Spectral content of cloud cavitation about a sphere

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 1.5×10⁶ with cavitation numbers, 𝜎, ranging from inception to supercavitation. Three distinct shedding regimes are identified: a uni-modal regime for 𝜎 > 0.9 and two bi-modal regimes for 0.9 > 𝜎 > 0.675 and 0.675 > 𝜎 > 0.3. For small cavity lengths (𝜎 > 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 (𝜎 < 0.9), 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 𝜎 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 𝜎 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 𝜎 < 0.3.