高超声速二元进气道脉冲起动双波结构的理论与数值研究

为了研究高超声速进气道的脉冲起动特性,对二元高超声速进气道在脉冲风洞中流场建立过程进行了数值模拟,并根据运动激波关系式建立了双波结构数学模型,分析了进气道脉冲起动过程中双波结构的产生及传播过程。研究表明通过理论计算得到的双波结构气动参数与仿真结果基本一致。结合仿真发现进气道脉冲起动过程流场的主要特征包括:运动激波与斜激波相交、激波/边界层相互干扰以及分离包的空间位置和自身体积变化、进气道自起动过程等。另外结合双波结构计算模型,通过仿真研究了影响双波结构流动参数及影响进气道脉冲起动特性的因素,发现初始条件影响进气道的脉冲起动特性主要是通过改变双波结构的强度实现的。

铝锂合金材料的极限应变率软化特性研究

通过系列准静态与动态力学实验得到了铝锂合金材料的基本力学参数。证实了该种材料具有独特的应变率软化特性 ,并揭示了材料应变率软化效应的极限性特征 ,提出了极限应变率软化材料的概念 ,从而对突加载荷下粘塑性材料中也会出现双波结构的现象给出了合理的解释。为便于实际应用 ,还对实验得到的材料应力 应变曲线进行了拟合 ,提出了一种便于实际应用的材料本构关系的具体形式 ,即双幂次应变硬化 极限幂次应变率软化本构关系。

Design and analysis of dual-mode structure repetitive control based hybrid current regulation scheme for active power filters

An all-digital hybrid current regulation scheme for the single-phase shunt active power filter （APF） is presented. The proposed hybrid current control scheme integrates the deadbeat control and the dual-mode structure repetitive control （DMRC） so that it can offer superior steady-state performance and good transient features. Unlike the conventional schemes, the proposed scheme-based APF can compensate both the odd and the even order harmonics in grid. The detailed design criteria and the stability analysis of the proposed hybrid current controller are presented. Moreover, an improved structure which incorporates the proposed hybrid controller and the resonant controller for tracking specific order harmonics is given. The relationships between the resonant controller and different repetitive control schemes are discussed. Experimental results verify the effectiveness and advantages of the proposed hybrid control scheme.

NOVEL MICROSTRIP DUAL-MODE BANDPASS FILTER WITH COUPLING ENHANCEMENT AND WIDEBAND SPURIOUS RESPONSE SUPPRESSION

The microstrip dual-mode filter （DMF） with conventional coupling structure has some limitations in- eluding the port coupling strength limited by fabrication tolerance and the existence of serious second order spuri- ous band. Therefore, a novel DMF with a offset-feed bended coupling structure and a stepped-impedance dual- mode resonator is proposed for coupling enhancement and spurious response suppression. Based on the analysis of the change of spur frequencies and the current distribution of spur resonant modes, all spurs near passband of the cascaded DMF can be fully suppressed by optimizing the structure parameters of parasite resonators, which bene- fits from the inherent well-controlled transmission zeros. Experimental results show that the proposed DMF ex- hibits lower insertion loss ,much sharper rate of cutoff and wider spur-free stop band compared with conventional DMF. This design is applicable for spur suppression in wideband communication.

Seasonal variability of the North Equatorial Current transport in the western Pacific Ocean

Seasonal variability of the North Equatorial Current （NEC） transport in the western Pacific Ocean is investigated with ECMWF Ocean Analysis/Reanalysis System 3 （eRA-S3）. The result shows that NEC transport （NT） across different longitudes in the research area shows a similar double-peak structure, with two maxima （in summer and winter）, and two minima （in spring and autumn）. This kind of structure can also be found in NEC geostrophic transport （NGT）, but in a different magnitude and phase. These differences are attributable to Ekman transport induced by the local meridional wind and transport caused by nonzero velocity at the reference level, which is assumed to be zero in the NGT calculation. In the present work, a linear vorticity equation governing a 1.5-layer reduced gravity model is adopted to examine the dynamics of the seasonal variability of NGT. It is found that the annual cycle of NGT is mainly controlled by Ekman pumping induced by local wind, and westward-propagating Rossby waves induced by remote wind. Further research demonstrates that the maximum in winter and minimum in spring are mostly attributed to wind east of the dateline, whilst the maximum in summer and minimum in autumn are largely attributed to that west of the dateline.

Experimental Research on Erbium-doped Superfluorescent Fiber Source

The effects of the variations in fiber length,fiber mirror reflectance on efficiency and output power are experimentally investigated for erbium-doped double pass backward superfluorescent fiber sources (SFSs).The influence of fiber length on mean wavelength stability (MWS) has also been demonstrated.By incorporating a short section of un-pumped erbium-doped fiber (EDF) at the output port,the pump power dependent on MWS becomes independent of pumped EDF length.This is a novel phenomenon that hasn’t been reported up to now,and should be helpful to SFS fabrication and theory analysis.By using a fiber Michelson interferometer as spectrum slicing component,a multi-wavelength fiber source (MWFS) with ～20 channels (from 1 542 nm to 1 559 nm) is got.The MWFS has a channel spacing of ～0.8 nm which satisfies ITU-standard.The intensity fluctuation among channels is less than 0.5 dB,and the extinction ratio of all channels is above 14 dB.This kind of MWFS should be useful to wavelength division multiplexing systems.

A novel denoising method for infrared image based on bilateral filtering and non-local means

This paper presents an image denoising method based on bilateral filtering and non-local means. The non-local region texture or structure of the image has the characteristics of repetition, which can be used to effectively preserve the edge and detail of the image. And compared with classical methods, bilateral filtering method has a better performance in denosing for the reason that the weight includes the geometric closeness factor and the intensity similarity factor. We combine the geometric closeness factor with the weight of non-local means, and construct a new weight. Experimental results show that the modified algorithm can achieve better performance. And it can protect the image detail and structure information better.

Nonlinear investigation of beam-wave interaction in double-groove loaded folded-waveguide traveling-wave tube

A W-band traveling-wave tube (TWT) with double-groove loaded folded waveguide structure (FWSWS) has been designed and numerically modelled. The nonlinear performance of such a TWT is investigated by a particle-in-cell code MAGIC3D. Simulation results indicate this TWT produces a saturated electromagnetic power of 170.2 W at 90 GHz, corresponding to 36.9 dB gain and 69.6 mm interaction distance. A comparison between the novel folded waveguide traveling-wave tube (FWTWT) and the conventional one is also carried out to verify the effect of groove loading on the large-signal performance of TWT. Within the same working conditions, the double groove-loaded FWTWT could obtain higher saturated output power and gain in a shorter interaction length. The maximum of output power and gain of this novel TWT is 58.6% and 10% higher than those of the conventional FWTWT, while the 3-dB bandwidth of TWT is reduced to 4 GHz. With the additional advantage of ease of fabrication based on micro-electro-mechanical systems (MEMS) technologies, the double-groove loaded FWSWS is suitable for a millimeter-wave TWT with high power capacity and gain.