Active Control of Separated Flows behind a Backward-Facing Step with Synthetic Jet (2nd Report, Study on Details of the Flow Structures and Consideration of Mechanism of Flow Modification from a Viewpoint of Coherent Structure)

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Flow fields with coherent turbulent structures behind a backward-facing step have been investigated with Laser Doppler Velocimetry to clarify a mechanism of flow control with synthetic jets. Synthetic jets with a frequency which is observed in the velocity fluctuation of the backward-facing step flow without the jet have been fed perpendicularly to the main flow from the vicinity of the step edge. Periodic component of velocity fluctuation with the same frequency of the jet is extracted with procedure of phase locked averaging. Vortices shed from the step edge have an organized structure which is synchronized with the jet and these vortices are advected immediately above the separated stream line at a speed of around 45% of the free stream velocity. Amplitude of non-periodic velocity fluctuation are larger than those of periodic fluctuations due to the vortices. The kinetic energy of the non-periodic velocity fluctuations comes from the main flow and the periodic velocity fluctuations are just the beginning of such taking in. The reattachment position closely correlates with total kinetic energy of the periodic and non-periodic velocity fluctuations. This implies the flow reattachment is promoted by the mixing of the fluids in the main stream and the reversed flow region.