Transient operations are commonly founded in fluid machineries such as the starting, stopping, and variations of rotor speeds, etc. Flow generated from a started fiat plate is of fundamental importance. Experiments ha...Transient operations are commonly founded in fluid machineries such as the starting, stopping, and variations of rotor speeds, etc. Flow generated from a started fiat plate is of fundamental importance. Experiments have been done to observe the flow evolution in current researches. And in order to explore the flow in more detailed scale, some vortex methods with high resolution and other numerical methods were developed to solve various related problems by some researchers. But the promotion of vortex method to engineering application is rare due to its complexity and difficulty in specifying the boundary conditions. In order to build up a method of numerical study for such problems, a simplified model is built up with a flat plate. The development of two-dimensional viscous incompressible flow generated from an impulsively started and uniformly accelerated infinitesimally thin flat plate is simulated numerically. A dynamic mesh（DM） method based on the spring analogue and local remeshing is applied to realize the mesh motion caused by the started plate. Researches show that the mesh quality will decline under large grid shear force during the updating process. To conquer this problem, a region near the plate is separated to guarantee the mesh quality at location of interest which is the innovation of the present paper. All computations at least cover a period during which the plate translates 6 times its length. The simulated instantaneous velocity profiles, flow structures and drag coefficients under several Reynolds numbers （20 ≤ Re ≤ 126） and accelerations （20 m/s2≤ a ≤ 152 m/s2） are presented and compared with existing results in literatures. Comparisons are found to be satisfactory, confirming the validity of the current proposed method（region separated DM）. The proposed DM method is firstly used to study the transient flow generated from a started flat plate and can be used in further study of transient characteristics during transient operations of turbo machineries.展开更多
目的比较PET-CT与MRI检查短暂性脑缺血发作(TIA)后脑缺血改变的灵敏度。方法随机抽取临床明确诊断为TIA的患者96例,分别做头部PET-CT及MRI检查,比较两组诊断TIA后脑缺血改变的阳性率。所得数据采用x^2检验。应用SPSS 13.0 for Windows...目的比较PET-CT与MRI检查短暂性脑缺血发作(TIA)后脑缺血改变的灵敏度。方法随机抽取临床明确诊断为TIA的患者96例,分别做头部PET-CT及MRI检查,比较两组诊断TIA后脑缺血改变的阳性率。所得数据采用x^2检验。应用SPSS 13.0 for Windows统计分析软件进行分析。结果应用PET-CT诊断TIA后脑缺血改变的阳性率为95.83%,应用MRI诊断的阳性率为8.33%,有明显差别(P<0.01)。结论PET-CT在检查TIA后脑缺血改变的方面上优于MRI检查。展开更多
基金supported by National Natural Science Foundation of China(Grant Nos. 50979095, 51176168, 50906074)China Postdoctoral Science Foundation(Grant Nos. 20100471697, 201104713)
文摘Transient operations are commonly founded in fluid machineries such as the starting, stopping, and variations of rotor speeds, etc. Flow generated from a started fiat plate is of fundamental importance. Experiments have been done to observe the flow evolution in current researches. And in order to explore the flow in more detailed scale, some vortex methods with high resolution and other numerical methods were developed to solve various related problems by some researchers. But the promotion of vortex method to engineering application is rare due to its complexity and difficulty in specifying the boundary conditions. In order to build up a method of numerical study for such problems, a simplified model is built up with a flat plate. The development of two-dimensional viscous incompressible flow generated from an impulsively started and uniformly accelerated infinitesimally thin flat plate is simulated numerically. A dynamic mesh（DM） method based on the spring analogue and local remeshing is applied to realize the mesh motion caused by the started plate. Researches show that the mesh quality will decline under large grid shear force during the updating process. To conquer this problem, a region near the plate is separated to guarantee the mesh quality at location of interest which is the innovation of the present paper. All computations at least cover a period during which the plate translates 6 times its length. The simulated instantaneous velocity profiles, flow structures and drag coefficients under several Reynolds numbers （20 ≤ Re ≤ 126） and accelerations （20 m/s2≤ a ≤ 152 m/s2） are presented and compared with existing results in literatures. Comparisons are found to be satisfactory, confirming the validity of the current proposed method（region separated DM）. The proposed DM method is firstly used to study the transient flow generated from a started flat plate and can be used in further study of transient characteristics during transient operations of turbo machineries.
文摘目的比较PET-CT与MRI检查短暂性脑缺血发作(TIA)后脑缺血改变的灵敏度。方法随机抽取临床明确诊断为TIA的患者96例,分别做头部PET-CT及MRI检查,比较两组诊断TIA后脑缺血改变的阳性率。所得数据采用x^2检验。应用SPSS 13.0 for Windows统计分析软件进行分析。结果应用PET-CT诊断TIA后脑缺血改变的阳性率为95.83%,应用MRI诊断的阳性率为8.33%,有明显差别(P<0.01)。结论PET-CT在检查TIA后脑缺血改变的方面上优于MRI检查。
