There are five most widely used contact angle schemes in the pseudopotential lattice Boltzmann(LB)model for simulating the wetting phenomenon:The pseudopotential-based scheme(PB scheme),the improved virtualdensity scheme(IVD scheme),the modified pseudopotential-based scheme with a ghost fluid layer constructed by using the fluid layer density above the wall(MPB-C scheme),the modified pseudopotential-based scheme with a ghost fluid layer constructed by using the weighted average density of surrounding fluid nodes(MPB-W scheme)and the geometric formulation scheme(GF scheme).But the numerical stability and accuracy of the schemes for wetting simulation remain unclear in the past.In this paper,the numerical stability and accuracy of these schemes are clarified for the first time,by applying the five widely used contact angle schemes to simulate a two-dimensional(2D)sessile droplet on wall and capillary imbibition in a 2D channel as the examples of static wetting and dynamic wetting simulations respectively.(i)It is shown that the simulated contact angles by the GF scheme are consistent at different density ratios for the same prescribed contact angle,but the simulated contact angles by the PB scheme,IVD scheme,MPB-C scheme and MPB-W scheme change with density ratios for the same fluid-solid interaction strength.The PB scheme is found to be the most unstable scheme for simulating static wetting at increased density ratios.(ii)Although the spurious velocity increases with the increased liquid/vapor density ratio for all the contact angle schemes,the magnitude of the spurious velocity in the PB scheme,IVD scheme and GF scheme are smaller than that in the MPB-C scheme and MPB-W scheme.(iii)The fluid density variation near the wall in the PB scheme is the most significant,and the variation can be diminished in the IVD scheme,MPB-C scheme andMPBWscheme.The variation totally disappeared in the GF scheme.(iv)For the simulation of capillary imbibition,the MPB-C scheme,MPB-Wscheme and GF scheme simulate the dynamics of the liq
To directly incorporate the intermolecular interaction effects into the discrete unified gas-kinetic scheme(DUGKS)for simulations of multiphase fluid flow,we developed a pseudopotential-based DUGKS by coupling the pseudopotential model that mimics the intermolecular interaction into DUGKS.Due to the flux reconstruction procedure,additional terms that break the isotropic requirements of the pseudopotential model will be introduced.To eliminate the influences of nonisotropic terms,the expression of equilibrium distribution functions is reformulated in a moment-based form.With the isotropy-preserving parameter appropriately tuned,the nonisotropic effects can be properly canceled out.The fundamental capabilities are validated by the flat interface test and the quiescent droplet test.It has been proved that the proposed pseudopotential-based DUGKS managed to produce and maintain isotropic interfaces.The isotropy-preserving property of pseudopotential-based DUGKS in transient conditions is further confirmed by the spinodal decomposition.Stability superiority of the pseudopotential-based DUGKS over the lattice Boltzmann method is also demonstrated by predicting the coexistence densities complying with the van der Waals equation of state.By directly incorporating the intermolecular interactions,the pseudopotential-based DUGKS offers a mesoscopic perspective of understanding multiphase behaviors,which could help gain fresh insights into multiphase fluid flow.
This paper presents a pseudopotential lattice Boltzmann analysis to show the deficiency of previous pseudopotential models,i.e.,inconsistency between equilibrium velocity and mixture velocity.To rectify this problem,there are two strategies:decoupling relaxation time and kinematic viscosity or introducing a system mixture relaxation time.Then,we constructed two modified models:a two-relaxationtime(TRT)scheme and a triple-relaxation-time(TriRT)scheme to decouple the relaxation time and kinematic viscosity.Meanwhile,inspired by the idea of a system mixture relaxation time,we developed three mixture models under different collision schemes,viz.mix-SRT,mix-TRT,and mix-TriRT models.Afterwards,we derived the advection-diffusion equation for the multicomponent system and derived the mutual diffusivity in a binarymixture.Finally,we conducted several numerical simulations to validate the analysis on these models.The numerical results show that these models can obtain smaller spurious currents than previous models and have a wider range for the accessible viscosity ratio with fourth-order isotropy.Compared to previous models,presentmodels avoid complex matrix operations and only fourth-order isotropy is required.The increased simplicity and higher computational efficiency of these models make them easy to apply to engineering and industrial applications.
Yong ZhaoGerald G.PereiraShibo KuangZhenhua ChaiBaochang Shi
The performances of the Color-Gradient(CG)and the Shan-Chen(SC)multicomponent Lattice Boltzmann models are quantitatively compared side-by-side on multiple physical flow problems where breakup,coalescence and contraction of fluid ligaments are important.The flow problems are relevant to microfluidic applications,jetting of microdroplets as seen in inkjet printing,as well as emulsion dynamics.A significantly wider range of parameters is shown to be accessible for CG in terms of density-ratio,viscosity-ratio and surface tension values.Numerical stability for a high density ratio O(1000)is required for simulating the drop formation process during inkjet printing which we show here to be achievable using the CG model but not using the SC model.Our results show that the CG model is a suitable choice for challenging simulations of droplet formation,due to a combination of both numerical stability and physical accuracy.We also present a novel approach to incorporate repulsion forces between interfaces for CG,with possible applications to the study of stabilized emulsions.Specifically,we show that the CG model can produce similar results to a known multirange potentials extension of the SC model for modelling a disjoining pressure,opening up its use for the study of dense stabilized emulsions.
Karun P.N.DatadienGianluca Di StasoHerman M.A.WijshoffFederico Toschi
The interaction between cavitation bubble and solid surface is a fundamental topic which is deeply concerned for the utilization or avoidance of cavitation effect.The complexity of this topic is that the cavitation bubble collapse includes many extreme physical phenomena and variability of different solid surface properties.In the present work,the cavitation bubble collapse in hydrophobic concave is studied using the pseudopotential multi-relaxation-time lattice Boltzmann model(MRT-LB).The model is modified by involving the piecewise linear equation of state and improved forcing scheme.The fluid-solid interaction in the model is employed to adjust the wettability of solid surface.Moreover,the validity of the model is verified by comparison with experimental results and grid-independence verification.Finally,the cavitation bubble collapse in a hydrophobic concave is studied by investigating density field,pressure field,collapse time,and jet velocity.The superimposed effect of the surface hydrophobicity and concave geometry is analyzed and explained in the framework of the pseudopotential LBM.The study shows that the hydrophobic concave can enhance cavitation effect by decreasing cavitation threshold,accelerating collapse and increasing jet velocity.
Minglei ShanYu YangXuemeng ZhaoQingbang HanCheng Yao
Understanding the quantum dynamics of spin defects and their coherence properties requires an accurate modeling of spinspin interaction in solids and molecules,for example by using spin Hamiltonians with parameters obtained from first principles calculations.We present a real-space approach based on density functional theory for the calculation of spin-Hamiltonian parameters,where only selected atoms are treated at the all-electron level,while the rest of the system is described with the pseudopotential approximation.Our approach permits calculations for systems containing more than 1000 atoms,as demonstrated for defects in diamond and silicon carbide.We show that only a small number of atoms surrounding the defect needs to be treated at the all-electron level,in order to obtain an overall all-electron accuracy for hyperfine and zero-field splitting tensors.We also present results for coherence times,computed with the cluster correlation expansion method,highlighting the importance of accurate spin-Hamiltonian parameters for quantitative predictions of spin dynamics.