Fluid-wall Interaction Research Articles

A BIOMECHANICAL COMPARISON OF BLOOD FLOW AND MECHANICAL CHARACTERISTICS OF THE EARLY ARTERIOSCLEROTIC AND HEALTHY ARTERIES USING A FLUID-WALL INTERACTION MODEL

A fluid-wall interaction model was introduced to investigate and compare the properties of blood flow, wall stress and deformation of the early arteriosclerotic and healthy arteries. The unsteady blood flow was assumed to be laminar, Newtonian, viscous and incompressible. The vessel wall was assumed to be linear-elastic, isotropic and incompressible. The fluid-wall interaction algorithm was constructed using a finite element method. The results show that the blood flow, wall stress and strain distributions of both arteries are similar. The blood flow characteristics of the diseased artery are similar to those of the healthy artery, which may be the main reason that the early arteriosclerosis could be reversible and treated by itself. The values of velocities, wall shear stresses, pressures and wall stresses of the diseased artery are approximately equal to those of the healthy artery, however the values of wall strains of both arteries are different and the cyclic strain ratio of the diseased artery is higher than that of the healthy artery. A high cyclic strain ratio is thought to be disadvantageous for arterial tissue. A new parameter anti-fatigue index (AFI) is introduced to evaluate mechanical fatigue of artery in the study. AFI is effective in characterizing the anti-fatigue ability of artery.

Structural quantities of quasi-two-dimensional fluids

Quasi-two-dimensional fluids can be generated by confining a fluid between two parallel walls with narrow separation. Such fluids exhibit an inhomogeneous structure perpendicular to the walls due to the loss of translational symmetry. Taking the transversal degrees of freedom as a perturbation to an appropriate 2D reference fluid we provide a systematic expansion of the m-particle density for arbitrary m. To leading order in the slit width this density factorizes into the densities of the transversal and lateral degrees of freedom. Explicit expressions for the next-to-leading order terms are elaborated analytically quantifying the onset of inhomogeneity. The case m = 1 yields the density profile with a curvature given by an integral over the pair-distribution function of the corresponding 2D reference fluid, which reduces to its 2D contact value in the case of pure excluded-volume interactions. Interestingly, we find that the 2D limit is subtle and requires stringent conditions on the fluid-wall interactions. We quantify the rapidity of convergence for various structural quantities to their 2D counterparts.

Molecular simulation study of triangle-well fluids confined in slit pores

Grand canonical Monte Carlo simulations are used to study the behaviour of triangle-well (TW) fluids with variable well widths confined inside slit pores. The effect of individual factors influencing the properties of confined fluids such as fluid–fluid interactions, pore size and pore wall–fluid interactions are obtained using simulations as it is difficult to experimentally determine the same. An interesting observation of this study is that inside the narrow pore of slit height h* = 5 at the high-pressure condition of P* = 0.8, for the TW fluid with long-range attraction or for the fluid at a low temperature for even a short-range attraction, the density profiles show layering such that there is a sticking tendency of the particles at centre, while there is a depletion of particles near the wall (as the layers at the centre have higher density peak heights than near the walls).

Computational modeling of blood flow in the aorta—insights into eccentric dilatation of the ascending aorta after surgery for coarctation

To assess whether combining a computational modeling technique with data from patient magnetic resonance imaging studies can detect different fluid dynamics and vascular biomechanical properties of the ascending and horizontal aorta in patients with angulated "gothic" arch geometry compared with those with normal "Romanesque" arch geometry after aortic coarctation repair. Advanced computational fluid dynamics techniques (coupled Navier-Stokes and elastodynamics equations) were used to predict the fluid-wall interactions in large arteries. We modeled the fluid dynamics and shear stress in the ascending and horizontal aorta in cases of "gothic" arch and normal "Romanesque" aortic arch geometry. A total of 30 patients after aortic coarctation repair prospectively underwent 3-dimensional magnetic resonance imaging angiography of the thoracic aorta. Measurements of the ascending and horizontal aorta were assessed using multiplanar reformatting images. Our computational model demonstrated that wall shear stress is greater in those with an angulated "gothic" aortic arch than in those with a "Romanesque" arch. In particular, wall shear stress affected the anterior and posterior segments of the ascending aorta and the inferior and superior segments of the horizontal aorta (vs the left and right segments). In vivo, a "gothic" arch was associated with dilatation of the ascending and horizontal aorta, which was eccentric rather than concentric (P<.05). Our results have shown that wall shear stress is eccentric and significantly increased in the ascending and horizontal aorta in patients with a "gothic" aortic arch after repair of coarctation. This suggests that patients with an angulated "gothic" aortic arch might warrant increased surveillance for aortic complications.

Molecular dynamics simulation of fluid containing gas in hydrophilic rough wall nanochannels

This study performed the molecular dynamic simulations to investigate the boundary behavior of liquid water with entrapped gas bubbles over various hydrophilic roughened substrates. A “liquid–gas–vapor coexistence setup” was employed to maintain a constant thermodynamic state during individual equilibrium simulations and corresponding non-equilibrium Poiseuille flow cases. The two roughened substrates (Si(100) and graphite) adopted in this study present similar contact angles and slip length with gas-free fluid. By considering the effects of argon molecules at the interface, we demonstrated that the boundary slip behavior differed dramatically between these two rough wall channels. This divergence can be attributed to differences in the morphology of argon bubble at the interface due to discrepancies in the atomic arrangement and wall–fluid interaction energy. Furthermore, the density of gas at the interface had a significant impact on the effective slip length of the roughened graphite substrate, whereas shear rate \(\dot{\gamma }\) presented no noticeable influence. On the roughened Si(100) surface, the morphology of the argon bubbles exhibited far higher meniscus curvature and unstable properties under hydrodynamic effects. Thus, this substrate exhibited no slip to slight negative slip and no remarkable influence from either the density of gas at the interface or shear rate. In the present study, we demonstrate that the morphology and behavior of interfacial gas bubbles are influenced by the parameters of wall–fluid interaction as well as the atomic arrangement of the substrate. Our results related to nanochannel flow reveal that different surfaces, such as Si(100) and graphite, may possess similar intrinsic wettability; however, properties of the interfacial gas bubbles can lead to noticeable changes in interfacial characteristics resulting in various degrees of boundary slippage.

Thermal transpiration through single walled carbon nanotubes and graphene channels

Thermal transpiration through carbon nanotubes (CNTs) and graphene channels is studied using molecular dynamics (MD) simulations. The system consists of two reservoirs connected by a CNT. It is observed that a flow is developed inside the CNT from the low temperature reservoir to the high temperature reservoir when the two reservoirs are maintained at different temperatures. The influence of channel size and temperature gradient on the mean velocity is analysed by varying the CNT diameter and the temperature of one of the reservoirs. Larger flow rate is observed in the smaller diameter CNTs showing an increase in the mean velocity with increase in the temperature gradient. For the flow developed inside the CNTs, slip boundaries occur and the slip length is calculated using the velocity profile. We examine the effect of fluid-wall interaction strength (ε(fw)), diffusivity (D), and viscosity of the fluid (μ) on the temperature induced fluid transport through the CNTs. Similar investigations are also carried out by replacing the CNT with a graphene channel. Results show that the mean velocity of the fluid atoms in the graphene channel is lower than that through the CNTs. This can be attributed to the higher degree of confinement observed in the CNTs.

Natural convection flow in a vertical microchannel with suction/injection

This study analyses the hydrodynamic and thermal behaviours of a fully developed natural convection flow in a vertical parallel-plate microchannel with suction/injection. The velocity slip and temperature jump at the walls are taken into account. The fully developed solutions of the velocity, temperature, volume flow rate, rate of heat transfer which is expressed as the Nusselt number and skin friction are derived analytically. The influence of each governing parameter on the microchannel hydrodynamic and thermal behaviours is discussed with the aid of graphs. Results show that as suction/injection on the channel surfaces increases, the volume flow rate increases and the rate of heat transfer decreases. In addition, it is observed that increase in the effects of rarefaction and fluid–wall interaction results in the increase in the volume flow rate and decrease in the heat transfer rate.

Density functional study of complete, first-order and critical wedge filling transitions

We present numerical studies of complete, first-order and critical wedge filling transitions, at a right angle corner, using a microscopic fundamental measure density functional theory. We consider systems with short-ranged, cut-off Lennard-Jones, fluid–fluid forces and two types of wall–fluid potential: a purely repulsive hard wall and also a long-ranged potential with three different strengths. For each of these systems we first determine the wetting properties occurring at a planar wall, including any wetting transition and the dependence of the contact angle on temperature. The hard wall corner is completely filled by vapour on approaching bulk coexistence and the numerical results for the growth of the meniscus thickness are in excellent agreement with effective Hamiltonian predictions for the critical exponents and amplitudes, at leading and next-to-leading order. In the presence of the attractive wall–fluid interaction, the corresponding planar wall–fluid interface exhibits a first-order wetting transition for each of the interaction strengths considered. In the right angle wedge geometry the two strongest interactions produce first-order filling transitions while for the weakest interaction strength, for which wetting and filling occur closest to the bulk critical point, the filling transition is second-order. For this continuous transition the critical exponent describing the divergence of the meniscus thickness is found to be in good agreement with effective Hamiltonian predictions.

Computational modelling of fluid structure interaction at nano-scale boundaries with modified Maxwellian velocity distribution

The soft collisions among fluid–fluid and fluid-wall molecules are modeled from first principles. In particular, the assumption of Maxwellian distribution of velocities for thermalized molecules, in both parallel and perpendicular directions to the wall, has been re-evaluated with supporting experimental and/or numerical evidence.It is proposed that the normal component of molecular velocity post collision is conserved for all fluid molecules. The slip effect at the wall boundary, introduced by the surface roughness, is accounted by an accommodation coefficient f. A moving least square method is used to calculate macroscopic velocity values. The influence of molecular interaction on the macroscopic velocity distribution is investigated at 40MPa and 300K for slit pore, inclined and stepped wall configurations. The accommodation coefficient values f=0, 0.07, 0.257, 0.45, 0.681 and 1; and acceleration values ranging from zero to 1×1011m/s2 and 250×1011m/s2 are used for comparison.The distribution of macroscopic velocity parallel to the wall is studied to observe the effect of the slip behaviour. The detailed study of average of velocity values at various magnitudes of acceleration has shown an evidence of characteristic low and high speed of molecular flows that is considered as significant and a comparison is sought with an equivalent laminar and turbulent flow style behaviour. The two dimensional vector and contour plots of macroscopic velocity provide further insights in understanding Continuum velocity distributions resulting from molecular fluid-wall interaction at nanoscale. The research has highlighted the need to develop molecular dynamics simulation techniques for non-periodic boundary conditions.

Experimental analysis on FEM definition of backfill-rectangular tank-fluid system

In the present study, the numerical and experimental investigations were performed on the backfill- exterior wall-fluid interaction systems in case of empty and full tanks. For this, firstly, the non-linear three dimensional (3D) finite element models were developed considering both backfill-wall and fluid-wall interactions, and modal analyses for these systems were carried out in order to acquire modal frequencies and mode shapes by means of ANSYS finite element structural analysis program. Secondly, a series of field tests were fulfilled to define their modal characteristics and to compare the results from proposed approximation in the selected structures. Finally, comparing the theoretical predictions from the finite element models to results from experimental measurements, a close agreement was found between theory and experiment. Thus, it can be easily stated that experimental verifications provide strong support for the finite element models and the proposed procedures themselves are the meritorious approximations to the real problem, and this makes the models appealing for use in further investigations.

Simulating Gas–Liquid Flows by Means of a Pseudopotential Lattice Boltzmann Method

Dispersed gas (vapor)–liquid flow through an inclined microchannel with bends has successfully been simulated, that is, without numerical difficulties, by means of a two-phase Lattice Boltzmann method. Combining in this method the Shan-Chen1 pseudopotential interaction model with the Yuan and Schaefer2 proposal for dealing with nonideal equations of state makes high density ratios achievable. This approach also allows simulation of gas–liquid flows without explicitly having to track the phase interfaces. Rather, a potential function related to the equation of state for vapor–liquid equilibrium, a coupling strength representing attraction or repulsion between species, and a relaxation time scale take care of microscale and mesoscale phenomena such as phase separation and interfacial tension as well as interphase transport and multiphase flow. In addition, fluid–wall interaction (contact angle) is taken into account by selecting proper potential functions and coupling strengths. As far as the phase behavior...

Wave propagation in an elastic tube: a numerical study

In this paper, a fluid–wall interaction model, called the elastic tube model, is introduced to investigate wave propagation in an elastic tube and the effects of different parameters. The unsteady flow was assumed to be laminar, Newtonian and incompressible, and the vessel wall to be linear-elastic, isotropic and incompressible. A fluid–wall interaction scheme is constructed using a finite element method. The results demonstrate that the elastic tube plays an important role in wave propagation. It is shown that there is a time delay between the velocity waveforms at two different locations and that the peak velocity increases while the low velocity decreases in the elastic tube model, contrary to the rigid tube model where velocity waveforms overlap each other. Compared with the elastic tube model, the increase of the wall thickness makes wave propagation faster and the time delay cannot be observed clearly, however, the velocity amplitude is reduced slightly due to the decrease of the internal radius. The fluid–wall interaction model simulates wave propagation successfully and can be extended to study other mechanical properties considering complicated geometrical and material factors.

Tunable surface free energies of functionalized molecular layers on Si surfaces for microfluidic immunosensor applications

Enhanced antigen–antibody interactions in microfluidic immunosensors can be effected by tailoring the surface free energies of the antibody immobilized surfaces to obtain the appropriate fluid–wall interactions. We report a systematic study to evaluate the surface free energies from contact angle measurements, using the LWAB method, of different antibody (anti-BSA, anti-PSA, and anti-CRP) surfaces, each immobilized separately on to non- and nanotextured Si surfaces via a stack of functionalized layers including aminosilanes of which three different types were used. The apolar surface free energy components were independent of the physical modification in the non-functionalized and the intermediate hydrolyzed surfaces where as they depended on the nature of the surface and the chemical modifications in the subsequent functionalized stages. Surface free energies of the different antibodies immobilized with the shorter chain length aminosilane (APTES) on non- and nanotextured surfaces were in the order of anti-BSA<anti-PSA<anti-CRP. A tunability of the surface free energy up to 9.6mJ/m2 was achieved which is reasonably significant when compared to the surface free energy window (Δγs=40mJ/m2) of biofunctionalized surfaces. This fundamental understanding of the surface energetics of the biofunctionalized surfaces can be utilized in modulating the surface properties to design efficient immunosensors.

Numerical investigation of bubble nucleation process using the lattice Boltzmann method

In this paper, the state of metastable equilibrium and the state of unstable equilibrium of water at a certain temperature are explored using an exact difference lattice Boltzmann model and the conditions of bubble (droplet) formation are investigated in the isothermal phase transition processes. From these simulation results, it is found that the model predictions are in good agreement with analytical results. Based on these works, a new model, which is based on exact difference lattice Boltzmann model and extended with an energy transfer equation to model heat transfer, is proposed to describe liquid-vapor phase transition process. The effects of the wall-fluid interaction strength on the bubble nucleation process in a pit are investigated using this new heterogeneous phase transition model. Simulation results accurately reproduce the characteristics of three stages of the bubble nucleation process. The changes of the contact angle, curvature radius, and volume with the bubble nucleation process are explored, and the relationship curve between curvature and bubble volume from the simulations is in qualitative agreement with the previous results.

Elastic properties of a confined fluid

Monte Carlo computer simulation is employed to determine the local, wave-number-dependent, high-frequency elastic properties of a Lennard-Jones fluid that is confined between two walls. In particular, the elastic constants are calculated from coarse-grained stress correlation functions and then related to the local fluid structure via planar radial distribution functions. We find that local fluid properties correlate with the inhomogeneous fluid density and the strength of the wall-fluid interaction. Finally, we discuss the utility of this analysis in the interpretation of experiments involving the characterization of confined fluids.

Wave propagation analysis of carbon nano-tube conveying fluid including slip boundary condition and strain/inertial gradient theory

This article addresses wave propagation in carbon nano-tube (CNT) conveying fluid. CNT structure is modeled by using size-dependent strain/inertia gradient theory of continuum mechanics, CNT wall-fluid flow interaction by slip boundary condition and Knudsen number (Kn). Complex-valued wave dispersion relations and corresponding characteristic equations are derived. Fluid viscosity, gyroscopic inertial force, flow velocity, wave number, wave frequency, and decaying ratio are among parameters that their variations are discussed and some remarkable results are drawn. It was observed Kn could impress complex wave frequencies at both lower and higher ranges of wave numbers, while small-size had impression at higher range.

Viscosity calculation of a nanoparticle suspension confined in nanochannels

The kinetic properties of the pressure-driven Poiseuille flow in nanochannels with and without nanoparticles were studied with a nonequilibrium molecular dynamics simulation. To allow the fluid to dissipate heat, the boundary was kept at a constant temperature. Pure fluid simulations were taken as references and also used to study the fluid-wall interfacial interaction effects. The viscosity profiles of the fluid were calculated on the basis of velocity profiles and known applied shear stress. We present the relationship between the viscosity increase and particle loading. The role of channel wall-fluid wetting properties on the flow and viscosity was also investigated.

Molecular dynamics simulation of heat transfer with effects of fluid–lattice interactions

The nonequilibrium molecular dynamics simulation is employed to investigate the thermal properties of fluid confined in different FCC nanochannels. The results show that fluid in different lattice channels appears diverse wetting characteristics at low temperature. Based on wall parameters, a ratio is defined to describe the fluid–lattice interaction. Wall attraction, number of absorbed particles and thermal conductivity are increased as the increase of this ratio as well as the location of particles get closer to the wall. Thermal resistance exists along with the fluid–wall interface and loses the dominant of heat transport as the system temperature gets raised. At the same time, the thermal conductivity of nanoscale experiences unconventional increase. The fluid thermal properties are influenced both by wall–fluid interaction and temperature.

Vapor–liquid phase diagrams and structure of a square-well fluid confined in weakly attractive cylindrical pores

Using canonical Monte Carlo simulations, the vapor-liquid phase diagrams of a square-well fluid confined in hard and weakly attractive cylindrical pores are calculated. The critical temperature shows a maximum as a function of the attraction strength between the fluid particle and the pore wall. This maximum is located where the fluid–fluid equals the wall–fluid interaction strengths. With the aid of combined density profiles in the axial and radial directions the maximum of the critical temperature is correlated with the structure of the confined fluid.

Repeated sorption of water in SBA-15 investigated by means of in situ small-angle x-ray scattering

The effect of repeated cycles of water adsorption/desorption on the structural stability of ordered mesoporous silica SBA-15 is studied by small-angle x-ray scattering (SAXS). In situ sorption measurements are conducted using a custom-built sorption apparatus in connection with a laboratory SAXS setup. Two striking irreversible changes are observed in the sorption isotherms as derived from the integrated SAXS intensity. First, the capillary condensation pressure shifts progressively to lower relative pressure values with increasing number of sorption cycles. This effect is attributed to chemisorption of water at the silica walls, resulting in a change of the fluid–wall interaction. Second, the sorption cycles do not close completely at vanishing vapour pressure, suggesting that progressively more water remains trapped within the porous material after each cycle. This effect is interpreted to be the result of an irreversible collapse of parts of mesopores, originating from pore wall deformation due to the large Laplace pressure of water acting on the pore walls at capillary condensation and capillary evaporation.