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101.
汽车尾部湍流场是汽车压差阻力的主要来源,在HD-2汽车模型风洞中,首先使用测力天平和测压系统,对横摆角工况下汽车模型的气动六分力和纵对称截面48个测点的表面压力进行了测量,然后利用PIV测量技术对模型在横摆角分别为0°、15°的尾部湍流场进行了测量,获得该模型尾流场的速度场、涡量场和雷诺应力流场信息,通过计算得出尾流场区域空间相关系数和湍流积分尺度。结果表明:在横摆角工况下,汽车模型尾部涡流的结构呈现向上发展的趋势;尾流场拖拽涡的范围和强度的增大导致了模型气动力出现较大的增加;湍流积分尺度的变化表明,尾部涡流区的分离噪声与涡流分离位置有关,在汽车尾部造型设计中,要尽量推迟尾部涡流的分离。 相似文献
102.
In this paper we test for the presence of bubbles in the Nasdaq stock market index over the period 1994–2003 applying fractional integration techniques and allowing for structural breaks and non-linear adjustments of prices to dividends. The results show a significant structural break in 1998 for all model specifications and data periodicity. Furthermore, we do not find evidence of asymmetric adjustment of prices to dividends when using M-TAR and TAR models. The evidence of bubbles varies depending on the data periodicity and model specification used in the analysis. Finally, the results show persistent deviations of stock prices to dividends in all cases considered, though we only find evidence of bubbles in the Nasdaq index when using weekly data for the time period after June 1998. 相似文献
103.
针对SILEX钛宝石激光器参数,采用PIC数值模拟程序VORPAL对激光尾波场加速进行了模拟,得到了电子轨迹及能量数据,进而通过理论计算得到了空泡机制下X射线辐射特性。结果表明,空泡机制下高能电子在空泡中做betatron振荡且多数电子被加速到170 MeV左右;加速能量较低的电子(约100 MeV), 其辐射谱为临界能量约3 keV的类同步辐射谱,发散角约为8 mrad,而能量较高的电子(约170 MeV)对应的光子临界能量约为10 keV。 相似文献
104.
For CSNS RCS tune measurement, tune value is measured by exciting the bunch with a strip-line kicker fed with white noise and using a FFT algorithm. This article simulates the strip-line kicker in RCS and the efficiency of the kicker is discussed in a Matlab environment. The parameters of the kicker with an arc electrode structure such as a VSWR, wake impedance, and thermal state are analyzed based on the advantages of this design. 相似文献
105.
Local and global instabilities are investigated of wakes of general two-dimensional bluff bodies placed near and parallel to a plane boundary or ground. A spatio-temporal linear stability analysis is first applied to a four-parameter family of local wake profiles to investigate the fundamental local stability characteristics of the wake in ground effect. The analysis shows significant dependencies of the stability characteristics of the wake on the distance from the wake centreline to the ground (normalised by the wake width), and also on the velocity ratio of the near- and far-ground sides of the wake. The analysis is then compared with earlier experiments on a circular cylinder to examine, according to the transition scenario of the steep global modes, the streamwise variation of the local stability characteristics of the wake in ground effect. The comparison indicates that the near wake region of the cylinder changes from being absolutely unstable to being convectively unstable when the cylinder comes down into the near-ground range in which the von Kármán-type vortex shedding from the cylinder is suppressed, being qualitatively consistent with the transition scenario for general wake-type flows. A possible explanation is also given for the counter-intuitive relation between the thickness of the boundary layer on the ground and the critical gap distance for the cessation of the von Kármán-type vortex shedding in ground effect. 相似文献
106.
We have experimentally and analytically studied transport of a passive scalar in the wake of a thin flat plate located at
the centerline of a planar contraction with flat walls. The constant Launder parameter in the contraction, K = 6.25 ×10 − 6, was twice the value required for a turbulent boundary layer to relaminarize. In addition to the mixing analysis inside the
contraction, layer mixing is also investigated downstream, where the flow continues inside a constant cross-section channel.
In order to generate the passive scalar, the airflow above the plate was heated and the temperature stratification in the
wake was traced by measuring the temperature field using constant current anemometry. Using different plate lengths, we found
that the degree of mixing, obtained at a given position in the straight channel, is a function of the distance from the plate
trailing edge to the contraction outlet. For a plate which does not protrude into the straight channel, we demonstrate the
existence of an optimal trailing edge-contraction outlet distance that results in the lowest possible degree of mixing at
a given downstream position in the straight channel. This finding is also supported by a semi-empirical relationship based
on our developed self-similar solution for mixing layers in planar contractions. 相似文献
107.
基于特征值方法的旋翼尾迹稳定性分析 总被引:1,自引:0,他引:1
给出一个旋翼尾迹线性化稳定性分析的方法.在该方法中,尾迹涡线被离散为直线涡段,尾迹的扰动归结为涡元端点的扰动,考虑了桨尖涡的自诱导和涡线的互诱导以及桨尖涡与桨叶的实际干扰.使用该方法,分别以UH-1H和AH-1G模型旋翼为例,对悬停和前飞状态的旋翼尾迹的稳定性进行计算和分析.结果表明:旋翼尾迹运动存在大于0的特征值,是内在不稳定的,且最大发散率随波数变化呈现出一定规律性;前飞与悬停状态不同,其最大发散率减小,不稳定性减弱. 相似文献
108.
109.
上层大气中石质流星尾迹的紫外光谱模拟 总被引:1,自引:0,他引:1
利用Sparrow的粒子碰撞理论,计算了海拔100~90 km范围内的大气中的不同初始地心坐标速度的石质流星的有效温度;分别采取不同的成分配比,给出了速度72 km·s-1的石质流星尾迹主要成分的粒子数密度;应用大型光谱合成程序Cloudy模拟了上述流星尾迹的240~400 nm波段紫外辐射,给出了若干预期的强谱线的相对强度;经比较发现,有效温度5 680 K、蒸汽分压0.1 Pa的尾迹光谱比较接近观测结果。 相似文献
110.