Abstract
Cavitation experiments performed in the near field of a 50 mm diameter (D) jet
at ReD = 5 × 105, 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 on the primary flow, the strength distribution
of the ‘streamwise’ vortices and the velocity profiles within the initial boundary layers
were obtained. The untripped case showed a direct transition to three-dimensional
flow in the near field (x/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, in the tripped jet the vortex sheet rolled up to the familiar
Kelvin–Helmholtz vortex rings with weak secondary vortices. 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 the measured
cavitation rates. Also, the Reynolds stresses in the near field of the jet show similar
trends and magnitudes to those of Browand & Latigo (1979) and Bell & Mehta
(1990) for a plane shear layer.