Near-field flow structure and cavitation inception in jets
Cavitation experiments performed in the near field of a 50-mm diameter (D) jet at Re(D)=5x105, showed inception in the form of inclined 'cylindrical' bubbles at axial distances (x/D) less than 0.55, with indices of 2.5. On tripping the boundary layer, cavitation inception occurred at x/D ~ 2, as distorted 'spherical' bubbles with inception indices of 1.7. To investigate these substantial differences, the near field of the jet was measured using PIV. Data was obtained to measure the primary flow, the strength and distributions of the 'streamwise' vortices and the velocity profiles within the initial boundary layers. The untripped case showed a direct transition to three-dimensional flow dominated by strong 'streamwise' vortices with strengths up to 25% of the jet velocity times the characteristic wavelength. Cavitation inception occurred in these vortices. Prominent vortex rings were seen beyond x/D = 0.7. In contrast, in the tripped jet the vortex sheet rolled up to the familiar Kelvin-Helmholtz vortex rings with weaker secondary vortices. Then, using the measured nuclei distribution, strengths and straining of the 'streamwise' structures the rates of cavitation events were estimated. The estimated results match very well with the measured cavitation rates.
Gopalan, S; Katz, J; Knio, O
Proceedings of the 1999 3rd Asme/Jsme Joint Fluids Engineering Conference, Fedsm'99, San Francisco, California, Usa, 18 23 July 1999 (Cd Rom)
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