Collision between high and low viscosity droplets: Direct Numerical Simulations and experiments

Binary droplet collisions are of importance in a variety of practical applications comprising dispersed two-phase flows. In the present work we focus on the collision of miscible droplets, where one droplet is composed of a high viscous liquid and the other one is of lower viscosity. This kind of collisions take place in, for instance, spray drying processes when droplets with different solid content collide in recirculation zones. The aim of this paper is to investigate the details of the flow inside the colliding droplets. For this purpose, two prototype cases are considered, namely the collision of equal sized droplets and the collision between a small and highly viscous droplet and a bigger low viscous droplet. A new experimental method has been developed in order to visualize the penetration and mixing process of two colliding droplets, where a fluorescence marker is added to one liquid and the droplets are excited by a laser. The results show a delay in the coalescence which takes place during the initial stage of a collision of droplets with different viscosities. Direct Numerical Simulations based on the Volume-of-Fluid method are used to study these collisions and to allow for a more detailed inspection of the mixing process. The method is extended to consider a second liquid with a different viscosity. In order to reproduce the delay of coalescence, an algorithm for the temporal suppression of the coalescence is applied. A predictive simulation of the delay is not possible, because the extremely thin air gap separating the droplets cannot be resolved by the numerics. This approach is validated by comparison with experimental data. The results provide local field data of the flow inside the collision complex, showing in particular a pressure jump at the liquid–liquid interface although no surface tension is present. The detailed analysis of the terms in the momentum balance show that the pressure jump results from the viscosity jump at the liquid–liquid interface.     

High-order Finite Difference and Finite Volume WENO Schemes and Discontinuous Galerkin Methods for CFD

In recent years, high order numerical methods have been widely used in computational fluid dynamics (CFD), to effectively resolve complex flow features using meshes which are reasonable for today's computers. In this paper, we review and compare three types of high order methods being used in CFD, namely the weighted essentially non-oscillatory (WENO) finite difference methods, the WENO finite volume methods, and the discontinuous Galerkin (DG) finite element methods. We summarize the main features of these methods, from a practical user's point of view, indicate their applicability and relative strength, and show a few selected numerical examples to demonstrate their performance on illustrative model CFD problems.     

Higher-order Upwind Distribution-formula Scheme for Structured and Unstructured Adaptive Flow Solvers

The development of inviscid and viscous flow solvers for both structured and unstructured meshes is presented in this paper. The solution method is the distribution-formula scheme. This is an explicit, cell-vertex, finite volume method which is essentially second-order accurate in both space and time. The Euler and Navier-Stokes equations are integrated over each finite volume cell to determine the change in flow properties (e.g. density) for the cell. Distribution formulas are then used to distribute such changes to the surrounding vertices. Increments in each vertex (which is a calculation point) thus consist of contributions from the surrounding cells. The original discretization technique involves central differencing and is simple, robust and computationally efficient. In this work, starting with inviscid flow simulations using the original scheme on structured grids, improvements are subsequently made to the scheme by replacing the central differencing portion with MUSCL type higher-order upwind differencing. Numerical investigations with the improved scheme are performed using inviscid flow simulations on structured grids. Upon establishing improved accuracy, stability and excellent shock capturing properties, further extension to viscous flow computations on unstructured adaptive meshes is implemented. Results are presented for laminar, viscous flow over a NACA 0012 airfoil.     

STEADY STRATIFIED CIRCULATION INDUCED BY CONSTANT COASTAL WIND

SUMMARY

The nearshore zone is the area where the action of waves and currents on the sea bed is most intense, and where thermal stratification plays an important role in the transport of momentum, heat and pollutants. Therefore, in recent years this topic has drawn the attention of various investigations using analytical, experimental or numerical methods. The coastal water surface can be thought to be agitated by the uniform wind shear stress under flow field with thermal stratified density if the on-shore and off-shore wind blowing over the coastal water surface is assumed to be uniform. The physical properties that enter into the governing equations, which both describe the problem and are also essential to the solution of the phenomena of interest, are the eddy viscosity and the eddy diffusivity. The nonlinear governing equation is simplified in this present work by using the Boussinesq Approximation. A quadratic upwind finite volume method employing contravariant velocities as dependent variables is applied to the simulation of a three-dimensional steady state laminar incompressible flow. The flux terms in the basic equations were discretized via the QUICK ¹ Leonard , B. P. ( 1979 ) “ A stable and accurate convective modelling procedure based on quadratic upstream interpolation ”, J. Comput. Meth. Appl. Mech. Engrg. , 19 , 59 – 98 .[Crossref], [Web of Science ®] , [Google Scholar] scheme for internal nodes and power-law scheme for the boundary nodes. The resulting algebraic equations were solved in a decoupled manner by the SIMPLEC ² Patankar , S. V. ( 1980 ) “ Numerical Heat Transfer and Fluid Flow ”, McGrawHill , New York . [Google Scholar] solution algorithm. A numerical method was then utilized in calculating the flow conditions for ratios of shear stress region and depth, and for different obstacles near the shoreline. Two circulation forms would be formed when the distance of the obstacle (Ex. breakwater) from the coast is equal to the depth. The cellular circulation in the lower layer would become gradually weakened with an increasing distance of the obstacle from the coast. If the eddy viscosity and eddy diffusivity are considered, the cellular circulation in the lower layer would also become gradually weakened. The cellular circulation would arise both behind and in front of the coastal structures, if the terrains with sedimentary obstacles are considered. The cellular circulation in the low layer would also tend to gradually disappear; however, the tendency of weakening is more apparent than the former. The effects of the cellular circulation in the lower layer would make it difficult to transport oxygen, heat and pollutants. Finally, the Coriolis force effect would be taken into consideration and therefore, its order of influence could be indicated.     

面向三维城市地理信息系统的体相交分析方法的实现与应用

提出一种应用于三维城市地理信息系统的空间分析方法:体相交分析。该方法先后输入两个体数据集, 经过分析计算, 输出一个关系集合, 集合中的每一条关系代表第一个输入数据集 中的一个体对象与第二个输入数据集中的一个或多个体对象之间存在的相交关系。基于 OO3D 数据模型, 实现了一种稳定、快速 的体相交分析算法,并将其应用到一个三维城市地理信息系统项目(TG-U-3D) 之中。在该项目中, 体相交分析用于辅助检测城市地下空间规划过程中的用地冲突, 以及查询某一城市设施所位于或者穿越的地层。     

Computation of Jump Coefficients for Momentum Transfer Between a Porous Medium and a Fluid Using a Closed Generalized Transfer Equation

The momentum transfer between a homogeneous fluid and a porous medium in a system analogous to the one used by Beavers and Joseph (J Fluid Mech 30:197–207, 1967) is studied using volume averaging techniques. In this article, we present a closed generalized momentum transport equation (GTE) that is valid everywhere and is expressed in terms of position-dependent effective transport coefficients, which are computed from the solution of associated closure problems previously reported. A combination of the velocity profiles from the GTE in the definition of the excess terms that define the jump coefficients allows their computation using numerical techniques. The calculations are in concordance with those resulting from the work of Goyeau et al. (Int J Heat Mass Transf. 46:4071–4081, 2003), showing a strong dependence with the porosity. In addition, the effects of the roughness of the boundary on the computation of the position-dependent permeability tensor in the inter-region are also analyzed.     

Modeling high density microholographic data storage: Using linear, quadratic, thresholding and hard clipping material characteristics

Crosstalk related raw signal-to-noise ratio (SNR) and bit error rate (BER) of high density bitwise microholographic data storage is investigated by numerical modeling. Scattering and diffraction of light is calculated in non-paraxial scalar approximation. A multiple thin slice implementation of the perturbative volume integral equation is used, which can be easily parallelized. The effect of bit and track spacing, and the different local characteristics of the holographic recording material on the SNR, BER and diffraction efficiency are investigated. The results show that these lateral spacing parameters have much more effect on crosstalk noise than the number of layers. Using two-photon, thresholding or hard clipping materials generates less crosstalk noise at the same data density than a linear material, and the dynamic range of these materials can be used more effectively resulting in higher single microhologram diffraction efficiencies.     

Two-Phase Inertial Flow in Homogeneous Porous Media: A Theoretical Derivation of a Macroscopic Model

The purpose of this article is to derive a macroscopic model for a certain class of inertial two-phase, incompressible, Newtonian fluid flow through homogenous porous media. Starting from the continuity and Navier–Stokes equations in each phase β and γ, the method of volume averaging is employed subjected to constraints that are explicitly provided to obtain the macroscopic mass and momentum balance equations. These constraints are on the length- and time-scales, as well as, on some quantities involving capillary, Weber and Reynolds numbers that define the class of two-phase flow under consideration. The resulting macroscopic momentum equation relates the phase-averaged pressure gradient to the filtration or Darcy velocity in a coupled nonlinear form explicitly given by

一种基于移动设备的医学图像体绘制方法

针对移动设备硬件局限性,提出一种新的纹理切片体绘制方案,利用最新支持移动设备的OpenGL ES 2.0接口的图形处理能力,精心地设计体绘制着色程序,完全避免复杂的计算和条件分支,较好地解决了实现医学图像交互式体绘制速度慢的问题。通过在不同的设备上和不同的场景下进行实验,表明本文的方法可以使绘制帧率提高一倍左右。     

Hydration and dehydration behavior of <Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide gel particles

Thermal response of N-isopropylacrylamide (NiPAM) gel particles of submicron size was investigated by means of ultrasonic velocity and density focusing on the hydration and dehydration behavior. Hydration number, defined as the number of water molecules bound to one NiPAM monomer unit, was quantitatively evaluated in the course of the volume phase transition. Hydration numbers at low and high temperatures were about 7.5 (20 °C) and 3 (40 °C), respectively. Hydration number decreases markedly near the volume phase transition temperature, and the decrease is responsible to the formation of hydrophobic bonding and dehydration. The hydration number is independent both of the size and the composition of gel particles in the shrunken state, although there exists a composition dependence in the swollen state. It was also found that the self-assemblies formed by hydrophobic interaction are more bulky compared to those in the hydrated state.