Publication Date:
2016-12-22
Description:
The physics and spectral content of cloud cavitation about a sphere are investigated in a variable-pressure water tunnel using dynamic surface pressure measurement and high-speed imaging. Experiments are conducted using a polyvinyl chloride sphere at a Reynolds number of with cavitation numbers, , ranging from inception to supercavitation. Three distinct shedding regimes are identified: a uni-modal regime for 0.9$]]〉 and two bi-modal regimes for unicode[STIX]{x1D70E}〉0.675$]]〉 and unicode[STIX]{x1D70E}〉0.3$]]〉. For small cavity lengths ( 0.9$]]〉), Kelvin-Helmholtz instability and transition to turbulence in the overlying separated boundary layer form the basis for cavity breakup and coherent vortex formation. At greater lengths ( 〈![CDATA[$unicode[STIX]{x1D70E}), larger-scale shedding ensues, driven by coupled re-entrant jet formation and shockwave propagation. Strong adverse pressure gradients about the sphere lead to accumulation and radial growth of re-entrant flow, initiating breakup, from which, in every case, a condensation shockwave propagates upstream causing cavity collapse. When the shedding is most energetic, shockwave propagation upstream may cause large-scale leading edge extinction. The bi-modal response is due to cavity shedding being either axisymmetric or asymmetric. The two bi-modal regimes correspond to ranges where the cavity and re-entrant jet either remain attached or become detached from the sphere. There is a distinct frequency offset at transition between regimes in both shedding modes. Despite the greater cavity lengths at lower values, the second bi-modal regime initially exhibits shorter shedding periods due to increased cavity growth rates. The second regime persists until supercavitation develops for 〈![CDATA[$unicode[STIX]{x1D70E}. © 2016 Cambridge University Press.
Print ISSN:
0022-1120
Electronic ISSN:
1469-7645
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
,
Physics
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