Investigation and Control of the Combined Three Dimensional Organized Structures in Turbulent Wakes behind Bluff Bodies

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The unsteady three dimensional flow structures behind a finite circular cylinder with an aspect ratio of 10 and a thin circular disc at various angles of attack have been investigated. The flow details in the frequency domain were determined using FFT analyzer and the features of large-scaled organized structures in the turbulent wakes were obtained with phase-locked averaging technique in the time domain. Three typical organized structures with different frequencies and different features were extracted from the complex turbulent wake flows. The results can be summarized as follows :1. The wake flows behind a finite circular cylinder typically consist of the following three large-scaled organized vortex structures with different origins, (1) low frequency component(tip component), which is a pair of longitudinal vortices shed from a tip of the cylinder, (2) mid-frequency component (mid-span component), which is hairpin typed organized vortices shed from a mid-span of the cylinder, (3) … More high frequency component (root component), which is shed from the root of the sylinder and quite similar to the two-dimensional Karman's vortices .2. The wake flow behind a finite circular cylinder consists of not only the typical three types of organized vortex structures described above but also various types of vortices with intermediate features and frequencies as well as small eddies with almost random configurations. Therefore, the turbulent wake behind a circular cylinder appears to be a very complicated flow at a glance.3. The wake flow behind a thin circular disc with longituninal vortices, which is a typical vortex features for unstalled conditions, indicates only a few organized structure. The wake flow with ring-type vortices, which is typical vortex features for stalled conditions, indicates a clear organized flow structures with well defined frequency. The Strouhal numbers of the frequency based on the height of the shed ring-type vortices are in the range from 0.19 to 0.22. Less