Linlin Fei

Last Name

Fei

First Name

Linlin

ORCID

0000-0002-4722-5093

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Publications1 - 10 of 21

Pore-scale study on shear rheology of wet granular materials
Fei, Linlin; He, Ya-Ling; Derome, Dominique; et al. (2024)
Physics of Fluids
We study pore-scale rheological phenomena in two-dimensional sheared wet granular materials. Simulations use a coupled cascaded lattice Boltzmann and discrete element method, to model the liquid-gas multiphase flows and multiple-solid-particle dynamics, respectively. The wet granular material is prepared by first filling a rectangular domain with solid particles and then partially filling the pores between the particles with the liquid phase. The material is then sheared based on standard Couette flow configuration, i.e., with lid-driven velocities U and -U on the top and bottom walls, respectively. The simulations show that the apparent viscosity of the system attains a minimum when the material is wet but not fully saturated, i.e., at a saturation of ∼0.10. Such an observation is coherent both for materials composed of monodisperse and polydisperse particles. Interestingly, this observation coincides with the experimental finding of the decrease in sliding friction on sand by adding a small amount of water. The underlying mechanism is elucidated based on the pore-scale study of liquid patch dynamics. It is shown that, with increasing liquid saturation, the rheology of the wet granular materials is affected by two competing effects: (i) a larger number of liquid patches appear leading to fluidization of the system and (ii) larger patches are formed, clogging the flow. The minimum apparent viscosity saturation of ∼0.10 coincides with the maximum of the product of the two factors: the number of liquid patches and ratio between the system height and largest patch height.
Pore-scale study on the effect of heterogeneity on evaporation in porous media
Fei, Linlin; Derome, Dominique; Carmeliet, Jan (2024)
Journal of Fluid Mechanics
The evaporation process in porous media typically experiences three main periods, among which the first period, named the constant rate period (CRP), performs most efficiently in removing liquid. We aim to prolong the CRP to very low degrees of saturation (S) and increase its evaporation rate by playing with heterogeneity in wettability and pore size. First, we show that a porous medium with a smaller contact angle at the surface and increasing contact angle towards the inside generally dries out faster compared with that with uniform contact angle. Second, a constant contact angle porous medium with smaller/larger pores in the surface/inside part dries out faster than a medium with uniform pore size. The underlying mechanism is the occurrence of a capillary pressure jump at the border between the two layers accompanied by enhanced capillary pumping, increasing/maintaining the interfacial area in the surface pores. Harnessing the potential of this mechanism, we propose an optimized strategy by combining two heterogeneity effects: increasing contact angle and pore size towards the inside. This strategy is found to be robust both for multilayer and larger systems. In this case, a small drying front first penetrates fast towards the inside and then expands, followed by a horizontal drying front moving back layer by layer to the surface. Quantitatively, compared with evaporation from a homogeneously porous medium with uniform contact angle where CRP stops at S = 0.64, our optimized design can extend the CRP down to S = 0.12, and decrease five-fold the drying time needed to reach S = 0.05.
Droplet impact on a heated porous plate above the Leidenfrost temperature: A lattice Boltzmann study
Wang, Geng; Fei, Linlin; Lei, Timan; et al. (2022)
Physics of Fluids
In the past few decades, the droplet impact on a heated plate above the Leidenfrost temperature has attracted immense research interest. The strong hydrophobicity caused by the Leidenfrost effect leads to the droplet bouncing from a flat plate at a given contact time predicted by the classical Rayleigh theory. Numerous investigations were conducted to break the theoretical Rayleigh's limit to reduce the interfacial contact time. Recently, a droplet was observed to form a pancake shape and bounce as it impacted nanotube or micropost surfaces above the Leidenfrost temperature. This led to a significant reduction in droplet contact time. However, this unique bouncing phenomenon is still not fully understood, such as the influence of the plate configuration and the relationship between the droplet rebound time and evaporation mass loss. In this study, we carry out a numerical study of the droplet impact dynamics on a heated porous plate above the Leidenfrost temperature, using a multiphase thermal lattice Boltzmann model. Our model is constructed within the unified lattice Boltzmann method framework and is first validated based on theoretical and experimental results. Then, a comprehensive parametric study is performed to investigate the effects of the impact Weber number, the plate temperature, and the plate configurations on the droplet bouncing dynamics. Results show that higher plate temperature, larger Weber number, and smaller pore intervals can accelerate the droplet rebound and promote the droplet pancake bouncing. We demonstrate that the occurrence of the pancake bouncing is attributed to the additional lift force provided by the vapor pressure due to the evaporation of liquid inside the pores. Moreover, the droplet maximum spreading time and maximum spreading factor can be described by a power law function of the impact Weber number. The droplet evaporation mass loss increases linearly with the impingement Weber number and the plate opening fractions. This study provides new insights into the Leidenfrost droplet impingement on porous plates, which may potentially facilitate the design of novel engineering surfaces and devices.
Coupled phase-field lattice Boltzmann method and discrete element method for gas-liquid-solid multiphase flows
Li, Ruixin; Fei, Linlin; Luo, Kai H.; et al. (2025)
Physics of Fluids
A methodology combining the lattice Boltzmann method (LBM) and discrete element method (DEM) is proposed to simulate gas-liquid-solid multiphase flows and interphase interactions. Specifically, the phase-field LBM is employed to simulate the fluid flow, incorporating the virtual density boundary method. This method effectively enables the realization of wetting phenomena with relatively small spurious velocities, in comparison with the pseudopotential LBM. The DEM is utilized to simulate the motion of multiple solid particles. As for the interaction between fluid and solid, leveraging the features of LBM, the calculation of fluid forces acting on solids can be achieved by going through all fluid nodes surrounding the solid walls, enabling a straightforward and efficient calculation process. The numerical stability and accuracy of the hybrid LBM-DEM are demonstrated via benchmark cases. It is then applied successfully to simulate the upward migration of leaked gas bubbles through a deformable porous medium composed of solid particles.
Competition between main meniscus and corner film flow during imbibition in a strongly wetting square tube
Zhao, Jianlin; Qin, Feifei; Fei, Linlin; et al. (2022)
Journal of Hydrology
Imbibition in a strongly wetting square tube with corner flow can be described by an interacting capillary bundle model, where the first sub-capillary describes the main meniscus flow while the others describe corner film flow. In this work, an advanced modified interacting capillary bundle model (MICBM) is developed to simulate imbibition dynamics in a strongly wetting square tube incorporating the viscous coupling effect. The flow conductances of each sub-capillary are obtained from two-phase lattice Boltzmann simulations with different viscosity ratios considering the viscous coupling effect. After verifying its accuracy, MICBM is used to analyze the imbibition dynamics of main meniscus and corner film flows under different conditions. The results show that, with increasing viscosity ratio between wetting and non-wetting fluids and increasing driving force, the wetting corner film development tends to be less significant compared with main meniscus flow. Interestingly, the corner film length first increases then decreases when the wetting fluid is less viscous than the non-wetting fluid in a long square tube. A phase diagram of dimensionless corner film length versus driving force and viscosity ratio is proposed to characterize the competition between main meniscus and corner film flow. Gravity and smaller contact angle make the corner film development more obvious. In addition, the tube length and width are shown to influence the interaction between main meniscus and corner film flow for a low viscosity ratio. The underlying physics of the above phenomena are explained in detail.
On the flow of soft suspensions through orifices
Fei, Linlin; Puglisi, Andrea; Succi, Sauro; et al. (2023)
Computers & Fluids
The behavior of confined suspensions of soft droplets under pressure-driven flow, passing an obstacle within a planar channel, is investigated by means of a mesoscopic lattice Boltzmann model capable of simulating soft non-coalescing droplets. The simulations reveal that the threshold of the pore size, below which the flux vanishes, is between 1 and 2 droplet diameters, and increases with the packing fraction. Moreover, we show that the classical Beverloo relation between the total flux and the pore size is not suitable for the soft suspensions considered here.
Lattice boltzmann investigation of droplet interactions with non-uniform chemically patterned surfaces
Song, Xiang; Fei, Linlin; Peng, Haonan; et al. (2024)
Computers & Fluids
In this study, we employed a non-orthogonal, three-dimensional, multi-relaxation-time pseudo-potential lattice Boltzmann method, and investigate the behaviors of droplets impacting chemically patterned surfaces. We considered two interfaces: a hydrophobic/neutral strip on a hydrophilic wall (surface A), and a hydrophilic strip on a hydrophobic wall (surface B). The dynamics of impact, including the evolution of the morphology and the droplet spreading factor, were investigated under the influence of the difference in wettability, Weber number (We), and strip width. An increase in the wettability difference of surface A delayed droplet detachment and reduced the amplitude of oscillations, where this can be attributed to the surface tension and viscous dissipation, which had more time to weaken the strength of the jet. The magnitude of droplet detachment time initially increased with We but eventually decreased. As We is further increased, the ratio of viscous loss to the initial kinetic energy of the droplet is decreased and resulted in a shorter detachment time. The unbalanced Young's force significantly affected the evolution of the droplet on surface B. The mass of the droplet accumulated near the borderline of the strip and expanded along the y-axis under the influence of the inertial force, where this led to a larger spreading factor along the y-axis. In addition, the mass of the droplet on the hydrophobic wall affected the strength of the unbalanced Young's force. As the strip width increased, the spreading factor initially increased but then decreased along the y-axis owing to the combined action of inertial forces.
Unified lattice Boltzmann method with improved schemes for multiphase flow simulation: Application to droplet dynamics under realistic conditions
Wang, Geng; Fei, Linlin; Luo, Kai H. (2022)
Physical Review E
As a powerful mesoscale approach, the lattice Boltzmann method (LBM) has been widely used for the numerical study of complex multiphase flows. Recently, Luo et al. [Philos. Trans. R. Soc. A: Math. Phys. Eng. Sci. 379, 20200397 (2021)10.1098/rsta.2020.0397] proposed a unified lattice Boltzmann method (ULBM) to integrate the widely used lattice Boltzmann collision operators into a unified framework. In this study, we incorporate additional features into this ULBM in order to simulate multiphase flow under realistic conditions. A nonorthogonal moment set [Fei et al., Phys. Rev. E 97, 053309 (2018)10.1103/PhysRevE.97.053309] and the entropic-multi-relaxation-time (KBC) lattice Boltzmann model are used to construct the collision operator. An extended combined pseudopotential model is proposed to realize multiphase flow simulation at high-density ratio with tunable surface tension over a wide range. The numerical results indicate that the improved ULBM can significantly decrease the spurious velocities and adjust the surface tension without appreciably changing the density ratio. The ULBM is validated through reproducing various droplet dynamics experiments, such as binary droplet collision and droplet impingement on superhydrophobic surfaces. Finally, the extended ULBM is applied to complex droplet dynamics, including droplet pancake bouncing and droplet splashing. The maximum Weber number and Reynolds number in the simulation reach 800 and 7200, respectively, at a density ratio of 1000. The study demonstrates the generality and versatility of ULBM for incorporating schemes to tackle challenging multiphase problems.
Three-dimensional modelling of cavitation bubble collapse using non-orthogonal multiple-relaxation-time lattice Boltzmann method
Peng, Haonan; Fei, Linlin; He, Xiaolong; et al. (2024)
Ocean Engineering
In this study, we employ a three-dimensional (3D) non-orthogonal multiple relaxation time (MRT) pseudo-potential lattice Boltzmann (LB) model to simulate the dynamics of cavitation bubble evolution. We benchmark the model against the Laplace law and the Rayleigh–Plesset (R–P) equation, confirming its efficacy in accurately capturing cavitation phenomena. We then apply the model to examine the collapse dynamics of a singular bubble located near a plane wall boundary and right-angled wall corner. Additionally, the dynamic interactions among five cross-shaped bubbles revealed the dimensionless jet volume Vj*, which is the ratio of the jet volume to the maximum bubble volume, exhibits a power relationship with the bubble distance δ. The simulation results demonstrate the accuracy of the model in discerning the effect of the wall boundary and the protective mechanisms inherent to multi-bubble interactions. These results further validate the aptness of the model for cavitation bubble dynamics simulations. Moreover, the tested case studies provide a foundational basis for future research into more complex cavitation behaviours. In summary, the developed 3D non-orthogonal MRT pseudo-potential LB model is capable of reproducing fluid flows, capturing pressure waves, and measuring wall pressures. Our work provides a deep insight into cavitation bubble dynamics and a solid basis for both applied and fundamental research.
Lattice Boltzmann modelling and study of droplet equatorial streaming in an electric field
Wang, Geng; Lei, Timan; Yang, Junyu; et al. (2024)
Journal of Fluid Mechanics
In 2017, Brosseau & Vlahovska (Phys. Rev. Lett, vol. 119, no. 3, 2017, p. 034501) found that, in a strong electric field, a weakly conductive, low-viscosity droplet immersed in a highly conductive, high-viscosity medium formed a lens shape, and liquid rings continuously detached from its equatorial plane and subsequently broke up into satellite droplets. This fascinating multiphase electrohydrodynamic (EHD) phenomenon is known as droplet equatorial streaming. In this paper, based on the unified lattice Boltzmann method framework proposed by Luo et al. (Phil. Trans. R. Soc. A Math. Phys. Engng Sci, vol. 379, no. 2208, 2021, p. 20200397), a novel lattice Boltzmann (LB) model is constructed for multiphase EHD by coupling the Allen-Cahn type of multiphase LB model and two new LB equations to solve the Poisson equation of the electric field and the conservation equation of the surface charge. Using the proposed LB model, we successfully reproduced, for the first time, the complete process of droplet equatorial streaming, including the continuous ejection and breakup of liquid rings on the equatorial plane. In addition, it is found that, under conditions of high electric field strength or significant electrical conductivity contrast, droplets exhibit fingering equatorial streaming that was unknown before. A power-law relationship is discovered for droplet total charge evolution and a theoretical model is then proposed to describe the droplet radius and height over time. The breakup of liquid rings is found to be dominated by capillary instability, while the breakup of liquid fingers is governed by the end-pinching mechanism. Finally, a phase diagram is constructed for fingering equatorial streaming and ring equatorial streaming, and a criterion equation is established for the phase boundary.

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