STABILITY OF TRANSONIC SHOCKS TO THE EULER-POISSON SYSTEM WITH VARYING BACKGROUND CHARGES

This paper is devoted to studying the stability of transonic shock solutions to the Euler-Poisson system in a one-dimensional nozzle of finite length.The background charge in the Poisson equation is a piecewise constant function.The structural stability of the steady transonic shock solution is obtained by the monotonicity argument.Furthermore,this transonic shock is proved to be dynamically and exponentially stable with respect to small perturbations of the initial data.One of the crucial ingredients of the analysis is to establish the global well-posedness of a free boundary problem for a quasilinear second order equation with nonlinear boundary conditions.

不均匀海底地形影响下柔性板与斜向来浪的相互作用

The present work analyzes the interaction of oblique waves by a porous flexible breakwater in the presence of a step-type bottom.The physical models for both scattering and trapping cases are considered and developed within the framework of small amplitude water-wave theory.Darcy’s law is used to model the wave interaction with the porous medium.It is assumed that the varying bottom extends over a finite interval,connected by a finite length of uniform bottom near an impermeable wall,and a semi-infinite length of bottom in the open water region.The boundary value problem is solved using the eigenfunction expansion method in the uniform bottom regions,while a modified mild-slope equation(MMSE)is used for the region with the varying bottom.Additionally,a mass-conserving jump condition is employed to handle the solution at slope discontinuities in the bottom.A system of equations is derived by matching the solutions at interfaces.The reflection coefficient and force on the breakwater and impermeable wall are plotted and analyzed for various parameters,such as the length of the varying bottom,depth ratio,angle of incidence,and flexural rigidity.It is observed that moderate values of flexural rigidity and depth ratio significantly contribute to an optimum reflection coefficient and reduce the wave force on the wall and breakwater.Remarkably,the outcomes of this study are assumed to be applicable in the construction of this type of breakwater in coastal regions.

Locally and globally uniform approximations for ruin probabilities of a nonstandard bidimensional risk model with subexponential claims

Consider a nonstandard continuous-time bidimensional risk model with constant force of interest,in which the two classes of claims with subexponential distributions satisfy a general dependence structure and each pair of the claim-inter-arrival times is arbitrarily dependent.Under some mild conditions,we achieve a locally uniform approximation of the finite-time ruin probability for all time horizon within a finite interval.If we further assume that each pair of the claim-inter-arrival times is negative quadrant dependent and the two classes of claims are consistently-varying-tailed,it shows that the above obtained approximation is also globally uniform for all time horizon within an infinite interval.

Efficient simulation of spatially correlated non-stationary ground motions by wavelet-packet algorithm and spectral representation method

Although the classical spectral representation method(SRM)has been widely used in the generation of spatially varying ground motions,there are still challenges in efficient simulation of the non-stationary stochastic vector process in practice.The first problem is the inherent limitation and inflexibility of the deterministic time/frequency modulation function.Another difficulty is the estimation of evolutionary power spectral density(EPSD)with quite a few samples.To tackle these problems,the wavelet packet transform(WPT)algorithm is utilized to build a time-varying spectrum of seed recording which describes the energy distribution in the time-frequency domain.The time-varying spectrum is proven to preserve the time and frequency marginal property as theoretical EPSD will do for the stationary process.For the simulation of spatially varying ground motions,the auto-EPSD for all locations is directly estimated using the time-varying spectrum of seed recording rather than matching predefined EPSD models.Then the constructed spectral matrix is incorporated in SRM to simulate spatially varying non-stationary ground motions using efficient Cholesky decomposition techniques.In addition to a good match with the target coherency model,two numerical examples indicate that the generated time histories retain the physical properties of the prescribed seed recording,including waveform,temporal/spectral non-stationarity,normalized energy buildup,and significant duration.

Probabilistic Global Maximum Power Point Tracking Algorithm for Continuously Varying Partial Shading Conditions on Autonomous PV Systems

A photovoltaic (PV) string with multiple modules with bypass diodes frequently deployed on a variety of autonomous PV systems may present multiple power peaks under uneven shading. For optimal solar harvesting, there is a need for a control schema to force the PV string to operate at global maximum power point (GMPP). While a lot of tracking methods have been proposed in the literature, they are usually complex and do not fully take advantage of the available characteristics of the PV array. This work highlights how the voltage at operating point and the forward voltage of the bypass diode are considered to design a global maximum power point tracking (GMPPT) algorithm with a very limited global search phase called Fast GMPPT. This algorithm successfully tracks GMPP between 94% and 98% of the time under a theoretical evaluation. It is then compared against Perturb and Observe, Deterministic Particle Swarm Optimization, and Grey Wolf Optimization under a sequence of irradiance steps as well as a power-over-voltage characteristics profile that mimics the electrical characteristics of a PV string under varying partial shading conditions. Overall, the simulation with the sequence of irradiance steps shows that while Fast GMPPT does not have the best convergence time, it has an excellent convergence rate as well as causes the least amount of power loss during the global search phase. Experimental test under varying partial shading conditions shows that while the GMPPT proposal is simple and lightweight, it is very performant under a wide range of dynamically varying partial shading conditions and boasts the best energy efficiency (94.74%) out of the 4 tested algorithms.

Adaptive Density-Based Spatial Clustering of Applications with Noise(ADBSCAN)for Clusters of Different Densities

Finding clusters based on density represents a significant class of clustering algorithms.These methods can discover clusters of various shapes and sizes.The most studied algorithm in this class is theDensity-Based Spatial Clustering of Applications with Noise(DBSCAN).It identifies clusters by grouping the densely connected objects into one group and discarding the noise objects.It requires two input parameters:epsilon(fixed neighborhood radius)and MinPts(the lowest number of objects in epsilon).However,it can’t handle clusters of various densities since it uses a global value for epsilon.This article proposes an adaptation of the DBSCAN method so it can discover clusters of varied densities besides reducing the required number of input parameters to only one.Only user input in the proposed method is the MinPts.Epsilon on the other hand,is computed automatically based on statistical information of the dataset.The proposed method finds the core distance for each object in the dataset,takes the average of these distances as the first value of epsilon,and finds the clusters satisfying this density level.The remaining unclustered objects will be clustered using a new value of epsilon that equals the average core distances of unclustered objects.This process continues until all objects have been clustered or the remaining unclustered objects are less than 0.006 of the dataset’s size.The proposed method requires MinPts only as an input parameter because epsilon is computed from data.Benchmark datasets were used to evaluate the effectiveness of the proposed method that produced promising results.Practical experiments demonstrate that the outstanding ability of the proposed method to detect clusters of different densities even if there is no separation between them.The accuracy of the method ranges from 92%to 100%for the experimented datasets.

Modes of multi-mechanistic gas diffusion in shale matrix at varied effective stresses:Observations and analysis

Gas diffusion in the shale matrix has a dominant effect on late-stage production from shale gas reservoirs.However,adequate research on the mechanisms and contributions of gas diffusion for varied pore size populations in shale matrix under recreated in situ stress is lacking.We report gas-diffusion measurements under constant in situ stress but variable gas pressures for contrasting non-adsorbent(helium(He))and adsorbed(methane(CH_(4)))gases to investigate the impact of effective stress on the evolution of dominant mechanisms of diffusion.An intact sample replicates true pore-network topology and diffusion paths.An integrated diffusion model is proposed that combines the effects of slip flow,Knudsen flow,and surface diffusion to constrain the evolution of these flow regimes and their respective contributions to the observational data.Finally,a probability density function(PDF)is employed to separate the gas content distributions of macropores and micropores from the total gas content and to investigate gas contributions in various pores.The results reveal that the diffusion coefficients of both He and CH_(4) in macropores and micropores increase with gas pressure but decrease with increasing effective stress.The diffusion coefficients of He and CH_(4) are different in macropores but remain nearly the same in micropores.The diffusion coefficients of slip flow and surface diffusion increase with decreasing effective stress except for CH_(4) diffusion in the micropores,while the evolution of Knudsen diffusion shows the opposite trend.Slip flow plays a dominant role in He and CH_(4) diffusion within macropores(pore size 45 nm).Knudsen diffusion gradually becomes significant for He diffusion in the micropores(pore size 4 nm),conversely,for CH_(4) diffusion in the micropores,surface diffusion becomes significant.Related to gas production from reservoirs,the contributions of the micropores will increase gradually with the duration of gas recovery,indicating the significant role of gas diffusion in micropores to steady supply during latestage production.

Investigations on Fluid Resonance Within A Narrow Gap Formed by Two Fixed Bodies with Varying Breadth Ratios

The resonant motion of the fluid inside a narrow gap between two fixed boxes induced by incident regular waves with various wave heights is investigated by adopting a two-dimensional numerical wave flume based on an opensourced CFD package,OpenFOAM.The two boxes have identical draft and height,but the upstream box has a variable breadth.This article focuses on the influences of the breadth ratio,defined as the ratio of the breadth of the upstream box to that of the downstream box,on the following three aspects of hydrodynamic characteristics of gap resonance:(1)the wave height amplifications inside the gap,and in front and at the rear of the structure system,(2)the reflection,transmission,and energy loss coefficients of the structure system,and(3)the response and damping time of the fluid resonance.It is found that the fluid resonant frequency,the amplification factor of the resonant wave height inside the gap and the maximum energy loss coefficient of the structure system are shown to gradually decrease with the increase of the breadth ratio.The response time of gap resonance is shown to first increase and then decrease with the breadth ratio overall,regardless of the incident wave height,and the configuration that the two boxes have the same breadth would bring the largest response time of gap resonance.

高密肋结构整体壁板VARI成型技术研究

高密肋壁板结构可采用真空辅助树脂渗透工艺(VARI)整体一次成型,但成型过程中存在加筋区预定型困难、树脂流动控制复杂等问题,导致制件尺寸精度和孔隙缺陷难以保证。为此,本文根据高密肋壁板多特征区域的特点,采用压实实验和注胶仿真模拟分析方法,优化预定型工艺参数、分区注胶方案。结果表明,选择定型剂浓度4%、温度120℃、压力0.1 MPa、保压时间60 min的压实参数,可有效保证纤维体积分数;采用线注射、线冒口与筋条点冒口结合的分级注胶方案,可实现壁板结构的有效浸润。优化设计的工艺方案与工装成功应用于某型号高密肋整体壁板VARI成型制造,验证了本文分析方法的有效性,研究结果对壁板类结构低成本整体成型技术在军民飞机上的应用与发展具有重要参考价值。

真空成型体育器材用碳纤维增强PES树脂材料力学性能研究

在碳纤维内加入聚醚砜(PES)树脂法,利用真空辅助成型(Vacuum Assisted Resin Infusion,VARI)工艺制备得到改性复合材料,研究真空成型压力参数对体育器材用碳纤维增强PES树脂材料力学性能和拉伸断面的影响。研究结果表明:相对于改性前的PES树脂材料拉伸强度,经过0.1 MPa、0.25 MPa与0.5 MPa处理得到的改性复合材料在拉伸强度方面依次提升了17.36%、41.39%与29.54%;拉伸模量依次提升1.61%、19.35%与10.48%。相对于原始纤维,经过改性形成了更加粗糙的表面结构,使纤维与树脂可以发生更加紧密的结合,经过改性的PES树脂材料形成的断口具有明显的韧性断裂特点。在0.25 MPa下得到的改性PES树脂材料形成了明显断口结构,此时的试样获得了最优的拉伸强度与模量。

The Speed of Light Is Not Constant in Basic Big Bang Theory

Starting from the basic assumptions and equations of Big Bang theory, we present a simple mathematical proof that this theory implies a varying (decreasing) speed of light, contrary to what is generally accepted. We consider General Relativity, the first Friedmann equation and the Friedmann-Lema?tre- Robertson-Walker (FLRW) metric for a Comoving Observer. It is shown explicitly that the Horizon and Flatness Problems are solved, taking away an important argument for the need of Cosmic Inflation. A decrease of 2.1 cm/s per year of the present-day speed of light is predicted. This is consistent with the observed acceleration of the expansion of the Universe, as determined from high-redshift supernova data. The calculation does not use any quantum processes, and no adjustable parameters or fine tuning are introduced. It is argued that more precise laboratory measurements of the present-day speed of light (and its evolution) should be carried out. Also it is argued that the combination of the FLRW metric and Einstein’s field equations of General Relativity is inconsistent, because the FLRW metric implies a variable speed of light, and Einstein’s field equations use a constant speed of light. If we accept standard Big Bang theory (and thus the combination of General Relativity and the FLRW metric), a variable speed of light must be allowed in the Friedmann equation, and therefore also, more generally, in Einstein’s field equations of General Relativity. The explicit form of this time dependence will then be determined by the specific problem.

真空辅助树脂注射成型(VARI)研究进展

复合材料低成本制造工艺中,VARI是一种很重要的成型技术,本文对VARI的优点、难点、专用树脂体系、成型方法、树脂分配系统、树脂流动模拟及其国内外应用进行了综述。研究表明,VARI作为一种复合材料的低成本成型技术,已得到广泛应用,尤其是大尺寸、大厚度制件的制造。发展VARI工艺对于降低复合材料的制造成本、扩大复合材料的应用范围有很大作用。由于VARI常用于大尺寸结构件的制造,树脂充模过程的精确模拟对制品质量至关重要,目前国内对此研究仍停留在初级阶段,该方面研究需要引起重视。

真空辅助成型技术及其配套基体树脂研究进展

综述了国内外关于真空辅助成型技术(VARI)及其配套基体树脂的研究和应用状况,介绍了VARI成型技术的主要原理、工艺流程及其对基体树脂的基本要求,并对国内外在VARI成型技术和配套基体树脂领域中的进展进行了适当的比较和分析。

树脂基复合材料成型工艺研究进展

主要综述了树脂基复合材料的几种成型工艺,包括RTM、VARTM、CRTM、LRTM、RFI、VARI、SCRIMP、SRIM、TERM,各自的发展现状、成型原理、特点等。

一类基于进入过程的风险模型的精细大偏差

Li和Kong提出了一类基于进入过程的具有多种独立保单的风险模型并在时间趋于无穷大时对保险公司盈余资产的渐进分布作了研究.该文探讨了这类模型的大偏差.当索赔尾部为C(consistently varying)族时,分别得到了具有单种保单和多种独立保单的模型的大偏差,所得结果丰富了相应的发射噪声过程的大偏差.

VARI工艺制备典型平面及曲面构件的流道设计模拟研究

对典型平面及典型曲面构件进行了VARI工艺模拟分析,分别采用不同的模拟方案对两类构件进行流道设计模拟研究,由此得出了两类构件工艺流道设计的异同点,旨在针对不同构件的结构特点,选择最佳的注射方式及合理的流道设计方案,从而为模具设计及构件制造提供重要依据。

低丘红壤茶园的磷素营养及转化(英文)

试验结果表明,在低丘红壤茶园土壤中的磷主要以无机矿物磷为主,其中又以闭蓄态磷占多数,铁磷和铝磷与茶园生产能力有密切关系。茶园土壤对磷的吸附符合Langmuir吸附曲线(r>0.9)。土壤pH及质地对磷吸附有明显影响。磷肥施入茶园以后转化和被固定很快,开始以铝磷为主,以后逐步向铁磷和闭蓄态磷转化。土壤有机酸对防止磷固定有明显的影响,其影响程度是:草酸>混合酸>柠檬酸>酒石酸>苹果酸>乳酸>琥珀酸。被固定的磷及土壤原生矿物磷,在土壤游离酸,有机酸等作用下,又逐渐释放,越是高产茶园释放量越多。所以茶园土壤中的磷始终处于有固定又有释放,有释放又有固定的动态变化中。

真空辅助成型(VARI)工艺研究进展

主要介绍了真空辅助成型的工艺原理、特点以及技术要求等,并从VARI专用树脂体系、树脂流动行为和计算机模拟等方面阐述了VARI工艺技术的研究进展,指出了当前VARI工艺研究中存在的一些问题,并提出了解决思路。

真空辅助树脂灌注配套基体树脂的制备及性能

针对真空辅助成型技术(VARI)对基体树脂的特殊要求,结合航空材料对性能的高要求,研制了BA9912中温固化环氧树脂体系。采用VARI工艺制备了G0827/BA9912复合材料,测试了BA9912树脂浇注料及其复合材料的力学性能和耐热性能,并与国内外同类树脂进行了适当的比较分析。分析测试结果表明,BA9912树脂具有良好的力学性能、耐热性能和工艺性能,能够满足VARI成型工艺要求,适合在航空航天领域中应用。

真空导入工艺参数对复合材料孔隙含量的影响

以舰船构件为对象,通过玻璃纤维/乙烯基酯树脂复合材料的真空导入成型实验,研究了真空度、树脂温度参数对制件孔隙含量的影响规律。结果表明:真空度大小对孔隙率含量有显著影响,当真空度为80～90kPa时孔隙含量较低;低于20℃时,温度对孔隙含量有明显的影响,温度高于20℃时孔隙含量变化不大;沿树脂流动方向,试件孔隙含量由低到高变化;沿试件从上到下的厚度方向,孔隙含量也呈由低到高增加的趋势。