Molecular dynamics–continuum hybrid simulation for condensation of gas flow in a microchannel

A molecular dynamics-continuum coupling method combining fluid flow and heat transfer is developed to study the condensation process of gas flow in a microchannel. The computational domain is decomposed into particle (P), continuum (C) and overlap (O) regions with solving approaches of molecular dynamics simulation, finite volume method and the developed coupling method, respectively. Continuities of momentum and energy in O region are ensured by constraint dynamics and the Langevin method. The validity of the developed method is confirmed by a good agreement between hybrid results and analytical solutions from two cases including the unsteady dynamical and thermal problems. For the condensation process of gas flow, the hybrid transient velocity and temperature fields indicate that the process does not progress smoothly but wavily with noticeable fluctuation, leading to oscillation in temperature field and recirculation flow in velocity field. Analysis based on heat and mass transfer is carried out in P region, and the Kapitza resistance and the thermal conductivity in liquid are obtained with the satisfying agreement with experimental data, which shows the availability of the developed model for the investigation on the thermal boundary resistance. The good performance had demonstrated that the developed coupling method and computational model are available to provide a multiscale overview in dynamical and thermal problems including phase-transition from nanoscale to microscale, which will show significantly potential in micro fluidics and thermal engineering.     

Modeling of micro/nano particle separation in microchannels with field-flow fractionation

The cyclical electrical field-flow fractionation (CyElFFF) is a very promising separation technique for particles and biological molecules such as proteins, nucleic acids, viruses, bacteria, yeast cells, mammalian cells. But a clear understanding of the mechanism and performance prediction of this system under different operating parameters is far from completed. This research focuses on a computational investigation of particle behavior in a CyElFFF system by taking into account both electrokinetic effects and particle dynamics. The model was validated with both theory and experimental results. The effects of key parameters such as applied electric field strength and frequency, solution fluid flow rate, particle size, particle shape on separation process are addressed in a systematic way. The developed model can also be utilized in studying the behavior of spherical or non-spherical particles (such as nanowire, nanorod, and nanofiber) in other microfluidic systems.     

Computational and performance analysis of a continuous magnetophoretic bioseparation chip with alternating magnetic fields

This paper presents the modeling and optimization of a magnetophoretic bioseparation chip for isolating cells, such as circulating tumor cells from the peripheral blood. The chip consists of a continuous-flow microfluidic platform that contains locally engineered magnetic field gradients. The high-gradient magnetic field produced by the magnets is spatially non-uniform and gives rise to an attractive force on magnetic particles flowing through a fluidic channel. Simulations of the particle–fluid transport and the magnetic force are performed to predict the trajectories and capture lengths of the particles within the fluidic channel. The computational model takes into account key forces, such as the magnetic and fluidic forces and their effect on design parameters for an effective separation. The results show that the microfluidic device has the capability of separating various cells from their native environment. An experimental study is also conducted to verify and validate the simulation results. Finally, to improve the performance of the separation device, a parametric study is performed to investigate the effects of the magnetic bead size, cell size, number of beads per cell, and flow rate on the cell separation performance.     

Computational modelling of titanium particles in warm spray

A warm spray system has been computationally investigated by introducing a centrally located mixing chamber into a HVOF thermal spray gun. The effects of injecting a cooling gas on the gas and particle dynamics are examined. The gas phase model incorporates liquid fuel droplets which heat, evaporate and then exothermically combust with oxygen within the combustion chamber producing a realistic compressible, supersonic and turbulent jet. The titanium powder is tracked using the Lagrangian approach including particle heating, melting and solidification. The results present an insight into the complex interrelations between the gas and particle phases, and highlight the advantage of warm spray, especially for the deposition of oxygen sensitive materials such as titanium. This work also demonstrates the effectiveness of a computational approach in aiding the development of thermal spray devices.     

Numerical Methods for Two-Dimensional Stem Cell Tissue Growth

Growth of developing and regenerative biological tissues of different cell types is usually driven by stem cells and their local environment. Here, we present a computational framework for continuum tissue growth models consisting of stem cells, cell lineages, and diffusive molecules that regulate proliferation and differentiation through feedback. To deal with the moving boundaries of the models in both open geometries and closed geometries (through polar coordinates) in two dimensions, we transform the dynamic domains and governing equations to fixed domains, followed by solving for the transformation functions to track the interface explicitly. Clustering grid points in local regions for better efficiency and accuracy can be achieved by appropriate choices of the transformation. The equations resulting from the incompressibility of the tissue is approximated by high-order finite difference schemes and is solved using the multigrid algorithms. The numerical tests demonstrate an overall spatiotemporal second-order accuracy of the methods and their capability in capturing large deformations of the tissue boundaries. The methods are applied to two biological systems: stratified epithelia for studying the effects of two different types of stem cell niches and the scaling of a morphogen gradient with the size of the Drosophila imaginal wing disc during growth. Direct simulations of both systems suggest that that the computational framework is robust and accurate, and it can incorporate various biological processes critical to stem cell dynamics and tissue growth.     

Computational simulation of gas flow and heat transfer near an immersed object in fluidized beds

To improve the understanding of the heat transfer mechanism and to find a reliable and simple heat-transfer model, the gas flow and heat transfer between fluidized beds and the surfaces of an immersed object is numerically simulated based on a double particle-layer and porous medium model. The velocity field and temperature distribution of the gas and particles are analysed during the heat transfer process. The simulation shows that the change of gas velocity with the distance from immersed surface is consistent with the variation of bed voidage, and is used to validate approximately dimensional analysing result that the gas velocity between immersed surface and particles is 4.6U_mf/ε_mf. The effects of particle size and particle residence time on the thermal penetration depth and the heat-transfer coefficients are also discussed.     

一种真实感雪场景模拟方法

探讨已有降雪和积雪模拟方法,针对粒子系统难以同时兼得真实性与实时性的问题,提出一种真实感较强的雪场景实时绘制算法。算法用矩形基本粒子进行纹理叠加映射来建模雪粒子,用温度调节控制雪粒子大小及雪密度大小,归一化生命周期并用于颜色混合。飘雪阶段,引入层次细节(LOD)技术,根据雪粒子真实受力特点模仿其运动过程,并将其受力全面综合考虑并简化,提高降雪模拟效果真实度,并降低计算复杂度。积雪阶段,获取场景模型暴露面及其高度场,以暴露面中点的高度及其点坐标为基础,得到雪粒子落地位置,进而模拟积雪高度变化效果;将整个地面作为地面雪粒子发射器,模拟积雪融化效果,提高真实性。     

Smoothed particle hydrodynamics: Applications to heat conduction

In this paper, we modify the numerical steps involved in a smoothed particle hydrodynamics (SPH) simulation. Specifically, the second order partial differential equation (PDE) is decomposed into two first order PDEs. Using the ghost particle method, consistent estimation of near-boundary corrections for system variables is also accomplished. Here, we focus on SPH equations for heat conduction to verify our numerical scheme. Each particle carries a physical entity (here, this entity is temperature) and transfers it to neighboring particles, thus exhibiting the mesh-less nature of the SPH framework, which is potentially applicable to complex geometries and nanoscale heat transfer. We demonstrate here only 1D and 2D simulations because 3D codes are as simple to generate as 1D codes in the SPH framework. Our methodology can be extended to systems where the governing equations are described by PDEs.     

Effects of temperature on particle trajectories inside hard disk drives

The presence of particles, which can intrude into the gas bearing, is one of the most common factors in the failure of hard disk drives (HDDs). Previous works investigated particle trajectories inside air-filled drives without considering temperature effects on the distribution of particles. Actually, especially for the submicron particle, particle trajectories and trapping status are affected by the temperature gradient since the thermophoretic force cannot be ignored. In this paper, considering major heat generation components such as the spindle motor and voice coil motor (VCM), trajectories and trapping status for Al₂O₃ particles inside a 2.5 inch helium-filled drive are simulated by the commercial computational fluid dynamics solver FLUENT with user-defined functions (UDFs). The trapping criterion for Al₂O₃ particles is used as boundary conditions for different colliding surfaces. The results reveal that particles in the air-filled drive will more likely degrade the head–disk interface (HDI) reliability. In addition, after considering the temperature, the particle trapping rate by the disk decreases both inside the air-filled drive and the helium-filled drive. And its reduction inside the air-filled drive is larger. Moreover, small particles will more likely degrade the HDI reliability since they can follow the rotatory flow well and have more chance to collide with the disk surface, and then easily attach onto the disk surface.

     

Development of the meshless finite volume particle method with exact and efficient calculation of interparticle area

The Finite Volume Particle Method (FVPM) is a meshless method based on a definition of interparticle area which is closely analogous to cell face area in the classical finite volume method. In previous work, the interparticle area has been computed by numerical integration, which is a source of error and is extremely expensive. We show that if the particle weight or kernel function is defined as a discontinuous top-hat function, the particle interaction vectors may be evaluated exactly and efficiently. The new formulation reduces overall computational time by a factor between 6.4 and 8.2. In numerical experiments on a viscous flow with an analytical solution, the method converges under all conditions. Significantly, in contrast with standard FVPM and SPH, error depends on particle size but not on particle overlap (as long as the computational domain is completely covered by particles). The new method is shown to be superior to standard FVPM for shock tube flow and inviscid steady transonic flow. In benchmarking on a viscous multiphase flow application, FVPM with exact interparticle area is shown to be competitive with a mesh-based volume-of-fluid solver in terms of computational time required to resolve the structure of an interface.     

Micropositioning and microscopic observation of individual picoliter-sized containers within SU-8 microchannels

We describe the fabrication and application of a bioanalytical chip, made of SU-8 photoresist, comprising integrated, high aspect-ratio microfluidic channels, suitable to manipulate and investigate vesicles, cell fragments and biological cells. A central micrometer-sized aperture allows electrical particle counting and planar membrane experiments, microfluids allow (sub)micrometer-sized objects to be transported and addressed with different chemicals. Here we show how lipid vesicles are positioned with micrometer precision within the micro-channels by means of pressure and electrophoretic movement. Our approach is suited for controlling and investigating (bio)chemical synthesis and cellular signalling processes in ultrasmall individual vesicles by electro-optical techniques.     

Continuous-flow particle and cell separations in a serpentine microchannel via curvature-induced dielectrophoresis

Particle and cell separations are critical to chemical and biomedical analyses. This study demonstrates a continuous-flow electrokinetic separation of particles and cells in a serpentine microchannel through curvature-induced dielectrophoresis. The separation arises from the particle size-dependent cross-stream dielectrophoretic deflection that is generated by the inherent electric field gradients within channel turns. Through the use of a sheath flow to focus the particle mixture, we implement a continuous separation of 1 and 5 μm polystyrene particles in a serpentine microchannel under a 15 kV/m DC electric field. The effects of the applied DC voltages and the serpentine length on the separation performance are examined. The same channel is also demonstrated to separate yeast cells (range in diameter between 4 and 8 μm) from 3 μm particles under an electric field as low as 10 kV/m. The observed focusing and separation processes for particles and cells in the serpentine microchannel are reasonably predicted by a numerical model.     

GPU-Assisted High Quality Particle Rendering

Visualizing dynamic participating media in particle form by fully solving equations from the light transport theory is a computationally very expensive process. In this paper, we present a computational pipeline for particle volume rendering that is easily accelerated by the current GPU. To fully harness its massively parallel computing power, we transform input particles into a volumetric density field using a GPU-assisted, adaptive density estimation technique that iteratively adapts the smoothing length for local grid cells. Then, the volume data is visualized efficiently based on the volume photon mapping method where our GPU techniques further improve the rendering quality offered by previous implementations while performing rendering computation in acceptable time. It is demonstrated that high quality volume renderings can be easily produced from large particle datasets in time frames of a few seconds to less than a minute.     

Simulations of subsonic vortex-shedding flow past a 2D vertical plate in the near-continuum regime by the parallelized DSMC code

A general-purpose Parallel Direct Simulation Monte Carlo Code, named PDSC, is used to simulate near-continuum subsonic flow past a 2D vertical plate for studying the vortex-shedding phenomena. An unsteady time-averaging sampling method and a post-processing procedure called DREAM (DSMC Rapid Ensemble Averaging Method) have also been implemented, reducing the overall computational expense and improving the sampling quality of time-dependent flow problems in the rarefied flow regime. Parametric studies, including the temporal variable time step (TVTS) factor, the number of particles per cell, the domain size, and the Reynolds number, have been conducted, obtaining the Strouhal number and various aerodynamic coefficients of the flow. Results are compared to experimental data in the continuum regime available in the literature, demonstrating the capacity of PDSC and DREAM to simulate near-continuum vortex-shedding problems within acceptable computational time.     

A microfluidic device for thermal particle detection

We demonstrate the use of heat to count microscopic particles. A thermal particle detector (TPD) was fabricated by combining a 500-nm-thick silicon nitride membrane containing a thin-film resistive temperature detector with a silicone elastomer microchannel. Particles with diameters of 90 and 200 μm created relative temperature changes of 0.11 and ?0.44 K, respectively, as they flowed by the sensor. A first-order lumped thermal model was developed to predict the temperature changes. Multiple particles were counted in series to demonstrate the utility of the TPD as a particle counter.     

Blood cell transport and aggregation using discrete ellipsoidal particles

A computational model is presented for efficient mesoscale simulation of the transport, collision and aggregation of blood cells, which can be applied to examine red blood cells (RBCs), leukocytes, or platelets in various types of blood flows in which the fluid length scale is substantially larger than the particle length scale. This method is intended to be intermediate between microscale models, which examine deformation and flow around a small number of individual blood cells, and more phenomenological continuum models. The computational model utilizes a particle approximation for the blood cells and introduces other physically-justifiable approximations in order to accommodate computations with large numbers of cells. For instance, the non-spherical RBC and platelet shape is incorporated into the model by use of ellipsoidal particles. A novel method based on particle level-surfaces is presented for rapid identification of particle collision. It is shown that receptor–ligand binding between the cells can be modeled under certain conditions using a formulation that is mathematically similar to van der Waals adhesion of particles, but in which the surface energy density is variable in time. The method is demonstrated to provide computations of the interaction and adhesion of over 13,000 red-blood-cell particles on an ordinary workstation. These computations exhibit formation of chain-like rouleaux aggregates, modification of rouleaux structure due to shear flow, and capture and/or breakup of colliding rouleaux. The model predictions are examined for rouleaux size distribution in channel flow in comparison to experimental data, as well as for the effect of RBC aggregation on margination of white blood cells and platelets in channel flows.     

Testing trivializing maps in the Hybrid Monte Carlo algorithm

We test a recent proposal to use approximate trivializing maps in a field theory to speed up Hybrid Monte Carlo simulations. Simulating the CPN−1 model, we find a small improvement with the leading order transformation, which is however compensated by the additional computational overhead. The scaling of the algorithm towards the continuum is not changed. In particular, the effect of the topological modes on the autocorrelation times is studied.     

On reduction of computational cost of imitation Monte Carlo algorithms for modeling rarefied gas flows

This article describes Monte Carlo methods and algorithms for the Boltzmann equation for rarefied gases problems in the case of large-scale flow areas. We consider imitation or continuous-time Monte Carlo methods where frequencies of interactions of pairs of particles depend on the difference of the coordinates of particles. The question about reduction of computational costs of algorithms is examined using the specificity of the problem. First, algorithms of an approximated method are constructed, analyzed, and implemented. This method is obtained by using splitting (over groups of particles) of the operator in master equations system. Second, we investigate the fictitious collisions technique, where the upper bound for the number of interacting pairs is specified. The plane Poiseuille flow (in the field of external forces) problem, the heat transfer problem, and the temperature discontinuity propagation problem are numerically solved using the developed algorithms. Asymptotical estimates of the computational costs are confirmed with the data of the computational processes and the comparative properties of the later are fixed. The suggested algorithms of the method with splitting allow parallelization of a certain type.     

A method for particle location and field interpolation on complex, three-dimensional computational meshes

A method for locating particles within arbitrary three-dimensional computational meshes is described. It is based on an iterative procedure which uses transformed coordinates defined by iso-parametric functions. The method also enables one to interpolate field values from the mesh nodes to the particle position. Example applications demonstrate how effective the method is. For very distorted computational cells special practices have to be introduced in order to keep the number of iterations to a minimum.     

Efficient computation of micro-particle dynamics including wall effects

This study describes an effective method for one-way coupled Eulerian-Lagrangian simulations of spherical micro-size particles, including particle-wall interactions and the quantification of near-wall stasis at possibly elevated concentrations. The focus is on particle-hemodynamics simulations where particle suspensions are composed of critical blood cells, such as monocytes, and the carrier fluid is non-Newtonian. Issues regarding adaptive time-step integration of the particle motion equation, relevant point-force model terms, and adaptation of surface-induced particle forces to arbitrary three-dimensional geometries are outlined. By comparison to available experimental trajectories, it is shown that fluid-element pathlines may be used to simulate non-interacting blood particles removed from wall boundaries under dilute transient conditions. However, when particle-wall interactions are significant, an extended form of the particle trajectory equation is required which includes terms for Stokes drag, near-wall drag modifications, or lubrication forces, pressure gradients, and near-wall particle lift. Still, additional physical and/or biochemical wall forces in the nano-meter range cannot be readily calculated; hence the near-wall residence time (NWRT) model indicating the probability of blood particle deposition is presented. The theory is applied to a virtual model of a femoral bypass end-to-side anastomosis, where profiles of the Lagrangian-based NWRT parameter are illustrated and convergence is verified. In order to effectively compute the large number of particle trajectories required to resolve regions of particle stasis, the proposed particle tracking algorithm stores all transient velocity field solution data on a shared memory architecture (SGI Origin 2400) and computes particle trajectories using an adaptive parallel approach. Compared to commercially available particle tracking packages, the algorithm presented is capable of reducing computational time by an order of magnitude for typical transient one-way coupled blood particle simulations in complex cyclical flow domains.