Abstract
The leading-edge separation bubble of a blunt circular cylinder was excited by sinuous disturbances with single-frequency components at Reynolds numbers of the order of 105. The disturbance was introduced by a woofer installed inside the cylinder through a thin slit along the separation edge. The level of the disturbances was defined by (i) the positive peak value of the sinuous velocity fluctuation at the middle of the slit without the main flow, and (ii) the r. m. s. value of the velocity fluctuation at a position immediately upstream of the separation edge with the main flow. Method (ii) was found to be more appropriate than method (i) although the latter was employed by previous investigators. In both methods the reattachment length was less than that for the unexcited separation bubble, attaining a minimum at a particular excitation frequency. This frequency for (ii) was approximately 5 times the frequency of shedding of large-scale vortices from the reattachment region. The vortex-shedding frequency was interpreted in terms of a feedback mechanism in which a disturbance produced by a large-scale vortex impinging on the surface propagates upstream to be accepted at the separation edge to modify the rolling-up of the separated shear layer.
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