Active Control of a Circular Cylinder Flow at Transitional Reynolds Numbers |
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Authors: | A Naim D Greenblatt A Seifert I Wygnanski |
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Institution: | (1) School of Mechanical Engineering, Faculty of Engineering, Tel-Aviv University, Tel-Aviv, 69978, Israel;(2) Institute of Fluid Dynamics and Engineering Acoustics, Berlin University of Technology, Berlin, Germany |
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Abstract: | Active and passive control of flow around a circular cylinder, at transitional Reynolds numbers was investigated experimentally
by measuring cylinder surface pressures and wake velocity profiles. Two- and three-dimensional passive boundary layer tripping
was considered and periodic active control using piezo-fluidic actuators was introduced from a two-dimensional slot that was
nearly tangential to the cylinder surface. The slot location was varied circumferentially by rotating the cylinder and this
facilitated either upstream- or downstream-directed actuation using sinusoidal or modulated wave-forms. Separation was controlled
by two distinct methods, namely: by forcing laminar-turbulent transition when applied at relatively small angles (30–60°)
from the forward stagnation point; and by directly forcing the separated shear-layer at larger angles. In the latter case,
actuation produced the largest load changes when it was introduced at approximately 90° from the forward stagnation point.
When the forcing frequency was close to the natural vortex-shedding frequency, the two frequencies “locked-in” creating clear
and persistent structures. These were examined and categorized. The “lock-in” effect lowered the base pressure and increased
the form-drag whereas delaying separation from the cylinder did the opposite. |
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Keywords: | Flow control Cylinder Boundary layer Transition Separation Shedding |
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