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|標題:||Convection velocity of vortex structures in the near wake of a circular cylinder|
|期刊/報告no：:||Journal of Engineering Mechanics-Asce, Volume 129, Issue 10, Page(s) 1108-1118.|
|摘要:||The convection velocity of vortex structures in the near wake of a circular cylinder was experimentally investigated over the region 1.6-2.5less than or equal tox/Dless than or equal to12.0 for R=160-12,000. Dye injection technique of flow visualization and two completely noninvasive laser Doppler velocimeters were employed for Rless than or equal to320 and greater than or equal to400, respectively. The convection velocity, U-c, is defined as the mean traveling velocity of vortex cores passing a streamwise separation during a mean elapsed time. For Rless than or equal to320, U-c was determined directly from the motion of dye-marked vortex cores filmed by a video camera. In the cases of Rgreater than or equal to400, the positions of peak vorticity and half of the half-velocity-defect width at each downstream section were first used to identify the mean path of vortex cores (i.e., the most probable trajectory of the vortex structures), along which spatial correlation measurements were then performed to determine the mean elapsed time corresponding to the maximum cross correlation. The present results show that, in laminar and transitional wakes, the ratio U-c/U-o increases from 0.53 to 0.84 over a region of 1.6less than or equal tox/Dless than or equal to6.0 and then tends to be a constant of 0.84 for x/Dgreater than or equal to6.0. In a turbulent wake, U-c/U-o also increases from a certain value at a point downstream from the position of vortex formation to a mean value of about 0.86 at x/Dgreater than or equal to5.0-6.0, and then changes little with the increase of x/D. In addition, it is found that the dependence of U-c/U-o on R almost disappears for x/Dgreater than or equal to5.0.|
|Appears in Collections:||土木工程學系所|
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