Effect of External Fluctuation Frequency Introduced in the Mean-Flow on the Characteristic Frequency of Separated Flow

The vortex structure in a separated flow region changes greatly owing to the introduced fluctuation in a mean flow. This study investigated experimentally the vortex structure in a separated flow. The periodic external fluctuation of velocity was introduced into the mean-flow using the disturbance generator set at the upstream of the separated region. In the experiment, two kinds of frequencies of the external fluctuation were chosen; the frequency measured in the separated shear layer and the frequency of vortex shedding without the external fluctuation. It was found that the separated region is reduced most effectively when the fluctuation frequency is multiples of the frequency of the vortex shedding of the separated region. The flow on the flat plate was found to become extremely periodic when the effective frequency fluctuation is introduced. This is due to the generation of the regular vortex structure on the flat plate.