纳米颗粒多相流体动力学研究及应用

纳米颗粒多相流研究是目前多相流研究中新的研究方向及重点发展领域.为探索纳米尺度多相流相间作用机理及内部存在机制,采用理论分析及数值计算手段,对一般动力学方程的封闭处理、颗粒碰撞率宏观模型的有效构建、颗粒凝并系统动力学演变特性的机理分析、非稀相问题碰撞率的求取、双变量问题求解方法的建立以及一些实际应用进行了系统研究,提出了新的针对纳米尺度颗粒动力学演变的一般动力学方程求解方法,并将其应用于实际工业过程问题的研究.该文对上述研究工作进行了综述.     

基于修正偶应力和高阶剪切变形理论的变截面微梁的自由振动

基于修正偶应力和高阶剪切理论建立了仅含有一个尺度参数的Reddy变截面微梁的自由振动模型,研究了变截面微梁自由振动问题的尺度效应和横向剪切变形对自振频率计算的影响。基于哈密顿原理推导了动力学方程与边界条件,并采用微分求积法求解了各种边界条件下的自振频率。算例结果表明,基于偶应力理论预测的变截面微梁的自振频率均大于经典梁理论的预测结果,即捕捉到了尺度效应。另外,梁的几何尺寸与尺度参数越接近,尺度效应就越明显,而梁的长细比越小,横向剪切变形对自振频率的影响就越明显。     

雾层两相耦合流场及气溶胶颗粒物的动力学性质

雾层气溶胶系统涉及复杂的动力学演变过程:碰撞、凝并、破碎、冷凝/蒸发、成核、沉积、表面化学反应等.因此,发展雾层与气相流场耦合的Eulerian-Lagrangian两相流模型、颗粒动力学及随机轨道模型,考虑重力、曳力、布朗力、Basset力等对颗粒相的作用.基于SIMPLE和多重Monte Carlo算法求解颗粒群平衡方程,自行开发了FAD程序首先对室内燃烧源细微颗粒物的扩散实验展开数值模拟,计算结果与实验数据吻合较好.将建立的模型和方法数值研究气溶胶污染物在雾环境中的输运过程,分析雾消散阶段颗粒相浓度、平均尺度的时空分布.结果显示:当时间演化至60 min,雾滴的平均尺度减小到初始的65.67％,而气溶胶颗粒最大数目对应的尺度为0.006 μm.     

基于格子Boltzmann方法的电流变液动力学建模及数值模拟

电流变悬浮液内部结构对外电场的快速响应发生在指定的控制空间中,在这一特指的时间和空间尺度上电流变悬浮液的物理行为特征主要为剪切速率低和流动阻尼大,即Mach和Reynolds数一般不大,可以视为微尺度流动来加以研究。针对这一流动特征,基于介观动理论的格子Boltzmann方法,建立了电流变悬浮液两相流动的离散颗粒运动模型,通过该模型进行了动力学模拟,结果表明,该模型解决了分子动力学模型难以描述的因颗粒运动造成局部流场流变特性改变的难题,以及该流场双向耦合过程中对颗粒运动的影响。     

冲击载荷作用下颗粒材料动态力学响应的近场动力学模拟

颗粒材料在冲击载荷作用下的动态力学行为是学术界关注的热点问题. 新近问世的近场动力学(peridynamics)理论将材料视为由大量有限体积和有限质量的物质点组成,基于非连续性和非局部作用假定建模,建立空间积分形式的运动方程,自然适应于颗粒材料动态力学行为的描述与分析. 发展了描述颗粒间接触作用的物质点尺度的排斥力模型,考虑近场动力学方法中非局部长程力特征,改进了近场动力学中的初始微观弹脆性(prototype microelastic brittle, PMB) 模型的本构力函数,并消除了原PMB 模型中存在的“边界效应” 问题. 计算分析了冲击载荷作用下碳化钨陶瓷颗粒体系的动态力学响应,得到了不同冲击速度下颗粒体系的冲击波速,PD计算结果与试验结果高度一致;通过颗粒物质点尺度作用描述单颗粒尺度的接触作用,很好地再现了颗粒的转动与平动、颗粒挤压变形以及颗粒破碎等现象;刚性冲击板附近同时存在严重的颗粒破碎与轻微的颗粒损伤,远离冲击板的部分颗粒出现破损,且颗粒破碎主要是由颗粒间挤压、碰撞以及相对滑动剪切作用造成的. 研究结果表明,所发展的计算模型和分析方法能很好地反映颗粒材料动态力学行为,具有广泛的应用价值.      

轻敲模式下 AFM 动力学模型及能量耗散机理研究

轻敲模式下探针从远离到间歇性接触样品表面,是一个连续的能量耗散过程.针对该连续过程的能量耗散机理研究仅零星存在于各个文献之中,对于连续过程中各个阶段的能量耗散机理也没有一个系统的解释和实验验证.本文提出了新的位移激励下原子力显微镜探针-样品系统简化模型并得到了一维振子系统等效阻尼的计算方法,并通过该方法计算了探针在远离样品表面时的空气黏性阻尼和靠近样品时的空气压膜阻尼,分析了探针从远离样品到间歇性接触样品表面这一过程中的环境耗散机理变化,得到了原子力显微镜系统理论品质因数与探针工作位置的关系曲线;在此基础上设计了轻敲模式下的微悬臂梁扫频实验,得到了系统实验品质因数与探针工作位置的关系曲线,进而验证了理论模型的准确性. 本文通过对轻敲模式下AFM环境耗散机理进行理论分析和实验验证,希望可以对轻敲模式下AFM动力学特性及其阻尼作用机理有更近一步的认识,同时对微纳米机电系统 (MEMS/NEMS) 能量耗散机理的研究提供理论参考和实验方法.      

剪切和扭转工况下微纳米薄壁蜂窝的等效剪切模量计算

分别针对剪切和扭转两种工况给出了微纳米薄壁蜂窝等效剪切模量的解析计算方法.该方法综合考虑了由面板对芯层的约束导致的高度效应和当蜂窝胞壁厚度进入微纳米量级时引起的尺度效应.首先对蜂窝各胞壁选取了可反映面板约束以及受力状态的三角级数位移场,然后在本构关系中引入修正偶应力理论以描述尺度效应,最后应用能量均匀化方法求得蜂窝的等效剪切模量.以典型六边形蜂窝为例,给出了完整的计算过程和结果.与文献中的等效剪切模量结果进行对比,讨论了不同工况下等效剪切模量随芯层高度和胞壁厚度的变化趋势,以及高度效应和尺度效应之间的相互影响.     

金属基纳米复合材料等效弹性模量的均匀化方法数值模拟

均匀化理论利用位移场双尺度渐近展开建立有限元列式，本文将其与有限元通用程序相结合，应用于金属基复合材料的弹性本构数值模拟。通过对不同尺度增强相金属基复合材料等效模量的数值模拟，考察了均匀化方法的适用情况。数值计算结果表明，对常规尺度增强相金属基复合材料，均匀化方法可以较准确地预测其等效弹性模量；对纳米增强相金属基复合材料，该方法仍可给出较好的预测，但存在某种程度的系统偏差。通过对纳米尺度增强机理的分析讨论，认为纳米增强相与基体材料问的界面效应可能有别于连续介质假设，指出可以考虑采用离散原子-连续介质耦合模型改进数值模拟结果。     

STUDY ON A DYNAMICS MODEL OF TAPPING MODE AFM AND ENERGY DISSIPATION MECHANISM$^{\bf 1)}$

轻敲模式下探针从远离到间歇性接触样品表面,是一个连续的能量耗散过程.针对该连续过程的能量耗散机理研究仅零星存在于各个文献之中,对于连续过程中各个阶段的能量耗散机理也没有一个系统的解释和实验验证.本文提出了新的位移激励下原子力显微镜探针-样品系统简化模型并得到了一维振子系统等效阻尼的计算方法,并通过该方法计算了探针在远离样品表面时的空气黏性阻尼和靠近样品时的空气压膜阻尼,分析了探针从远离样品到间歇性接触样品表面这一过程中的环境耗散机理变化,得到了原子力显微镜系统理论品质因数与探针工作位置的关系曲线;在此基础上设计了轻敲模式下的微悬臂梁扫频实验,得到了系统实验品质因数与探针工作位置的关系曲线,进而验证了理论模型的准确性. 本文通过对轻敲模式下AFM环境耗散机理进行理论分析和实验验证,希望可以对轻敲模式下AFM动力学特性及其阻尼作用机理有更近一步的认识,同时对微纳米机电系统 (MEMS/NEMS) 能量耗散机理的研究提供理论参考和实验方法.     

Biot-Cosserat连续体-离散颗粒集合体模型的非饱和土连接尺度方法

耦合了非饱和多孔多相介质有限元模型和颗粒介质离散元(DEM)模型,提出了以宏、细观尺度分别耦合Biot-Cosserat连续体模型和离散颗粒集合体模型的连接尺度方法来分析非饱和含液颗粒材料的力学渗流耦合问题。根据被动空气压力假定和对空间离散孔隙水质量守恒方程的约化,从非饱和土有限元控制方程的基本未知量中消去了孔隙水压力,而将其取作有限元积分点上定义取值的内状态变量,进而建立了节点未知量仅包含固相线位移和转角的非饱和Cosserat多孔连续体约化有限元数值模型。基于连接尺度方法(BSM)宏、细观尺度数值过程的解耦计算的特点,对宏、细观两尺度数值模型的时域积分分别采用隐式Newmark方法和显式中心差分法,且取不同时间步长以提高计算效率。与全域采用DEM的精细分析方法相比,本文BSM在保证计算精度的前提下可大幅节省计算时间。在不考虑湿化效应的边坡稳定算例中,在得到类似计算精度条件下它比全域采用DEM节省计算时间高达86.65%。二维边坡稳定算例结果验证了本文连接尺度方法的有效性,以及在揭示含液颗粒结构细观破坏机理上的优点。数值算例结果显示,边坡承载能力因降雨大幅下降约50%,这表明本文发展的计及伴随湿化过程的颗粒材料结构中饱和度及吸力分布演变及其对结构破坏失效影响的非饱和颗粒材料多尺度计算模型是很有必要的。     

Solution of Smoluchowski coagulation equation for Brownian motion with TEMOM

The particle number density in the Smoluchowski coagulation equation usually cannot be solved as a whole, and it can be decomposed into the following two functions by similarity transformation: one is a function of time (the particle k-th moments), and the other is a function of dimensionless volume (self-preserving size distribution). In this paper, a simple iterative direct numerical simulation (iDNS) is proposed to obtain the similarity solution of the Smoluchowski coagulation equation for Brownian motion from the asymptotic solution of the k-th order moment, which has been solved with the Taylor-series expansion method of moment (TEMOM) in our previous work. The convergence and accuracy of the numerical method are first verified by comparison with previous results about Brownian coagulation in the literature, and then the method is extended to the field of Brownian agglomeration over the entire size range. The results show that the difference between the lognormal function and the self-preserving size distribution is significant. Moreover, the thermodynamic constraint of the algebraic mean volume is also investigated. In short, the asymptotic solution of the TEMOM and the self-preserving size distribution form a one-to-one mapping relationship; thus, a complete method to solve the Smoluchowski coagulation equation asymptotically is established.     

Nanoparticle dispersion and coagulation in a turbulent round jet

Nanoparticle dispersion and coagulation behaviors in a turbulent round jet were studied in this article. An experimental system was designed to generate a uniformly distributed air–nanoparticle two-phase flow in a turbulent round jet. The particle size distribution (PSD) was measured by a scanning mobility particle sizer (SMPS) in the near field of the jet. The particle diameters were nearly constant in the potential core due to the high carrying velocity and laminar characteristic of the flow but grew larger in the region of high turbulence intensities because the vortex structures in the mixing layer promoted coagulation. Furthermore, the migration property of small-sized nanoparticles forced them to be preserved in the potential core also leading to the diameter increase. The comparison of the particle concentration distributions at different sections indicated that the shear layer is the major region for the mixing of particle-laden stream and ambient air. The particle diameters in the axial direction experienced three stages including a slightly changed stage, an increasing stage and a constant stage. The diameter increase should be attributed to turbulence coagulation.     

Wall temperature effects on shock unsteadiness in a reattaching compressible turbulent shear layer

In this study, the effect of heat transfer on the compressible turbulent shear layer and shockwave interaction in a scramjet has been investigated. To this end, highly resolved Large Eddy Simulations (LES) are performed to explore the effect of wall thermal conditions on the behavior of a reattaching free shear layer interacting with an oblique shock in compressible turbulent flows. Various wall-to-recovery temperature ratios are considered, and results are compared to the adiabatic wall. It is found that the wall temperature affects the reattachment location and the shock behavior in the interaction region. Furthermore, fluctuating heat flux exhibits a strong intermittent behavior with severe heat transfer compared to the mean, characterized by scattered spots. The distribution of the Stanton number shows a strong heat transfer and complex pattern within the interaction, with the maximum thermal (heat transfer rates) and dynamic loads (root-mean-square wall pressure) found for the case of the cold wall. The analysis of LES data reveals that the thermal boundary condition can significantly impact the wall pressure fluctuations level. The primary mechanism for changes in the flow unsteadiness due to the wall thermal condition is linked to the reattaching shear layer, which agrees with the compressible turbulent boundary layer theory.     

Numerical and experimental investigation of turbulent characteristics in a drag-reducing flow with surfactant additives

Cetyltrimethyl ammonium chloride (CTAC) surfactant additives, because of their long-life characteristics, can be used as promising drag-reducers in district heating and cooling systems. In the present study we performed both numerical and experimental tests for a 75 ppm CTAC surfactant drag-reducing channel flow. A two-component PIV system was used to measure the instantaneous streamwise and wall-normal velocity components. A Giesekus constitutive equation was adopted to model the extra stress due to the surfactant additives, with the constitutive parameters being determined by well-fitting apparent shear viscosities, as measured by an Advanced Rheometric Expansion System (ARES) rheometer. In the numerical study, we connected the realistic rheological properties with the drag-reduction rate. This is different from previous numerical studies in which the model parameters were set artificially. By performing consistent comparisons between numerical and experimental results, we have obtained an insight into the mechanism of the additive-induced drag-reduction phenomena.

Our simulation showed that the addition of surfactant additives introduces several changes in turbulent flow characteristics: (1) In the viscous sublayer, the mean velocity gradient becomes gentler due to the viscoelastic forces introduced by the additives. The buffer layer becomes expanded and the slope of the velocity profile in the logarithmic layer increases. (2) The locations where the streamwise velocity fluctuation and Reynolds shear stress attain their maximum value shifted from the wall region to the bulk flow region. (3) The root-mean-square velocity fluctuations in the wall-normal direction decrease for the drag-reducing flow. (4) The Reynolds shear stress decreases dramatically and the deficit of the Reynolds shear stress is mainly compensated by the viscoelastic shear stress. (5) The turbulent production becomes much smaller and its peak-value position moves toward the bulk flow region. All of these findings agree qualitatively with experimental measurements.

Regarding flow visualization, the violent streamwise vortices in the near wall region become dramatically suppressed, indicating that the additives weaken the ejection and sweeping motion, and thereby inhibit the generation of turbulence. The reduction in turbulence is accomplished by additive-introduced viscoelastic stress. Surfactant additives have dual effects on frictional drag: (1) introduce viscoelastic shear stress, which increases frictional drag; and (2) dampen the turbulent vortical structures, decrease the turbulent shear stress, and then decrease the frictional drag. Since the second effect is greater than the first one, drag-reduction occurs.     

可压缩均匀剪切湍流的二阶矩模拟

基于对可压缩湍流中脉动压力场和脉动速度场特征的理论分析以及ＤＮＳ结果,建立了可均匀剪切湍流中压力－变形率关联的压缩性修正模式,应用这个模式,加上Ｓａｒｋａｒ等建立的脉动体胀率项（ｄｉｌａｔａｔｉｏｎａｌ ｔｅｒｍｓ）的模式,预测可压缩均匀剪切湍流随时间的发展,所得雷诺应力各是性张量的平衡值与Ｂｌａｉｓｄｅｌｌ等的ＤＮＳ数据非常一致。这个模式准确地预测出均匀剪切湍流中压缩性导致的雷诺应力结构的“流向     

Direct numerical simulation of the turbulent energy balance and the shear stresses in power-law fluid flows in pipes

The results of direct numerical simulation of turbulent flows of non-Newtonian pseudoplastic fluids in a straight pipe are presented. The data on the distributions of the turbulent stress tensor components and the shear stress and turbulent kinetic energy balances are obtained for steady turbulent flows at the Reynolds numbers of 10⁴ and 2×10⁴. As distinct from Newtonian fluid flows, the viscous shear stresses turn out to be significant even far from the wall. In power-law fluid flows the mechanism of the energy transport from axial to transverse component fluctuations is suppressed. It is shown that with decrease in the fluid index the turbulent transfer of the momentum and the velocity fluctuations between the wall layer and the flow core reduces, while the turbulent energy flux toward the wall increases. The earlier-proposed models for the average viscosity and the non-Newtonian one-point correlations are in good agreement with the data of direct numerical simulation.     

Solution of general dynamic equation for nanoparticles in turbulent flow considering fluctuating coagulation

A new averaged general dynamic equation (GDE) for nanoparticles in the turbulent flow is derived by considering the combined effect of convection, Brownian diffusion, turbulent diffusion, turbulent coagulation, and fluctuating coagulation. The equation is solved with the Taylor-series expansion moment method in a turbulent pipe flow. The experiments are performed. The numerical results of particle size distribution correlate well with the experimental data. The results show that, for a turbulent nanoparticulate flow, a fluctuating coagulation term should be included in the averaged particle GDE. The larger the Schmidt number is and the lower the Reynolds number is, the smaller the value of ratio of particle diameter at the outlet to that at the inlet is. At the outlet, the particle number concentration increases from the near-wall region to the near-center region. The larger the Schmidt number is and the higher the Reynolds number is, the larger the difference in particle number concentration between the near-wall region and near-center region is. Particle polydispersity increases from the near-center region to the near-wall region. The particles with a smaller Schmidt number and the flow with a higher Reynolds number show a higher polydispersity. The degree of particle polydispersity is higher considering fluctuating coagulation than that without considering fluctuating coagulation.     

An LES Turbulent Inflow Generator using A Recycling and Rescaling Method

The present paper describes a recycling and rescaling method for generating turbulent inflow conditions for Large Eddy Simulation. The method is first validated by simulating a turbulent boundary layer and a turbulent mixing layer. It is demonstrated that, with input specification of mean velocities and turbulence rms levels (normal stresses) only, it can produce realistic and self-consistent turbulence structures. Comparison of shear stress and integral length scale indicates the success of the method in generating turbulent 1-point and 2-point correlations not specified in the input data. With the turbulent inlet conditions generated by this method, the growth rate of the turbulent boundary/mixing layer is properly predicted. Furthermore, the method can be used for the more complex inlet boundary flow types commonly found in industrial applications, which is demonstrated by generating non-equilibrium turbulent inflow and spanwise inhomogeneous inflow. As a final illustration of the benefits brought by this approach, a droplet-laden mixing layer is simulated. The dispersion of droplets in the near-field immediately downstream of the splitter plate trailing edge where the turbulent mixing layer begins is accurately reproduced due to the realistic turbulent structures captured by the recycling/rescaling method.     

Compressibility effects due to turbulent fluctuations

This paper deals with intrinsic effects of compressibility, i.e. with dilatation fluctuations in response to pressure fluctuations. Three different types of turbulent flows are considered in more detail: homogeneous turbulent shear flow, wall-bounded turbulent shear flow and shock/turbulence interaction. A survey of the present knowledge in this field, mainly based on DNS data, is given. Using the linear inviscid perturbation equations a direct link between fluctuations of dilatation and of velocity in the direction of mean shear is presented for homogeneous shear flow. This relation might form the basis for a more universal pressure-dilatation model. It is conjectured that the insignificance of intrinsic compressibility effects in wall-bounded supersonic shear flow is mainly due to the impermeability constraint of the wall. To this end, a linear stability analysis of supersonic channel flow along cooled, but permeable walls has been performed based on Coleman et al.'s [5] mean flow data. It shows an increase in the moduli of eigenfunctions related to compressibility, like pressure, and in moduli of quantities derived from eigenfunctions such as ‘pressure dilatation’ and squared dilatation. Although these results do not prove our hypothesis they provide hints in this direction. Shock/turbulence interaction is viewed as a source of compressibility. Former DNS data of Hannappel and Friedrich [10] for shock/isotropic turbulence interaction showing the effect of compressibility on the amplification of fluctuations are interpreted based on linear perturbation equations.     

A structural model for the time-dependent recovery of mineral suspensions

A thixotropic recovery model has been developed that is based on consideration of the microstructural interactions that occur between particles within a suspension particle network. The model is based on Smoluchowski coagulation rate theory, utilizing second order kinetics to describe the thixotropic recovery behavior. The model is applied to Na-montmorillonite-based coal tailings suspensions and is also shown to be applicable to brown coal and bauxite residue suspensions. The model describes all the recovery data well, especially at intermediate to large recovery times. The recovery of the montmorillonite suspensions at short times was faster than predicted, indicating the existence of additional factors in early-time structure development. The discrepancy may have also been due to the highly anisotropic nature of the clay platelets. The recovery rate constant, K _r , increases with increasing solids concentration (for constant surface chemical conditions) as would be expected from the basis of the model. Received: 22 September 2000 Accepted: 16 March 2001