Home Books eBooks Journals References & Proceedings Authors, Editors, Reviewers A-Z Product Index Awards
Turbulence and Shear Flow Phenomena -1 First International Symposium

ISBN:
1-56700-135-1 (Print)

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

Shridhar Gopalan
Department of Mechanical Engineering, The Johns Hopkins University Baltimore, MD 21218, USA

Joseph Katz
Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, Maryland, USA

Omar Knio
Department of Mechanical Engineering, The Johns Hopkins University Baltimore, MD 21218, USA

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.