A von Karman vortex street generated in the usual way was subjected to a deceleration, thereby changing the ratio of longitudinal to lateral spacing between the vortices. Distortion of the individual vortices followed which resulted in annihilation of concentrated vortex regions and creation of a stationary wake flow. This wake flow was itself dynamically unstable and developed into a new vortex street of a different frequency from the initial one. The breakdown of the initial vortex street is qualitatively explained by considering the convection of a concentrated vortex region due to the motion imposed by all the other vortices.
Flow Flow past past the the opening opening of of a a cavity cavity can can excite excite acoustic acoustic resonance. resonance. TheThe periodic periodic velocity velocity field field in in the the region region of of the the cavity cavity opening opening has has been been measured measured for for various various flow flow conditions conditions and and the the vorticity vorticity distribution distribution has has been been computed computed from from the the measured measured data data using using numerical numerical differentiation. differentiation. The The results results indicate indicate that that the the shear shear layer layer rolls rolls up up into into discrete discrete vortices, vortices, which which travel travel across across the the cavity cavity opening. opening. Two Two resonances resonances were were found. found. The The first first is is characterized characterized by by a a single single vortex vortex being being present present and and results results in in the the greatest greatest amplitude. amplitude. The The second second is is characterized characterized by by the the presence presence of of two two vortices vortices which which excite excite a a distinct distinct but but lesser lesser amplitude amplitude resonance. resonance. As As the the flow flow velocity velocity changes, changes, the the position position of of these these vortices vortices shifts shifts relative relative to to the the phase phase of of the the acoustic acoustic cycle. cycle. TheThe timing timing of of the the interaction interaction between between the the moving moving vortices vortices and and the the acoustic acoustic particle particle velocity velocity determines, determines, to to a a large large extent, extent, the the intensity intensity of of the the excitation, excitation, and and therefore therefore also also the the oscillation oscillation amplitude. amplitude. TheThe measurements measurements also also indicate indicate that that the the boundary boundary layer layer upstream upstream of of the the leading leading edge edge pulsates pulsates considerably. considerably.
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