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131.
针对非线性、非平稳航空发动机双发差异信号,提出利用瞬时能量密度谱凸显微波动信号这一方法获得双发差异波动值出现的时间。通过总体平均经验模式分解(EEMD)结合希尔伯变换(HT)求解信号的HT谱与瞬时能量密度水平,从频率、幅值与时间三方面给出信号特征。以燃油流量为例,分析了航空发动机气路参数差值在巡航段的波动情况。结果表明:在2095s、3600s左右发动机燃油流量差值(ΔFF)出现微弱波动,且后者波动幅值为前者的1/5,频率范围集中在0-0.2Hz之间。2095s信号波动是由飞机飞行高度、航向变化及加速度变化引起的,3600s信号波动是加速度变化导致的。由瞬时能量密度谱获得航空发动机参数微波动发生的精确时间,并与QAR记录的同时刻其他相关数据进行对比,追溯发动机FF变化的原因,判断航空发动机是否存在故障。 相似文献
132.
基于杭州城西9个街区1 365份大型超市出行数据,利用居民个体属性、出行距离和土地利用特征3组变量,构建了多项逻辑斯蒂克决策模型,分析了街区土地利用特征对大型超市出行模式的影响.结论为大城市街区的“可达性提高”和“商住混合度增加”,可以明显提高大型超市出行的步行(2 km内)、自行车(4 km内)选择概率;公交服务水平的提升可鼓励大型超市出行的非机动化,但公交密度本身不足以抑制小汽车的选择.提出在“社区商圈”尺度中,有必要遵循空间组织的渐进规划思路,通过“土地利用-交通”一体化措施,优化大型超市的出行结构. 相似文献
133.
Sound relaxational absorption spectroscopy of excitable gas mixtures is potentially applied for gas composition detection. The relaxation of vibrational modes of gas molecules determines the sound relaxational absorption. However, to our knowledge, the contribution of each vibrational mode available in gas mixtures to sound multi-relaxation absorption has not been calculated in existing literature. In this paper, based on the decoupled expression of the effective isochoric molar heat for a gas mixture, a sound multi-relaxation absorption spectrum is decomposed into the sum of single-relaxation spectra. From this decomposable characteristic, the contribution of each vibrational mode available in the gaseous medium to the multi-relaxation absorption is obtained at room temperature. For various gas compositions including carbon dioxide, methane, nitrogen etc., the calculated contributions of vibrational modes are verified by the comparison with experiment data. We prove the following views with quantifiable outcomes that the primary molecular relaxation process associated with the lowest mode plays the major role in acoustic relaxational absorption of gas mixtures; the mode with lower vibrational frequency provides higher contribution to the primary relaxation process. This work could provide a deeper insight into the relationship between the sound relaxational absorption spectroscopy and gas molecules. 相似文献
134.
Prestressed stayed columns are an innovative type of structural system where the compressive load-carrying capacity can be enhanced through prestressed external cable stays. A nonlinear analytical model for prestressed stayed columns with multiple crossarm systems along the column length, based on the Rayleigh–Ritz method, is presented that captures modal interactions for perfect geometries explicitly for the first time. It is demonstrated that the theoretical compressive strength enhancements under certain configurations can only be obtained at the expense of triggering a sequence of destabilizing bifurcations. This can give rise to dangerously unstable interactive post-buckling behaviour including so-called ‘mode jumping’ and ‘snaking’ phenomena. Parametric spaces where the system is most susceptible to the modal interactions are identified and it is for those configurations that the system is likely to be highly sensitive to initial imperfections. The model is validated using a nonlinear finite element model formulated within the commercial code ABAQUS and excellent comparisons are obtained. 相似文献
135.
We investigate numerically the linear vibrations of inclined risers using the Galerkin approach. The riser is modeled as an Euler–Bernoulli beam accounting for the nonlinear mid-plane stretching and self-weight. After solving for the initial deflection of the riser due to self-weight, we use a Galerkin expansion employing 15 axially loaded beam mode shapes to solve the eigenvalue problem of the riser around the static equilibrium configuration. This yields the riser natural frequencies and corresponding exact mode shapes for various values of inclination angles and tension. The obtained results are validated against a boundary-layer analytical solution and are found to be in good agreement. This constitutes a basis to study the nonlinear forced vibrations of inclined risers. 相似文献
136.
培养警务英语应用型人才是涉外警务工作的要求。警务英语课程的开设,是警务英语人才培养的有效途径。当前警务英语专业课程的开设仍然存在一些问题亟待解决。借鉴ESL课程结构,以培养学生运用警务英语的实践操作能力为目标,在教学过程中,以学生为主角,增强学生在警务工作中的英语运用能力。 相似文献
137.
138.
《Photonics and Nanostructures》2014,12(5):493-500
Solar cell nano-structured anti-reflection coatings based on graded index (GI) and mode coupling are all proposed to significantly reduce the reflection from semiconductor–air interface. In this work, it is shown that purely graded index approach can lead to degradation of long wavelength absorption by eliminating quasi-guided mode excitation. The reason is that the physically graded layer not only provides low reflectance path from air to semiconductor, but also from semiconductor to air, leading to photon escape. This results in out-coupling of photons from the semiconductor to air. On the other hand, anti-reflection coating based on mode coupling does not suffer from degraded long wavelength absorption and it is capable of acting as one-way photon pass coating. It is found that the sidewall thickness of mode coupling anti-reflection coating has significant impact on its effectiveness for anti-reflection, and therefore the selection of process methods is critical for its low reflectance. It is proposed that the purely graded index coating is more suitable for wafer-based photovoltaics where full absorption is possible by two photon traces. The mode coupling coating is suitable for both wafer-based photovoltaics and thin-film photovoltaics since it provides not only low reflectance but also long wavelength quasi-guided mode excitations. In the end, new types of anti-reflection coating and light trapping structure are proposed to further enhance the performance. 相似文献
139.
《Optik》2014,125(24):7116-7119
The 2D FDTD method is used to design a tapered photonic crystal fiber, where large mode area is prominently coupled to optical fiber. For the first time, we observed that the mode area is stabilized with respect to possible fabrication tolerance by implementing a secondary surrounding waveguide with configuration of large air holes in the outer ring. The mode field area can be obtained in tapered region using a full vectorial H field technique. Beside these, the coupling coefficient can be optimizing by using least square boundary residual technique. 相似文献
140.
When a bounded beam is incident on an immersed plate Lamb waves or Rayleigh waves can be generated. Because the amplitude of a bounded beam is not constant along its wave front, a specific beam profile is formed that influences the local efficiency of energy conversion of incident sound into Lamb waves or Rayleigh waves. Understanding this phenomenon is important for ultrasonic immersion experiments of objects because the quality of such experiments highly depends on the amount of energy transmitted into the object. This paper shows by means of experiments based on monochromatic Schlieren photography that the area within the bounded beam responsible for Lamb wave generation differs from that responsible for Rayleigh wave generation. Furthermore it provides experimental verification of an earlier numerical study concerning Rayleigh wave generation. 相似文献