An Investigation on Karman-type Vortex Shedding from a Finite Square Cylinder

The phenomenon of Karman-type vortex shedding from finite square cylinders of the aspect ratios 2, 4 and 6 at Reynolds number of 1.9 × 105 were examined with the employment of a three-component force balance to measure the aerodynamic forces. The signals of the lateral force measured were first analyzed by the HHT (Hilbert-Huang Transformation) together with a conditional sampling technique for identifying the time periods during which the vortex shedding frequency component was prominent. Meanwhile, the force measured in the vertical direction was analyzed by the same procedure to identify the events of pronounced unsteady downwash motion induced by the flow over the finite end of the model. Therefore, the unsteady flow motions around a finite cylinder model could be categorized into four patterns. Namely, the patterns 11 and 10 denote the situations of pronounced Karman-type vortex shedding with and without strong downwash motion, respectively; and the patterns 01 and 00 denote the situations of no pronounced vortex shedding with and without strong downwash motion, respectively. The results obtained show that the pattern 00 occupied more than 60% of the time sampled, apparently dominant over the other three patterns, whereas the second popular pattern 10 (Karman type vortex shedding) occupied no more than 30% of the time sampled. Further experiments were made for the square cylinder of the aspect ratio 6, with a hot-wire situated near either side of the cylinder. By analyzing the unsteady lateral forces experienced by the cylinder and the hot-wire velocity data with the data reduction scheme employed, it is unveiled that the Karman-type vortex shedding induced an anti-symmetric, unsteady flow motion around the cylinder, although this component was not the dominant one. On the other hand, it is found that a large portion of the fluctuating energy was resided in the low-frequency component featuring a symmetric unsteady flow motion around the cylinder. This finding further supports the earlier observation that the flow pattern 00 is most commonly seen in the unsteady flow motions around a finite square cylinder.