Abstract
Flow-structure interaction of separated shear flow from the sphere and a flat plate was investigated by using dye visualization and the particle image velocimetry technique. Later, a passive control method was applied with 2mm o- ring located on the sphere surface at 55 o from front stagnation point. The experiments were carried out in open water channel for Reynolds number value of Re=5000. Flow characteristics have been examined in terms of the 2-D instantaneous and time-averaged velocity vectors, patterns of vorticity, streamlines, rms of velocity fluctuations and Reynolds stress variations and discussed from the point of flow physics, vortex formation, lengths of large-scale Karman Vortex Streets and Kelvin-Helmholtz vortices depending on the sphere locations over the flat plate. It is demonstrated that the gap flow occurring between the sphere bottom point and the flat plate surface has very high scouring effect until h/d=0.25 and then unsymmetrical flow structure of the wake region keeps up to h/D=1.0 for smooth sphere. For the sphere with o-ring, the wake flow structure becomes symmetrical at smaller gap ratios and reattachment point on the flat plate surface occurs earlier. Moreover, o-ring on the sphere diminishes peak magnitudes of the flow characteristics and thus it is expected that the flow-induced forces will be lessened both on the sphere and flat plate surface. Vortex formation lengths and maximum value occurring points become closer locations to the rear surface of the sphere with o-ring.
Highlights
Flow around sphere has many engineering applications in single and two phase flows such as air pollution, nuclear and thermal power plants, towed sonars, swimming bodies in the water, pneumatic and hydraulic conveying, chemical and food processing, conveying of sediments in the river, rain drops, combustion systems and sport balls
1) G/D ratio has a strong influence on the flow structure of the wake-boundary layer interaction for the smooth sphere and the sphere with 2mm o-ring
2) The non-uniform velocity profile of the boundary layer flow causes a difference in the strength of the separated shear layers from the periphery of the sphere
Summary
Flow around sphere has many engineering applications in single and two phase flows such as air pollution, nuclear and thermal power plants, towed sonars, swimming bodies in the water, pneumatic and hydraulic conveying, chemical and food processing, conveying of sediments in the river, rain drops, combustion systems and sport balls. Their results could enhance understanding of the vortical flow structure and shear-layer instability inside the near-wake of a sphere [3] It is well-known that in uncontrolled flows around a sphere, the drag coefficients rapidly decrease down to approximately 0.07 and this phenomenon is called the drag crisis occurred around Re!2x105 [4] (Achenbach, 1972). EFM11 rapidly grow along the separated shear layer, and high momentum fluids in the free stream are entrained toward the bluff-body surface This causes the reattachment of the flow ( forming a separation bubble above the surface) and generates strong nearwall momentum, resulting in the delay of main separation. A passive control method is applied with 2 mm o-ring located on the front surface of the sphere at 55o degree for the same positions
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