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为了研究小口径武器的膛口气动噪声特性,采用CFD-CAA耦合算法对7.62 mm枪的射流噪声场进行了数值模拟。由于膛口流场结构复杂,在目前的计算发展水平下还不足以采用CAA直接法,因而本文中采用混合方法,即首先采用CFD方法计算7.62 mm枪的膛口流场,然后利用所得结果,采用声学方程计算射流噪声,具体为膛口近场采用LES进行计算,远场声场采用FW-H声拟法计算。通过对比验证实验,验证了该计算方法的可行性。然后,对7.62 mm枪射流噪声进行了数值模拟,分析了噪声指向性,绘制了声压级云图。研究表明:在本文的计算条件下,射流噪声强度主要集中在近膛口区域;且射流最大噪声主要分布在与轴线方向成30°~60°范围内。
相似文献2.
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允许卖空的资本市场中存在非负均衡价格向量的充要条件 总被引:1,自引:0,他引:1
For the capital market satisfying standard assumptions that are widely adopted in the equilibrium analysis,a necessary and sufficient condition for the existence and uniqueness of a nonnegative equilibrium price vector that clears the mean-variance capital market with short sale allowed is derived. Moreover, the given explicit formula for the equilibrium price shows clearly the relationship between prices of assets and statistical properties of the rate of return on assets, the desired rates of return of individual investors as well as other economic quantities.The economic implication of the derived condition is briefly discussed. These results improve the available results about the equilibrium analysis of the mean-variance market. 相似文献
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Measurements of NO2 mixing ratios with topographic target light scattering-differential optical absorption spectroscopy system and comparisons to point monitoring technique 下载免费PDF全文
A topographic target light scattering-differential optical absorption spectroscopy(ToTaL-DOAS) system is developed for measuring average concentrations along a known optical path and studying surface-near distributions of atmospheric trace gases.The telescope of the ToTaL-DOAS system points to targets which are located at known distances from the measurement device and illuminated by sunlight.Average concentrations with high spatial resolution can be retrieved by receiving sunlight reflected from the targets.A filed measurement of NO2 concentration is performed with the ToTaL-DOAS system in Shijiazhuang in the autumn of 2011.The measurement data are compared with concentrations measured by the point monitoring technique at the same site.The results show that the ToTaL-DOAS system is sensitive to the variation of NO2 concentrations along the optical path. 相似文献
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本文利用一种具有H_2O_2催化活性的Cu-MOF[Cu_3(BTC)_2(H_2O)_3,简称HKUST-1],构建了以邻苯二胺(OPD)为颜色指示分子的比色传感体系,实现了对H_2O_2和多巴胺(DA)的快速灵敏检测。HKUST-1起到催化H_2O_2氧化OPD的作用,反应体系能够呈现出显著的颜色变化。在优化条件下,415nm处的吸收峰强度与H_2O_2浓度呈双线性关系,线性范围分别为10~50 mmol/L和50~100 mmol/L,相对标准偏差分别为0.9947和0.9995,最低检出限为1.29mmol/L。由于DA能抑制H_2O_2氧化OPD,因此比色传感体系还可以用于快速检测DA,线性范围分别为0.25~5μmol/L和2.5~25μmol/L,相对标准偏差分别为0.9783和0.9705,最低检出限为0.262μmol/L。该项工作拓展了Cu-MOFs材料在生物分子催化和生物传感方面的应用。 相似文献
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采用偏微分方程方法研究了彩虹障碍期权的定价问题,推导出它满足的偏微分方程,通过求解这个偏微分方程得出了八种彩虹障碍期权的定价公式及四个看涨——看跌平价公式. 相似文献
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固体板背覆薄层特性的低频超声反演方法研究 总被引:3,自引:0,他引:3
研究了应用低频超声的反射频谱对固体板厚度及背覆薄层的各特征参数(包括厚度、声速、密度、衰减、声阻抗率、声时)进行反演的方法。从灵敏度函数、目标函数及误差传递方程的角度,对应用反射频谱的谐振频率或幅度谱反演模型系统的单参数和双参数的方法进行了分析讨论。实验使用中心频率为7MHz的探头检测铝板/聚合物薄层样品,在同样的条件下,谐振频率法和幅度谱法反演的单参数反演效果相当,谐振频率方法的双参数反演效果优于幅度谱方法,前者薄层声阻抗率和声时同时反演的平均误差为3.4%和4.7%,而后者没有得到结果。 相似文献
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Effect of Fe3O4 segregation at grain boundaries on the electrical transport and magnetic properties of La0.67Ca0.33MnO3 is investigated. The experimental results show that the Fe3O4 segregation not only shifts the paramagnetic-ferromagnetic transition temperature of La0.6TCa0.33MnO3 to a lower temperature region but also induces a new transition in a lower temperature region. Meanwhile, the transition processes observed in both the resistivity and magnetization curves are obviously widened. Compared to pure La0.67Ca0.33MnO3, we assume that the Fe3O4 segregation level at the grain boundaries can modify the electrical transport and magnetic properties of La0.67Ca0.33MnO3. 相似文献
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In this work, we use a 3-nm-thick Al_(0.64)In_(0.36)N back-barrier layer in In_(0.17)Al_(0.83) N/Ga N high-electron mobility transistor(HEMT) to enhance electron confinement. Based on two-dimensional device simulations, the influences of Al_(0.64)In_(0.36) N back-barrier on the direct-current(DC) and radio-frequency(RF) characteristics of In AlN/GaN HEMT are investigated, theoretically. It is shown that an effective conduction band discontinuity of approximately 0.5 eV is created by the 3-nm-thick Al_(0.64)In_(0.36) N back-barrier and no parasitic electron channel is formed. Comparing with the conventional In AlN/GaN HEMT, the electron confinement of the back-barrier HEMT is significantly improved, which allows a good immunity to short-channel effect(SCE) for gate length decreasing down to 60 nm(9-nm top barrier). For a 70-nm gate length, the peak current gain cut-off frequency( f_T) and power gain cut-off frequency( f_(max)) of the back-barrier HEMT are172 GHz and 217 GHz, respectively, which are higher than those of the conventional HEMT with the same gate length. 相似文献