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Turbulence and Shear Flow Phenomena -1 First International Symposium

ISBN Print: 1-56700-135-1

ON THE NEAR-FIELD FLOW STRUCTURE, TURBULENCE AND RESULTING CAVITATION IN JETS

Abstract

Cavitation experiments performed in the near field of a 50-mm diameter (D) jet at ReD=5×l05, 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. The cavitation event rates were measured using a piezoelectric pressure transducer. To investigate these substantial differences, the near field of the jet was measured using PIV. Three types of PIV measurements were performed in the near field - (a) 2-D velocity, vorticity and strain in the shear layer at specific phases. 50-60 such realizations were obtained for each of the two cases. Phase averaged velocity, vorticity, strain rate and Reynolds stresses were then calculated, (b) High magnification (~4) images of the separating boundary layer (at x/D ~ 0.007) were obtained to measure the velocity profile (i.e. inlet boundary conditions). Momentum thickness (Θ), displacement thickness (δ*) were then estimated for the two cases, (c) The velocity field in planes parallel to the jet axis but offset from the center at two locations: rcosΦ=0.53D and 0.55D (Φ is the azimuthal angle measured from the horizontal) were also obtained. This plane gives data on the "streamwise" vortices. 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 only seen beyond x/D~0.7. In contrast in the tripped jet the vortex sheet rolled up to familiar Kelvin-Helmholtz vortex rings with weaker "streamwise" vortices. 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.
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