Overshoot phenomena:Observation and simulation

High-power O-mode radio waves can excite artificial instabilities in the F region,according to experiments conducted at the European Incoherent Scatter Science Association(EISCAT)heating facility.The main instabilities include the parametric decay instability(PDI),oscillating two-stream instability(OTSI),and thermal parametric instability(TPI).The PDI and OTSI not only compete with each other,but also compete with the TPI,leading to a two-stage overshoot phenomenon:a miniovershoot occurs on a millisecond time scale after pump-on,followed by the main overshoot.We gain insight into the miniovershoot via a generalized Zakharov model,whereas the main overshoot can be observed as an enhanced plasma line overshoot phenomenon in incoherent scatter radar spectra.We can also observe that the zero-frequency ion line exists only in the initial heating period after a cold start and that the upshifted and downshifted ion lines behave irregularly in the spectra.The simulation results show that competition between the PDI and OTSI leads to an initial peak,which we named the pre-miniovershoot.The following processes,namely ion density caviton generation,and collapse and cascade in the development of the PDI,contribute to the miniovershoot phenomenon.

ANALOG PREDISTORTION LINEARIZER ON REDUCING CORRECTED AMPLITUDE OVERSHOOT

This paper presents a dual-nonlinear branch linearizer for reducing the corrected amplitude overshoot of conventional single nonlinear branch linearizer. Theoretical analysis is carried out, the analysis is verified by simulation, and a prototype of Ka band 25.28~26.08 GHz dual nonlinear branch linearizer is achieved. It indicates that the corrected amplitude overshoot is less than 0.5 dB, the C/I3 improvement is more than 10 dB related to a single carrier IBO 9 dB, when it is linked and tested for 50 W spacebrone Travelling Wave Tube Amplifier(TWTA).

Analysis of Energy Overshoot of High Frequency Waves with Wavelet Transform

A study is made on the overshoot phenomena in wind-generated waves. The surface displacements of time-growing. waves are measured at four fetches in a wind wave channel. The evolution of high frequency waves is displayed with wavelet transform. The results are compared with Sutherland's. It is found that high frequency wave components experience much stronger energy overshoot in the evolution. The energy of high frequency waves decreases greatly after overshoot.

Erratum to“A note on overshoot estimation in pole placements”

There is a mild flaw in the statement of Proposition 2.1 in the above paper （cf. [1] ）. We restate it as follows.

Causes and solutions of overshoot and undershoot and end swing in Hilbert-Huang transform

There are overshoot and undershoot phenomenon and end swing phenomenon in the cubic spline fitting in Hil- bert-Huang transform. The two problems influence data quality of the empirical mode decomposition seriously. The cubic spline fitting has been analysed, and the causes of producing the overshoot and undershoot phenomenon and the end swing phenomenon have been pointed out in this paper. Two new methods of cubic spline fitting and sine spline fitting and the new technique of handling the end points of the original data curve can completely re- move the overshoot and undershoot phenomenon and the end swing phenomenon on the condition of unchanging original data, and have the advantages of the continuous fitting functions and its continuous one order derivative, the simple and convenient calculations, the small calculation amount and the easy work on it.

Velocity overshoot of start-up flow for a Maxwell fluid in a porous half-space

Stokes＇ first problem has been investigated for a Maxwell fluid in a porous half-space for gaining insight into the effect of viscoelasticity on the start-up flow in a porous medium. An exact solution was obtained by using the Fourier sine transform. It was found that at large values of the relaxation time the velocity overshoot occurs obviously and the system exhibits viscoelastic behaviours. On the other hand, for short relaxation time the velocity overshoot disappears and the system exhibits viscous behaviours. A critical value of the relaxation time was obtained for the emergence of the velocity overshoot. Furthermore, it was found that the velocity overshoot is caused by both the viscoelasticity of the Maxwell fluid and the Darcy resistance resulting from the structure of the micropore in the porous medium.

A further note on overshoot estimation in pole placements

Recently, it is proved in the literature that for a given controllable pair （A, B） with A ∈R^n×n, B ∈R^n×m, and any λ ≥ 1, a gain matrix K can be designed so that ‖e^（A＋BK）t‖ ≤Mλ^Le^-λt, where M and L are constants independent of λ. Here, we show that M and L can be chosen much smaller than that proposed above. As a consequence, the estimation on overshoot of a transition matrix can be bounded more precisely. This can be regarded as a complement to the existing result.

Non-overshooting and Non-undershooting Cubic Spline Interpolation for Empirical Mode Decomposition

To suppress the overshoots and undershoots in the envelope fitting for empirical mode decomposition （EMD）, an alternative cubic spline interpolation method without overshooting and undershooting is proposed. On the basis of the derived slope constraints of knots of a non-overshooting and non-undershooting cubic interpolant, together with ＂not-a-knot＂ conditions the cubic spline interpolants are constructed by replacing the requirement for equal second order derivatives at every knot with Brodlie＇ s derivative formula. Analysis and simulation experiments show that this approach can effectively avoid generating new extrema, shifting or exaggerating the existing ones in a signal, and thus significantly improve the decomposition performance of EMD.

Characteristics of convection and overshooting in RGB and AGB stars

Based on the turbulent convection model （TCM） of Li ＆ Yang, we have studied the characteristics of turbulent convection in the envelopes of 2 and 5M⊙ stars at the red giant branch and asymptotic giant branch phases. The TCM has been successfully applied over the entire convective envelopes, including the convective unstable zone and the overshooting regions. We find that the convective motions become progressively stronger when the stellar models are located farther up along the Hayashi line. In the convective unstable zone, we find that the turbulent correlations are proportional to functions of a common factor （V - V^d）T, which explains similar distributions in those correlations. For the TCM we find that if the obtained stellar temperature structure is close to that of the mixing length theory （MLT）, the convective motion will have a much larger velocity and thus be more violent. However, if the turbulent velocity is adjusted to be close to that of the MLT, the superadiabatic convection zone would be much more extended inward, which would lead to a lower effective temperature of the stellar model. For the overshooting distance, we find that the e-folding lengths of the turbulent kinetic energy k in both the top and bottom overshooting regions decrease as the stellar model is progressively located farther up along the Hayashi line, but both the extents of the decrease are not obvious. The overshooting distances of the turbulent correlation /u＇rT＂ are almost the same for the different stellar models with the same set of TCM parameters. For the decay modes of the kinetic energy k, we find that they are very similar for different stellar models based on the same set of TCM parameters, and there is a nearly linear relationship between lg k and In P for different stellar models. When Cs or α increases while the other parameters are fixed, the obtained linearly decaying distance will become longer.

Spatio-temporal measurements of overshoot phenomenon in pulsed inductively coupled discharge

Pulse inductively coupled plasma has been widely used in the microelectronics industry,but the existence of overshoot phenomenon may affect the uniformity of plasma and generate high-energy ions,which could damage the chip.The overshoot phenomenon at various spatial locations in pulsed inductively coupled Ar and Ar/CF_(4) discharges is studied in this work.The electron density,effective electron temperature,relative light intensity,and electron energy probability function(EEPF) are measured by using a time-resolved Langmuir probe and an optical probe,as a function of axial and radial locations.At the initial stage of pulse,both electron density and relative light intensity exhibit overshoot phenomenon,i.e.,they first increase to a peak value and then decrease to a convergent value.The overshoot phenomenon gradually decays,when the probe moves away from the coils.Meanwhile,a delay appears in the variation of the electron densities,and the effective electron temperature decreases,which may be related to the reduced strength of electric field at a distance,and the consequent fewer high-energy electrons,inducing limited ionization and excitation rate.The overshoot phenomenon gradually disappears and the electron density decreases,when the probe moves away from reactor centre.In Ar/CF_(4) discharge,the overshoot phenomenon of electron density is weaker than that in the Ar discharge,and the plasma reaches a steady density within a much shorter time,which is probably due to the more ionization channels and lower ionization thresholds in the Ar/CF_(4) plasma.

Influence of Velocity Overshoot Effect on High Frequency Perform- ance of AlGaAs / GaAs HBT's

The semiclassical transport equations are used to study the high frequency performance of AlGaAs / GaAs HBTs. Electron velocity overshoot effect and its influence on the cut off frequency characteristics of AlGaAs / GaAs HBTs with different collector design parameters are analyzed and discussed.

Favorable Environments for the Occurrence of Overshooting Tops in Tropical Cyclones

Based on Multifunctional Transport Satellite data and the infrared window-texture detection algorithm, the level of overshooting top（OT） activity within a tropical cyclone（TC）, which is defined as the hourly mean number of OT occurrence,was statistically investigated in the western North Pacific basin for the period 2005–12. Based on the level of OT activity,the samples were divided into OT and non-OT cases or high-activity-OT（HA-OT） and low-activity-OT（LA-OT） cases. The differences in large-scale environmental variables between OT（HA-OT） and non-OT（LA-OT） cases were examined 12 hours prior to the OT occurrence. Statistical analysis showed that environmental differences did exist between the OT and non-OT cases. The OTs were more skewed towards the early stage of the TC life cycle, and mostly concentrated in low latitudes.Meanwhile, a sufficiently deep warm-water layer, large temperature difference between the upper- and lower-level troposphere, large humidity at the middle and upper levels, and large atmospheric instability, were favorable for OT occurrence.The differences in large-scale environmental characteristics between HA-OTs and LA-OTs were not as significant as those between OTs and non-OTs, but the HA-OT samples tended to occur when the vertical shear was weak and the TC intensity was low. Finally, statistical models were designed to predict the OT and HA-OT. When at least three OT（HA-OT） predictor thresholds were satisfied, the Peirce skill score reached a maximum value of 0.49（0.30）.

Does overshoot in leaf development of ponderosa pine in wet years leads to bark beetle outbreaks on fine-textured soils in drier years?

Background： Frequent outbreaks of insects and diseases have been recorded in the native forests of western North America during the last few decades, but the distribution of these outbreaks has been far from uniform. In some cases, recent climatic variations may explain some of this spatial variation along with the presence of expansive forests composed of dense, older trees. Forest managers and policy makers would benefit if areas especially prone to disturbance could be recognized so that mitigating actions could be taken. Methods： We use two ponderosa pine-dominated sites in western Montana, U.S.A. to apply a modeling approach that couples information acquired via remote sensing, soil surveys, and local weather stations to assess where bark beetle outbreaks might first occur and why. Although there was a general downward trend in precipitation for both sites over the period between 1998 and 2010 （slope =-1.3, R2 = 0.08）, interannual variability was high. Some years showed large increases followed by sharp decreases. Both sites had similar topography and fire histories, but bark beetle activity occurred earlier （circa 2000 to 2001） and more severely on one site than on the other. The initial canopy density of the two sites was also similar, with leaf area indices ranging between 1.7-2.0 m2. m-2. We wondered if the difference in bark beetle activity was related to soils that were higher in clay content at site I than at site II. To assess this possibility, we applied a process-based stand growth model （3-PG） to analyze the data and evaluate the hypotheses. Results： We found that when wet years were followed by drier years, the simulated annual wood production per unit of leaf area, a measure of tree vigor, dropped below a critical threshold on site I but not on site II. Conclusion： We concluded that the difference in vulnerability of the two stands to beetle outbreaks can be explained largely by differences in gross photosynthesis attributed to the fact that an equivalent amount of stored water in the rooting zone （100 mm） is extracted less efficiently from fine-textured soils than from coarse-textured ones.

货币政策、资产价格与消费者价格波动——基于庇古效应和超调理论的研究

首先基于价格超调理论和庇古财富效应理论,全面解析了货币政策对消费者物价指数(CPI)的动态影响机制,结果发现,货币政策不仅会直接影响CPI,而且会通过资产价格的超调效应间接影响到CPI.其次,本文进一步引入马尔科夫区制转移模型,分析了货币政策的不同区制特征,并从参数时变角度测算了货币政策对资产价格和CPI的动态影响关系.实证研究表明:第一,货币政策具有明显的扩张和收缩二区制特征.第二,短期内货币政策对资产价格较之CPI有超调效应,长期看货币政策通过资产价格的庇古中介效应间接影响CPI.第三,货币政策对资产价格和CPI均存在非对称影响,扩张性货币政策效应小于紧缩性货币政策效应,同时资产价格对CPI同样存在非对称性影响,“损失规避”效应明显.

基于BPNN-PID控制策略的果蔬保鲜环境参数调控优化

【目的】开发新的控制策略,用于解决传统控制方法果蔬保鲜环境参数因时变性、非线性、滞后性强和惯性大等特点导致的控制精度低、鲁棒性弱等问题。【方法】将传统比例−积分−微分(Proportional-integral-derivative,PID)和BP神经网络(Back-propagation neural network,BPNN)算法相结合,开发一种基于BPNN-PID的控制策略,通过自主搭建的果蔬保鲜环境调控试验平台和自主设计的控制系统,研究不同控制策略对保鲜环境参数调控效果的影响。【结果】基于BPNN-PID控制策略的果蔬保鲜环境控制系统,环境温度超调量为1.7℃、稳定时间为80 min、稳态误差为±0.2℃,环境相对湿度超调量为2.8%、稳定时间为55 min,相对湿度稳定维持在80%~90%范围内。与传统PID控制策略相比,BPNN-PID控制策略环境温度超调量减小了2.1℃、稳态误差减小了0.3℃、稳定时间缩短了25 min,环境相对湿度超调量减小了2.2%、稳定时间缩短了25 min,环境参数波动幅度均有所降低。【结论】本文开发的果蔬保鲜环境控制系统呈现出良好的动态调整能力,具有较强的鲁棒性,控制性能明显提升,实现了保鲜环境参数的精准控制,满足果蔬保鲜贮藏要求。研究结果为果蔬保鲜环境参数调控提供了参考。

孤岛微电网多虚拟同步发电机频率无差协调控制策略

传统虚拟同步发电机的频率控制为有差控制,且虚拟惯量的引入使得虚拟同步发电机呈现二阶振荡特性,导致孤岛微电网多虚拟同步发电机并联系统在受到负荷扰动时面临频率偏移和有功超调的问题。针对此,提出了一种多虚拟同步发电机频率无差协调控制方法。该方法包含了兼具有功精确分配的频率无差调节控制和阻尼改进控制两部分,通过利用母线频率全局一致的“通信”效果,以分散式的形式协调系统内各台虚拟同步发电机,可主动消除微电网频率稳态偏差且实现各台虚拟同步发电机按照容量承担负荷有功功率,并抑制系统的有功超调。最后,通过仿真和实验验证了所提控制方法的有效性。

低排放情景下全球极端气候事件变化在温升过冲前后达到1.5℃的差异

基于耦合模式比较计划第六阶段(CMIP6)全球气候模式在SSP1-1.9温室气体排放情景下的模拟结果,预估了全球平均气温相对于工业化前达到1.5℃温升(P1阶段)后继续增暖然后再次返回(过冲)到1.5℃温升时的(P2阶段)全球气温、降水及极端气候指数的可能变化,并且分析其预估不确定性。结果表明:P1和P2两阶段间气温、降水及极端气候的多模式一致的差异在全球各分区广泛出现,且区域性和局地性特征明显。各指标表征的多模式一致的极端温度变化普遍接近或者超过全球陆地面积的15%。极端低温的P1和P2两阶段差异的空间分布与冬季平均气温差异的空间分布有一定相似度,极端高温变化的分布则更凸显局地性。全球范围内低温和高温多模式一致增加的面积都高于其减少的面积,预计欧亚大陆中高纬的西部、北美洲、中国东北等区域的低温风险以及青藏高原、中国东部、南亚、东非、北美洲、南极洲等区域的高温风险会升高。各指标表征的多模式一致的极端降水变化普遍超过全球陆地面积的20%,其中增加与减少的面积接近。强降水差异的分布特征与年降水的有部分相似,一致增多主要分布在中国南方、南亚、东南亚、南美洲东部和西南端、北美、澳洲和中东欧的部分地区等,一致减少主要分布在中国华北到东北、青藏高原南麓、非洲南部、南美洲北部和澳洲北部等。连续干旱日数在多数区域表现为增多,集中且连续地分布在中亚、南亚、青藏高原、俄罗斯中北部、非洲撒哈拉以北和中部、澳洲中部、南极洲部分地区等。这些都表明即使全球在2030年前达到碳排放峰值,并立即开始减少碳排放,但由于各个区域气候对全球变暖的响应有明显差异,温升过冲后部分区域的极端事件发生频率并未及时回退到过冲前的水平,需警惕区域和局地尺度气象灾害及其影响的增加。

基于模型参考自适应的水肥一体化控制系统设计

针对水肥混合过程中水肥EC值精准控制的问题,设计一个基于模型参考自适应的水肥一体化决策和控制系统。建立母液动态调控模型,并通过该模型设计模型参考自适应控制器。仿真试验结果显示,模型参考自适应控制相较于模糊PID控制,在跟踪试验中超调量降低1.2%,调整时间减少12.6 s,阶跃试验中超调量降低14.4%,调整时间减少超过2 s,抗干扰能力优于模糊PID控制。通过3种方式进行田间试验,对比模糊PID控制和人工灌溉方式,采用模型参考自适应控制能够使番茄株高分别提高15.7%和35.6%,茎粗分别增长11.1%和18.6%,产量分别提升23.3%和48.1%,结果表明,模型参考自适应控制优于模糊PID控制和人工灌溉方式,从而验证模型参考自适应控制的有效性,为精准施肥灌溉提供技术支持。

Implementation of a particle-in-cell method for the energy solver in 3D spherical geodynamic modeling

The thermal evolution of the Earth’s interior and its dynamic effects are the focus of Earth sciences.However,the commonly adopted grid-based temperature solver is usually prone to numerical oscillations,especially in the presence of sharp thermal gradients,such as when modeling subducting slabs and rising plumes.This phenomenon prohibits the correct representation of thermal evolution and may cause incorrect implications of geodynamic processes.After examining several approaches for removing these numerical oscillations,we show that the Lagrangian method provides an ideal way to solve this problem.In this study,we propose a particle-in-cell method as a strategy for improving the solution to the energy equation and demonstrate its effectiveness in both one-dimensional and three-dimensional thermal problems,as well as in a global spherical simulation with data assimilation.We have implemented this method in the open-source finite-element code CitcomS,which features a spherical coordinate system,distributed memory parallel computing,and data assimilation algorithms.

基于FOA优化PID参数的永磁同步电机转速控制

为提高永磁同步电机转速控制的效果,提出一种基于果蝇优化算法(Fruit Fly Optimization Algorithm,FOA)优化比例积分微分控制(proportional-integral-derivative control)的方法。其中,以PMSM调速系统为背景,构建调速系统的PID方法,然后构建FOA优化PID参数的PID控制器,以实现永磁同步电机转速系统的自适应控制。仿真结果表明,FOA-PID方法具有响应速度快、超调小、抗干扰能力和速度调节能力强的特点,在固定负载改变转速条件下,仅需0.02 s即可达到稳定状态,有效改善了永磁同步电机转速控制系统的控制性能;相较于标准PID和RBF-PID方法,FOA-PID方法在正向起动转速情况下的超调量为1.36%,分别低5.73%和1.12%;在负向起动转速情况下,FOA-PID方法的最大超调量为0.56%,分别低15.13%和8.22%。由此得出,本FOA-PID方法可行,具有一定的优越性。