致密油藏微纳米喉道中的边界层特征

通过引入吸引力修正耗散粒子动力学（DPD）方法,实现流体和固体的相互吸引作用,模拟纳米喉道中的微尺度流动,探讨边界层的产生机理,结合微圆管实验,定量表征微纳米喉道中边界层的特征,明确微纳米喉道中边界层的影响因素.研究发现：分子尺度,热运动对速度影响很大;超过分子尺度,压差占主导作用.热运动使粒子在原位置振动,不改变粒子的整体移动方向.随着喉道半径的增大,泊肃叶流动的抛物线特征越来越明显.边界层厚度受压力梯度、喉道半径和流体粘度的影响.当压力梯度增大或流体粘度减小时,边界层厚度增大;当喉道半径减小时,边界层厚度先增大后减小.边界层厚度是导致非线性渗流特征的根本原因.随着边界层厚度增大,非线性渗流特征越来越明显.     

Thermal boundary layer for non-Newtonian fluid

Summary Analytical and numerical solutions for the momentum and thermal boundary layer equations of a non-Newtonian power law fluid are presented. The flow is assumed to be under the influence of an external magnetic fieldB (x) applied perpendicular to the surface and an electric fieldE(x) perpendicular toB(x) and the direction of the longitudinal velocity in the boundary layer. For the power law fluid it is assumed that the shear stress is proportional to then-th power of the velocity gradient andn is called the flow index. The variations of the velocity fieldf′, the temperature field θ, the shear stress on the surfaceτ _W , the displacement thicknessδ ₁ and the momentum thicknessδ ₂ with the magnetic-field parameter γ, the flow indexn, the heat transfer indexS and the Prandtl number Pr are studied. It is found that, if the outer flow velocityU(x) (potential flow) is proportional to the arc lengthx raised to a powerm, then the similarity solution for the thermal boundary layer equation is possible only whenm=1/3, which represents flow past a wedge of included angle π/2. It is established that the temperature of the wedge increases with the increase of γ, Pr,S and the decrease ofn. In general the magnetic field can be used as a heater for the surface of the wedge.     

Effect of periodic blowing/suction through sequentially located annular slots on the turbulent boundary layer on a body of revolution

The effect of local periodic forcing in the form of blowing/suction through sequentially located annular slots on the features of the turbulent boundary layer formed on an axisymmetric body of revolution in an incompressible flow is studied experimentally. The Reynolds number based on the momentum thickness of the boundary layer ahead of the annular slot is 1362. The dimensionless amplitude of the forcing signal A ₀ is set to be 0.4. The frequency of the forcing signal in the law-of-the-wall units is f ⁺ = 0.0048. Beginning from the distance upstream from the slot equal approximately to half-thickness of the displacement thickness of the boundary layer δ* and further downstream to 18 δ*, a stable decrease in local friction is observed; the maximum value of this decrease reaches 50 %. Each next slot favors reduction of friction, though the effectiveness of blowing/suction becomes noticeably lower in the downstream direction.     

Heat transfer and diffusion-controlled kinetics of liquid–solid phase in titanium matrix composite during selective laser melting

This study describes a self-consistent theoretical model of simulating diffusion-controlled kinetics on the liquid–solid phase boundary during high-speed solidification in the melt pool after the selective laser melting (SLM) process for titanium matrix composite based on Ti–TiC system. The model includes the heat transfer equation to estimate the temperature distribution in the melt pool and during crystallization process for some deposited layers. The temperature field is used in a micro region next to solid–liquid boundary, where solute micro segregation and dendrite growth are calculated by special approach based on transient liquid phase bonding. The effect of the SLM process parameters (laser power, scanning velocity, layer thickness and substrate size) on the microstructure solidification is being discussed.     

Effect of the electron beam and ion flux parameters on the rate of plasma nitriding of an austenitic stainless steel

The method of nitriding of metals in an electron beam plasma is used to change the current density and energy of nitrogen ions by varying the electron beam parameters (5–20 A, 60–500 eV). An electron beam is generated by an electron source based on a self-heated hollow cathode discharge. Stainless steel 12Kh18N10T is saturated by nitrogen at 500°C for 1 h. The microhardness is measured on transverse polished sections to obtain the dependences of the nitrided layer thickness on the ion current density (1.6–6.2 mA/cm²), the ion energy (100–300 eV), and the nitrogen-argon mixture pressure (1–10 Pa). The layer thickness decreases by 4–5 μm when the ion energy increases by 100 V and increases from 19 to 33 μm when the ion current density increases. The pressure dependence of the layer thickness has a maximum. These results are in conflict with the conclusions of the theory of the limitation of the layer thickness by ion sputtering, and the effective diffusion coefficient significantly exceeds the well-known reported data.     

DNS study of swirling intensity effect on flow pattern of a circular jet

Abstract  

The figures show the 3D flow pattern of a circular jet with different swirling intensity. Reynolds number is approximately 4300 computed based on the nozzle diameter (d), jet velocity (U), and air fluid property at 1 atm and 300 K. The overall computational domain is set to be 4 × 4 × 12 d in spanwise, height, and streamwise direction. The governing equations are the fully compressible Navier–Stokes equations, firstly differenced by eighth-order explicit scheme and then advanced temporarily by using the fourth-order explicit Runge–Kutta method. 3D characteristics non-reflecting boundary condition including transverse source contribution is imposed on all other boundaries except the inflow boundary handled by assigning fixed profiles of temperature and velocity. To ensure the simulation resolution, here over 16 million grids are employed in sum, combined with a handful of grids located at buffer zones of outflow boundaries. To correctly represent the vortex in the flow field, velocity gradient tensor invariant Q is used here. And ψ refers to the swirling intensity defined as the ratio of tangential momentum to axis momentum. As shown in velocity profile, the flow pattern of the jet changes from a close mode to a totally open mode as ψ increases from 0.4 to 1.5. Accordingly, the recirculation zone gradually moves upstream and backflow velocity is enlarged as well. It is inteseting to found that the obvious drops of the momentums in two shown directions always occur at the same position downstream, no matter how large the ψ value is. Therefore, a momentum compensatory mechanism is expected to exist in the vortex-abundant zone. With the increase of ψ value, the increased strain rate in tangential direction can induce vortex more quickly, intensifying the entrainment and velocity-attenuation, which can be observed in Q value profile.     

金属熔体近壁面流动剪切模型及其对金属凝固影响的理论研究

本文建立了金属熔体近壁面流动剪切模型, 并分析了流动剪切对金属凝固的影响. 针对A356合金计算结果表明:层流流动的熔体内部剪应力随垂直斜板表面距离的增大而减小, 随着流动长度的增加先急剧下降之后趋于稳定; 紊流流动的熔体所受的剪应力随着垂直倾斜板表面距离的增大先急剧下降之后趋于稳定, 随着流动长度的增加而不断增大; 斜板倾角越大, 斜板上相同位置的熔体层受到的剪应力越大; 熔体垂直斜板表面距离越小, 柱状晶所承受的弯曲应力越大; 斜角越大, 斜板上相同位置的柱状晶的弯曲应力越大; 随着熔体在倾斜板表面流动长度的增加, 在层流阶段, 倾斜板表面柱状晶根部所受的弯曲应力先急剧下降之后趋于平稳, 而在紊流阶段, 弯曲应力是缓慢增加的; 理论分析表明柱状晶在熔体近壁面流动过程受到的最大弯曲应力低于αup -Al晶粒的屈服强度, 所以斜板上熔体流动产生的弯曲力不能将柱状晶折断, 只能将晶粒冲刷游离到熔体中使晶粒增殖, 与实验结果相符合. 所以本模型可以很好地解释熔体近壁面流动过程中的剪切本构关系以及剪应力对凝固组织的影响.     

微圆管进口区气体流动与换热特性研究

对微圆管进口区运用一阶速度滑移和温度跳跃边界,考察了Kn、动量调和及热调和系数对流动与换热特性的影响机理和规律.模拟结果表明:流动进口段长度随Kn增加而增加,但随动量调和系数减小而减小;热进口段长度随Kn增加而增加,但随动量调和系数及热调和系数减小而减小;Nu数随Kn增加及热调和系数减小而减小,但随动量调和系数减小而增加.     

抽拉速度对SCN-DC共晶生长形貌的影响

本文采用类金属透明模型合金丁二腈-23.6 wt%樟脑 (SCN-23.6 wt%DC) 合金, 研究了棒状共晶定向凝固组织的演化行为, 考察了抽拉速度对棒状共晶合金组织形貌演化的影响规律. 结果表明, 在共晶生长初期, 共晶组织首先起源于晶粒晶界或者试样盒型壁处, 随后沿液/固界面和平行于热流方向生长; 在较小的抽拉速度 (0.064–0.44 μm/s)下, 棒状共晶界面前沿呈现平界面形态, 内部两相棒状组织平行生长, 并且随着抽拉速度的增大,棒状共晶逐渐细化, 棒状间距减小; 而在较大的抽拉速度 (0.67–1.56 μm/s)下, 共晶界面前沿呈现胞状生长形貌, 胞内的棒状共晶呈放射状生长, 同样, 随着抽拉速度的增大, 胞内棒状共晶逐渐细化, 棒状间距减小. 关键词： 定向凝固 共晶形貌 抽拉速度 共晶间距     

Quasi-one-dimensional foam drainage

Foam drainage is considered in a froth flotation cell. Air flow through the foam is described by a simple two-dimensional deceleration flow, modelling the foam spilling over a weir. Foam microstructure is given in terms of the number of channels (Plateau borders) per unit area, which scales as the inverse square of bubble size. The Plateau border number density decreases with height in the foam, and also decreases horizontally as the weir is approached. Foam drainage equations, applicable in the dry foam limit, are described. These can be used to determine the average cross-sectional area of a Plateau border, denoted A, as a function of position in the foam. Quasi-one-dimensional solutions are available in which A only varies vertically, in spite of the two-dimensional nature of the air flow and Plateau border number density fields. For such situations the liquid drainage relative to the air flow is purely vertical. The parametric behaviour of the system is investigated with respect to a number of dimensionless parameters: K (the strength of capillary suction relative to gravity), α (the deceleration of the air flow), and n and h (respectively, the horizontal and vertical variations of the Plateau border number density). The parameter K is small, implying the existence of boundary layer solutions: capillary suction is negligible except in thin layers near the bottom boundary. The boundary layer thickness (when converted back to dimensional variables) is independent of the height of the foam. The deceleration parameter α affects the Plateau border area on the top boundary: weaker decelerations give larger Plateau border areas at the surface. For weak decelerations, there is rapid convergence of the boundary layer solutions at the bottom onto ones with negligible capillary suction higher up. For strong decelerations, two branches of solutions for A are possible in the K = 0 limit: one is smooth, and the other has a distinct kink. The full system, with small but non-zero capillary suction, lies relatively close to the kinked solution branch, but convergence from the lower boundary layer onto this branch is distinctly slow. Variations in the Plateau border number density (non-zero n and h) increase individual Plateau border areas relative to the case of uniformly sized bubbles. For strong decelerations and negligible capillarity, solutions closely follow the kinked solution branch if bubble sizes are only slightly non-uniform. As the extent of non-uniformity increases, the Plateau border area reaches a maximum corresponding to no net upward velocity of foam liquid. In the case of vertical variation of number density, liquid content profiles and Plateau border area profiles cease to be simply proportional to one another. Plateau border areas match at the top of the foam independent of h, implying a considerable difference in liquid content for foams which exhibit different number density profiles. Received 3 July 2001     

Isothermal liquid epitaxy of variable-gap AlxGa1−xAs films

A mathematical diffusion model is proposed for the isothermal liquid epitaxy which arises when a layer of molten gallium is in contact on its two sides with a substrate of gallium arsenide and aluminum. Computer calculations showed that the thickness of the variable-gap films increases with increasing temperature of the process and increasing thickness of the layer of melt, and that the Al concentration in the films increases monotonically in the direction of growth. The maximal concentration of Al in the solid solution is determined by the amount of Al brought into contact with the melt. Experimental results confirm the theoretical calculations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 38–41, April, 1980.     

Dynamics of twist effect in a dual-frequency nematic liquid crystal

The dynamics of the electrooptical 90° twist effect in a dual-frequency nematic liquid crystal is investigated for wavelengths of 0.65 and 1.55μ m. It is shown that the boundary conditions of the interaction between the phases affect the optical threshold of the twist effect, the contrast, and the working voltage range. The switching time of the twist effect from the off to the on state upon a variation of the amplitude of a rectangular dc voltage pulse from 15 to 50 V changes from 1.5 to 0.3 ms for a thickness of the nematic crystal layer of about 7 μ m. The minimal time of switching from the “on” to the “off” state was 3 ms in the case when relaxation of molecules in a cell with asymmetric boundary conditions was controlled electrically. The dynamic range of transmittance variation at a wavelength of 1.55 μm extended to 30 dB.     

Pulsed electron-beam melting of a copper — steel 316 system: Evolution of the chemical composition,microstructure, and properties

The surface topography, chemical composition, microstructure, nanohardness, and tribological characteristics of a Cu (film, 512 nm)-stainless steel 316 (substrate) system subjected to pulsed melting by a low-energy (20–30 keV), high-current electron beam (2–3 μs, 2–10 J/cm²) were investigated. The film was deposited by sputtering a Cu target in the plasma of a microwave discharge in argon. To prevent local exfoliation of the film due to cratering, the substrate was multiply pre-irradiated with 8–10 J/cm². On single irradiation, the bulk of the film survived, and a diffusion layer containing the film and substrate components was formed at the interface. The thickness of this layer was 120–170 nm irrespective of the energy density. The diffusion layer consisted of subgrains of γ-Fe solid solution and nanosized particles of copper. In the surface layer of thickness 0.5–1 μm, which included the copper film quenched from melt and the diffusion layer, the nanohardness and the wear resistance nonmonotonicly varied with energy density, reaching, respectively, a maximum and a minimum in the range 4.3–6.3 J/cm². As the number of pulsed melting cycles was increased to five in the same energy density range, there occurred mixing of the film-substrate system and a surface layer of thickness ∼2 μm was formed which contained ∼20 at. % copper. Displacement of the excess copper during crystallization resulted in the formation of two-phase nanocrystal interlayers separating the γ-phase grains. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 6–13, December, 2005.     

An investigation of strong backflow events at the interface of air–water systems using large-eddy simulation

A large-eddy simulation of a counter-current gas–liquid flow is performed. At the flat interface where the different fluids meet, continuity of momentum and momentum fluxes are enforced following the work of Lombardi et al. [Direct numerical simulation of near-interface turbulence in coupled gas-liquid flow. Phys Fluids. 1996;8(6):1643–1665]. The increase in vertical vorticity fluctuations near the interface increases mixing, reducing the thickness of the inner region of the boundary layer. Such increase reduces shear while allowing for more frequent backflow motions in the inner region, being this phenomenon stronger on water. Due to the higher inertia of water these backflow motions are ultimately responsible for the streaky structure of shear stresses seen along the interface. The present study shows that such bimodality in the streamwise velocities is also seen in the angle distribution of vorticity relative to the interface, where such angles are linked to the presence of interface-connected and quasi-streamwise vortex cores. Finally, it is shown that backflow events on the interface shear stresses correlate with coupled ‘strong’ ejections in the near interface region despite the disparagingly different near-interface streamwise velocity distributions on the near interface boundary layers.     

Production of slow muonic hydrogen isotopes in vacuum

Muonic hydrogen isotopes (μ⁻ p, μ⁻ d, and μ⁻t) are simple quantum mechanical systems ideally suited for studies of numerous fundamental phenomena in electroweak and strong interactions as well as in applied areas such as muon chemistry or muon catalyzed fusion. Emission of muonic hydrogen isotopes into vacuum helps to overcome the limitations which are normally imposed on conventional investigations with gaseous and liquid targets. A proof of principle experiment for this new technique was performed at TRIUMF last year. Negative muons with 30 MeV/c momentum were stopped in a thin film of solid hydrogen and produced very low energy μ⁻d in vacuum. The distribution center of the normal velocity components of emitted μ⁻d atoms was measured to be ∼1 cm/μs. The yield of μ⁻d in vacuum is an increasing function of H₂ film thickness δ up to a value of δ≥1 mm.     

Mass transfer due to nonlinear capillary-gravitational waves on the surface of a viscous liquid

An analytical expression of the second order of smallness in wave amplitude-to-wavelength ratio is derived for a horizontal flow arising in a finite-depth layer of a viscous liquid under the action of a periodic nonlinear capillary wave. It is found that the liquid flow is determined by the nonlinear component of the velocity field vortex part and the flow rate increases with increasing viscosity and decreasing wavelength irrespective of the layer thickness. In thin layers, the flow rate rapidly drops from its maximal value with increasing viscosity, wavelength, and surface charge density. If the liquid surface is charged, the horizontal liquid flow decreases rapidly as the surface charge density approaches the threshold of the Tonks-Frenkel instability.     

Quasilinear model of deformation of a stratified three-dimensionally inhomogeneous flow above a randomly inhomogeneous surface

We study the deformation of the wind velocity profile due to resonant interactions with waves radiated by the flow over a statistically homogeneous topography. The wind whose velocity vector changes its direction within a layer of finite thickness is considered. Quasilinear equations for the velocity components of the mean flow are derived under large Richardson, numbers and small Froude numbers. It is shown that the modulus of the wind velocity is constant in time and its direction angle satisfies the Riemann equation for simple waves. The flow deformation is determined by the average wave resistance force per unit square. The deformation of the wind velocity profile takes place within the layer between the Earth’s surface and the level where the wind change its direction to the opposite one. At large time scales, the wind velocity vector in this layer approaches the direction opposite to the near-surface one. Institute of Applied Physics, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 3, pp. 255–265, March 1999.     

Effects of dimples on laminar boundary layers

A series of direct numerical simulations of the flow past a flat plate with two and eight rows of dimples in a staggered arrangement is carried out. The Reynolds number based on the boundary layer thickness and freestream velocity near the inflow plane is 1000 and the dimples are spherical with a depth to diameter ratio of 0.1. The incoming flow is laminar and the boundary layer thickness before the dimples is half the dimple depth. At this low Reynolds number the flow is expected to remain laminar over a smooth flat plate. The presence of the dimples triggers instabilities that cause significant momentum transport. It is shown that the shear layer that forms as the flow separates over the first two rows of dimple becomes unstable and sheds coherent vortex sheets. The vortex sheets become unstable and are transformed into packets of horseshoe vortices. When these vortices evolve over a flat plate or over a series of dimples the flow dynamics are very different with important changes in momentum transport across the boundary layer.     

Quantitative measurement of dynamic flow induced by <Emphasis Type="Italic">Tetrahymena pyriformis</Emphasis> (<Emphasis Type="Italic">T. pyriformis</Emphasis>) using micro-particle image velocimetry

Abstract  

In-depth quantitative visualization studies are required to understand the flow induced by swimming micro-organisms and find potential applications. The present study visualized the flow induced by Tetrahymena pyriformis of size 45–50 μm, which swam freely and via stimulation by galvanotaxis in a PDMS micro-chamber using a micro-particle image velocimetry system. The results showed that the maximum velocity of the induced flow was around 430 μm/s for free swimming and 700 μm/s for galvanotactic-controlled swimming. Due to the applied electric field, the electro-osmosis flow led to increased velocity of roughly 135 μm/s at 3 V/mm. The increased velocity stems from the increased motility of the cell under the electric field. Therefore, it was demonstrated that galvanotaxis can control the swimming direction and increase the induced velocity.     

加载形式对涡轮叶尖泄漏损失的影响

本文通过改变90%叶高截面叶型的安装角和厚度分布获得了不同的加载形式,并采用数值模拟方法对比分析了加载形式对叶尖泄漏流动和损失的影响。结果表明,叶尖加载形式影响了泄漏流的流量和法向/流向速度差沿轴向的分布,进而影响泄漏流动损失。随着叶尖负荷向前缘移动,叶尖泄漏总流量增大,当泄漏量和动量差沿轴向分布相对均匀,当地峰值减小,叶中附近所占比重增加;在泄漏量与动量差共同作用下,泄漏涡卷起位置向上游移动,但与主流掺混强度减弱,损失减小;采用均匀加载和前加载形式能有效降低泄漏流与主流的动量差,减小泄漏损失,提高涡轮性能。