panidiomorphic textures with plagioclase, olivine, and augite constituting the porphyritic phase in a fine groundmass of the same composition. Rutile, titanite, apatite, fluorite, graphite, calcite, allanite, autunite and Fe-Ti oxides are accessory minerals. Kaolinite, chlorite and epidote are secondary minerals. Carbonitization and hematitization are common. Rounded to sub-rounded porphyritic and zoned ocelli with radiate or brush-like shapes are generally common and represent physical traps for mineralization. The ocellar features are interpreted to represent the late stage of magmatic segregation or magmatic crystallization involving two immiscible magmatic liquids. Mineralogical studies of altered lamprophyre samples, based on X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM), indicate the presence of secondary uranium minerals (autunite and uranophane), associated with gold, silver, nickel, atacamite, molybdenum, pyrite and zincite. The presence of kaolinite indicates a high temperature environment. The mixing of volatile fluids with meteoric water and fluid-wall rock interaction result in changes of pH and oxygen activity and deposition of base metals in reducing environs (graphite and pyrite). Precipitation of hematite probably decreased the pH of the solution and rose acidic fluids. The sudden change in pH and temperature of the fluids would lead to destabilization of base metal complexes favoring their deposition. The magma source of the Nugrus lamprophyre is determined to be between calc-alkaline and alkaline and it has HKCAB and shoshonitic nature and may be derived from decomposition melting of such a metasomatised lithospheric mantle. Olivine is a major fractional phase to account for a rapid decrease in Ni and Fe2O3 and an increase in SiO2. Also, clinopyroxene fractionation is important in accounting for the positive correlations of CaO and Cr vs. MgO, whereas plagioclase differentiation is insignificant according to the negative correlations between MgO and Al2O3 and Sr. Crystal fractionation of accessory minerals such as apatite and Fe-Ti oxides is insignificant because of the less variable P2O5 and TiO2 contents.

Excess-entropy scaling of dynamics for a confined fluid of dumbbell-shaped particles

We use molecular simulation to study the ability of excess entropy scaling relationships to describe the kinetic properties of a confined molecular system. We examine a model for a confined fluid consisting of dumbbell-shaped molecules that interact with atomistically detailed pore walls via a Lennard-Jones potential. We obtain kinetic, thermodynamic, and structural properties of the system at three wall-fluid interaction strengths and over a temperature range that includes sub- and super-critical conditions. Four dynamic properties are considered: translational and rotational diffusivities, a characteristic relaxation time for rotational motion, and a collective relaxation time stemming from analysis of the coherent intermediate scattering function. We carefully consider the reference state used to define the excess entropy of a confined fluid. Three ideal-gas reference states are considered, with the cases differentiated by the extent to which one-body spatial and orientational correlations are accounted for in the reference state. Our results indicate that a version of the excess entropy that includes information related to the one-body correlations in a confined fluid serves as the best scaling variable for dynamic properties. When adopting such a definition for the reference state, to a very good approximation, bulk and confined data for a specified dynamic property at a given temperature collapse onto a common curve when plotted against the excess entropy.

On the kinetics of the capillary imbibition of a simple fluid through a designed nanochannel using the molecular dynamics simulation approach

A molecular dynamics (MD) approach was employed to simulate the imbibition of a designed nanopore by a simple fluid (i.e., a Lennard-Jones (LJ) fluid). The length of imbibition as a function of time for various interactions between the LJ fluid and the pore wall was recorded for this system (i.e., the LJ fluid and the nanopore). By and large, the kinetics of imbibition was successfully described by the Lucas-Washburn (LW) equation, although deviation from it was observed in some cases. This lack of agreement is due to the neglect of the dynamic contact angle (DCA) in the LW equation. Two commonly used models (i.e., hydrodynamic and molecular-kinetic (MK) models) were thus employed to calculate the DCA. It is demonstrated that the MK model is able to justify the simulation results in which are not in good agreement with the simple LW equation. However, the hydrodynamic model is not capable of doing that. Further investigation of the MD simulation data revealed an interesting fact that there is a direct relationship between the wall-fluid interaction and the speed of the capillary imbibition. More evidence to support this claim is presented.

Simulation of boiling heat transfer in small heaters by a coupled cellular and geometrical automata

Automata are entities defined by mathematical states that change following iterative rules representing neighborhood interactions. A model of automata for pool boiling heat transfer simulation consisting in collections of virtual spheres that change their volumes and move around a certain environment is presented. The approach is an alternative technique to describe the turbulent features of boiling phenomena, such as interfacial topological transitions and fluid-wall interaction. The novel computer model presented here is able to capture the essential features underlying boiling heat transfer and crisis above a small heater, showing good agreement with experimental data reported in the open literature.

Monte Carlo study of structural ordering of Lennard-Jones fluids confined in nanochannels

We investigate quantitatively the ordering of Lennard-Jones fluids confined in a thin and infinitely long nanochannel with square cross section. The most probable spatial configurations of the atoms were examined by Monte Carlo simulations, and the order parameter was calculated. The effect of the various parameters, such as the wall-fluid attractive interaction, the size of constriction, and the temperature, was studied. The results indicate that for strong wall-fluid interactions and small constrictions, the ordering of the fluid particles is almost perfect. Geometrical mismatch, as well as increasing the system's temperature, deteriorates the ordering phenomenon, even for very small openings. We observe a nontrivial trend in the dependence of the order parameter on the size of the opening of the channel with a linear size smaller than five atomic layers. We also examined the rearrangements of the fluid's atoms in more symmetrical pores--slitlike pores and cylindrical nanopores--and discuss their similarities and differences with the square channels.

Multiple sources for mineralizing fluids in the Charmitan gold(-tungsten) mineralization (Uzbekistan)

Mineral assemblages present within the Charmitan gold(-tungsten) quartz-vein mineralization have been investigated for their cathodoluminescence behaviour, chemical composition and noble gas isotope systematics. This inventory of methods allows for the first time a systematic reconstruction of the paragenetic relationships of quartz, scheelite, sulphides and native gold within the gold mineralization at Charmitan and provides the basis to utilise noble gas data in the discussion of sources and evolution of ore-forming fluids. The vein quartz is classified into four generations based on microscopic and cathodoluminescence investigations. Quartz I shows intense brittle deformation as associated scheelite I. Undeformed scheelite II overgrows scheelite I and has lower light rare earth element and higher intermediate rare earth element contents as well as higher strontium concentrations. Scheelite II is associated with the economic gold mineralization and formed during re-crystallisation and re-precipitation of material which was partly re-mobilised from early scheelite I during infiltration of gold-bearing fluids. Early stage native gold inclusions are often associated with stage 2 sulphides, scheelite II and bismuth tellurides and contain Ag (3.6–24.4 wt.%), Hg (≤1.0 wt.%) and Bi (≤0.2 wt.%). Later stage electrum grains occur in association with stage 3 sulphides and sulphosalts and contain Hg (<0.8 wt.%) and elevated Sb concentrations (up to 3.0 wt.%). Noble gas isotope data (3He/4He: 0.2-0.4 Ra) for hydrothermal ore fluids trapped in the gold-related sulphides and sulphosalts (stage 2 pyrite and arsenopyrite; stage 3 pyrite, sphalerite, galena and lead sulphosalts) suggest that diverse fluid sources were involved in the formation of the Charmitan gold deposit. These data are indicative of a small, but significant input of fluids from external, deep-seated (mantle and possibly lower crust) sources. A decrease in the input of mantle helium and an increasing role of crustal helium from early to later stages of the mineralization is suggested by the measured 3He/4He and 40Ar*/4He ratios. Sulphides from ore veins in meta-sedimentary rocks contain higher portions of meteoric fluids than those in intrusive rock types as indicated by their lower 3He/36Ar ratios. The 3He/36Ar ratios in the meta-sedimentary rocks agree well with ratios typical of gold mineralizations in the Tien Shan gold province completely hosted by meta-sedimentary sequences, indicating intense fluid-wall rock interaction.