Fluid-wall Interaction Research Articles

Globally time-reversible fluid simulations with smoothed particle hydrodynamics

This paper describes an energy-preserving and globally time-reversible code for weakly compressible smoothed particle hydrodynamics (SPH). We do not add any additional dynamics to the Monaghan's original SPH scheme at the level of ordinary differential equation, but we show how to discretize the equations by using a corrected expression for density and by invoking a symplectic integrator. Moreover, to achieve the global-in-time reversibility, we have to correct the initial state, implement a conservative fluid-wall interaction, and use the fixed-point arithmetic. Although the numerical scheme is reversible globally in time (solvable backwards in time while recovering the initial conditions), we observe thermalization of the particle velocities and growth of the Boltzmann entropy. In other words, when we do not see all the possible details, as in the Boltzmann entropy, which depends only on the one-particle distribution function, we observe the emergence of the second law of thermodynamics (irreversible behavior) from purely reversible dynamics.

Computing Thermodynamic Properties of Fluids Augmented by Nanoconfinement: Application to Pressurized Methane.

Nanoconfined fluids exhibit remarkably different thermodynamic behavior compared to the bulk phase. These confinement effects render predictions of thermodynamic quantities of nanoconfined fluids challenging. In particular, confinement creates a spatially varying density profile near the wall that is primarily responsible for adsorption and capillary condensation behavior. Significant fluctuations in thermodynamic quantities, inherent in such nanoscale systems, coupled to strong fluid-wall interactions give rise to this near-wall density profile. Empirical models have been proposed to explain and model these effects, yet no first-principles based formulation has been developed. We present a statistical mechanics framework that embeds such a coupling to describe the effect of the fluid-wall interaction in amplifying the near-wall density behavior for compressible gases at elevated pressures such as pressurized methane in confinement. We show that the proposed theory predicts accurately the adsorbed layer thickness as obtained with small-angle neutron scattering measurements. Furthermore, the predictions of density under confinement from the proposed theory are shown to be in excellent agreement with available experimental and atomistic simulations data for a range of temperatures for nanoconfined methane. While the framework is presented for evaluating the near-wall density, owing to its rigorous foundation in statistical mechanics, the proposed theory can also be generalized for predicting phase-transition and nonequilibrium transport of nanoconfined fluids.

Methane flow in nanopores: Analytical approximation based on MD simulations

In the current study by means of molecular dynamics (MD) computer simulation, we have investigated pressure-driven flow of methane at high temperature and pressures (398.15 K and 400–500 bar) in circular, square, and triangular nanopores of different size and different fluid–wall interaction strength. To generalize the results obtained in MD simulation, we have proposed an analytical approximation (ansatz) of fluid flow dependence on pore size, while the numerical coefficients have been derived from the fitting of computer simulation data. The formulae obtained take into consideration such nanoflow features as adsorption of fluid on the pore walls and high slip velocity, which are usually neglected in petrophysical simulators operating with continuum fluid approach. These analytical formulae can be then incorporated into various larger scale models used for calculation of fluid flow in nanoporous media. This study is aiming the investigation of general phenomena occurring near the fluid–wall interfaces at nanoscale.

Couette flow of pentane in clay nanopores: Molecular dynamics simulation

Shear flow of n-pentane in slit nanopores between clay surfaces is investigated by molecular dynamics simulations. Pyrophyllite and hydrated Na-montmorillonite are considered as representative examples of hydrophobic and hydrophilic clay surfaces, respectively. The viscosity-density relations and slip lengths are calculated for both pentane-clay interfaces for different pore widths. The results show that the viscosity-density dependencies for n-pentane are not changed by the confinement in pores with sizes from 3 to 7nm, compared to the bulk liquid. At the pyrophyllite-pentane interface the slip length is 0.29nm on average for all studied densities and pore sizes. However, the slip length is negligible at the montmorillonite-pentane interface, likely due to the microscopic roughness of the interface between pentane and the adsorbed water layer. The orientation analysis shows some preference for pentane molecules ordering parallel to the wall surfaces, which is stronger in pyrophyllite pores compared to the montmorillonite, suggesting an influence of the details of fluid-wall interaction on the liquid structure in nanopores.

Bridging confined phase behavior of CH[formula omitted]-CO[formula omitted] binary systems across scales

Phase behavior of confined fluids may deviate significantly from that of the bulk fluid due to the fluid–wall interactions being a significant portion of all intermolecular interactions under confinement. Despite recent advancements in understanding confined phase behavior of pure fluids, confined phase behavior of mixtures remains an understudied topic. In this work, we examine the confined phase behavior of a CH4-CO2 binary system by combining Monte Carlo (MC) simulations, a cubic equation of state (EoS), and the lattice Boltzmann method (LBM). First, the effects of confinement on density and phase distribution in nano-size pores are established using Gibbs Ensemble MC calculations, which produce precise results of liquid and vapor confined pressures and account for the modification of the phase change location. By comparing the phase envelopes of bulk and confined mixtures at a fixed temperature, it is observed that the phase envelopes shrink with reductions in pore size. Based on this observation, we extend a modified Peng–Robinson EoS, which was originally developed for pure fluids under confinement, to mixtures via van-der-Waals-type mixing rules and by accounting for shifts in the critical properties of confined CH4-CO2. The resulting phase envelopes are in good agreement with the MC data. In addition, a local density model is used in combination with the confined EoS to calculate adsorption isotherms of CH4-CO2 mixtures and to characterize the behavior of confined matter in nanopores. Finally, we incorporate this EoS in a multicomponent multiphase LBM that uses a pseudopotential model to represent intermolecular forces. This workflow utilizes multiscale simulation techniques to bridge the behavior of multicomponent systems across scales and to shed light on the confined phase behavior of CH4-CO2 binary systems.

Molecular Insights into the Wall Slip Behavior of Pseudoplastic Polymer Melt in Nanochannels during Micro Injection Molding

Wall slip directly affects the molding quality of plastic parts by influencing the stability of the filling flow field during micro injection molding. The accurate modeling of wall slip in nanochannels has been a great challenge for pseudoplastic polymer melts. Here, an effective modeling method for polymer melt flow in nanochannels based on united-atom molecular dynamics simulations is presented. The effects of driving forces and wall–fluid interactions on the behavior of polyethylene melt under Poiseuille flow conditions were investigated by characterizing the slip velocity, dynamics information of the flow process, and spatial configuration parameters of molecular chains. The results indicated that the united-atom molecular dynamics model could better describe the pseudoplastic behavior in nanochannels than the commonly used finitely extensible nonlinear elastic (FENE) model. It was found that the slip velocity could be increased with increasing driving force and show completely opposite variation trends under different orders of magnitude of the wall–fluid interactions. The influence mechanism was interpreted by the density distribution and molecular chain structure parameters, including disentanglement and orientation, which also coincides with the change in the radius of gyration.

New Investigation of Asymmetric Wall Temperature and Fluid-Wall Interaction on Radiative Steady MHD Fully Developed Natural Convection in Vertical Micro-Porous–Channel

New Investigation of Asymmetric Wall Temperature and Fluid-Wall Interaction on Radiative Steady MHD Fully Developed Natural Convection in Vertical Micro-Porous–Channel

Heat transfer analysis of arrhenius-controlled free convective hydromagnetic flow with heat generation/absorption effect in a micro-channel

The study of a chemically reacting fluid in a vertical micro-channel limited to an applied transverse magnetic field that has fully developed steady natural convection flow under the impact of internal heat generation or absorption effects is the focus of this research. The governing equations are solved in dimensionless form using the homotopy perturbation method, HPM. Temperature, velocity, volume flow rate, and frictional drag force fundamental flow attributes are investigated as a function of controlling parameters such as heat generating or absorbing parameters, chemical reaction parameter, rarefaction parameter, fluid-wall interaction parameter and wall-ambient temperature difference ratio. The results are thoroughly analyzed and graphically depicted in a variety of plots. It is worth noting that raising the chemical reaction and rarefaction parameters increases fluid flow and volume flow rate, respectively. Furthermore, the action of heat absorption is observed to suppress fluid flow whereas heat generation establishes the opposite condition. Additionally, numerical data was generated and tabulated to compare the findings of this study to those of Jha et al. (2014) when the chemical reactant and heat generating/absorbing parameters were neglected, and an excellent agreement was discovered.

Dynamic behavior of miscible binary fluid mixtures in nanopores: Implications for CO2-enhanced oil flow in shale reservoirs

Dynamic behavior of miscible CO2-oil mixtures is the heart of CO2 enhanced oil recovery (EOR) and CO2 storage in shale reservoirs. However, the omnipresent nanopores not only raise the difference in CO2-reduced oil viscosity through thermodynamic effect from that of bulk condition but also add additional kinetic resistance resulting from a pore surface. In this work, a novel thermo-kinetic model is proposed to characterize the miscible mixtures flow in nanopores with involving changes in activation free energy induced by thermodynamic effect and kinetic effect, where the former is correlated with Helmholtz free energy by a modified Soave-Redlich-Kwong equation of state, while the latter is determined by a solid–fluid potential model with assuming a miscible mixture as a single pseudo-fluid. Subsequently, CO2-enhanced oil flow in nanopores and its deviation from bulk results are demonstrated. Moreover, the roles of different microscopic interactions are clarified, and the effects of a pore size, CO2 content as well as the strength of fluid-wall interactions are discussed. It is found that CO2 can simultaneously weaken the fluid–fluid interactions and fluid-wall interactions, jointly leading to a reduction in local fluid viscosity and an enhancement in oil flow. The increase in wall-oil affinity and CO2 content can strengthen the enhancement effect. Moreover, our results indicate that the relationship between enhancement effect and pore size strongly depends on the strength of interfacial interactions. For a nanopore with low wall energy (εs/kb < 10 K), a reduction in the enhancement factor is obtained with a decrease in pore size, while the opposite phenomenon is observed for a pore wall with a strong affinity to the oil. Our study lay a theoretical foundation for the reservoir simulation of CO2-EOR and CO2 storage in shale reservoirs.

Hydrogen storage in nanoporous media: Molecular dynamics simulations of the confinement effects

Hydrogen has attracted considerable interest as a clean substitute for fossil fuels. Especially, substantial efforts have been dedicated into the development of nanoporous materials for mobile H2 storage. However, the potential of this approach is significantly limited by the generally low H2 loading capacity. Moreover, the fundamental understanding of the H2 behavior in confined spaces is still lacking. In this study, the enhancement of the fluid-wall interaction with decreasing pore sizes was first investigated using a simple numerical model. Subsequently, molecular dynamics simulations were conducted to discern the influencing factors on confined H2 density. The results confirmed the importance of pore size and fluid-wall interaction in determining the H2 storage capacity. Specifically, the confined density of H2 increased with shrinking pore sizes only when the nanoporous material met the energy requirement. The reverse trend of the optimal pore size below and above the threshold H2-wall interaction energy was attributed, respectively, to the absence and presence of the adsorbed H2 layers. It was also found that the benefit of confinement decays with increasing pressure due to the disproportional density change in the adsorption layers. The findings in this study shed light on the defining factors influencing the H2 storage in nanoporous media and provide guidance on the selection and synthesis of the H2 hosting materials.

Thermodynamic modeling for confined fluids in nanopores using an external potential-advanced equation of state

Quantitatively understanding thermodynamic properties of confined fluids in nanoporous media is of great significance to the development of shale gas. Due to the complex intermolecular forces in the nanopore, it is difficult to accurately predict the thermodynamic properties of fluid molecules. Focusing on the fluid-fluid and fluid-wall molecules interactions in shale reservoir system, the thermodynamic model of fluid molecules in pores is constructed, and a modified equation of state is proposed by systematically coupling the original Soave-Redlich-Kwong equation of state with Tjatjopoulos-Feke-Mann potential model in this work. The advanced EoS could facilitate a good prediction on thermodynamic properties of confined fluids without any introduction of new empirical parameters. For verification, fluid density as the important thermodynamic property was targeted and pure methane at a wide pressure range was employed to represent the fluid. The results indicated that the calculated densities accord well with the reported ones in the free gas zone. The deviation of discrete density ranges from 0.239% to 1.7329%. The fluid density distribution in the nanopores is found to be nonuniform, exhibiting a greater value near the wall than that in the pore center, which would be ascribed to the more dominant fluid wall molecule interaction. For example, the local density is 16.90 kg/m3 in the pore center, while it increases to 26.67 kg/m3 in the region which is 0.76 nm to the wall at 350 K, 3 MPa, and 5 nm (radius). Moreover, effects of other critical factors on fluid density distribution were also conducted, and it was indicated that higher pressure, lower temperature, and smaller pore size could be favorable for the occurrence of confined fluid. In general, the novel EoS could provide a quantitative and simple method in predicting the thermodynamic properties of confined fluids relating to applications of shale gas storage and exploitation.

Viscoelastic response of fluid trapped between two dissimilar van der Waals surfaces

Employing grand canonical Monte-Carlo and molecular dynamics simulations, the viscoelastic response of trapped fluid under molecularly thin confinement by walls having different wall–fluid interaction strengths, is investigated. With increase in slit asymmetry, given by the ratio of interaction strengths of the wall having strong wall–fluid interaction to that of the wall with weak wall–fluid interaction, a crossover in effective density of the fluid film, from rarer (R) to denser (D) than the bulk density is observed. Upon increasing asymmetry further, the dense fluid (F) layers undergo bond-orientational (S) ordering. The variation of viscoelastic relaxation time with scaled asymmetry shows a universal behavior, independent of slit width, with two distinct regimes. Below a critical value of asymmetry, the viscoelastic relaxation time is a slowly varying function of asymmetry, comparable with the structural relaxation time. Beyond the critical asymmetry, on the other hand, viscoelastic response time shows a sharp increase upon increasing asymmetry, deviating markedly from the structural relaxation time. Interestingly the critical asymmetry value is found to correlate with R to D crossover. The microscopic origin of the two-regime universal behavior of viscoelastic response time is found to stem from the fact that below critical asymmetry, the overall viscoelastic behaviour of the slit is dominated by that of the fast relaxing layer close to the weakly attracting surface, while above the critical asymmetry, the relaxation behaviour is guided by the dense fluid layer adjacent to the strongly attracting wall. In vicinity of fluid to ordering transition, the loss and storage moduli merge for low frequencies as in gel-like mechanical behaviour. The storage modulus takes over the loss modulus in the phase co-existence region even before the long ranged order sets in. Our findings bear important implications for fluid transport in hetero-structured geometry in nanotechnology.

Simplified Lumped Parameter Model for Oscillatory Flow in an Elastic Tube: A Hierarchical Approach

Abstract In this article, the significant characteristics of wave transmission and boundary layer influence are studied for an incompressible fluid flowing through a long elastic tube using lumped parameter model. An ameliorated version of the lumped parameter model is derived, where the flow impedance is linked with Womersley number to understand the phenomena of fluid wall interaction, especially in human airways. To analyze the flow phenomena through tubes, the flow governing equations are transformed through the Bessel series. We have observed that for the low value of Womersley number, there is a phase difference between pressure and flowrate, which produces a frequency-dependent flow impedance. The physiological relevance of this result is especially shown in the nasal-pharynx during breathing. The results are validated by ansys-based numerical simulation as well as by experimental results.

Non-equilibrium characteristics of mass and heat transfers in the slip flow

Slip flow is a common phenomenon in micro-/nano-electromechanical systems. It is well known that the mass and heat transfers in slip flow show many unique behaviors, such as the velocity slip and temperature jump near the wall. However, the kinetic understanding of slip flow is still an open problem. This paper first clarifies that the Thermodynamic Non-Equilibrium (TNE) flows can be roughly classified into two categories: near-wall TNE flows and TNE flows away from the wall. The origins of TNE in the two cases are significantly different. For the former, the TNE mainly results from the fluid–wall interaction; for the latter, the TNE is primarily due to the considerable (local) thermodynamic relaxation time. Therefore, the kinetic modeling methods for the two kinds of TNE flows are significantly different. Based on the Discrete Boltzmann Modeling (DBM) method, the non-equilibrium characteristics of mass and heat transfers in slip flow are demonstrated and investigated. The method is solidly verified by comparing with analytic solutions and experimental data. In pressure-driven flow, the DBM results are consistent with experimental data for the Knudsen number up to 0.5. It is verified that, in the slip flow regime, the linear constitutive relations with standard viscous or heat conduction coefficients are no longer applicable near the wall. For the Knudsen layer problem, it is interesting to find that a heat flux (viscous stress) component in the velocity (temperature) Knudsen layer approximates a hyperbolic sinusoidal distribution. The findings enrich the insights into the non-equilibrium characteristics of mass and heat transfers at micro-/nano-scales.

Force Measurement with a Strain Gauge Subjected to Pure Bending in the Fluid–Wall Interaction of Open Water Channels

An experimental method to measure forces of small magnitude with a strain gauge as a force sensor in the fluid–wall interaction of open water channels is presented. Six uniaxial strain gauges were employed for this purpose, which were embedded across the entire sensing area and subjected to pure bending, employing two-point bending tests. Sixteen two-point bending tests were performed to determine the existence of a direct relationship between the load and the instrument signal. Furthermore, a regression analysis was used to estimate the parameters of the model. A data acquisition system was developed to register the behavior of the strain gauge relative to the lateral displacement induced by the loading nose of the universal testing machine. The results showed a significant linear relationship between the load and the instrumental signal, provided that the strain gauge was embedded between 30% and 45% of the central axis in the sensing area of the sensor (R2 &gt; 0.99). Thus, the proposed sensor can be employed to measure forces of small magnitude. Additionally, the linear relationship between the load and the instrumental signal can be used as a calibration equation, provided that the strain gauge is embedded close to the central axis of the sensing area.

Integral Methodology for the Multiphysics Design of an Automotive Eddy Current Damper

The present work reports an integrated (experimental and numerical) methodology that combines the development of a finite element multiphysics model with an experimental strategy to optimally design an eddy current damper for automotive suspensions. The multiphysics model couples the whole set of time-dependent electromagnetic, thermal, mechanical, and fluid–wall interaction (CFD) partial differential equations. The developed FE model was validated against both literature model predictions and in-house experimental data. The electromagnetic model takes into account the magnetic material characteristics of the ferromagnetic material and iron poles. Loss separation and the Jiles–Atherton hysteresis models were invoked to determine the heat generated in the soft iron parts. The computation of the fluid–wall interaction phenomena in the air gap allowed for the prediction of the temperature field across the solid materials, including the magnets. The design of the EC damper addresses the effects of the geometries of the stator and rotor, as they are the most critical geometries for maximizing the functions of an eddy current damper. The magneto-thermal simulations suggested that the heating of the permanent magnets remains within a safe region over the investigated operational frequency range of the eddy current damper.

Non-equilibrium molecular dynamics simulation to evaluate the effect of confinement on fluid flow in silica nanopores

The flow of fluids in nano-confinement has applications in separations, water purification, medical systems and in the recovery of fluids in petroleum systems. It is believed that fluids do not obey continuum laws when flowing in nanopores (or in confinement). This paper attempts to shed light on various nano confinement effects such as fluid-wall interactions, pore size and molecular geometry, using molecular dynamic simulation. Water, hexane, and methanol flow behaviors were simulated for pore diameters ranging from 1 to 8 nm. In addition to the density analysis of the confined fluids, the fluid flows in saturated nanopores were simulated by the sectional flow method. Water and methanol molecules were completely stabilized in the 1 nm pore. Hexane molecules could not enter the 1 nm pore, due to geometric considerations. For the 2 – 8 nm pores, all fluids showed reduced flow rate compared to the Hagen–Poiseuille flow, due to an interfacial molecular layer stabilized at the pore surface. Flow reduction observed in these studies is contrary to significant flow enhancements observed for fluid flow in carbon nanotubes of similar dimensions. Water flow showed almost constant stick length for all the pore sizes. Methanol flow had the largest stick length. Hexane flow was reduced because of overcrowding of molecules at the pore surface. The effect of confinement diminished with an increase in pore diameter.

Investigation on the influence mechanism of wall-fluid interaction on flow around nanoscale circular cylinder

The influence of atomic interaction strength between solid wall and fluid on the flow around a nanoscale circular cylinder was numerically probed with the molecular dynamics method. The results show that when the flow velocity is 2.2 and Reynolds number is 23, vortices are periodically formed behind the circular cylinder at the nanoscale, and go through the development and shedding. With the weakening of wall-fluid interaction between cylinder surface and fluid atoms, the vortex shedding period decreases, the Strouhal number increases, the weak wall-fluid interaction is beneficial for increasing vortex shedding frequency. It reveals that the length of time-averaged vortex behind the nanoscale cylinder decreases slightly with the decrease of wall-fluid interaction. Furthermore, the fluid density and velocity are symmetrically distributed along the central axis of nanoscale cylinder. As the wall-fluid interaction decreases, the binding force of cylinder surface to the fluid atoms decreases, which results in the decrease of the minimum value of fluid density in the upper right and lower right regions near the cylinder. And, the argon fluid is more prone to occur velocity slip on the cylinder surface, the corresponding velocity near the cylinder surface increases significantly.

Influence of Heat Generation and Thermal Radiation on MHD Flow in a Vertical Micro-Porous-Channel in the Presence of Viscous Dissipation

This article analyses the influence of heat generation and thermal radiation on steady hydromagnetic fully developed natural convection flow in a vertical micro-porous–channel in the presence of viscous dissipation. The governing ordinary differential equations, exhibiting the physics of the flow formation are displayed. Using the relevant non–dimensional variables, the governing equations are transformed into their corresponding non–dimensional form and then solved exactly by employing the perturbation technique. The influence of different admissible parameters such as fluid wall interaction parameter, Knudsen number, heat generation parameter, thermal radiation parameter, magnetic parameter, Eckert number, and permeability parameter on the fluid velocity, temperature, skin friction coefficient, and heat transfer coefficient at the micro-porous–channel surfaces is discussed with the aid of line graphs and tables.

Extension of PC-SAFT equation of state to include mineral surface effect in fluid properties using molecular dynamic simulation

In the vicinity of fluid-mineral interfaces a transition zone exist in which the order and packing of the molecules differ from that of the bulk phase where the distribution of intermolecular forces exhibit a more homogenous form. To develop an understanding of the thermodynamic properties in the fluid-mineral interface molecular dynamic (MD) simulation was conducted for the water-calcite system. To predict the water properties near the calcite wall, we have defined a contribution for Helmholtz energy extended from PC-SAFT equation of state (EOS). The new energy contribution depends on the confinement parameters i.e. potential of fluid-wall interaction, confinement degree, bulk density, and fraction of confined molecules estimated by MD simulation. The outcomes of MD simulation exhibit the layering transition of water on the water-calcite interface. In addition, MD simulation confirm the energy deviation within the layering transition zone, where the calcite adsorbs the water molecules. In this approach, the modified PC-SAFT showed a good agreement with MD observations. The results of this study can contribute to a better understanding of fluid behavior at the fluid-mineral interface. In addition, this technique is a valuable tool that can be used to estimate solubility limits in multicomponent fluid processing and pipeline transport.