Numerical simulation of separated boundary-layer flow

The numerical simulation of time-dependent, 2-D compressible boundary-layer flow containing a region of separation is studied. The separation is generated by the introduction of an adverse pressure gradient along the freestream boundary. In order to validate the numerical method, a low Mach-number laminar separation bubble flow is considered, which enables an extensive comparison with incompressible results. The generation of an adverse pressure gradient along the freestream boundary can be realized in various ways. An imposed decelerating flow boundary is compared with a suction technique. The effects of the strength of the pressure gradient and the presence of small upstream perturbations on the separation bubble are also investigated. The time-averaged characteristics of the flow are in good quantitative agreement with incompressible approximate theories predicting the condition for separation. The appearance of self-excited vortex shedding in unperturbed flows under a sufficiently strong adverse pressure gradient is consistent with incompressible flow simulations reported in the literature. The satisfactory result achieved in the calculation of the low-Mach-number flow encourages the application of the numerical method to flows with strong compressibility effects.     

Numerical solution of a multi-ion one-potential model for electroosmotic flow in two-dimensional rectangular microchannels

A new more general numerical model for the simulation of electrokinetic flow in rectangular microchannels is presented. The model is based on the dilute solution model and the Navier-Stokes equations and has been implemented in a finite-element-based C++ code. The model includes the ion distribution in the Helmholtz double layer and considers only one single electrical' potential field variable throughout the domain. On a charged surface(s) the surface charge density, which is proportional to the local electrical field, is imposed. The zeta potential results, then, from this boundary condition and depends on concentrations, temperature, ion valence, molecular diffusion coefficients, and geometric conditions. Validation cases show that the model predicts accurately known analytical results, also for geometries having dimensions comparable to the Debye length. As a final study, the electro-osmotic flow in a controlled cross channel is investigated.     

Imaging of electroosmotic flow in plastic microchannels

We have characterized electroosmotic flow in plastic microchannels using video imaging of caged fluorescent dye after it has been uncaged with a laser pulse. We studied flow in microchannels composed of a single material, poly(methyl methacrylate) (acrylic) or poly(dimethylsiloxane) (PDMS), as well as in hybrid microchannels composed of both materials. Plastic microchannels used in this study were fabricated by imprinting or molding using a micromachined silicon template as the stamping tool. We examined the dispersion of the uncaged dye in the plastic microchannels and compared it with results obtained in a fused-silica capillary. For PDMS microchannels, it was possible to achieve dispersion similar to that found in fused silica. For the acrylic and hybrid microchannels, we found increased dispersion due to the nonuniformity of surface charge density at the walls of the channels. In all cases, however, electroosmotic flow resulted in significantly less sample dispersion than pressure-driven flow at a similar velocity.     

Numerical simulation of gas flow in pneumatic components

The flow through pneumatic components is characterized by very complex flow phenomina. In general the flow is viscous, transonic (0≦M≦4) and turbulent. Small geometrical dimensions of pneumatic components make flow measurement difficult or sometimes impossible. Hence the accurate numerical prediction of the flow field becomes of great importance. In this paper we present the theoretical framework and the numerical capabicities of the commercial Navier-Stokes CFD code TASC flow. We solve some test problems which reflect many features of the numerical flow simulation in pneumatic components. For the test cases considered here, TASC flow was found to be an excellent tool for fast and accurate flow simulation.     

Numerical simulation of fresh SCC flow: applications

Numerical simulation of self-compacting concrete (SCC) flow shows great potential for developing into a powerful tool for prediction of SCC form filling. Numerical simulation is also of interest for modelling small scale material phenomena. This paper presents three different applications useful for modelling different phenomena on different scales: (i) particles, each representing an aggregate in the concrete, (ii) fluid, modelling concrete as a homogeneous liquid and (iii) particle in fluid, studying details of flow. The methods are compared and evaluated in order to give the reader a quick guidance into the world of possibilities that open up with numerical simulation.     

Microfluidic T-form mixer utilizing switching electroosmotic flow

This paper presents a microfluidic T-form mixer utilizing alternatively switching electroosmotic flow. The microfluidic device is fabricated on low-cost glass slides using a simple and reliable fabrication process. A switching DC field is used to generate an electroosmotic force which simultaneously drives and mixes the fluid samples. The proposed design eliminates the requirements for moving parts within the microfluidic device and delicate external control systems. Two operation modes, namely, a conventional switching mode and a novel pinched switching mode, are presented. Computer simulation is employed to predict the mixing performance attainable in both operation modes. The simulation results are then compared to those obtained experimentally. It is shown that a mixing performance as high as 97% can be achieved within a mixing distance of 1 mm downstream from the T-junction when a 60 V/cm driving voltage and a 2-Hz switching frequency are applied in the pinched switching operation mode. This study demonstrates how the driving voltage and switching frequency can be optimized to yield an enhanced mixing performance. The novel methods presented in this study provide a simple solution to mixing problems in the micro-total-analysis-systems field.     

透平膨胀机导流器的数值模拟

介绍了透平膨胀机直线圆弧叶型导流器数值模拟研究.运用标准k-ε湍流模型对几种不同叶片数的导流器进行了单个通道的计算流体力学(CFD)模拟.并对模拟结果进行了分析,得到了导流器的性能随着叶片数改变的变化规律,最终将模拟计算结果和试验数据进行了对比,得到了较为一致的结论,验证了理论模型的可靠性.对导流器的设计及叶片数的选择具有理论指导意义.     

Numerical simulation of the gas flow in a circuit-breaker

A computational methodology for the solution of unsteady two-dimensional/axisymmetric Euler equations within geometries with moving boundaries is presented. The flow simulation is carried out by applying a finite-volume method which makes use of a Lagrangian-Eulerian version of Roe's approximate Riemann solver. The domain discretization is handled via unstructured triangular grids. Grid adaptation is applied on the basis of geometric and physical requirements. The importance of the implicit treatment of the space conservation laws, based on geometric analysis, is evoked. The procedure for reconstructing Roe's method for moving meshes is described and validated. Finally, the ability of the method for the prediction of the transient flow in a circuit-breaker during its opening phase is illustrated.     

Numerical simulation of particle flow in a sand trap

Sand traps are used to measure Aeolian flux. Since they modify the surrounding wind velocity field their gauging represents an important challenge. We use numerical simulations under the assumption of homogeneous turbulence based on FLUENT to systematically study the flow field and trapping efficiency of one of the most common devices based on a hollow cylinder with two slits. In particular, we investigate the dependence on the wind speed, the Stokes number, the permeability of the membrane on the slit and the saltation height.     

环槽流量计的数值模拟与优化

研究流量计内部流场和结构优化,对改善流量计的测量性能和提高测量精度,具有重要的现实意义。将计算流体力学(CFD)仿真试验应用于一种新型差压流量计——环槽流量计,考查不同等效直径比β、前端和尾部长度、等直径段长度以及雷诺数对环槽流量计的流出系数和压力损失的影响。结果表明:随着雷诺数的增加,流出系数逐渐增大并达到稳定值;随着β增大,流出系数先增大后减小;前端及尾部长度对流出系数影响不大,但尾部长度越大,永久压损越小;等直径段长度越小,永久压损越小。根据结果拟合出环槽流量计流出系数的公式,CFD数值模拟作为一种辅助设计和标定手段,有助于指导环槽流量计的现场测试。     

热声系统内交变流动特性的数值模拟

对热声系统中交变流动的声场与流场分布进行了数值模拟,得到了热声谐振管和回热器(板叠)内的压力和速度分布及二者之间的相位关系.数值模拟表明:在谐振管内,在模拟的边界条件下,径向速度分布出现"环形效应",流动分为核心区的完全湍流流动和湍流边界层内的粘性流动;对于水力半径和粘性渗透层深度相当的热声板叠,其内速度分布逐步出现"环形效应",流动介于层流和湍流的过渡区,为过渡态流动,在流道的大部分区域压力和速度振荡的相位差趋向于π/2;对于水力半径和粘性渗透层深度之比较小的热声回热器,其内交变流动的流动特性与稳态流动的相似,速度的径向分布为抛物线形,类似于定常流动的层流速度分布.     

逆流填料式溶液除湿器性能的数值模拟

对溶液除湿器中传热传质过程进行热力学分析,根据除湿塔的结构及溶液与空气的流动方式,建立除湿器的热质交换物理和数学模型,模拟计算除湿器人口空气和溶液参数对除湿器出口空气参数的影响,得到各入口参数对出口空气温度和含湿量的影响曲线。结果表明：空气出口参数与空气人口含湿量、温度和流量、溶液人口温度和浓度几乎呈线性变化;当溶液入口流量达到2．5kg／s后,空气出口参数的变化趋于平缓。     

高层建筑周围定常风场的数值模拟

通过对高层建筑周围定常风场的数值模拟与原型试验结果的比较，分析了在进行数值模拟时，入口边界条件和湍流模型等因素对计算结果的影响。模拟结果表明，入口边界条件和湍流模型均对模拟结果具有较大影响。其中，入口边界条件中速度分布对计算结果的影响大于湍流强度分布对计算结果的影响，而RNGk-s湍流模型的计算精度优于标准κ-ε湍流模型。     

变压吸附系统气流分布器结构的数值模拟计算及分析

建立分布器结构的数学模型,并采用此模型对目前小型医用制氧中应用的基本型吸附柱内部的速度场分布进行了数值计算分析。计算结果表明：吸附柱入口物流分配存在严重的不均匀性,同时发现改变分布器的结构可以有效改善吸附柱入口的物流分配。在此基础上,对两种不同型分布器结构进行了数值模拟,结果表明改进效果十分显著,采用这种现代数值图型研究方法所得出的结论对吸附柱气体分布器结构优化设计具有一定的指导意义。     

Dynamic aspects of electroosmotic flow in a cylindrical microcapillary

Dynamic aspects of the electroosmotic flow (EOF) in a cylindrical capillary are analysed. An analytical solution for electrostatic potential of the double layer has been derived by solving the complete Poisson–Boltzmann equation for arbitrary zeta-potentials under an analytical scheme. Transient EOF field in response to the application of time dependent electric field is obtained analytically by using Green’s function method. Specifically, sinusoidally alternating (AC) electric fields are used to study the effect of frequency-dependent oscillation on the EOF. Limiting cases of zero frequency and pulsed electric field are also discussed.     

膜式燃气表内部流动数值模拟研究

采用CFD数值模拟方法对有柔性膜片的膜式燃气表内部流动进行数值模拟研究,解决了皮膜和旋转阀之间的联动关系以及旋转阀转动与皮膜运动的仿真实现等技术问题;运用滑移网格、动网格及用户自定义函数(UDF)技术实现了膜式燃气表内部流动的动态数值模拟,获得了旋转阀与阀座存在1mm间隙的情况下膜式燃气表内部流场和仪表压损数据.结果表明,数值模拟技术用于膜式燃气表这种复杂结构的内部流动研究是可行的.     

Numerical simulation of unsteady viscous flow around a circular cylinder

A finite difference solution is obtained for the time-dependent viscous incompressible 2-dimensional flow past a circular cylinder by direct integration of the Navier-Stokes equations expressed in a general curvilinear coordinate system. The solution describes the development of the vortex street developed behind the cylinder. Evolution of flow configuration is studied by means of streamlines, pressure contours, and vorticity contours for different Reynolds numbers. The time-dependent lift and drag coefficients are also obtained.     

Numerical simulation of multilayer deposition in an obstructed channel flow

Simulation of multilayer deposition of dry aerosol particles in turbulent flows has gained a growing interest in various industrial and research applications. The multilayer deposition of carbonaceous aerosol particles in a turbulent channel flow obstructed by a succession of square ribs is here numerically investigated. The multilayer particle bed growth on the various wall surfaces affects the air flow, which in turn affects the overall deposition rate. An iterative numerical procedure is therefore suggested to simulate the evolution of the graphite layer. The iterative process used to reproduce the layer build-up is decomposed as follows: Reynolds-Avergared Navier Stokes is employed to generate the flow field. The turbulent dispersion of the particles is reproduced through the use of a continuous random walk model. After statistically sufficient deposition of particulate matter, the layer build-up is computed using mechanics of dry granular material. The layer build-up model shows good agreement with data obtained from experimental tests carried out on-site.     

Numerical simulation of 2D granular flow entrainment using DEM

To understand the entrainment process in granular flow, numerical experiments have been conducted using a Discrete Element Method model. A flow channel of 8 m long with \(15^\circ \) slope is setup with monitoring points located in an erodible bed. Particles, ranging from 3 to 4 mm in diameters, are used in the simulations. In the simulations, translational, rotational and average velocities, total volume, shear stresses are calculated in the measurement circles. The sizes of the measurement circles have been varied to see their effects on the results. It is found the minimum size of the measurement circles should include 20–30 particles. An new analytical model has been developed to calculate entrainment in granular flow. Results of the numerical experiment are compared with analytical model. Shear stresses at the interface between flowing particles in motion and the immobile particles in the channel bed, change of depth of erosion and entrainment rate are used to verify the analytical model. It is found that the calculated shear stresses in the PFC model agree well with the shear stresses calculated using Mohr–Coulomb frictional relationship in the analytical model. The calculated depth of erosion using the new analytical model is also compared with that from dynamic and static entrainment model. The results indicates that the analytical model is able to capture the mechanism of erosion and it can be used in granular flow analysis.