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Abstract
The turbulent drag of a 2-D bluff body with a blunt trailing edge is investigated for Reynolds numbers in the range $5\cdot 10^3 < \text{Re} <5 \cdot 10^4$. The study focusses on the wake when the drag is reduced by 17.5% by means of a smaller cylinder located behind the bluff body. Mean pressure and PIV measurements are investigated in the wake. It is shown that the bubble recirculation length and the pressure inside are increased by the presence of the control cylinder. The turbulent wake is also found to have less diffusion compared to that of the natural wake. The global Benard /von Karman instability and the corresponding large-scale coherent structures in the far wake are investigated. The global mode envelope of the instability, measured with a hot wire anemometer, is found to be strongly inhibited by the presence of the control cylinder. The maximum amplitudes of the envelope are shifted further downstream the recirculation bubble end and the Strouhal number is slightly increased by 4.3%. Phase averaging of the vorticity field shows that the circulation of the large-scale structures is not significantly affected. A plausible mechanism for the drag reduction is described.
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