INVESTIGATION OF LOW-FREQUENCY VARIATIONS EMBEDDED IN VORTEX SHEDDING PROCESS

• Main Authors: Chin-Tsan Wang, 王金燦
• Other Authors: Jiun-Jih Miau
• Format: Others
• Language: en_US
• Published: 2000
• Online Access: http://ndltd.ncl.edu.tw/handle/48134667574804811619

博士 === 國立成功大學 === 航空太空工程學系 === 88 === Experiments were made to study the flow characteristics in the near wake region of a two-dimensional bluff body, namely a trapezoidal cylinder or a circular cylinder. The instantaneous velocity signals obtained at the inner edge of the separated shear layer and in the neighborhood of the rear end of the vortex formation region show the presence of low-frequency variations at the Reynolds numbers of 104. The low frequency variations noted in the velocity signals measured in the near wake and the instantaneous base pressure measured on the bluff body appear to be well correlated. These experimental observations suggest a physical picture that the variations of vortex formation length and base pressure are closely related in a real time manner. Further experiments were made to perform a quantitative analysis on low-frequency variations embedded in the vortex shedding process behind a trapezoidal cylinder. Firstly, the low-passed base pressure fluctuations were examined with different cut-off frequencies. It is interesting to point out that the normalized integral time scales obtained with respect to the non-dimensional cut-off frequencies are collapsed into a single curve, irrespective of the Reynolds numbers and aspect ratio studied. Further, this curve can be expressed in an exponential form. Secondly, time-lag correlation between the base-pressure fluctuations measured at rear surface of bluff body and the velocity fluctuations detected in the near wake region was performed for the signal traces low-passed at different cut-off frequencies. The optimal cut-off frequency was then determined as the one at which the maximum correlation coefficient was occurred. The optimal cut-off frequencies found vary with Reynolds numbers in a linear fashion roughly, hence can be non-dimensionalized and represented by a single value. Finally, the integral time scales associated with the spanwise velocity fluctuations measured at different streamwise locations in the separated shear layer appear to be very close to the integral time scale of the low frequency variations. This strongly suggests that spanwise unsteady motions of the separated shear layers are closely related to mentioned previously the low frequency variations embedded in the vortex shedding process. Meanwhile, the water-tunnel flow visualization photographs reveal that the three-dimensional pattern of the wake flow varies globally in time. Based on these understanding, a physical picture proposed is that the low frequency variations in the vortex shedding process are associated with the global change of the three-dimensional flow pattern with respect to time.