An efficient block parallel AMR method for two phase interfacial flow simulations

The direct numerical simulation of two phase interfacial flows can be computationally challenging, as the strong resolution needed to follow the deformations of the interface leads to a lot of time spent solving the whole computation domain. Efficient solution of such problems requires an adaptive mesh refinement capability to concentrate computational effort where it is most needed. In this paper a parallel adaptive algorithm to solve incompressible two-phase flows with surface tension is presented: the AMR is handled with the help of the PARAMESH package. The free interface between fluids is tracked via Level Set approach; the jump conditions at the interface for pressure and velocity are imposed by the Ghost Fluid method. A multigrid preconditioned BiCG-stab solver adapted to the AMR data structure has been developed to allow high density ratio computations (up to 1:1000). Special treatment has been done at the refinement jumps to maintain the fine mesh accuracy. Computational results are compared in different test cases with analytical solutions or literature, and show very good agreement with the references. The effectiveness of PARAMESH parallelization has been quite well maintained, as shown in the strong and weak scaling tests. Speed-up capabilities of the AMR are demonstrated.     

Particle and thermohydraulic maldistribution of nanofluids in parallel microchannel systems

A deep understanding of fluidic maldistribution in microscale multichannel devices is necessary to achieve optimized flow and heat transfer characteristics. A detailed computational study has been performed using an Eulerian–Lagrangian twin-phase model to determine the concentration and thermohydraulic maldistributions of nanofluids in parallel microchannel systems. The study reveals that nanofluids cannot be treated as homogeneous single-phase fluids in such complex flow situations, and effective property models drastically fail to predict the performance parameters. To comprehend the distribution of the particulate phase, a novel concentration maldistribution factor has been proposed. It has been observed that the distribution of particles does not entirely follow the fluid flow pattern, leading to thermal performance that deviates from those predicted by homogeneous models. Particle maldistribution has been conclusively shown to be due to various migration and diffusive phenomena such as Stokesian drag, Brownian motion and thermophoretic drift. The implications of particle distribution on the cooling performance have been illustrated, and smart fluid effects (reduced magnitude of maximum temperature in critical zones) have been observed for nanofluids. A comprehensive mathematical model to predict the enhanced cooling performance in such flow geometries has been proposed. The article clearly highlights the effectiveness of discrete phase approach in modeling nanofluid thermohydraulics and sheds insight on the specialized behavior of nanofluids in complex flow domains.     

Numerical calculation of a laminar two dimensional straight cascade flow

The laminar and incompressible flow in a straight cascade is investigated. Numerical solutions of the full Navier-Stokes equations are obtained using the vorticity-stream function formulation and body fitted coordinate system. The numerical method includes a special force balance for the determination of the downstream boundary condition and a double sweep deferred correction which allows a second order accuracy but with the stability properties of an upwind first order scheme. Results for cylindrical, elliptical and NACA 0012 airfoils are presented including separated flow regions. Good agreement with experiments and previous computations is obtained.     

Simulation of carbon dioxide absorption process by aqueous monoethanolamine in a microchannel in annular flow pattern

The process of carbon dioxide absorption by aqueous monoethanolamine solvent was simulated in a microchannel in an annular flow pattern. This simulation has been carried out as a multiphase and three-dimensional process. The effects of different operating parameters such as temperature, superficial gas and liquid velocities, aspect ratio, and concentrations of solvent and solute have been investigated on the mass transfer flux and carbon dioxide conversion. The results of simulating mass transfer flux based on the calculated mass transfer coefficient were well consistent with the experimental data. The result of this study indicated that the mass transfer flux shall increase with the superficial gas and liquid velocities, temperature, concentration of solvent, and increment in the aspect ratio. It also revealed that increasing the concentration of solute would lead to an increase in the mass transfer flux and a decrease in the conversion.     

The impact of surface roughness on flow through a rectangular microchannel from the laminar to turbulent regimes

Modifications of fluid flow within microscale flow passages by internal surface roughness is investigated in the laminar, transitional, and turbulent regimes using pressure-drop measurements and instantaneous velocity fields acquired by microscopic particle-image velocimetry (micro-PIV). The microchannel under study is rectangular in cross-section with an aspect ratio of 1:2 (depth: width) and a hydraulic diameter of D_h = 600 \upmu m.D_{\rm h} =600\,\upmu \hbox{m}. Measurements are first performed under smooth-wall conditions to establish the baseline flow characteristics within the microchannel followed by measurements for two different rough-wall cases [with RMS roughness heights of 7.51 \upmu m7.51\,\upmu \hbox{m} (0.0125D _h) and 15.1 \upmu m15.1\,\upmu \hbox{m} (0.025D _h)]. The roughness patterns under consideration are unique in that they are reminiscent of surface irregularities one might encounter in practical microchannels due to imperfect fabrication methods. The pressure-drop results reveal the onset of transition above Re_cr=1,800Re_{\rm cr}=1{,}800 for the smooth-wall case, consistent with the onset of transition at the macroscale, along with deviation from laminar behavior at progressively lower Re with increasing roughness. Mean velocity profiles computed from the micro-PIV ensembles at various Re for each surface condition confirm these trends, meaning Re_crRe_{\rm cr} is a strong function of roughness. The ensembles of velocity fields at each Re and surface condition in the transitional regime are subdivided into fields embodying laminar behavior and fields containing disordered motions. This decomposition reveals a clear hastening of the flow toward a turbulent state due both to the roughness dependence of Re _cr and an enhancement in the growth rate of the non-laminar fraction of the flow when the flow is in the early stages of transition. Nevertheless, the range of Re relative to Re _cr over which the flow transitions from a laminar to a turbulent state is found to be essentially the same for all three surface conditions. From a structural viewpoint, instantaneous velocity fields embodying disordered behavior in the transitional regime are found to contain large-scale motions consistent with hairpin-vortex packets irrespective of surface condition. These observations are in accordance with the characteristics of transitional and turbulent flows at the macroscale and therefore indicate that the overall structural paradigm of the flow is relatively insensitive to roughness. From a quantitative viewpoint, however, the intensity of both the velocity fluctuations and structural activity appear to increase substantially with increasing roughness, particularly in the latter stages of transition. These differences are further supported by the trends of single-point statistics of the non-laminar ensembles and quadrant analysis in which an intensification of the velocity fluctuations by surface roughness is noted in the region close to the wall, particularly for the wall-normal fluctuations.     

A parallel algorithm for molecular dynamics simulation of branched molecules

To get an insight into the effects of molecular architecture in the behaviour of thin lubricant films we have devised an algorithm for simulation of branched molecules. We have used this algorithm successfully to simulate branched isomers of C₃₀. However the algorithm is flexible enough to be used for the simulation of more complex branched molecules. The resulting algorithm can be used in molecular dynamics simulation of branched molecules and could be helpful in designing new materials at the molecular level.     

Timing of traffic lights and phase separation in two-dimensional traffic flow

In this paper, we study the effects of traffic light period in two-dimensional Biham-Middleton-Levine (BML) traffic flow model. It is found that a phase separation phenomenon, in which the system separates into coexistence of free flow and jam, could be observed in intermediate vehicle density range when traffic light period T?4. We have explained the reason of occurrence of phase separation and investigated its behavior in different traffic light period.     

Electrokinetic transport and separation of droplets in a microchannel

This work presents theoretical, numerical and experimental investigations of electrokinetic transport and separation of droplets in a microchannel. A theoretical model is used to predict that, in case of micron-sized droplets transported by electro-osmotic flow, the drag force is dominant as compared to the dielectrophoretic force. Numerical simulations were performed to capture the transient electrokinetic motion of the droplets using a two-dimensional multi-physics model. The numerical model employs Navier–Stokes equations for the fluid flow and Laplace equation for the electric potential in an Arbitrary Lagrangian–Eulerian framework. A microfluidic chip was fabricated using micromilling followed by solvent-assisted bonding. Experiments were performed with oil-in-water droplets produced using a cross-junction structure and applying electric fields using two cylindrical electrodes located at both ends of a straight microchannel. Droplets of different sizes were produced by controlling the relative flow rates of the discrete and continuous phases and separated along the channel due to the competition between the hydrodynamic and electrical forces. The numerical predictions of the particle transport are in quantitative agreement with the experimental results. The work reported here can be useful for separation and probing of individual biological cells for lab-on-chip applications.     

Real-time parallel computation of disparity and optical flow using phase difference

A phase-difference-based algorithm for disparity and optical flow estimation is implemented on a TI-C40-based parallel DSP system. The module performs real-time computation of disparity maps on images of size 128 × 128 pixels and computation of optical flows on images of size 64 × 64 pixels. This paper describes the algorithm and its parallel implementation. Processing times required for the computation of disparity maps and velocity fields and measures of the algorithm's performance are reported in detail.     

Investigation of temperature-induced flow stratification and spiral flow in T-shaped microchannel

In this study, the heat and fluid flows were investigated when fluids at varying temperatures are mixed in a T-shaped microchannel. A temperature gradient was formed using two Peltier modules in the junction of a T-shaped microchannel, and the velocity fields were measured using microparticle image velocimetry (µ-PIV). Measurements were obtained at five planes in the direction of the depth by changing the focal position of the objective lens. Under the operation of the Peltier modules, the flow velocity on the heated side was increased and the velocity on the cooled side was decreased in the upper area in the vertical direction. Furthermore, in the lower area, the flow velocity on the cooled side was increased and the velocity on the heated side was decreased. The velocity difference between the two inlets depends on the applied power of the Peltier modules. We also investigated the mixing behavior downstream of the channel, and a strong spiral flow was clearly observed. The spiral flow should enlarge the contact interface area, and it should be useful for application to a micromixer. We first observed the fluid stratification induced by the temperature in the microchannel. This phenomenon is useful for application in microfluidic devices as a contactless micromixer.     

Measurement and modeling of pulsatile flow in microchannel

An experimental study of pulsatile flow in microchannel is reported in this paper. Such a study is important because time-varying flows are frequently encountered in microdevices. The hydraulic diameter of the microchannel is 144 μm and deionized water is the working fluid. The pressure drop across the microchannel as a function of time is recorded, from which the average and r.m.s. pressure drops are obtained. The experiments have been performed in the quasi-steady flow regime for a wide range of flow rate, frequency of pulsations, and duty cycle. The results suggest that the pressure with pulsations lies between the minimum and maximum steady state pressure values. The average pressure drop with pulsation is approximately linear with respect to the flow rate. The theoretical expression for pressure has also been derived wherever possible and the experimental data is found to lie below the corresponding theoretical values. The difference with respect to the theoretical value increases with an increase in frequency and a decrease in flow rate, with a maximum difference of 32.7%. This is attributed to the small size of the microchannel. An increase in frequency of square waveform leads to a larger reduction in pressure drop as compared to rectangular waveform, irrespective of the duty cycle. The results can be interpreted with the help of a first-order model proposed here; the model results are found to compare well against the experimental results. A correlation for friction factor in terms of the other non-dimensional governing parameters is also proposed. Experimental study of mass-driven pulsatile flow in microchannel is being conducted for the first time at these scales and the results are of both fundamental and practical importance.     

Simulation of gas separation effect in microchannel with moving walls

Numerical simulation of a free-molecular gas flow through plane microchannel with walls performing forced curving motion according to sine law is presented. It is shown that the probability of gas molecules to pass through the channel significantly depends on relation between wave speed of walls harmonic oscillations and characteristic thermal speed of gas molecules. It is then shown how this effect can be utilized for gas separation, and the comprehensive study of the influence of the main parameters (channel width and length, wave amplitude and length, etc.) on the magnitude of effect is performed.     

一种测量气--液两相流的线列阵传感器设计

针对气—液两相流研究对含气率测量的需求,基于线列阵测量技术原理,设计了一种可移动式线列阵两相流测量传感器,该传感器具有较高的空间分辨率（3 mm）和极高的时间分辨率（2500 Hz）,设计了线列阵传感器标定和含气率算法,实现了瞬时二维局部含气率的测量。经过射流冲击试验验证表明：该线列阵传感器结构稳定,基于原始测量数据,采用标定和含气率求解算法,可计算气泡夹带现象在水平截面的平均含气率分布情况。     

Process planning optimization for parallel drilling of blind holes using a two phase genetic algorithm

Determining the optimal process parameters and machining sequence is essential in machining process planning since they significantly affect the cost, productivity, and quality of machining operations. Process planning optimization has been widely investigated in single-tool machining operations. However, for the research reported in process planning optimization of machining operations using multiple tools simultaneously, the literature is scarce. In this paper, a novel two phase genetic algorithm (GA) is proposed to optimize, in terms of minimum completion time, the process parameters and machining sequence for two-tool parallel drilling operations with multiple blind holes distributed in a pair of parallel faces and in multiple pairs of parallel faces. In the first phase, a GA is used to determine the process parameters (i.e., drill feed and spindle speed) and machining time for each hole subject to feed, spindle speed, thrust force, torque, power, and tool life constraints. The minimum machining time is the optimization criterion. In the second phase, the GA is used to determine the machining sequence subject to hole position constraints (i.e., the distribution of the hole locations on each face is fixed). The minimum operation completion time is the optimization criterion in this phase. Simulation results are presented to demonstrate the effectiveness of the proposed algorithm in solving the process planning optimization problem for parallel drilling of blind holes on multiple parallel faces. In order to evaluate the performance of proposed algorithm, the simulation results are compared to a methodology that utilizes the exhaustive method in the first phase and a sorting algorithm.     

A collocation finite element-finite difference solution for developing isothermal flow between two parallel plates

This paper presents a finite element-finite difference method for the solution of the boundary layer equations for developing flow between two parallel plates. Due to the parabolic nature of the equations it was possible to discretize the transverse flow direction with one-dimensional Hermite cubic finite elements and the axial flow direction with a backward finite difference approximation. The collocation finite element-finite difference approximation was found to be appropriate for the modeling of the non-linear convection terms in the axial momentum equation. The resulting system of mixed linear and non-linear algebraic equations was solved using the Newton-Raphson method. Several numerical experiments were conducted to study the behavior of the solution with respect to the element size and number, order of finite difference approximation, and the marching step size.     

Generating high-pressure sub-microliter flow rate in packed microchannel by electroosmotic force: potential application in microfluidic systems

We have developed a liquid delivery pump, known as an electroosmotic pump (EOP), based on the electrically induced osmosis principle, which is mainly made of one or several microchannels packed with porous fine dielectric material and connected in parallel. The EOP is tested with methanol, phosphate sodium buffer and their mixture, which can generate pressures from 0.1 to 15 MPa and flow rates of tens of nanoliters per minute to several microliters per minute. Constant and pulsation-free flow from the EOP adapts well to microfluidic systems.     

Scheduling problems in parallel systems for telecommunications

The scheduling of the large number of tasks with various cooperation modes poses interesting theoretical problems in parallel systems used in telecommunications. Such systems are fully distributed ones, preventing coherent observability. In such systems no absolute time-space reference exists. The theoretically optimal set of local references is given as the limiting factor for the incoherent observability. The failures and/or modifications inherent in the massively parallel systems may change their configurations dynamically. Thus no optimal algorithms can be developed for the effective utilization of the resources available in such a system. The influence of the granularity level and of the cooperation modes of the processes on scheduling are investigated in order to identify areas for further study.     

A two phase optimization method for Petri net models of manufacturing systems

Optimization is a key issue in the design of large manufacturing systems. An adequate modeling formalism to express the intricate interleaving of competition and cooperation relationships is needed first. Moreover, robust and efficient optimization techniques are necessary. This paper presents an integrated tool for the automated optimization of DEDS, with application to manufacturing systems. After a very quick overview of optimization problems in manufacturing systems, it presents the integration of two existing tools for the modeling and evaluation with Petri nets and a general-purpose optimization package based on simulated annealing. The consideration of a cache and a two phase technique for optimization allows to speed-up the optimization by a factor of about 35. During the first preoptimization phase, a rough approximation of the optimal parameter set is computed based on performance bounds. Two application examples show the benefits of the proposed technique.     

PIV measurements of flow within plugs in a microchannel

The use of two-phase flow in lab-on-chip devices, where chemical and biological reagents are enclosed within plugs separated from each other by an immiscible fluid, offers significant advantages for the development of devices with high throughput of individual heterogeneous samples. Lab-on-chip devices designed to perform the polymerase chain reaction (PCR) are a prime example of such developments. The internal circulation within the plugs used to transport the reagents affects the efficiency of the chemical reaction within the plug, due to the degree of mixing induced on the reagents by the flow regime. It has been hypothesised in the literature that all plug flows produce internal circulation. This work demonstrates experimentally that this is false. The particle image velocimetry (PIV) technique offers a powerful non-intrusive tool to study such flow fields. This paper presents micro-PIV experiments carried out to study the internal circulation of aqueous plugs in two phase flow within 762 μm internal diameter FEP Teflon tubing with FC-40 as the segmenting fluid. Experiments have been performed and the results are presented for plugs ranging in length from 1 to 13 mm with a bulk mean flow velocity ranging from 0.3 to 50 mm/s. The results demonstrate for the first time that circulation within the plugs is not always present and requires fluidic design considerations to ensure their generation.