Stochastic fluctuation induced the competition between extinction and recurrence in a model of tumor growth

We investigate the phenomenon that stochastic fluctuation induced the competition between tumor extinction and recurrence in the model of tumor growth derived from the catalytic Michaelis–Menten reaction. We analyze the probability transitions between the extinction state and the state of the stable tumor by the Mean First Extinction Time (MFET) and Mean First Return Time (MFRT). It is found that the positional fluctuations hinder the transition, but the environmental fluctuations, to a certain level, facilitate the tumor extinction. The observed behavior could be used as prior information for the treatment of cancer.     

Novel optical techniques for measurements of light extinction,scattering and absorption by single aerosol particles

Aerosol particles play important roles in a broad range of scientific disciplines, from atmospheric chemistry and physics, to the delivery of fuels for combustion and drugs to the lungs, and extending to industrial processes such as spray drying. Measurements of the light extinction, scattering and absorption by ensembles of aerosol particles can be used to non‐intrusively characterise aerosol particle samples. However, such measurements often lead to ambiguity in interpreting the properties and processes occurring on individual particles. In this review, recent developments in the use of laser based techniques to isolate and manipulate single particles and to characterise them will be highlighted. In particular, the use of cavity ring down spectroscopy, Bessel beams and optical tweezers for investigating light extinction, scattering and absorption, respectively, will be considered. The prospects for using optical techniques to interrogate the fundamental processes occurring in aerosol at the single particle level are discussed.     

An analytical prediction of the oscillation and extinction thresholds of a clarinet

This paper investigates the dynamic range of the clarinet from the oscillation threshold to the extinction at high pressure level. The use of an elementary model for the reed-mouthpiece valve effect combined with a simplified model of the pipe assuming frequency independent losses (Raman's model) allows an analytical calculation of the oscillations and their stability analysis. The different thresholds are shown to depend on parameters related to embouchure parameters and to the absorption coefficient in the pipe. Their values determine the dynamic range of the fundamental oscillations and the bifurcation scheme at the extinction.     

Population turnover and adaptation in heterogeneous environments

We study adaptive dynamics in a structured population model of asexual individuals which takes into account environmental heterogeneity among the subpopulations. The key purpose of the present work is to address how population turnovers, i.e. extinction events followed by recolonization, affect the rate of fixation of advantageous mutations. This model is a generalization of our previous model to address the interplay between environmental correlation and evolutionary forces on the adaptive process. The incorporation of population turnovers into the model enables us to make a direct correspondence between the model and host-parasite dynamics (epidemiological models). Strikingly, contrary to the intuitive and usual deleterious effect associated to extinction events, it is observed that population turnovers can in fact speed up adaptation as heterogeneity rises. On the other side, in nearly homogeneous population turnovers have a neutral effect on fixation rates, but a detrimental outcome is also achieved when extinction events become very common. In resume, population turnover outcomes on fixation rates of advantageous mutations are strongly influenced by the selective correlation among the subpopulations (demes).     

多波段激光雷达颗粒物质量浓度探测方法

为了获得大气颗粒物的质量浓度廓线,提出一种基于多波段激光雷达回波信号的大气气溶胶消光系数与颗粒物质量消光效率相结合的新型算法。该方法利用覆盖紫外到近红外波段的激光雷达作为遥感探测工具,获取气溶胶的消光与后向散射系数,反演得到气溶胶粒子谱分布;同时,根据米散射理论算出气溶胶消光效率,结合粒子谱分布,提出颗粒物质量消光效率模型,从而建立基于消光系数与质量消光效率相结合的反演颗粒物质量浓度的新型数学模型与算法。采用该算法对两组不同天气条件多波段激光雷达实测数据进行反演,并与地表采用的颗粒物浓度对比,证明该方法的可行性,为实现颗粒物质量浓度空间分布的探测提供科学依据和方法论。     

Quantitative lidar at 532 nm for vertical extinction profiles and the effect of relative humidity

An analysis of 532 nm lidar data is presented for the retrieval of vertical extinction profiles. The strong influence of the relative humidity on the extinction-to-backscatter ratio is parametrized for this purpose. A comparison is made between remotely sensed and locally measured extinction coefficients, using reference values in aircraft and at ground level.     

内陆地区近地面大气消光系数统计特征及气象要素影响分析

使用能见度仪测量了合肥地区2005年6月到2007年7月期间近地面的大气消光系数.给出了大气消光系数的时间变化特征和频数分布特征以及大气消光系数与相应时间段的水汽、温度、气压、风速的相关性.结果表明：局地大气消光系数具有明显的日变化、月变化、季节变化和频数特征|不同季节的消光系数与气象要素的相关性有所不同|其中水汽、温度是两个较为重要的影响消光系数特征分布的因素.分析结果可以作为局地大气消光系数模型建立的基础.     

Perfect extinction in subwavelength dual metallic transmitting gratings

We investigate the strong electromagnetic coupling that settles in dual metallic grating structures. This coupling is evidenced to lead to a perfect optical extinction in the transmission spectrum. The behavior of this perfect extinction that strongly depends on the longitudinal space and the lateral displacement between the two gratings can be explained by a simple model that describes the interference between a propagating mode and a couple of evanescent modes. The results show that the electromagnetic transmission of the structure can be tuned by controlling the position of this perfect transmission extinction and thus pave the way to new types of infrared tunable filters.     

基于PCA-BP神经网络的光谱消光法颗粒粒径反演算法研究

光谱消光法广泛应用于颗粒粒径测量领域,在利用光谱消光法对颗粒粒径进行反演的过程中,由于颗粒的消光系数存在理论复杂、计算繁琐、收敛速度慢以及求解不稳定等问题,很大程度上影响了整个反演过程的快速性和准确性。且在众多波长的消光数据中,存在较多重复冗余的信息,也很大程度上增加了反演算法的时间。针对光谱消光法粒径反演算法计算繁琐、反演效率低的问题,提出了基于主成分分析（PCA）和BP神经网络的光谱消光颗粒粒径分析方法。基于Mie散射理论对不同粒径、不同波长下的光谱消光值进行了仿真计算,通过对光谱消光数据集的主成分分析及各个波长综合载荷系数的计算,实现了最优特征波长的选取,利用降维后的光谱消光数据训练了PCA-BP神经网络模型,并利用该网络模型计算了粒径颗粒分布。通过仿真计算,比较了PCA-BP神经网络模型与传统的BP神经网络模型的预测精度,并分析了波长数目对两种神经网络模型预测结果的影响。针对训练得到的PCA-BP神经网络模型开展光谱消光法粒径参数反演算法的验证实验,搭建了光谱消光法颗粒粒径参数测量实验系统,测量了粒径范围在0.5～9.7 μm内的6种不同粒径参数的聚苯乙烯标准颗粒。仿真和实验结果表明：基于主成分分析方法可确定各个波长向量之间的相关性,利用综合载荷系数选取最优特征波长对应的消光值对整体的光谱数据具有较好的代表性,可实现光谱数据的降维。相比传统的BP神经网络模型,基于PCA-BP神经网络模型的颗粒粒径分布的分析方法预测精度更高,对于较分散颗粒系的分布参数的预测有更加明显的优势。而且,被选取的波长数较少时,PCA-BP神经网络模型依然有较高的预测精度。利用训练好的PCA-BP神经网络模型对颗粒粒径参数进行实验验证,预测结果可瞬时输出,颗粒粒径分布误差在5%以内,验证了该算法的可行性。     

Modeling extinction in forced opposed-flow smolder

We address a discrepancy between smolder extinction theory and experimental work regarding whether extinction can occur under oxygen-depleted conditions. Single heterogeneous (solid/gas) reaction models that model the complex reaction process of smoldering with a single reaction predict extinction for large head winds only when oxygen remains (fuel-limited smolder). Experimental evidence, however, shows that oxygen concentrations are essentially zero for smolder waves close to the extinction limit. We consider a two-step sequential reaction scheme in which the solid fuel is converted to char, which also reacts with oxygen to produce ash. This generic scheme leads to extinction with oxygen depletion. The model suggests that oxygen depletion is caused by the second reaction while extinction is driven by the first. Solid fuel must be completely converted to char for extinction to occur while oxygen consumption in the secondary reaction induces oxygen depletion. Spatial separation of the localized reactions is predicted to increase with head wind as a precursor to extinction.     

色交叉关联噪声作用下集合种群的稳定性和平均灭绝时间

在Levins模型的基础上研究了色交叉关联噪声对集合种群稳定性的影响, 应用Fokker-Plank方程得到了系统的稳态概率密度函数, 运用最快下降法得到了平均灭绝时间的解析式. 结果表明: 两噪声色关联时, 加性噪声强度和乘性噪声强度均弱化集合种群的稳定性; 噪声关联强度强化集合种群的稳定性. 两噪声之间负关联时, 平均灭绝时间是加性噪声强度和乘性噪声强度的减函数, 是噪声关联时间的增函数; 两噪声之间正关联时, 平均灭绝时间是加性噪声强度和噪声关联时间乘性噪声强度的减函数, 是乘性噪声强度的非单调函数. 关键词： 集合种群 色交叉关联噪声 稳定性 平均灭绝时间     

Ecological competition and the role of an apex predator

In this work we propose an agent-based model to study the process of ecological competition between two species coexisting with a common predator, including inter and intra-specific competition. Without the predator, the ecosystem always reaches a configuration in which one of the prey species faces extinction and the other survives, as is well known. However, we find that there is an optimum value for the amount of food consumed by the prey species for which the mean extinction time (MET) is maximum. In the presence of predators, the average extinction time may increase or decrease with respect to the non-predator case, depending on the food availability. It is also observed that the average extinction time has an optimum value depending on the efficiency of prey of the predator.     

交叉增益调制的全光波长转换的消光比特性分析

建立了基于半导体光放大器交叉增益调制的波长转换理论模型,讨论了放大器的偏置电流、抽运光参数以及探测光参数对转换后信号消光比特性的影响,结果表明,大的抽运光功率小的探测光功率以及合适的转换波长间隔有利于获得大的消光比。     

纳米碳纤维导电性对红外消光性能的影响

纳米碳纤维是一种新型吸波材料。为探讨烟幕粒子的红外消光特性,利用矩量法建立了纳米碳纤维感应电流和散射场的计算模型,在此基础上得出了纳米碳纤维吸收、散射和消光截面的计算表达式。通过Matlab编程计算分析了纳米碳纤维红外消光截面与电导率的关系。结果表明,纳米碳纤维红外消光特性与电导率密切相关,随着电导率的增加,消光截面迅速增加,当波长大于纤维本身长度时,消光截面随电导率的变化出现峰值,峰值电导率与波长有关。     

二氧化钒纳米点阵红外光学特性研究

针对二氧化钒纳米点阵从半导体到金属的可逆相变,考虑到点阵中各个点之间散射光的交互作用,基于VO2在不同温度和波长下的折射率和消光系数,以及小颗粒的吸收和散射特性,建立了VO2纳米颗粒的数学模型,研究了VO2纳米颗粒的相变光学特性.结果表明,随着波长变化,吸收截面相对散射截面占主导,金属相在980 nm附近出现吸收峰值|随着温度变化,可见光区域的消光系数变化较小,而红外区域较大,其中在近红外区域的消光系数变化最大.在纳米点阵中,消光截面随着颗粒间距变化,当颗粒间距增大时,消光峰值出现红移,且峰值大小也会随之增大|当间距超过一定数值后,峰值反而会逐渐减小.采用多孔氧化铝掩模的方法,通过磁控反应溅射制备VO2纳米点阵,测试结果表明其透过率比薄膜的透过率高.     

基于Mie理论的水雾粒子多光谱消光特性研究

基于Mie散射理论,建立了水雾粒子的消光模型,计算了水雾粒子的多光谱消光特性,分析了水雾的消光特性与水雾粒子粒径之间的关系。结果表明:不同波长条件下,最佳水雾消光性能所对应的水雾粒子粒径也不同;半径在0.22~0.53μm的水雾粒子对可见光的消光性能最好;半径在0.81~8.15μm的水雾粒子对中、远红外辐射的消光性能最好;对于1.06μm和10.6μm两个军用激光常见波长,水雾粒子半径应分别为0.89μm和8.15μm,才能达最佳消光效果。     

Radiation transfer in absorbing-scattering liquids—II. Comparisons of measurements with predictions

Comparisons have been made between measured and predicted results for the radiation field in both highly absorbing and scattering aqueous suspensions with a highly reflecting or absorbing bottom. Predictions were based on a discrete ordinate solution to the equation of transfer, with the requisite properties (extinction coefficient, absorption coefficient and scattering phase function) obtained from measurements performed on samples extracted from the suspensions. All trends in the data were predicted by the model, although poor numerical agreement was obtained for suspensions of large albedo and extinction coefficient. The disagreement is attributed to the effect of multiple scattering on the property measurements.     

The two-dimensional structure of low strain rate counterflow nonpremixed-methane flames in normal and microgravity

The structure and extinction of low strain rate nonpremixed methane–air flames was studied numerically and experimentally. A time-dependent axisymmetric two-dimensional (2D) model considering buoyancy effects and radiative heat transfer was developed to capture the structure and extinction limits of normal gravity (1-g) and zero gravity (0-g) flames. For comparison with the 2D modelling results, a one-dimensional (1D) flamelet computation using a previously developed numerical code was exercised to provide information on the 0-g flames. A 3-step global reaction mechanism was used in both the 1D and 2D computations to predict the measured extinction limit and flame temperature. Photographic images of flames undergoing the process of extinction were compared with model calculations. The axisymmetric numerical model was validated by comparing flame shapes, temperature profiles, and extinction limits with experiments and with the 1D computational results. The 2D computations yielded insight into the extinction mode and flame structure. A specific maximum heat release rate was introduced to quantify the local flame strength and to elucidate the extinction mechanism. The contribution by each term in the energy equation to the heat release rate was evaluated to investigate the multi-dimensional structure and radiative extinction of the 1-g flames. Two combustion regimes depending on the extinction mode were identified. Lateral heat loss effects and multi-dimensional flame and flow structure were also found. At low strain rates in 1-g flames (‘regime A’), the flame is extinguished from the weak outer edge of the flame, which is attributed to a multi-dimensional flame structure and flow field. At high strain rates, (‘regime B’), the flame extinction initiates near the flame centreline owing to an increased diluent concentration in the reaction zone, similar to the extinction mode of 1D flames. These two extinction modes can be clearly explained by consideration of the specific maximum heat release rate.     

相对湿度对大气气溶胶消光系数的影响

消光系数是气溶胶颗粒物光学性质的关键参数之一。基于Mie散射理论和气溶胶颗粒粒径及折射率的湿度增长模型,本文详细研究了气溶胶吸湿性对其消光系数的影响。计算结果表明:当入射光波长为808nm、相对湿度在60%~95%之间增加时,因颗粒粒径的增长及折射率的减小,导致颗粒物消光系数曲线右移且其波动性更加明显;而对于同一球形颗粒,小粒径颗粒的消光系数随相对湿度的增加呈指数规律增大,较大粒径颗粒的则表现为波动减小趋势。此外,在不同入射光条件下,小颗粒消光系数与相对湿度之间的关系保持不变。本文的计算结果对气溶胶光学性质与大气环境质量的研究具有一定的参考价值。     

Extinction phase transitions in a model of ecological and evolutionary dynamics

We study the non-equilibrium phase transition between survival and extinction of spatially extended biological populations using an agent-based model. We especially focus on the effects of global temporal fluctuations of the environmental conditions, i.e., temporal disorder. Using large-scale Monte-Carlo simulations of up to 3 × 10⁷ organisms and 10⁵ generations, we find the extinction transition in time-independent environments to be in the well-known directed percolation universality class. In contrast, temporal disorder leads to a highly unusual extinction transition characterized by logarithmically slow population decay and enormous fluctuations even for large populations. The simulations provide strong evidence for this transition to be of exotic infinite-noise type, as recently predicted by a renormalization group theory. The transition is accompanied by temporal Griffiths phases featuring a power-law dependence of the life time on the population size.