纳米通道粗糙内壁对流体流动行为的影响

纳米流动系统具有高效、经济等优势,在众多领域具有广泛的应用前景.因该类系统具有极高的表面积体积比,致使界面滑移效应对流动具有显著影响.本文采用分子动力学方法以两无限大平行非对称壁面组成的Poiseuille流动为对象,分析了壁面粗糙度与润湿性变化对通道内流体流动的影响.对于不同结构类型的壁面,需要通过水动力位置来确定固液界面位置,准确计算固液界面位置有助于更好地分析界面滑移效应.研究结果表明,上下壁面不对称会引起通道内流场参数分布的不对称,壁面粗糙度及润湿性的变化会影响近壁面附近流体原子的流动特性,由于壁面凹槽的存在,粗糙壁面附近的数密度分布低于光滑壁面一侧.壁面粗糙度及润湿性的变化会影响固液界面位置,肋高变化及壁面润湿性对通道中速度分布影响较大,界面滑移速度及滑移长度随肋高和润湿性的增大而减小;肋间距变化对通道内流体流动影响较小,界面滑移速度和滑移长度基本保持恒定.     

Viscous Slip MHD Flow over a Moving Sheet with an Arbitrary Surface Velocity

The magnetohydrodynamic(MHD) flow induced by a stretching or shrinking sheet under slip conditions is studied.Analytical solutions based on the boundary layer assumption are obtained in a closed form and can be applied to a flow configuration with any arbitrary velocity distributions. Seven typical sheet velocity profiles are employed as illustrating examples. The solutions to the slip MHD flow are derived from the general solution and discussed in detail. Different from self-similar boundary layer flows, the flows studied in this work have solutions in explicit analytical forms. However, the current flows require special mass transfer at the wall, which is determined by the moving velocity of the sheet. The effects of the slip parameter, the mass transfer at the wall, and the magnetic field on the flow are also demonstrated.     

Crossover from hydrodynamics to the kinetic regime in confined nanoflows

We present an experimental study of a confined nanoflow, which is generated by a sphere oscillating in the proximity of a flat solid wall in a simple fluid. Varying the oscillation frequency, the confining length scale, and the fluid mean free path over a broad range provides a detailed map of the flow. We use this experimental map to construct a scaling function, which describes the nanoflow in the entire parameter space, including both the hydrodynamic and the kinetic regimes. Our scaling function unifies previous theories based on the slip boundary condition and the effective viscosity.     

Drag reduction on a patterned superhydrophobic surface

We present an experimental study of a low-Reynolds number shear flow between two surfaces, one of which has a regular grooved texture augmented with a superhydrophobic coating. The combination reduces the effective fluid-surface contact area, thereby appreciably decreasing the drag on the surface and effectively changing the macroscopic boundary condition on the surface from no slip to limited slip. We measure the force on the surface and the velocity field in the immediate vicinity on the surface (and thus the wall shear) simultaneously. The latter facilitates a direct assessment of the effective slip length associated with the drag reduction.     

Advanced Study of Unsteady Heat and Chemical Reaction with Ramped Wall and Slip Effect on a Viscous Fluid

This paper investigate the effect of slip boundary condition, thermal radiation, heat source, Dufour number,chemical reaction and viscous dissipation on heat and mass transfer of unsteady free convective MHD flow of a viscous fluid past through a vertical plate embedded in a porous media. Numerical results are obtained for solving the nonlinear governing momentum, energy and concentration equations with slip boundary condition, ramped wall temperature and ramped wall concentration on the surface of the vertical plate. The influence of emerging parameters on velocity,temperature and concentration fields are shown graphically.     

Velocity profiles of water flowing past solid glass surfaces using fluorescent nanoparticles and molecules as velocity probes

Measurements of the velocity profile of water flowing on a glass surface using fluorescent nanoparticles and single fluorescent molecules as velocity probes show that the no slip boundary condition holds down to at least 10 nm from the surface. For water flowing on a hydrophobic solid surface, silanized glass, the no slip boundary condition fails, and a slip length of 45 nm is measured. These velocity measurements are complemented with atomic force microscopy measurements of dissipation on a small sphere oscillating near the surface with results in agreement with the velocity profiles.     

Immersed boundary method for the simulation of 2D viscous flow based on vorticity–velocity formulations

A new immersed boundary method based on vorticity–velocity formulations for the simulation of 2D incompressible viscous flow is proposed in present paper. The velocity and vorticity are respectively divided into two parts: one is the velocity and vorticity without the influence of the immersed boundary, and the other is the corrected velocity and the corrected vorticity derived from the influence of the immersed boundary. The corrected velocity is obtained from the multi-direct forcing to ensure the well satisfaction of the no-slip boundary condition at the immersed boundary. The corrected vorticity is derived from the vorticity transport equation. The third-order Runge–Kutta for time stepping, the fourth-order finite difference scheme for spatial derivatives and the fourth-order discretized Poisson for solving velocity are applied in present flow solver. Three cases including decaying vortices, flow past a stationary circular cylinder and an in-line oscillating cylinder in a fluid at rest are conducted to validate the method proposed in this paper. And the results of the simulations show good agreements with previous numerical and experimental results. This indicates the validity and the accuracy of present immersed boundary method based on vorticity–velocity formulations.     

Numerical simulation of cavitating flow around a slender body with slip boundary condition

In this paper, we perform a numerical simulation of the cavitating flow around an underwater hemispherical-head slender body running at a high speed. For the first time, the slip boundary condition is introduced into this problem, and we find that the slip boundary condition has a big influence on the cavitation in the flow-separation zone. By simulating the cavitating flow under different cavitation numbers, we demonstrate that the slip boundary condition can effectively reduce the intensity of cavitation, as represented by the length of cavitation bubbles. The present paper provides a new method for utilization of new surface materials to control the cavitation on the underwater moving objects.     

Three-dimensional lattice Boltzmann method for simulating blood flow in aortic arch

The three-dimensional （3D） lattice Boltzmann models, 3DQ15, 3DQ19 and 3DQ27, under different wall boundary conditions and lattice resolutions have been investigated by simulating Poiseuille flow in a circular cylinder for a wide range of Reynolds numbers. The 3DQ19 model with improved Fillippova and Hanel （FH） curved boundary condition represents a good compromise between computational efficiency and reliability. Blood flow in an aortic arch is then simulated as a typical haemodynamic application. Axial and secondary fluid velocity and effective wall shear stress profiles in a 180° bend are obtained, and the results also demonstrate that the lattice Boltzmann method is suitable for simulating the flow in 3D large-curved vessels.     

Tunable-slip boundaries for coarse-grained simulations of fluid flow

On the micro- and nanoscale, classical hydrodynamic boundary conditions such as the no-slip condition no longer apply. Instead, the flow profiles exhibit "slip" at the surface, which is characterized by a finite slip length (partial slip). We present a new, systematic way of implementing partial-slip boundary conditions with arbitrary slip length in coarse-grained computer simulations. The main idea is to represent the complex microscopic interface structure by a spatially varying effective viscous force. An analytical equation for the resulting slip length can be derived for planar and for curved surfaces. The comparison with computer simulations of a DPD (dissipative particle dynamics) fluid shows that this expression is valid from full slip to no slip.     

微尺度振荡Couette流的格子Boltzmann模拟

采用有效多松弛时间-格子Boltzmann方法(Effective MRT-LBM)数值模拟了微尺度条件下的振荡Couette和Poiseuille流动. 在微流动LBM中引入Knudsen边界层模型,对松弛时间进行修正. 模拟时平板或外力以正弦周期振动,Couette流中考虑了单平板振动、上下板同相振动这两类情况. 研究结果表明,修正后的MRT-LBM模型能有效用于这类非平衡的微尺度流动模拟;对于Couette流,随着Kn数的增大,壁面滑移效应变得越明显. St越大,板间速度剖面的非线性特性越剧烈;两板同相振荡时,若Kn,St均较小,板间流体受到平板拖动剪切的影响很小,板间速度几乎重叠在一起;在振荡Poiseuille流动中,St数增大到一定值时,相位滞后现象减弱;相对于Kn数,St数对振荡Couette 和Poiseuille流中不同位置处速度相位差的产生有较大影响. 关键词： 格子Boltzmann方法 有效MRT模型 Knudsen层 振荡流     

微通道疏水表面滑移的耗散粒子动力学研究

了解疏水表面的滑移规律对其在流动减阻方面的应用至关重要.利用耗散粒子动力学(dissipative particle dynamics, DPD)方法研究了微通道疏水表面的滑移现象.采用固定住的粒子并配合修正的向前反弹机制,构建了DPD固体壁面边界模型,利用该边界模型模拟了平板间的Couette流动.研究结果表明,通过调整壁面与流体间排斥作用强度,壁面能实现从无滑移到滑移的转变,壁面与流体间排斥作用越强,即疏水性越强,壁面滑移越明显,并且滑移长度与接触角之间存在近似的二次函数关系.无滑移时壁面附近密度分布均匀,有滑移时壁面附近存在低密度区域,低密度区域阻碍了动量传递,致使壁面产生滑移.     

表征体元尺度渗流的离散统一动理学格式

结合表征体元尺度的通用渗流模型,提出离散统一动理学格式（DUGKS）渗流方法,分别用均匀网格和非均匀网格计算二维Poiseuille、Couette、方腔流等经典渗流问题,检验DUGKS渗流方法的有效性和非均匀网格应用的优势,将DUGKS渗流方法应用到裂缝系统中.     

General Solution for Unsteady Natural Convection Flow with Heat and Mass in the Presence of Wall Slip and Ramped Wall Temperature

This work is focused on the effect of heat and mass transfer with unsteady natural convection flow of viscous fluid along with ramped wall temperature under the assumption of the slip wall condition at the boundary. Analytical solutions are obtained by using Laplace transformation to the non-dimensional set of governing equations containing velocity, temperature and concentration. Moreover, the expression for skin-friction is derived by differentiating the analytical solutions of fluid velocity. Numerical tables for Skin-friction, Sherwood number and Nusselt-number are examined. For the physical aspects of the flow, we use various values of involved physical parameters such as Prandtl number (Pr), slip parameter ($\eta$), Schmidt number (Sc), buoyancy ratio parameter ($N$), Sherwood number (Sh), and time $(t)$. Additionally, the general solutions are plotted graphically and a comprehensive theoretical section of numerical discussions is included.     

Direct experimental evidence of slip in hexadecane: solid interfaces

The boundary condition for the flow velocity of a Newtonian fluid near a solid wall has been probed experimentally with a novel setup using total internal reflection-fluorescence recovery after photobleaching leading to a resolution from the wall of the order of 80 nm. For hexadecane flowing on a hydrocarbon/lyophobic smooth surface, we give what we think to be the first direct experimental evidence of noticeable slip at the wall. We show that the surface roughness and the strength of the fluid-surface interactions both act on wall slip, in antagonist ways.     

双尺度疏油结构纳米通道滑移效应的研究

针对双尺度结构表面疏油特性的优异性,采用分子动力学的方法建立油液流体正十六烷烃分子模型,研究双尺度结构壁面润湿性影响下的纳米通道内流体的流动特性,通过对通道壁面亲疏油性下的双尺度结构的构建,与光滑壁面和单尺度壁面进行比较来探究双尺度纳米通道表面结构影响下油液流体在纳米通道内密度分布、速度分布、速度滑移和滑移长度的影响.模拟结果表明:对于亲油通道壁面,双尺度结构壁面亲油性明显加强,主流区域流体密度、流体速度和速度滑移都减小,甚至出现负滑移;而对于疏油通道壁面,双尺度分层结构能加强壁面的疏油性,通道内壁面形成稳定的气层使流体主流区域的密度增大,并且通道内流体的速度、速度滑移和滑移长度明显大于光滑和单尺度结构壁面.因此,纳米通道内双尺度结构能改变通道壁面的润湿性,并且能够加强流体在纳米疏油通道内的滑移减阻效应,为纳米通道内油液运输以及润滑薄膜减阻提供了设计基础.     

Surface roughness and hydrodynamic boundary slip of a newtonian fluid in a completely wetting system

The influence of surface roughness on the boundary condition for the flow of a Newtonian fluid near a hard wall has been investigated by measurement of the hydrodynamic drainage force. The degree of slip is found to increase with surface roughness. This leads to the conclusion that in most practical situations boundary slip takes place, leading to a reduction of the drainage force.     

The effects of porosity and permeability on fluid flow and heat transfer of multi walled carbon nano-tubes suspended in oil (MWCNT/Oil nano-fluid) in a microchannel filled with a porous medium

The forced convection heat transfer and laminar flow in a two-dimensional microchannel filled with a porous medium is numerically investigated. The nano-particles which have been used are multi walled carbon nano-tubes (MWCNT) suspended in oil as the based fluid. The assumption of no-slip condition between the base fluid and nano-particles as well as the thermal equilibrium between them allows us to study the nanofluid in a single phase. The nanofluid flow through the microchannel has been modeled using the Darcy–Forchheimer equation. It is also assumed that there is a thermal equilibrium between the solid phase and the nanofluid for energy transfer. The walls of the microchannel are under the influence of a fluctuating heat flux. Also, the slip velocity boundary condition has been assumed along the walls. The effects of Darcy number, porosity and slip coefficients and Reynolds number on the velocity and temperature profiles and Nusselt number will be studied in this research.     

Effects of wettability and interfacial nanobubbles on flow through structured nanochannels: an investigation of molecular dynamics

Solid–fluid boundary conditions are strongly influenced by a number of factors, including the intrinsic properties of the solid/fluid materials, surface roughness, wettability, and the presence of interfacial nanobubbles (INBs). The interconnected nature of these factors means that they should be considered jointly. This paper employs molecular dynamics (MD) simulation in a series of studies aimed at elucidating the influence of wettability in boundary behaviour and the accumulation of interfacial gas. Specifically, we examined the relationship between effective slip length, the morphology of nanobubbles, and wettability. Two methods were employed for the promotion of hydrophobicity between two structured substrates with similar intrinsic contact angles. We also compared anisotropic and isotropic atomic arrangements in the form of graphite and Si(100), respectively. A physical method was employed to deal with variations in surface roughness, whereas a chemical method was used to adjust the wall–fluid interaction energy (?_wf). We first compared the characteristic properties of wettability, including contact angle and fluid density within the cavity. We then investigated the means by which variations in solid–fluid interfacial wettability affect interfacial gas molecules. Our results reveal that the morphology of INB on a patterned substrate is determined by wettability as well as the methods employed for the promotion of hydrophobicity. The present study also illustrates the means by which the multiple effects of the atomic arrangement of solids, surface roughness, wettability and INB influence effective slip length.     

Three-Dimensional Hybrid Nanofluid Flow and Heat Transfer past a Permeable Stretching/Shrinking Sheet with Velocity Slip and Convective Condition

The present work deals with the three-dimensional hybrid Cu-Al₂O₃/water nanofluid flow towards a stretching/shrinking sheet with the presence of velocity slip and convective conditions. A permeable sheet is considered to maintain the shrinking flow through an adequate wall mass suction. The nonlinear governing boundary layer coupled with energy equations are transformed into the ordinary differential equations using similarity transformation. Numerical computations are performed with the aid of boundary value problem solver (bvp4c) in the Matlab software and the results are presented in the tables and graphs. The boundary layer separation occurs in the shrinking flow region. An upsurge of slip and copper nanoparticle volume fraction parameters can increase the range of first and second solutions whereas Biot parameter give zero impact on delaying the boundary layer separation. However, an increase of Biot and slip parameters can boost the heat transfer rate while opposite result is obtained with the augmentation of the copper solid volume fraction. The stability of both solutions are examined, and it is validated that the first (upper branch) solution is more stable than second solution.