Oldroyd-B黏弹性液滴碰撞过程的数值模拟

复杂的流变特性使凝胶推进剂的雾化过程存在一定困难,这制约了它的发展.聚合物胶凝剂的加入使凝胶推进剂具有黏弹性,从而在雾化时会产生黏弹性液滴,因此为了进一步认识凝胶推进剂的雾化机理、提高凝胶推进剂的雾化性能,对黏弹性液滴的碰撞行为进行数值模拟研究.针对凝胶推进剂雾化过程中出现的液滴撞击现象,考虑流体具有的黏弹性效应,采用...     

液滴在激波冲击下的破裂过程

对液滴在入射激波作用下的变形破碎过程进行了实验研究和数值模拟,得知数值模拟结果与实验结果基本吻合,以及在什么情况下两者出现分歧。结果显示,液滴在激波的作用下要经历从压缩变形、RM不稳定性变形、细小液雾剥离到全部雾化破碎等过程。结果还表明,不同液滴直径、入射激波马赫数和液滴介质等参数下的液滴变形破碎的发展趋势是一致的,而其发展速度明显则不同。其中Weber数的增加加速了液滴的破碎,而Ohnesorge数和黏性的增加则抑制了液滴的破碎。     

缩放喷管内正激波位置的数值模拟

数值模拟缩放喷管内正激波位置，需联立迭代求解一组方程．本文利用５个独立方程模拟了由背压与入口滞止压力的比值的变化而形成的不同流动工况．有利于学生对该部分教学内容的深刻理解及促进计算机辅助教学工作     

气体-燃料液滴两相系统爆轰的数值模拟

用两相流体力学模型对气体 燃料液滴系统进行了研究。数值模拟了点火后两相系统爆轰波的发展过程，得到爆轰波的结构和参数。数值模拟结果表明气体 燃料液滴系统爆轰波有较宽的反应区，因而两相爆轰波的曲率对爆速的影响效应十分明显。进行了燃料液滴尺寸对爆轰波的结构和参数的影响的数值模拟。除了很小的液滴外，燃料液滴在爆轰波前导激波面和ＣＪ面间不能完全气化。随着液滴尺寸的增加，燃料液滴在爆轰波前导激波面和ＣＪ面间释放出的能量随之减少，爆轰参数也随之下降。     

W/O液滴绕流微柱阵列的数值模拟研究

基于COMSOL Multiphysics 4.1层流两相流模型,通过数值模拟研究了连续相流速一定的情况下大接触角(?=160°)的油包水(W/O)液滴绕流微柱阵列的二维运动。通过相场方法考虑表面张力的影响,实现对油-水界面的追踪,获得了W/O液滴的运动状态,重点考查了液滴尺寸对运动特性的影响。模拟结果表明,初始尺寸不同的液滴变形后,长度上存在差异,但厚度基本一致,尺寸介于100?m~150?m液滴的运动速度与液滴尺寸无关。进一步研究了利用尖楔对液滴进行分割的过程,发现液滴脱离末端微柱时,其垂向位置高度与液滴尺寸密切相关,呈先线性增加后趋于定值的特点,利用这一预定位功能及尖楔结构可以获得尺寸可控的均匀子液滴。数值方法与已有实验结果基本一致,且可以很好地表现W/O液滴与微柱的相互作用,揭示液滴的流动细节和运动规律,为设计更为复杂的液滴微流控芯片提供了支撑。     

基于数据驱动的薄板结构多裂纹反演方法

采用比例边界有限元法(scaled boundary finite element methods,SBFEM)模拟薄板结构内Lamb波的传播过程，将SBFEM和最大信息系数相结合，研究了缺陷参数与观测点位置的相关性，为缺陷反演时传感器布置位置的选取提供了依据。在此基础上建立了基于SBFEM数据集和深度学习的结构内部多裂纹反演方法，将多裂纹反演归类为分类和回归预测问题，可在未知裂纹数量的情况下反演出裂纹的数量、位置和大小，并通过数值算例验证了该方法能够较好地进行裂纹状缺陷数量和参数的反演。     

液滴二次雾化破碎模式数值模拟

在许多现代工程领域中,二次雾化对于增强液体雾化以及提高混合率都起到了关键的作用．基于SIMPLE方法对这个问题进行了数值模拟,VOF方法与Level-Set方法耦合捕捉界面,自适应网格的运用平衡了对提高计算精度以及降低计算成本两方面的要求．在Oh数较低(Oh < 0.1)的情况下,通过数值模拟得到了四种典型的液滴分裂模式,并且对其中相对复杂的第二第三种分裂模式进行了详细的分析与比较．得出了该模式形成的条件与相关特性．最后,将以上结果与前人的实验结果作了对比,在大多数重要特征方面,两者有非常好的一致性．     

A STUDY ON DROPLET VELOCITY OF ETHANOL DURING ELECTROSPRAYING PROCESS AT CONE-JET MODE

研究液滴在静电喷雾下的速度特性是理解喷雾形态的形成及演化的关键.结合锥射流模式下乙醇静电喷雾实验数据,建立了静电喷雾二维轴对称模型.基于离散相液滴运动方程、连续相空气运动方程、电场方程以及用户自定义函数,进行了数值求解,获得了锥射流模式下的乙醇静电喷雾形态、空间电场分布以及液滴速度场分布.考虑了不同空气入口流速的影响,得到了乙醇/空气同轴射流静电喷雾形态的变化规律.结果表明,喷雾外围液滴与空气流场有较强的相互作用,导致喷雾中轴线附近的液滴速度分布变化较小,而在喷雾外围处的液滴速度分布沿径向剧烈变化;随着空气入口速度的增大,乙醇/空气同轴射流静电喷雾形态先趋于发散,当空气入口速度大于喷雾外围液滴轴向速度时,喷雾形态则趋于聚拢.因此,除改变施加电压、液体流量和电极结构外,通过控制空气入口速度来影响喷雾液滴速度场,也可获得不同的静电喷雾效果.     

锥射流模式下乙醇静电喷雾液滴速度特性分析

研究液滴在静电喷雾下的速度特性是理解喷雾形态的形成及演化的关键.结合锥射流模式下乙醇静电喷雾实验数据,建立了静电喷雾二维轴对称模型.基于离散相液滴运动方程、连续相空气运动方程、电场方程以及用户自定义函数,进行了数值求解,获得了锥射流模式下的乙醇静电喷雾形态、空间电场分布以及液滴速度场分布.考虑了不同空气入口流速的影响,得到了乙醇/空气同轴射流静电喷雾形态的变化规律.结果表明,喷雾外围液滴与空气流场有较强的相互作用,导致喷雾中轴线附近的液滴速度分布变化较小,而在喷雾外围处的液滴速度分布沿径向剧烈变化;随着空气入口速度的增大,乙醇/空气同轴射流静电喷雾形态先趋于发散,当空气入口速度大于喷雾外围液滴轴向速度时,喷雾形态则趋于聚拢.因此,除改变施加电压、液体流量和电极结构外,通过控制空气入口速度来影响喷雾液滴速度场,也可获得不同的静电喷雾效果.     

用格子Boltzmann方法模拟液滴撞击固壁动力学行为

首次用格子Boltzmann方法中的伪势模型对液滴撞击固壁的动力学行为进行了数值模拟.详细研究了液滴在壁面上的流动状态以及各种因素对撞击过程的影响.通过数值模拟得到:壁面的可润湿性越小,液滴越容易发生反弹,液滴的回缩速度越快;液滴的撞击速度越大,所得到的相对直径越大,回缩速度越快;液滴的粘性越小,所得到的相对直径越大;液滴的表面张力越大,液滴越容易发生反弹现象.另外,液滴的最大相对直径与We数满足一定的线性关系,这些结果与前人的理论预测和实验结果完全吻合.     

Modeling and staged assessments of the controllability of spread for repeated outbreaks of COVID-19

SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) has been causing an outbreak of a new type of pneumonia globally, and repeated outbreaks have already appeared. Among the studies on the spread of the COVID-19, few studies have investigated the repeated outbreaks in stages, and the quantitative condition of a controllable spread has not been revealed. In this paper, a brief compartmental model is developed. The effective reproduction number (ERN) of the model is interpreted by the ratio of net newly infectious individuals to net isolation infections to assess the controllability of the spread of COVID-19. It is found that the value of the ERN at the inflection point of the pandemic is equal to one. The effectiveness of the quarantine, even the treatment, is parametrized in various stages with Gompertz functions to increase modeling accuracy. The impacts of the vaccinations are discussed by adding a vaccinated compartment. The results show that the sufficient vaccinations can make the inflection point appear early and significantly reduce subsequent increases in newly confirmed cases. The analysis of the ERNs of COVID-19 in the United States, Spain, France, and Peru confirms that the condition of a repeated outbreak is to relax or lift the interventions related to isolation and quarantine interventions to a level where the ERN is greater than one.

     

Deformation and breakup of liquid drops in a gas stream at nearly critical Weber numbers

Experimentally determined values of the critical Weber number available from the literature are scattered over a very wide range ofWe _c from 2.2 to 99.6. To study one possible source of these discrepancies an experimental investigation was made of the deformation and breakup of water droplets at nearly critical Weber numbers. Experiments were conducted in a small horizontal wind tunnel. A continuous stream of uniform water droplets was allowed to fall perpendicularly to the continuous stream of air. The time histories of water droplets were recorded by using a high speed camera. Five different basic behaviours of water droplets were recorded in the range ofWe = 11 to 14. It was found that an increase in the Weber number in this region resulted in an increased percentage of droplets with regular bag type breakup.     

The impact of asymptomatic individuals on the strength of public health interventions to prevent the second outbreak of COVID-19

Nonlinear Dynamics - The pandemic of coronavirus disease 2019 (COVID-19) has threatened the social and economic structure all around the world. Generally, COVID-19 has three possible transmission...     

Are official confirmed cases and fatalities counts good enough to study the COVID-19 pandemic dynamics? A critical assessment through the case of Italy

As the COVID-19 outbreak is developing the two most frequently reported statistics seem to be the raw confirmed case and case fatalities counts. Focusing on Italy, one of the hardest hit countries, we look at how these two values could be put in perspective to reflect the dynamics of the virus spread. In particular, we find that merely considering the confirmed case counts would be very misleading. The number of daily tests grows, while the daily fraction of confirmed cases to total tests has a change point. It (depending on region) generally increases with strong fluctuations till (around, depending on region) 15–22 March and then decreases linearly after. Combined with the increasing trend of daily performed tests, the raw confirmed case counts are not representative of the situation and are confounded with the sampling effort. This we observe when regressing on time the logged fraction of positive tests and for comparison the logged raw confirmed count. Hence, calibrating model parameters for this virus’s dynamics should not be done based only on confirmed case counts (without rescaling by the number of tests), but take also fatalities and hospitalization count under consideration as variables not prone to be distorted by testing efforts. Furthermore, reporting statistics on the national level does not say much about the dynamics of the disease, which are taking place at the regional level. These findings are based on the official data of total death counts up to 15 April 2020 released by ISTAT and up to 10 May 2020 for the number of cases. In this work, we do not fit models but we rather investigate whether this task is possible at all. This work also informs about a new tool to collect and harmonize official statistics coming from different sources in the form of a package for the R statistical environment and presents the “COVID-19 Data Hub.”

     

A spread model of COVID-19 with some strict anti-epidemic measures

In the absence of specific drugs and vaccines, the best way to control the spread of COVID-19 is to adopt and diligently implement effective and strict anti-epidemic measures. In this paper, a mathematical spread model is proposed based on strict epidemic prevention measures and the known spreading characteristics of COVID-19. The equilibria (disease-free equilibrium and endemic equilibrium) and the basic regenerative number of the model are analyzed. In particular, we prove the asymptotic stability of the equilibria, including locally and globally asymptotic stability. In order to validate the effectiveness of this model, it is used to simulate the spread of COVID-19 in Hubei Province of China for a period of time. The model parameters are estimated by the real data related to COVID-19 in Hubei. To further verify the model effectiveness, it is employed to simulate the spread of COVID-19 in Hunan Province of China. The mean relative error serves to measure the effect of fitting and simulations. Simulation results show that the model can accurately describe the spread dynamics of COVID-19. Sensitivity analysis of the parameters is also done to provide the basis for formulating prevention and control measures. According to the sensitivity analysis and corresponding simulations, it is found that the most effective non-pharmaceutical intervention measures for controlling COVID-19 are to reduce the contact rate of the population and increase the quarantine rate of infected individuals.

     

Dynamics and control of COVID-19 pandemic with nonlinear incidence rates

World Health Organization (WHO) has declared COVID-19 a pandemic on March 11, 2020. As of May 23, 2020, according to WHO, there are 213 countries, areas or territories with COVID-19 positive cases. To effectively address this situation, it is imperative to have a clear understanding of the COVID-19 transmission dynamics and to concoct efficient control measures to mitigate/contain the spread. In this work, the COVID-19 dynamics is modelled using susceptible–exposed–infectious–removed model with a nonlinear incidence rate. In order to control the transmission, the coefficient of nonlinear incidence function is adopted as the Governmental control input. To adequately understand the COVID-19 dynamics, bifurcation analysis is performed and the effect of varying reproduction number on the COVID-19 transmission is studied. The inadequacy of an open-loop approach in controlling the disease spread is validated via numerical simulations and a robust closed-loop control methodology using sliding mode control is also presented. The proposed SMC strategy could bring the basic reproduction number closer to 1 from an initial value of 2.5, thus limiting the exposed and infected individuals to a controllable threshold value. The model and the proposed control strategy are then compared with real-time data in order to verify its efficacy.

     

Assessing the potential impact of immunity waning on the dynamics of COVID-19 in South Africa: an endemic model of COVID-19

We developed an endemic model of COVID-19 to assess the impact of vaccination and immunity waning on the dynamics of the disease. Our model exhibits the phenomenon of backward bifurcation and bi-stability, where a stable disease-free equilibrium coexists with a stable endemic equilibrium. The epidemiological implication of this is that the control reproduction number being less than unity is no longer sufficient to guarantee disease eradication. We showed that this phenomenon could be eliminated by either increasing the vaccine efficacy or by reducing the disease transmission rate (adhering to non-pharmaceutical interventions). Furthermore, we numerically investigated the impacts of vaccination and waning of both vaccine-induced immunity and post-recovery immunity on the disease dynamics. Our simulation results show that the waning of vaccine-induced immunity has more effect on the disease dynamics relative to post-recovery immunity waning and suggests that more emphasis should be on reducing the waning of vaccine-induced immunity to eradicate COVID-19.

     

Geospatial model of COVID-19 spreading and vaccination with event Gillespie algorithm

We have developed a mathematical model and stochastic numerical simulation for the transmission of COVID-19 and other similar infectious diseases that accounts for the geographic distribution of population density, detailed down to the level of location of individuals, and age-structured contact rates. Our analytical framework includes a surrogate model optimization process to rapidly fit the parameters of the model to the observed epidemic curves for cases, hospitalizations, and deaths. This toolkit (the model, the simulation code, and the optimizer) is a useful tool for policy makers and epidemic response teams, who can use it to forecast epidemic development scenarios in local settings (at the scale of cities to large countries) and design optimal response strategies. The simulation code also enables spatial visualization, where detailed views of epidemic scenarios are displayed directly on maps of population density. The model and simulation also include the vaccination process, which can be tailored to different levels of efficiency and efficacy of different vaccines. We used the developed framework to generate predictions for the spread of COVID-19 in the canton of Geneva, Switzerland, and validated them by comparing the calculated number of cases and recoveries with data from local seroprevalence studies.

     

THE MECHANISM OF THE INTERACTION BETWEEN NANOPARTICLE AND LIPIDS MONOLAYER OF PULMONARY SURFACTANT 1)

人体在吸入纳米颗粒时肺部的第一道生物屏障是肺表面活性剂(lung surfactant, LS),吸入的纳米颗粒可以到达肺部深处,从而干扰肺部的生物物理性能。目前关于金纳米粒子与LS单层的相互作用的机制研究及金纳米粒子对肺部功能的影响尚不清楚。本文使用粗粒化的分子动力学方法,研究了金纳米粒子与LS单层膜在纳米尺度上的相互作用,观察到金纳米粒子的存在使得单层膜的结构变形,改变了膜的生物物理性能,并且不同形状的纳米粒子对单层膜的影响也不相同。这些发现有助于识别空气中的纳米粒子对肺部的潜在影响。     

Cyclic variations of fuel-droplet distribution during the early intake stroke of a lean-burn stratified-charge spark-ignition engine

Lean-burn spark-ignition engines exhibit higher efficiency and lower specific emissions in comparison with stoichiometrically charged engines. However, as the air-to-fuel (A/F) ratio of the mixture is made leaner than stoichiometric, cycle-by-cycle variations in the early stages of in-cylinder combustion, and subsequent indicated mean effective pressure (IMEP), become more pronounced and limit the range of lean-burn operation. Viable lean-burn engines promote charge stratification, the mixture near the spark plug being richer than the cylinder volume averaged value. Recent work has shown that cycle-by-cycle variations in the early stages of combustion in a stratified-charge engine can be associated with variations in both the local value of A/F ratio near the spark plug around ignition timing, as well as in the volume averaged value of the A/F ratio. The objective of the current work was to identify possible sources of such variability in A/F ratio by studying the in-cylinder field of fuel-droplet distribution during the early intake stroke. This field was visualised in an optical single-cylinder 4-valve pentroof-type spark-ignition engine by means of laser-sheet illumination in planes parallel to the cylinder head gasket 6 and 10 mm below the spark plug. The engine was run with port-injected isooctane at 1500 rpm with 30% volumetric efficiency and air-to-fuel ratio corresponding to both stoichiometric firing (A/F=15, Φ =1.0) and mixture strength close to the lean limit of stable operation (A/F=22, Φ =0.68). Images of Mie intensity scattered by the cloud of fuel droplets were acquired on a cycle-by-cycle basis. These were studied in order to establish possible correlations between the cyclic variations in size, location and scattered-light intensity of the cloud of droplets with the respective variations in IMEP. Because of the low level of Mie intensity scattered by the droplets and because of problems related to elastic scattering on the walls of the combustion chamber, as well as problems related to engine “rocking” at the operating conditions close to the misfire limit, the acquired images were processed for background subtraction by using a PIV-based data correction algorithm. After this processing, the arrival and leaving timings of fuel droplets into the illuminated plane were found not to vary significantly on a cycle-by-cycle basis but the recorded cycle-by-cycle variations in Mie intensity suggested that the amount of fuel in the cylinder could have been 6–26% greater for the “strong” cycles with IMEP 115% higher than the average IMEP, than the ones imaged for “weak” cycles at less than 85% the average IMEP. This would correspond to a maximum cyclic variability in the in-cylinder equivalence ratio Φ of the order of 0.17.