A STUDY OF THE MECHANISM OF VORTEX BREAKDOWN

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The features of the vortex breakdown are that there are three types of breakdown, and it has a stagnation point on the vortex axis and the prominent frequencies of oscillating velocities. We presented an analytical model to explain these all features consistently under the idea that the small velocity disturbances of any wave length lying within a certain region would be amplified and unsteady velocity would produce acoustic streaming. We analyzed the stability and the velocity fields, and compared the calculated streak lines with the observed flow pattern for the three types of vortex breakdown.The three types of vortex breakdown were observed in water flows through a straight pipe by a visualization technique : bubble, spiral and double-helix types. Laser-Doppler velocimeter measurements upstream and in the wakes of breakdown of bubble type are described. Spectra of the fluctuating velocities, as well as timeaveraged swirl and axial velocity profile both inside and outside the recirc … More ulation zone, have been obtained. Spectral analysis of the data indicates that the prominent frequencies of oscillating velocities are constant in the steady recirculation.We made a spinning wave generator(SWG), which is a device to produce, at a specified frequency, a sinusoidal wave disturbance spinning in the circumferential direction, in order to confirm the idea that the azimuthal wave number of the disturbance would determine the type of breakdown. SWG is composed of four pistons, which operate in the cylinders attached to the test pipe wall, and creates four kinds of spinning sinusoidal wave disturbances : four azimuthal wave numbers m = 0, +1, -1, 2. The four kinds of sinusoidal waves with azimuthal wave number were imposed to the swirl flow and the transition process of the breakdown type and location was investigated. It was shown that the m = 0, +1, and 2 sinusoidal wave disturbances produce the bubble, spiral type breakdown, and two-tailed bubble or flattened bubble respectively. Less