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Unsteady drag on a sphere by shock wave loading   总被引:2,自引:0,他引:2  
The dynamic drag coefficient of a sphere by shock wave loading is investigated numerically and experimentally. The diameter of the sphere is varied from 8 m to 80 mm in numerical simulation. The axisymmetric Navier-Stokes equations are solved on a fine grid, and the grid convergence of the drag coefficient is achieved. The numerical result is validated by comparing the experimental data of a 80 mm sphere, measured by the accelerometer in a vertical shock tube. It is found that the sphere experiences in the early interaction one order higher drag than in the steady state. A transient negative drag, mainly resulting from the focusing of shock wave on the rear side of the sphere, is observed only for high Reynolds number flows, and the drag becomes positive because of increased skin friction for low Reynolds number flows.Received: 10 March 2004, Accepted: 24 May 2004, Published online: 20 August 2004[/PUBLISHED]M. Sun: Send offprints requests to  相似文献
BCC晶体中韧位错运动特性的分子动力学模拟   总被引:2,自引:2,他引:0  
采用镶嵌原子法 (EAM) ,采用沿 <111>方向插入两层 (2 11)半原子面的方法形成位错 ,模拟金属Mo中韧位错的运动特性 .模拟大于Peierls Nabarro应力时不同剪切力下的韧位错运动速度及相同剪切力下不同温度时韧位错运动速度 ,结果表明 :剪切力越大 ,韧位错运动速度越大 ;温度对韧位错运动有明显的阻碍作用 ,在相同剪切力下 ,随着温度的升高 ,韧位错运动速度减小 ,即拖动系数B(T)随温度升高而增大 .随剪应力增大 ,B(T)变化趋势减缓 .  相似文献
The aerodynamic characteristics of automobiles are greatly influenced by the unsteady change in the direction of relative airflow. The aim of this paper is to analyse how such a change influences vehicle wake flow patterns. An analysis was conducted on a simplified model capable of reproducing the typical structures encountered under the aerodynamic conditions of an automobile. The results were processed by mapping the steady and unsteady total pressure losses around the model. The findings should enable automobile development engineers inter alia to identify and analyse the physical phenomena that occur when a vehicle is subjected to a sudden gust of side wind.List of symbols B rod - l length of rod B (m) - angle of rod B (degrees) - D disk - P connection point between disk D and rod B - M o drive motor of disk D - O centre of rotation of disk D - e radius of disk D (m) - angle of disk (degrees) - t moment of time t (s) - o angle (degrees) at instant of time t=0 - d diameter of model (m) - C centre of rotation of model - x c abscissa of centre of rotation C of model (m) - M connection point between model and rod B - pi value=3.14159 - incidence of model (degrees) - M maximum value of incidence (degrees) - m minimum value of incidence (degrees) - angular amplitude (degrees) - c critical angle of incidence (degrees) for steady evolutions - critical angle of incidence (degrees) for the unsteady evolutions - ̄ mean angle of incidence (degrees) - angle (degrees) of the model such that =+ - pulse (rad s–1) - T period (s) - f frequency (Hz) - R radius of model in meter (m) - velocity vector of incident airflow - V o intensity of velocity vector (m s–1) - P io total pressure associated with upstream airflow velocity (Pa) - P i local total pressure (Pa or J m–3) - density (kg m–3) - C x drag coefficient - total pressure coefficient - (m, n) dimensions of grid: lines m, columns n - x X coordinate of sampling plane (m) - y j Y coordinate of point of index j for j[1,n] - z k Z coordinate of point of index k for k[1,m] - P i (x,y j ,z k ,(t)) continuous data of unsteady total pressure (Pa) - discrete data of unsteady total pressure (Pa) - N number of tomographic images, from 1 upwards over an oscillation period T - maximum value of total pressure coefficients for steady evolutions - maximum value of total pressure coefficients for unsteady evolutions and increasing incidences - maximum value of total pressure coefficients for unsteady evolutions and decreasing incidences - differential between and - differential between and - phase shifting (degrees)  相似文献
平板大攻角绕流升力和阻力系数的计算   总被引:1,自引:0,他引:1  
二维平板或二维对称薄翼型大攻角绕流升力和阻力系数与攻角之间存在的函数关系一般用数据表格的形式给出。本文根据垂直平板绕流阻力实验数据和对称薄翼型全攻角绕流实验数据,分析得到了平板大攻角绕流总压力及其升力分量和阻力分量系数的近似计算公式。结果表明:平板总压力系数约等于攻角正弦值的2倍;总压力的阻力分量系数约等于攻角正弦值平方的2倍;升力分量系数约为攻角2倍的正弦值。计算结果与两组试验数据具有较好的一致性。  相似文献
爆轰驱动对钨珠终点弹道性能的影响   总被引:1,自引:0,他引:1       下载免费PDF全文
通过实验比较了理想球形钨珠和经历爆轰驱动的钨珠(初始为球形)主要终点弹道性能的差异,结果表明:(1) 钨珠经爆轰驱动后,发生轻微的质量损失和变形现象,直径6.0 mm和7.5 mm钨珠的质量损失平均为8.4%;(2) 对于理想球形钨珠,在同一初始速度条件下,衰减系数为常数,空气阻力系数仅与初始速度有关,两者成线性关系,而对于经历爆轰驱动的钨珠,衰减系数不再为常数,空气阻力系数与飞行速度有关,两者成线性关系;(3) 爆轰驱动明显降低了钨珠的穿甲能力,但长距离飞行后,钨珠仍具有较强的穿甲能力,是破片式战斗部或杀爆战斗部较理想的选择。  相似文献
砂粒的连续冲击使输气管路弯头内壁面连续不均匀地发生着冲蚀进化现象. 本文作者基于3D成像技术精确描述了R/D=1.5弯头内壁冲蚀进化过程, 并采用CFD方法对该过程进行了数值模拟研究. 结果显示, 随着颗粒冲击,磨损严重的区域向弯头圆心角高角度扩散的速度较大; 通过修正Schiller Naumann拽力系数模型可较准确地模拟弯头的冲蚀进化过程(平均误差小于0.15 mm). 以上工作对管路的完整性评价具有重要意义.  相似文献
In view of the fact that large scale vortices play the substantial role of momentum transport in turbulent flows, large eddy simulation(LES) is considered as a better simulation model. However, the sub-grid scale(SGS) models reported so far have not ascertained under what flow conditions the LES can lapse into the direct numerical simulation. To overcome this discrepancy, this paper develops a swirling strength based the SGS model to properly model the turbulence intermittency, with the primary characteristics that when the local swirling strength is zero, the local sub-grid viscosity will be vanished. In this paper, the model is used to investigate the flow characteristics of zero-incident incompressible turbulent flows around a single square cylinder(SC)at a low Reynolds number range Re ∈ [103, 104]. The flow characteristics investigated include the Reynolds number dependence of lift and drag coefficients, the distributions of time-spanwise averaged variables such as the sub-grid viscosity and the logarithm of Kolmogorov micro-scale to the base of 10 at Re = 2 500 and 104, the contours of spanwise and streamwise vorticity components at t = 170. It is revealed that the peak value of sub-grid viscosity ratio and its root mean square(RMS) values grow with the Reynolds number. The dissipation rate of turbulent kinetic energy is larger near the SC solid walls.The instantaneous factor of swirling strength intermittency(FSI) exhibits some laminated structure involved with vortex shedding.  相似文献
双柱体绕流中高度变化对其阻力的影响   总被引:1,自引:1,他引:0  
对三维槽道内双柱体可压绕流进行了大涡模拟,揭示了绕流过程中柱体表面分离涡的生成、扩散与相互作用过程,并且数值模拟了两个柱体高度(阻塞比)的变化对整个流场的影响以及两柱体阻力系数的变化情况。结果表明,当两个柱体高度同时增加时,上游柱体阻力系数的大小以及下游柱体阻力系数的振幅都急剧变大,这是工程领域中所不期望的。而当仅增加下游柱体高度时,上游柱体阻力系数会略有降低,下游柱体阻力系数虽有增加,但仍小于前种情况的上游柱体阻力系数,且其振幅相对较小,因而有利于改善两柱体总体受力情况。  相似文献
不同高度柔性植被对坡面流水动力特性的影响   总被引:1,自引:0,他引:1       下载免费PDF全文
基于系列坡面流试验,研究了柔性植被的高度(4 cm, 8 cm 和15 cm) 对坡面流水动力特性的影响. 研究表明:覆盖率相同时,4 cm 植被阻水效果最好,15cm 最差,而该差异随着坡度增大而减小. 4 cm 和8 cm 植被覆盖时,水流阻力系数随雷诺数增加先减小后增加,这是由于雷诺数较小时水流阻力主要以床面阻力为主,雷诺增大时主要以植被阻力为主. 另外,通过染色剂法测得光滑床面平均流速修正系数为0.2~0.4,有植被覆盖床面为0.4~0.8,且修正系数随平均流速增大而增大.  相似文献
双圆柱绕流特性的模拟研究   总被引:1,自引:0,他引:1       下载免费PDF全文
采用格子Boltzmann方法对低雷诺数下气体绕流圆柱的规律进行了研究. 对比计算了双 圆柱在不同圆心距、不同Re数、不同来流速度与双圆柱圆心连线角度的情况下,各个圆柱的 受力大小和曳力系数. 结果表明,若Re数为20, 改变圆柱间距,圆柱间距在1.2d和1.4d 之间时,下游圆柱所受曳力有极小值;双圆柱间距为1.6d时,双圆柱受到总曳力最小;圆 柱间距大于2d时,上游颗粒受到的曳力不再受到下游颗粒的影响. 若圆柱间距 为1.2d, 改变雷诺数,Re数在30和40之间,下游圆柱所受曳力有极小值. 另外, 来流速度角度对圆柱的受力影响很大. 上述规律为低Re数下圆柱绕流的深入研究 与应用打下基础.  相似文献
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