| Summary: | 碩士 === 國立中興大學 === 機械工程學系 === 88 === This study investigates the wake characteristics behind the rotating circular cylinder in the uniform flow. The formation and the suppression of the wake behind the rotating circular cylinder, velocity distribution, auto-correlation, and the alteration of the mean drag coefficient will be emphasized. The experiments are performed in a recirculation water channel. Qualitative flow visualization and quantitative velocity measurements by FLDV and PIV systems are employed to study the details of the wake behind the rotating circular cylinder in a uniform inflow. The Reynolds number base on cylinder diameter is about 1500, the aspect ratio of the circular cylinder is 23, and the dimensionless rotational speed ranges from =0~3.
It is found that as the rotational speed increases, the formation length of the wake vortex structure becomes shorter, and the average angle of the upward deflected wake centerline becomes larger. Furthermore the wavelength of shear layer instability becomes shorter, the periodicity of the wake structure becomes worse, and the separation point on the circular cylinder surface moves to upstream at the upper side surface and moves to downstream at the lower side surface. We also found the higher the rotating speed, the lower the drag coefficient. At the cross section near the separation location of the cylinder, the velocity distribution presents an “overshoot” within the near surface region. On both sides of the max overshoot location, there appears different characteristic frequencies. Namely, the frequency of the rotating circular cylinder within the near surface region and the shedding frequency in the region away from the cylinder, respectively. When equals 1.5, the wake behind the circular cylinder becomes organized and stronger. With the spectrum analysis, it is found that the rotating frequency of the circular cylinder is just twice as the shedding frequency at this rotational speed.
Through the PIV technique, the vortex structure behind the circular cylinder can be studied via the instantaneous vorticity contour map. The vortex structure behind the rotating cylinder shows a destructive structure as the rotating speed increases. The circulation measured behind the cylinder at a local section (X=2~4cm) is found to decrease in the amplitude with the increasing rotational speed. Further, the periodicity of the wake become worse as the rotating speed increases.
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