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31.
The flames of forest fuels form a weakly ionized gas. Assuming a Maxwellian velocity distribution of flame particle and collision frequencies much higher than plasma frequencies, the propagation of microwaves through forest fuel flames is predicted to have attenuation and phase shift. A controlled fire burner was constructed where various natural vegetation materials could be used as fuel. The burner was equipped with thermocouples and used as a cavity for microwaves with a laboratory quality network analyzer to measure phase and attenuation. The controlled fires had temperatures in the range of 500-1000 K and microwave attenuation of 1.0-4.5 dB m−1 was observed across the 0.5 m diameter cavity. Attenuations of this magnitude could affect active remote sensing systems signals at microwave frequencies in forest fire environments where flame depths of up to 50 m are possible. In the experiment, temperature was not the only controlling parameter for the ionisation; type of fuel burnt also influenced it. Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) analysis of the composition of the fuel confirmed that a higher content of alkali (with low ionization potential) lead to higher electron densities. Electron densities in the range of 0.32-3.21×1016 m−3 and collision frequencies of 1.1-4.0×1010 s−1 were observed for flames with temperature in the range of 730-1000 K. 相似文献
32.
利用双流辐射理论,导出了信号光辐射照度的表达式。从理论上计算了同步扫描系统的信号光和后向散射光传到探测器的辐照度与衰减长度的定量关系。减小视场角和增大光源到探测器的距离可增成像距离。 相似文献
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为了确定岩体内部的孔隙及流体性质,运用地震衰减分析来区分地震信号谱与振幅特征的变化,分析在包含孔隙和裂隙的复杂环境下岩石属性特征和波的传播特征,乌审召地区主要目标层为上古生界二叠系石盒子组盒8段储层.该目标砂岩发育,但储层横向变化快,通过对乌审召实际地震资料处理结果表明,在频率域吸收衰减异常与岩石中含气有关,通过方法实践,采用该地震分析方法计算的衰减模式与高产区的关系比较确定,能够帮助确定新的潜在区域和勘探目标。 相似文献
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C. Ramadas Krishnan Balasubramaniam Avinash Hood Makarand Joshi C.V. Krishnamurthy 《Composite Structures》2011
In this study, the Lamb mode attenuation constants were derived in terms of the attenuation coefficient, group velocity and central frequency of excitation of the Lamb mode, using the Rayleigh damping model. Attenuation of Lamb waves, both fundamental symmetric and anti-symmetric modes, propagating through viscoelastic media (cross-ply glass/epoxy laminate) was modelled using the Finite Element Method. Numerically simulated attenuation of Lamb waves using Lamb mode attenuation constants was found to be in good agreement with the assumed attenuation. Experiments were performed on a quasi-isotropic laminate, employing air-coupled ultrasonic transducers, to measure the attenuation coefficient. Lamb mode attenuation constants, computed using the attenuation coefficient, were used to model the attenuation of the Lamb mode in quasi-isotropic laminates. Numerically simulated amplitude variation was found to be in good agreement with that computed from experiments. 相似文献
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Results from an experimental study of ultrasonic, through-transmission, wave propagation on fresh cementitious material are discussed in this paper. The propagation characteristics of different frequency tone-bursts revealed the strong dispersive behavior of this type of materials while using sine-sweep excitation, the attenuation up to about 1 MHz was examined. Sand content and size exercise significant influence on wave parameters while entrapped air bubbles seem to dominate the attenuation at low frequencies since cement paste specimens, containing no sand, exhibit strong attenuation. The possibility of mortar composition characterization within 10 min after mixing is also addressed leading to encouraging results. 相似文献
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位置条件对爆破震动信号分析中小波包时频特征的影响 总被引:1,自引:0,他引:1
爆心至测点的位置条件是影响爆破震动时频分布的主要因素之一。在考虑到段药量变化的情况下,位置条件由比例距离和高差系数来进行量化。运用小波包分析方法对监测获得的爆破震动信号进行时频特征分析,探讨时频分布与比例距离及高差系数之间的关系。试验研究发现,位置条件对震动波形小波包细节信号时频特征的影响主要体现在峰值质点振速、优势频率大小及波形衰减阻尼比的变化上。其中峰值质点振速随比例距离的增加呈指数衰减,其与高差系数的相关规律性不强;优势频率随比例距离增加也呈衰减趋势,高差系数对其有较大影响,但规律不明显;衰减阻尼比在不同的主振频带内与比例距离及高差系数有不同的关系,较低频率部分其阻尼比随着比例距离的增加而增大,较高频率部分则基本保持不变。 相似文献