Direct position determination in the presence of model errors—known waveforms

The performance of the direct position determination (DPD) approach in the presence of model errors is examined. DPD was recently introduced as a promising technique for localization of multiple radio frequency emitters with superior accuracy under low signal-to-noise ratio conditions. We analyze the performance of DPD in the presence of model errors caused by multipath, calibration errors, mutual coupling, etc. The analysis is general enough to encapsulate various sources of errors. Monte Carlo simulations are used to validate the analysis. We show that in many cases of interest DPD should be selected as the preferred method of localization.     

Physically based wall boundary condition for dissipative particle dynamics

In this paper, we present a novel wall boundary condition model, which stands just on the physical facts, for the dissipative particle dynamics (DPD) method. After the validation of this model by means of the common benchmarks such as the Couette and the Poiseuille flows, we study the effects of this model on the diffusion coefficient in a wide variety of different coarse-graining levels. The obtained results show that the proposed model preserves the thermodynamics of the system, also eliminates the spurious effects of the wall, and consequently is able to preserve the accurate structural characteristics of the working DPD fluid in the wall’s vicinity. We also study the fluid flow through a channel with the polymer-coated walls. A comprehensive investigation into the wall–solvent–polymer interactions is presented for the solvents with different qualities. Since the working DPD fluid’s structure is heterogeneous, the working fluid’s structure, its density variations and the force field that is experienced by the working DPD fluid particles near the wall cannot be predicted before the simulation. Hence, all of these data have to be determined systematically during the simulation. We show that the force field experienced by the particles near the wall depends substantially upon the solvent quality. We also show that the force fields experienced by the particles from different types (solvent/polymer) in the wall’s vicinity are significantly different from each other except in the athermal solvent.     

A new algorithm for simulating flows of conducting fluids in the presence of electric fields

We propose an algorithm based on dissipative particle dynamics (DPD) for simulations of conducting fluids in the presence of an electric field. In this model, the electrostatic equations are solved in each DPD time step to determine the charge density at the fluid surfaces. These surface charges are distributed on a thin layer of fluid particles near the interface, and the corresponding interfacial electric forces are added to other DPD forces. The algorithm is applied to the electrospinning process at the Taylor cone formation stage. It is shown that, when the applied voltage is sufficiently high, the algorithm captures the formation of a Taylor cone with analytical apex angle 98.6°. Our results demonstrate the potential of the presented DPD algorithm for simulating two-phase problems in the presence of an electric field with non-periodic boundary conditions.     

Migration of DNA molecules through entropic trap arrays: a dissipative particle dynamics study

Dissipative particle dynamics (DPD) simulations of worm-like chain bead-spring models are used to explore the electrophoresis migration of DNA molecules traveling through narrow constrictions. The DPD is a relatively new numerical approach that is able to fully incorporate hydrodynamic interactions. Two mechanisms are identified that cause the size-dependent trapping of DNA chains and thus mobility differences. First, small molecules are found to be trapped in the deep region due to higher Brownian mobility and crossing of electric field lines. Second longer chains have higher probability to form hernias at the entrance of the gap and can pass the entropic barrier more easily. Consequently, longer DNA molecules have higher mobility and travel faster than shorter chains. The present DPD simulations show good agreement with existing experimental data as well as published numerical data.     

Parallel implementation of isothermal and isoenergetic Dissipative Particle Dynamics using Shardlow-like splitting algorithms

A parallel implementation of the Shardlow splitting algorithm (SSA) for Dissipative Particle Dynamics (DPD) simulations is presented. The isothermal and isoenergetic SSA implementations are compared to the DPD version of the velocity-Verlet integrator in terms of numerical stability and performance. The integrator stability is assessed by monitoring temperature, pressure and total energy for both the standard and ideal DPD fluid models. The SSA requires special consideration due to its recursive nature resulting in more inter-processor communication as compared to traditional DPD integrators. Nevertheless, this work demonstrates that the SSA exhibits stability over longer time steps that justify its regular use in parallel, multi-core applications. For the computer architecture used in this study, a factor of 10–100 speedup is achieved in the overall time-to-solution for isoenergetic DPD simulations and a 15–34 speedup is achieved for the isothermal DPD simulations.     

A numerical investigation of the stability of expanding liquid sheets and the influence of boundary conditions

Incompressible flow solutions are found numerically for a radially expanding liquid sheet in order to confirm analytical results for inviscid flow and to investigate viscous and non-linear effects. An hp-finite element method is used to perform the numerical simulations. In our unsteady simulations, we observe that forced sinuous pulses cause two different speed waves to travel downstream for Weber numbers greater than one. We also witness wave deceleration for Weber numbers approaching one, confirming the predictions of inviscid linear stability analysis. Comparisons are also made to theoretical predictions of the radius where the sheet becomes unstable. To determine the critical radius, the inlet Weber number is reduced until the theoretical critical radius is within the simulated domain. Surprisingly, instead of leading to breakup, this causes the sheet to change from a stable symmetric shape to a stable asymmetric shape. The transition between these shapes occurs by both supercritical and subcritical bifurcations when the Weber number based on the sheet thickness approaches one, in agreement with the theoretical work of Taylor. The absence of breakup in our simulations appears to be a direct result of allowing the interface to span the entire domain. To verify this, we examine the dependence of the solutions on domain aspect ratio, shape, and exit boundary conditions. No boundary conditions are found that allows the sheet to break-up.     

Coherent summation of multiple short-time signals for direct positioning of a wideband source based on delay and Doppler

We consider identifying the source position directly from the received source signals. This direct position determination (DPD) approach has been shown to be superior in terms of better estimation accuracy and improved robustness to low signal-to-noise ratios (SNRs) to the conventional two-step localization technique, where signal measurements are extracted first and the source position is then estimated from them. The localization of a wideband source such as a communication transmitter or a radar whose signal should be considered deterministic is investigated in this paper. Both passive and active localization scenarios, which correspond to the source signal waveform being unknown and being known respectively, are studied. In both cases, the source signal received at each receiver is partitioned into multiple non-overlapping short-time signal segments for the DPD task. This paper proposes the use of coherent summation that takes into account the coherency among the short-time signals received at the same receiver. The study begins with deriving the Cramér–Rao lower bounds (CRLBs) of the source position under coherent summation-based and non-coherent summation-based DPDs. Interestingly, we show analytically that with coherent summation, the localization accuracy of the DPD improves as the time interval between two short-time signals increases. This paper also develops approximate maximum likelihood (ML) estimators for DPDs with coherent and non-coherent summations. The CRLB results and the performance of the proposed source position estimators are illustrated via simulations.     

Temporal analysis of power law liquid jets

In this paper we investigate the breakup mechanisms of power law liquid jets. The viscosity of the liquid is represented the Carreau-Yasuda model, and the surface tension of the liquid jet has a variation (gradient) along the jet axial direction. The surface tension gradient may be introduced by the thermal disturbance of the jet surface as it comes of out an orifice. The Carreau-Yasuda fluid has a power law viscosity bounded by two plateaus, the higher plateau at zero strain rate, μ₀, and the lower plateau at the infinite strain rate, μ_∞. The governing equation for the surface profile of the liquid jet is derived in the forms of a partial differential equation (PDE), as well as an ordinary differential equation (ODE). The PDE and ODE are solved for various cases of Carreau-Yasuda fluid to study the effect of fluid properties on jet breakup. The effects of various parameters on the instability behavior are studied in comparison with two Newtonian jets with upper and lower bound viscosities, μ₀ and μ_∞. A number of quantitative conclusions and sensitivities on the instability behavior of non-Newtonian jets are investigated. It is found that the jet breakup mechanism depends on the properties of the fluid as well as the wave number of the thermal disturbance that causes the surface tension gradient. In contrast to the Newtonian liquid where the jet surface profile has the same frequency as the surface tension gradient, the nonlinear nature of the Carreau-Yasuda constitutive behavior may enable the jet surface profile at frequencies higher than that of the surface tension gradient. This leads to significant surface profile oscillation within one wavelength of the surface tension gradient and the generation of small satellite drops. It is worth noting that at a small wave number the breakup time for the Carreau-Yasuda fluid maybe shorter than that of the Newtonian jet with μ_∞, although the Newtonian jet has a lower viscosity.     

Parametric study of fluid–solid interaction for single-particle dissipative particle dynamics model

In this paper, a parametric study of fluid–solid interaction for single-particle dissipative particle dynamics (DPD) model is conducted to describe the hydrodynamic interactions in a large range of particle sizes. To successfully reproduce the hydrodynamics for different particle sizes, and overcome the problem that effective radius of solid sphere does not match its real radius, the cut-off radius and conservative force coefficient of single-particle DPD model have been modified. The cut-off radius and conservative force coefficient are related to the drag force and radial distribution function, so that, for each particle size, they can be determined by DPD simulations. Through numerical fitting, two empirical formulas as a function of spherical radius are developed to calculate the cut-off radius and conservative force coefficient. Numerical results indicate that the single-particle DPD model is, indeed, capable of capturing low Reynolds number hydrodynamic interactions for different particle sizes by selecting these model parameters reasonably. Specifically, the model can not only insure that drag force and torque are quantitatively consistent with theoretical results, but also guarantee the effective radius matches well its real radius. In addition, the shear dissipative force is the major part of drag force and should not be ignored. This study will help to improve the application range of single-particle DPD model to make it suitable for different particle sizes and provide parameter guidance for studying fluid–solid interaction using single-particle DPD model.     

基于Intel MIC平台大规模耗散粒子动力学模拟的设计与优化

耗散粒子动力学(DPD)模拟是一种重要的研究流体动力学特性的计算模拟方法,基于Intel MIC平台设计实现了面向大规模耗散粒子动力学模拟,充分结合了DPD模拟本身的特性和MIC平台的特征。对DPD模拟中的近邻列表构建和短程作用力关键代码实现了向量化优化,在CPU和MIC协处理器之间采用任务计算负载平衡机制,支持MPI进程内线程数量负载平衡控制。分别在原型程序上和LAMMPS集成中做了性能对比分析,实验结果显示了引入相关优化技术的有效性,为进一步研究面向MIC众核平台的分子动力学相关工作奠定了基础。     

Accelerating dissipative particle dynamics with multiple GPUs

Dissipative particle dynamics (DPD) simulation is implemented on multiple GPUs by using NVIDIA’s Compute Unified Device Architecture (CUDA) in this paper. Data communication between each GPU is executed based on the POSIX thread. Compared with the single-GPU implementation, this implementation can provide faster computation speed and more storage space to perform simulations on a significant larger system. In benchmark, the performance of GPUs is compared with that of Material Studio running on a single CPU core. We can achieve more than 90x speedup by using three C2050 GPUs to perform simulations on an 80∗80∗80 system. This implementation is applied to the study on the dispersancy of lubricant succinimide dispersants. A series of simulations are performed on lubricant–soot–dispersant systems to study the impact factors including concentration and interaction with lubricant on the dispersancy, and the simulation results are agreed with the study in our present work.     

Parallel implementation of the fluid particle model for simulating complex fluids in the mesoscale

Dissipative particle dynamics (DPD) and its generalization—the fluid particle model (FPM)—represent the ‘fluid particle’ approach for simulating fluid‐like behavior in the mesoscale. Unlike particles from the molecular dynamics (MD) method, the ‘fluid particle’ can be viewed as a ‘droplet’ consisting of liquid molecules. In the FPM, ‘fluid particles’ interact by both central and non‐central, short‐range forces with conservative, dissipative and Brownian character. In comparison to MD, the FPM method in three dimensions requires two to three times more memory load and a three times greater communication overhead. Computational load per step per particle is comparable to MD due to the shorter interaction range allowed between ‘fluid particles’ than between MD atoms. The classical linked‐cells technique and decomposing the computational box into strips allow for rapid modifications of the code and for implementing non‐cubic computational boxes. We show that the efficiency of the FPM code depends strongly on the number of particles simulated, the geometry of the box and the computer architecture. We give a few examples from long FPM simulations involving up to 8 million fluid particles and 32 processors. Results from FPM simulations in three dimensions of the phase separation in binary fluid and dispersion of the colloidal slab are presented. A scaling law for symmetric quench in phase separation has been properly reconstructed. We also show that the microstructure of dispersed fluid depends strongly on the contrast between the kinematic viscosities of this fluid phase and the bulk phase. This FPM code can be applied for simulating mesoscopic flow dynamics in capillary pipes or critical flow phenomena in narrow blood vessels. Copyright © 2002 John Wiley & Sons, Ltd.     

Multiscale simulations of primary atomization

A liquid jet upon atomization breaks up into small droplets that are orders of magnitude smaller than its diameter. Direct numerical simulations of atomization are exceedingly expensive computationally. Thus, the need to perform multiscale simulations. In the present study, we performed multiscale simulations of primary atomization using a Volume-of-Fluid (VOF) algorithm coupled with a two-way coupling Lagrangian particle-tracking model to simulate the motion and influence of the smallest droplets. Collisions between two particles are efficiently predicted using a spatial-hashing algorithm. The code is validated by comparing the numerical simulations for the motion of particles in several vortical structures with analytical solutions. We present simulations of the atomization of a liquid jet into droplets which are modeled as particles when away from the primary jet. We also present the probability density function of the droplets thus obtained and show the evolution of the PDF in space.     

The aggregation and diffusion of asphaltenes studied by GPU-accelerated dissipative particle dynamics

The heavy crude oil consists of thousands of compounds and much of them have large molecular weights and complex structures. Studying the aggregation and diffusion behavior of asphaltenes can facilitate the understanding of the heavy crude oil. In previous studies, the fused aromatic rings were treated as rigid bodies so that dissipative particle dynamics (DPD) integrated with the quaternion method can be used to study asphaltene systems. In this work, DPD integrated with the quaternion method is implemented on graphics processing units (GPUs). Compared with the serial program, tens of times speedup can be achieved when simulations performed on a single GPU. Using multiple GPUs can provide faster computation speed and more storage space for simulations of significant large systems. By using large systems, simulations of the asphaltene–toluene system at extremely dilute concentrations can be performed. The determined diffusion coefficients of asphaltenes are similar to that in experimental studies. At last, the aggregation behavior of asphaltenes in heptane was investigated, and the simulation results agreed with the modified Yen model. Monomers, nanoaggregates and clusters were observed from the simulations at different concentrations.     

液丝破裂的研究进展和展望

总结有关液丝破裂研究的实验工作,分析Weber数、流体粘度及初始扰动对液丝破裂的影响。简要介绍牛顿流体和非牛顿流体的一维、二维数学模型,并指出该研究的实际应用和展望。一维模型在最近几年被广泛使用,极大地增强了人们对液滴形成过程中界面破裂的理解,但不能同时得到精确的宏观特征和微观特征;三维模型的计算量庞大。为了研究的可行性和结果的精确性,应对二维模型进行更多研究。     

Micro-droplet formation in non-Newtonian fluid in a microchannel

Micro-droplet formation from an aperture with a diameter of micrometers is numerically investigated under the cross-flow conditions of an experimental microchannel emulsification process. The process involves dispersing an oil phase into continuous phase fluid through a microchannel wall made of apertured substrate. Cross-flow in the microchannel is of non-Newtonian nature, which is included in the simulations. Micro-droplets of diameter 0.76–30 μm are obtained from the simulations for the apertures of diameter 0.1–10.0 μm. The simulation results show that rheology of the bulk liquid flow greatly affects the formation and size of droplets and that dispersed micro-droplets are formed by two different breakup mechanisms: in dripping regime and in jetting regime characterized by capillary number Ca. Relations between droplet size, aperture opening size, interfacial tension, bulk flow rheology, and disperse phase flow rate are discussed based on the simulation and the experimental results. Data and models from literature on membrane emulsification and T-junction droplet formation processes are discussed and compared with the present results. Detailed force balance models are discussed. Scaling factor for predicting droplet size is suggested.     

Image content adaptive color breakup index for field sequential color displays using a dominant visual saliency method

An index that can predict the perceptual visibility of color breakup for varying image content is valuable in field sequential color displays, whereas the current indices are usually for fixed patterns. To solve this problem, an image database containing 25 diverse reference images and 125 test cases with various color breakup visibility was first established. Next, visual experiments using a 240‐Hz liquid crystal display were performed to acquire the subjective color breakup scores of the test cases. A theorem based on visual saliency theory was proposed that the color breakup perception is mainly determined by the image regions with visual saliency values higher than a certain threshold, called the dominant visual saliency regions. A computational model based on this theorem was developed to obtain objective color breakup scores of the test cases from retinal images with and without color breakup. An analysis of the objective and subjective results revealed a Pearson linear correlation coefficient as high as 0.82, which matches the top‐level image quality assessment algorithms. Finally, the proposed color breakup index was used to benchmark against several mainstream field sequential color algorithms to determine their performances in color breakup suppression.     

A comparative numerical study between dissipative particle dynamics and smoothed particle hydrodynamics when applied to simple unsteady flows in microfluidics

Laminar flows through channels, pipes and between two coaxial cylinders are of significant practical interest because they often appear in a wide range of industrial, environmental, and biological processes. Discrete particle modeling has increasingly been used in recent years and in this study we examined two of these methods: dissipative particle dynamics (DPD) and smoothed particle hydrodynamics (SPH) method when applied to (a) time-dependent, plane Poiseuille flow and (b) flow between two coaxial cylinders at low Reynolds numbers. The two examples presented in this paper give insight into different features of the two discrete particle methods. It was found that both methods give results with high accuracy, but CPU time is much larger (of order 10²–10³ in the second example) for DPD than for SPH model. This difference is due to the fact that the number of time steps for the DPD model is much greater than for the SPH model (since thermal fluctuations are taken into account in the DPD model).     

Narrow Band FLIP for Liquid Simulations

The Fluid Implicit Particle method (FLIP) for liquid simulations uses particles to reduce numerical dissipation and provide important visual cues for events like complex splashes and small‐scale features near the liquid surface. Unfortunately, FLIP simulations can be computationally expensive, because they require a dense sampling of particles to fill the entire liquid volume. Furthermore, the vast majority of these FLIP particles contribute nothing to the fluid's visual appearance, especially for larger volumes of liquid. We present a method that only uses FLIP particles within a narrow band of the liquid surface, while efficiently representing the remaining inner volume on a regular grid. We show that a naïve realization of this idea introduces unstable and uncontrollable energy fluctuations, and we propose a novel coupling scheme between FLIP particles and regular grid which overcomes this problem. Our method drastically reduces the particle count and simulation times while yielding results that are nearly indistinguishable from regular FLIP simulations. Our approach is easy to integrate into any existing FLIP implementation.     

Numerical investigations in Rayleigh breakup of round liquid jets with VOF methods

Simulations of the growth of a capillary instability and of the breakup of a jet were carried out using a one-fluid model to describe the two-phase flow motion and a VOF approach to capture the interface. The model considered each phase as fictitious sub-domains and accounted implicitly for jump conditions at the interface through a unique set of equations for which a source term of surface tensions appeared in momentum equations. The predominance of capillary effects in the breakup mechanism required to accurately describe the surface tension contribution. The Brackbill surface model was chosen because of its simplicity to represent tension forces, although it was known to generate parasitic currents susceptible to limit its precision. The flow incompressibility was ensured with an augmented Lagrangian method in case of sequential calculations and by a predictor/corrector approach for 3D simulations that required parallel computations. As a first step, the numerical methods were validated by simulating the growth of a capillary instability and comparing results to those predicted by the Rayleigh theory for capillary instabilities. The consistency of the Brackbill surface tension model and the accuracy of the methods were evaluated via a convergence study. As a second step, the simulation of a jet breakup was carried out using water as injected liquid and compressed carbon dioxide as surrounding medium. It was shown that the simulation predicted accurately the breakup length and the droplet size evidenced experimentally in literature.