The influence of nucleation on the spectral content of cloud cavitation about a hydrofoil

The influence of nucleation on the spectral content of cloud cavitation is investigated using unsteady force measurements and simultaneous high-speed imaging of a rectangular planform NACA-0015 section hydrofoil, with and without upstream nuclei injection. Experiments were conducted for cavitation numbers over the range of 0.60 to 0.45, at a chord-based test Reynolds number of 1.4 × 10⁶, and at an incidence of 6°. The introduction of freestream nuclei was found to significantly alter the cavity topology and spectral content of the force response. Three shedding modes were observed in the case without nuclei injection, while two modes were present with the injection of abundant freestream nuclei. In the nuclei deplete case the dominant shedding frequency is associated with upstream shock propagation and was found to decrease with test cavitation number from a Strouhal number of 0.311 to 0.243, while the sub-harmonic and first harmonic were associated with localized tip and root activity. With the introduction of nuclei the dominant of two distinct shedding frequencies was found to be the lower frequency mode at approximately half that of the corresponding nuclei deplete case, with Strouhal number decreasing from 0.159 to 0.148. The higher frequency mode was shown to be due the propagation of an initial shockwave that only partially condenses the cavity prior to the passage of a second shockwave that leads to full span-wise condensation.