| 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 with 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. |
| S. Gopalan, J. Katz, O.
Knio, "The flow structure
in the near field of jets and its effect on cavitation inception",
Journal
of Fluid Mechanics, 398, 1-43, 1999.
S. Gopalan, J. Katz, O. Knio, “On the
near-field flow structure, turbulence and resulting cavitation in jets”,
First International Symposium on Turbulence and Shear Flow Phenomena, Santa
Barbara, September 1999.
S. Gopalan, J. Katz, O. Knio, "Near-field
flow structure and cavitation inception in jets", 1999 ASME/JSME International
Symposium on Cavitation Inception, San Francisco, July 1999.
S. Gopalan, J. Katz, O. Knio, "Effect
of boundary layer tripping on the onset of cavitation in jets", Third International
Symposium on Cavitation, Grenoble, France, April 1998.
S. Gopalan, J. Katz, O. Knio, "The
near field flow structure and its effect on cavitation inception in jets",
1998 ASME Fluids Engineering Division Summer Meeting, Washington D.C.,
June 1998.
L. Bertuccioli, S. Gopalan, J. Katz,
"Image shifting for PIV using birefringent and ferroelectric liquid crystals",
ASME Fluids Engineering Division, Laser Anemometry 1995, Hilton Head, SC,
August 1995. |
