arXiv subjects

Binghai Wen

Publications and source records attributed to Binghai Wen.

16 recordsLinked to original sources

arXiv2025

Influence of Membrane Characteristics on Efficiency of Vacuum Membrane Distillation: a Lattice Boltzmann Study

With increasing water scarcity, membrane distillation technology has gained widespread attention as an innovative method for seawater desalination.However,existing studies often overlook the influence of membrane characteristics on mass transfer efficiency. This study, based on the lattice Boltzmann method,proposes a model for a novel Poly(tetraethynylpyrene) membrane material to reveal the influence of membrane characteristics on the performance of vacuum membrane distillation. The model considers the factors such as porosity, tortuosity, membrane thickness, pore size, membrane surface wettability and temperature difference on the permeate flux. The results show that the permeate flux increases linearly with the porosity and decreases exponentially with the tortuosity factor. There is an optimal membrane thickness range (2μm) beyond which the permeate flux decreases exponentially. In addition, the permeate flux increases exponentially with increasing temperature difference and pore size. Further analysis of the effect of membrane surface wettability shows that permeate flux increases with increasing hydrophobicity. Finally, the feed temperature and tortuosity factor have the largest effect on permeate flux,followed by membrane thickness and, subsequently, pore size. The model can be further extended to study other configurations of membrane distillation technologies.

physics.flu-dyn↗

arXiv2025

Quantitative thermodynamic analyses of nucleation, evolution and stabilization of surface nanobubbles

Surface nanobubbles are complex micro- and nanoscale fluid systems. While thermodynamics is believed to dominate nanobubble dynamics, the precise mechanism by which nanobubble evolution is driven by thermodynamics remains unclear. It is essential to understand how nanobubble nucleation and growth, nanoscale contact line movement, and gas diffusion across the liquid-bubble interface are simultaneously driven by the change in free energy, leading to the ultimate thermodynamic equilibrium of surface nanobubble systems. In this paper, we first propose a quantitative theoretical model to elucidate the thermodynamic dominance behind the dynamics and stability of the fluid system with surface nanobubbles. The present model demonstrates that thermodynamic non-equilibrium drives the gas diffusion and the contact line motion of surface nanobubbles. Overcoming the nucleation energy barrier is crucial for bubble nucleation and growth. Surface nanobubbles evolve towards the reduction of the system's free energy and stabilize at the state with minimum free energy. The thermodynamic equilibrium is accompanied by the mechanical equilibrium at the contact line and the gas diffusion equilibrium at the liquid-bubble interface, and the theoretical results are in excellent agreement with the nanobubble morphology observed in experiments. The study highlights the significant influence of gas properties and ambient conditions in promoting bubble nucleation and stability.

cond-mat.soft↗

arXiv2022

Spurious currents suppression by accurate difference schemes in multiphase lattice Boltzmann method

Spurious currents, which are often observed near a curved interface in the multiphase simulations by diffuse interface methods, are unphysical phenomena and usually damage the computational accuracy and stability. In this paper, the origination and suppression of spurious currents are investigated by using the multiphase lattice Boltzmann method driven by chemical potential. Both the difference error and insufficient isotropy of discrete gradient operator give rise to the directional deviations of nonideal force and then originate the spurious currents. Nevertheless, the high-order finite difference produces far more accurate results than the high-order isotropic difference. We compare several finite difference schemes which have different formal accuracy and resolution. When a large proportional coefficient is used, the transition region is narrow and steep, and the resolution of finite difference indicates the computational accuracy more exactly than the formal accuracy. On the contrary, for a small proportional coefficient, the transition region is wide and gentle, and the formal accuracy of finite difference indicates the computational accuracy better than the resolution. Furthermore, numerical simulations show that the spurious currents calculated in the 3D situation are highly consistent with those in 2D simulations; especially, the two-phase coexistence densities calculated by the high-order accuracy finite difference are in excellent agreement with the theoretical predictions of the Maxwell equal-area construction till the reduced temperature 0.2.

physics.comp-ph↗

arXiv2022

Multiphase curved boundary condition in lattice Boltzmann method

The boundary treatment is fundamental for modeling fluid flows; especially, in the lattice Boltzmann method, the curved boundary conditions effectively improve the accuracy of single-phase simulations with complex-geometry boundaries. However, the conventional curved boundary conditions usually cause dramatic mass leakage or increase when they are directly used for multiphase flow simulations. We find that the principal reason is the absence of nonideal effect in the curved boundary conditions, followed by the calculation error. In this paper, incorporating the nonideal effect into the linear interpolation scheme and compensating for the interpolating error, we propose a multiphase curved boundary condition to treat the wetting boundaries with complex geometries. A series of static and dynamic multiphase simulations with large density ratio verify that the present scheme is accurate and ensures mass conservation.

physics.flu-dyn↗

arXiv2022

Active control of particle position by boundary slip in inertial microfluidics

Inertial microfluidic is able to focus and separate particles in microchannels based on the characteristic geometry and intrinsic hydrodynamic effect. Yet, the vertical position of suspended particles in the microchannel cannot be manipulated in real time. In this study, we utilize the boundary slip effect to regulate the parabolic velocity distribution of fluid in the microchannel and present a scheme to active control the vertical position of particles in inertial microfluidics. The flow field of a microchannel with a unilateral slip boundary is equivalent to that of the microchannel widened by the relevant slip length, and the particle equilibrium positions in the two microchannels are consistent consequently. Then, we simulate the lateral migrations of three kinds of typical particles, namely circle, ellipse, and rectangle in the microchannel. Unlike the smooth trajectories of circular particles, the motions of the elliptical and rectangular particles are accompanied by regular fluctuations and non-uniform rotations due to their non-circular geometries. The results demonstrate that the unilateral slip boundary can effectively control the vertical equilibrium position of particles. Thus, the present scheme enables to active manipulate the particles positions in vertical direction and can promote more accurate focusing, separating, and transport in inertial microfluidics.

physics.flu-dyn↗

arXiv2022

Contact angle measurement on curved wetting surface in multiphase lattice Boltzmann method

Contact angle is an essential physical quantity that characterizes the wettability of a substrate. Although it is widely used in the studies of surface wetting, capillary phenomena and moving contact lines, measuring contact angles in experiments and simulations is still complicated and time-consuming. In this paper, we present an efficient scheme for the real-time and on-the-spot measurement of contact angles on curved wetting surfaces in lattice Boltzmann simulations. The measuring results are in excellent agreement with the theoretical predictions by the spherical cap method without considering the gravity effect. A series of the simulations with various drop sizes and surface curvatures confirm that the present scheme is grid-independent. Then, it is verified in gravitational environments by simulating the deformations of sessile and pendent droplets on the curved wetting surface. The numerical results are highly consistent with experimental observations and support the theoretical analysis that the microscopic contact angle is independent of gravity. Furthermore, the scheme is applied to capture the dynamic contact angle hysteresis on homogeneous or chemically heterogeneous curved surfaces. Importantly, the accurate contact angle measurement enables the mechanical analysis at moving contact lines. The present measurement is simple and efficient, and can be extended to implement in various multiphase lattice Boltzmann models.

physics.flu-dyn↗

arXiv2021

A unified theory to describe the transition of stable nanobubbles to unstable microbubbles on homogeneous surface

Experiments have not only revealed the remarkably long lifetime of nanobubbles, but also demonstrated the diffusive instability of bubbles above micrometers, thus a full-scale physical understanding on the stability of bubbles is in urgent need. Herein, we develop a model that captures the state transition from the stable nanobubbles to the unstable microbubbles on homogeneous surfaces. The transition explains the typical long lifetime, limited height and small contact angle of surface nanobubbles observed in experiments. The consequent phase diagram shows that the bubble size and gas dissolving saturation determine the dynamic behaviors of surface bubbles, namely growth, stability, shrinkage or dissolution.

cond-mat.soft↗

arXiv2021

Numerical simulation of pulmonary airway reopening by the EOS-based multiphase lattice Boltzmann method

The aerosol formation is associated with the rupture of the liquid plug during the pulmonary airway reopening. The fluid dynamics of this process is difficult to predict because the rupture involved complex liquid-gas transition. Equation of state (EOS) plays a key role in the thermodynamic process of liquid-gas transition. Here, we propose an EOS-based multiphase lattice Boltzmann model, in which the nonideal force is directly evaluated by EOSs. This multiphase model is used to model the pulmonary airway reopening and study aerosol formation during exhalation. The numerical model is first validated with the simulations of Fujioka et al.(2008). and the result is in reasonable agreement with their study. Furthermore, two rupture cases with and without aerosol formation are contrasted and analyzed. It is found that the injury on the epithelium in the case with aerosol formation is essentially the same that of without aerosol formation even while the pressure drop in airway increases by about 67%. Then extensive simulations are performed to investigate the effects of pressure drop, thickness of liquid plug and film on aerosol size and the mechanical stresses. The results show that aerosol size and the mechanical stresses increase as the pressure drop enlarges and thickness of liquid plug become thicken, while aerosol size and the mechanical stresses decrease as thickness of liquid film is thicken. The present multiphase model can be extended to study the generation and transmission of bioaerosols which can carry the bioparticles of influenza or coronavirus.

physics.flu-dyn↗

arXiv2021

Aqueous humor dynamics in human eyes: a lattice Boltzmann study

This paper presents a lattice Boltzmann model to simulate the aqueous humor (AH) dynamics in the human eyes by involving incompressible Navier-Stokes flow, heat convection and diffusion, and Darcy seepage flow. Verifying simulations indicate that the model is stable, convergent and robust. Further investigations were carried out, including the effects of heat convection and buoyancy, AH production rate, permeability of trabecular meshwork, viscosity of AH and anterior chamber angle on intraocular pressure (IOP). The heat convection and diffusion can significantly affect the flow patterns in the healthy eye, and the IOP can be controlled by increasing the anterior chamber angle or decreasing the secretion rate, the drainage resistance and viscosity of AH. However, the IOP is insensitive to the viscosity of the AH, which may be one of the causes that the viscosity would not have been considered as a factor for controlling the IOP. It's interesting that all these factors have more significant influences on the IOP in pathologic eyes than healthy ones. The temperature difference and the eye-orientation have obvious influence on the cornea and iris wall shear stresses. The present model and simulation results are expected to provide an alternative tool and theoretical reference for the study of AH dynamics.

physics.flu-dyn↗

arXiv2021

Lateral Drop Rebound on Hydrophobic and Chemically Heterogeneous Surface

A drop rebounding from a hydrophobic and chemically heterogeneous surface is investigated using the multiphase lattice Boltzmann method. The behaviors of drop rebounding are dependent on the degrees of the hydrophobicity and heterogeneity of the surface. When the surface is homogeneous, the drop rebounds vertically and the height is getting higher and higher with increases of the surface hydrophobicity. When the surface consists of two different hydrophobic surfaces, the drop rebounds laterally towards the low hydrophobic side. The asymmetrical rebounding is because the unbalanced Young's force exerted on the contact line by the high hydrophobic side is greater than that by the low hydrophobic surface. A set of contours of momentum distribution illustrate the dynamic process of drop spreading, shrinking and rebounding. This work promotes the understanding of the rebound mechanism of a drop impacting the surface and also provides a guiding strategy for precisely controlling the lateral behavior of rebounding drops by hydrophobic degrees and heterogeneous surfaces.

physics.comp-ph↗

arXiv2020

Chemical-potential Multiphase Lattice Boltzmann Method with Superlarge Density Ratios

The liquid-gas density ratio is a key property of multiphase flow methods to model real fluid systems. Here, a chemical-potential multiphase lattice Boltzmann method is constructed to realize extremely large density ratios. The simulations show that the method reaches very low temperatures, at which the liquid-gas density ratio is more than 10^14, while the thermodynamic consistency is still preserved. Decoupling the mesh space from the momentum space through a proportional coefficient, a smaller mesh step provides denser lattice nodes to exactly describe the transition region and the resulting dimensional transformation has no loss of accuracy. A compact finite-difference method is applied to calculate the discrete derivatives in the mesh space with high-order accuracy. These enhance the computational accuracy of the nonideal force and suppress the spurious currents to a very low level, even if the density ratio is up to tens of thousands. The simulation of drop splashing verifies that the present model is Galilean invariant for dynamic flow field. An upper limit of the chemical potential is used to reduce the influence of nonphysical factors and improve the stability.

physics.comp-ph↗

arXiv2018

Contact Angle Measurement in Lattice Boltzmann Method

Contact angle is an essential characteristic in wetting, capillarity and moving contact line; however, although contact angle phenomena are effectively simulated, an accurate and real-time measurement for contact angle has not been well studied in computational fluid dynamics, especially in dynamic environments. Here, we design a geometry-based mesoscopic scheme to onthesport measure the contact angle in the lattice Boltzmann method. The computational results without gravity effect are in excellent agreement with the benchmarks from the spherical cap method. The qualities of the scheme are further verified in gravitational environments by simulating sessile and pendent droplets on smooth solid surfaces and contact angle hysteresis on chemically heterogeneous surfaces. This scheme is simple, efficient and requires only the local data. It is independent of multiphase models and can be easily extended to 3D dynamic contact angle measurements.

physics.comp-ph↗

arXiv2017

A pseudopotential multiphase lattice Boltzmann model based on high-order difference

The hyperbolic tangent function is usually used as a reliable approximation of the equilibrium density distributions of a system with phase transitions. However, analyzing the accuracies of the numerical derivatives, we find that its numerical derivatives computed by central difference method (CDM) may deviate significantly from its analytical solutions, while those computed by high-order difference method (HDM) can agree very well. Therefore, we introduce HDM to evaluate the interparticle interactions instead of popular CDM, and propose a pseudopotential multiphase lattice Boltzmann model based on high-order difference method. The present model not only retains the advantages of the pseudopotential model, such as easy implementation, high efficiency, full parallelism and so on, but also achieves higher accuracies. To verify the performances of this model, several multiphase flow simulations are conducted, including both stationary and dynamic situations. Firstly, full thermodynamic consistencies for the popular equations of state have been achieved in large temperature range and at large density ratio, without any combining interaction and any additional adjustable parameter of interaction. Secondly, with high-order difference, either the interparticle interaction proposed by Shan-Chen or by Zhang-Chen can equally depict the phase transitions of the fluids with all selected equations of the state. These numerical agreements based on HDM are consistent to the theoretical analysis that the two models are mathematically identical. Thirdly, the present model is stable and accurate at a wider temperature range. Lastly, our newly proposed model can be easily and reliably applied to various practical simulations and expected to obtain some more interesting results.

physics.comp-ph↗

arXiv2017

The roles of wettability and surface tension in droplet formation during inkjet printing

This paper describes a lattice Boltzmann-based binary fluid model for inkjet printing. In this model, a time-dependent driving force is applied to actuate the droplet ejection. As a result, the actuation can be accurately controlled by adjusting the intensity and duration of the positive and negative forces, as well as the idle time. The present model was verified by reproducing the actual single droplet ejection process captured by fast imaging. This model was subsequently used to investigate droplet formation in piezoelectric inkjet printing. It was determined that wettability of the nozzle inner wall and the surface tension of the ink are vital factors controlling the print quality and speed. Increasing the contact angle of the nozzle inner delays the droplet breakup time and reduces the droplet velocity. In contrast, higher surface tension values promote earlier droplet breakup and faster drop velocity. These results indicate that the hydrophilic modification of the nozzle inner wall and the choice of inks with high surface tensions will improve printing quality.

physics.comp-ph↗

arXiv2017

Chemical-potential-based Lattice Boltzmann Method for Nonideal Fluids

Chemical potential is an effective way to drive phase transition or express wettability. In this letter, we present a chemical-potential-based lattice Boltzmann model to simulate multiphase flows. The nonideal force is directly evaluated by a chemical potential. The model theoretically satisfies thermodynamics and Galilean invariance. The computational efficiency is improved owing to avoiding the calculation of pressure tensor. We have derived several chemical potentials of the popular equations of state from the free-energy density function. An effective chemical-potential boundary condition is implemented to investigate the wettability of a solid surface. Remarkably, the numerical results show that the contact angle can be linearly tuned by the surface chemical potential.

physics.comp-ph↗

arXiv2013

Galilean Invariant Fluid-Solid Interfacial Dynamics in Lattice Boltzmann Simulations

Galilean invariance is a fundamental property; however, although the lattice Boltzmann equation itself is Galilean invariant, this property is usually not taken into account in the treatment of the fluid-solid interface. Here, we show that consideration of Galilean invariance in fluid-solid interfacial dynamics can greatly enhance the computational accuracy and robustness in a numerical simulation. Surprisingly, simulations are so vastly improved that the force fluctuation is very small and a time average becomes unnecessary.

physics.flu-dyn↗