| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 1995: ¥600,000 (Direct Cost: ¥600,000)
Fiscal Year 1994: ¥1,400,000 (Direct Cost: ¥1,400,000)
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| Research Abstract |
The objective of this study is to investigate the feedback mechanism of aerodynamic instability phenomenon of the bluff body and its control. The rectangular cylinder with various ratio of depth (streamwise dimension) to height (cross-stream dimension) having a fundamental bluff body configuration is adopted. The rectangular cylinder is forced into rotary oscillation at angular displacement up to <plus-minus>10ﾟ and transverse oscillation at a maximum amplitude of 10% of the length of the front face. Measurements are made of the time-averaged and fluctuating fluid forces, vortex-shedding frequency, flow visualized observation and behavior of separated shear layr, impinging leading-edge vortices, trailing edge vortices and wake vortices. The main results can be summarized as follow
(1) In case of the rectangular cylinder with fully separated flow, in which the ratio of depth to height is smaller than 3.0, the Karman vortices is fairly strong in lock-in region, in which the vortex sheddin
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g is perfectly synchronized with the force frequency, and then the vortex-induced oscillation is generated. As a result, the fluid forces is greatly large, because the rolling-up of the separated shear layr is intensified by the feedback process.
(2) In case of the rectangular cylinder with the impinging leading-edge vortices, in which the ratio of depth to height is larger than 3.0, it is recognized that the vortex-induced oscillation is strongly dependent on the impinging leading-edge vortices rather than the wake vortices. In particular, the vortex-induced oscillation is fairly strong because the impinging leading-edge vortices is fairly strong in lock-in region due to the feedback phenomenon.
(3) The suppression of the vortex-induced oscillation is done by passive control of the flow around the rectangular cylinder which is forced into rotary oscillation. The flow control is established by introducing a flat plate upstream of the rectangular cylinder. As a result, the time-averaged and fluctuating fluid forces is reduced markedly below the plain rectangular cylinder, and then the vortex shedding is perfectly suppressed. In particular, the generation of the aerodynamic instability phenomenon is perfectly suppressed because the work done by the fluctuating fluid forces changes to negative work. Less
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