A improved incompressible lattice Boltzmann model for time-independent flows

It is well known that the lattice Boltzmann equation method (LBE) can model the incompressible Navier-Stokes (NS) equations in the limit where density goes to a constant. In a LBE simulation, however, the density cannot be constant because pressure is equal to density times the square of sound speed, hence a compressibility error seems inevitable for the LBE to model incompressible flows. This work uses a modified equilibrium distribution and a modified velocity to construct an LBE which models time-independent (steady) incompressible flows with significantly reduced compressibility error. Computational results in 2D cavity flow and in a 2D flow with an exact solution are reported.     

Artificial compressibility, characteristics-based schemes for variable density, incompressible, multi-species flows. Part I. Derivation of different formulations and constant density limit

The paper presents various formulations of characteristics-based schemes in the framework of the artificial-compressibility method for variable-density incompressible flows. In contrast to constant-density incompressible flows, where the characteristics-based variables reconstruction leads to a single formulation, in the case of variable density flows three different schemes can be obtained henceforth labeled as: transport, conservative and hybrid schemes. The conservative scheme results in pseudo-compressibility terms in the (multi-species) density reconstruction. It is shown that in the limit of constant density, the transport scheme becomes the (original) characteristics-based scheme for incompressible flows, but the conservative and hybrid schemes lead to a new characteristics-based variant for constant density flows. The characteristics-based schemes are combined with second and third-order interpolation for increasing the computational accuracy locally at the cell faces of the control volume. Numerical experiments for constant density flows reveal that all the characteristics-based schemes result in the same flow solution, but they exhibit different convergence behavior. The multigrid implementation and numerical studies for variable density flows are presented in Part II of this study.     

Synthesis and direct interactions of silver colloidal nanoparticles with pollutant gases

Silver nanoparticles (NPs) were synthesized in organic solvents. Spontaneous reduction of silver salts takes place in N,N′-dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) at room temperature. The formed colloids are not stable without a stabilizing agent, hence rarely used, and inexpensive organic molecules (β-cyclodextrin and cholic acid) were used as surface modifiers in DMF. The stabilization was successful; the Ag NPs remained stable for more than 3 months. Additionally, Ag NPs were prepared using Ag-2-ethylhexanoate and Na-citrate as capping agent in DMSO. The resulting NPs are stable, of 4.4 nm average size, and at the same time reactive for catalytic purposes. The interaction of Ag NPs with pollutant atmospheric gases (NO and SO₂) was studied. UV–visible spectra show the oxidation of silver and the very efficient reduction of NO at room temperature. SO₂ molecules are adsorbed on the NPs surface, causing their aggregation and precipitation.     

Coupling of discrete-element method and smoothed particle hydrodynamics for liquid-solid flows

Particle based methods can be used for both the simulations of solid and fluid phases in multiphase medium, such as the discrete-element method for solid phase and the smoothed particle hydrodynamics for fluid phase. This paper presents a computational method combining these two methods for solid-liquid medium. The two phases are coupled by using an improved model from a reported Lagrangian-Eulerian method. The technique is verified by simulating liquid-solid flows in a two-dimensional lid-driven cavity.     

Fluid mechanics of turbomachines: a review

Future development of more efficient turbomachinery depends on improving understanding of the fluid mechanics. This paper reviews advances made in the last five years in calculation methods for steady flow, unsteady flows and stability, and instrumentation and its applications. Fundamental problems which need further study are also indicated     

高黏性流动有限元模拟的迭代稳定分步算法

分步算法已被广泛应用于数值求解不可压缩N-S方程. Guermond等认为时间步长必须大于 某个临界值方能使算法稳定. 然而在高黏性流动模拟中,已有的显式和半隐式分步算法由于 其显式本质,必须采用小时间步长计算,不但降低了计算效率,同时也常与为使分步算法稳 分步算法已被广泛应用于数值求解不可压缩N-S方程. Guermond等认为时间步长必须大于 某个临界值方能使算法稳定. 然而在高黏性流动模拟中,已有的显式和半隐式分步算法由于 其显式本质,必须采用小时间步长计算,不但降低了计算效率,同时也常与为使分步算法稳 定必须满足的最小时间步长要求冲突. 本文目的是构造一种含迭代格式的分步算法,它能在 保证精度的前提下大幅度地增大时间步长. 方腔流和平面Poisseuille流数值计算结果证实 了此特点,该方法被有效应用于充填流动过程的数值模拟.     

平板混合层流动中两相脉动关联矩的大涡模拟

在气粒两相平板混合层流动中,对气相流动采用大涡模拟,对颗粒相流动采用轨道模拟,研究了两相脉动关联矩。由两相各自瞬时速度出发可以直接获得两相脉动关联矩的统计结果。模拟获得的颗粒相一阶矩、二阶矩以及两相脉动关联矩与实验结果定量符合,表明基于细观模拟是不通过模型获得和研究两相脉动关联矩的可行途径。     

Transient flows of a second grade fluid

Exact analytical solutions for a class of unsteady unidirectional flows of an incompressible second-order fluid are constructed. The flows are generated impulsively from rest by motion of a plate or two plates or by sudden application of a pressure gradient. Expressions for velocity, flux and skin friction are obtained for both large and small times. It is found that large and small times solutions are dependent on the coefficient of viscoelasticity. The solutions corresponding to Newtonian fluids can be easily obtained from those for fluids of second order by letting the viscoelastic parameter to be zero.     

Analysis and Modeling of the Turbulent Diffusion of Turbulent Kinetic Energy in Natural Convection

Buoyant flows often contain regions with unstable and stable thermal stratification from which counter gradient turbulent fluxes are resulting, e.g. fluxes of heat or of any turbulence quantity. Basing on investigations in meteorology an improvement in the standard gradient-diffusion model for turbulent diffusion of turbulent kinetic energy is discussed. The two closure terms of the turbulent diffusion, the velocity-fluctuation triple correlation and the velocity-pressure fluctuation correlation, are investigated based on Direct Numerical Simulation (DNS) data for an internally heated fluid layer and for Rayleigh–Bénard convection. As a result it is decided to extend the standard gradient-diffusion model for the turbulent energy diffusion by modeling its closure terms separately. Coupling of two models leads to an extended RANS model for the turbulent energy diffusion. The involved closure term, the turbulent diffusion of heat flux, is studied based on its transport equation. This results in a buoyancy-extended version of the Daly and Harlow model. The models for all closure terms and for the turbulent energy diffusion are validated with the help of DNS data for internally heated fluid layers with Prandtl number Pr = 7 and for Rayleigh–Bénard convection with Pr = 0.71. It is found that the buoyancy-extended diffusion model which involves also a transport equation for the variance of the vertical velocity fluctuation gives improved turbulent energy diffusion data for the combined case with local stable and unstable stratification and that it allows for the required counter gradient energy flux.     

固相微萃取-高效液相色谱联用技术的进展

固相微萃取（SPME）技术是近年来发展起来的一种无溶剂,集采样、萃取、浓缩、进样于一体的样品预处理新技术,它与同效液相色谱技术的联用已受到人们的瞩目;该文评述了SPME-HPLC联用技术的进展,并展望这一技术的应用前景。