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Third Symposium on Turbulence and Shear Flow Phenomena
June, 25-27, 2003, International Center, Sendai, Japan

DOI: 10.1615/TSFP3

DIRECT NUMERICAL SIMULATION OF OBLIQUE VORTEX SHEDDING FROM A CYLINDER IN SHEAR FLOW

pages 735-740
DOI: 10.1615/TSFP3.1240
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ABSTRAKT

The vortex dynamics of a shear flow over a circular cylinder is studied by means of Direct Numerical Simulation. A numerical code based on Cartesian grids and accurate schemes is used in combination with an immersed boundary method. The flow configurations are selected in order to consider the influence of three physical parameters: the vertical extension of the shear zone, the vertical domain size and the shear intensity. Different combinations of these three parameters are considered by maintaining constant the median Reynolds number. For each case, the non-uniform character of the upstream flow leads to the formation of complex Karman streets behind the cylinder. The analysis of the animations shows the occurrence of oblique vortex shedding driven through complex synchronization processes. The shear imposes strong distortions on the Karman vortices and dislocations are currently observed. Depending on the local Reynolds number (associated to the inflow velocity profile), highly three-dimensional secondary structures are observed in the high-speed region of the flow. The simple observation of the animations does not allow a unambiguous identification of the cellular pattern of vortex shedding. This phenomenon can be more rigorously described by a frequency analysis presented in this paper. It is shown that the main frequency selection is mainly conditioned by a local adjustment of oblique vortex shedding on the upstream velocity. However, due to the preservation of the spatial coherence of the flow, the variation of the main flow frequency occurs by jump along the cylinder axis direction, the distance between two jumps corresponding to the size of a cell. The Karman vortex formation is triggered in the high-speed region of the flow, but paradoxically, the local vortex shedding frequency found in this zone seems to be strongly influenced by the dynamics of the slow part of the flow.

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