The flow structure in the near field of jets and its effect on cavitation inception

Cavitation experiments performed in the near field of a 50mm diameter (D) j et at Re-D = 5 x 10(5), showed inception in the form of inclined 'cylindric al' bubbles at axial distances (x/D) less than 0.55, with indices of 2.5. O n tripping the boundary layer, cavitation inception occurred at x/D approxi mate to 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 on the primary flow, the strength distribution of the 'streamwise' vortices and the velocity profiles within the initial bou ndary layers were obtained. The untripped case showed a direct transition t o three-dimensional flow in the near field (w/D < 0.7) even before rolling up to distinct vortex rings. Strong 'streamwise' vortices with strengths up to 25% of the jet velocity times the characteristic wavelength were seen. Cavitation inception occurred in the core of these vortices. In contrast, i n the tripped jet the vortex sheet rolled up to the familiar Kelvin-Helmhol tz vortex rings with weak secondary vortices. Using the measured nuclei dis tribution, strengths and straining of the 'streamwise' structures, the rate s of cavitation events were estimated. The estimated results match very wel l the measured cavitation rates. Also, the Reynolds stresses in the near fi eld of the jet show similar trends and magnitudes to those of Browand & Lat igo (1979) and Bell & Mehta (1990) for a plane shear layer.